International Journal of Medicinal Chemistry

Volume 2013 (2013), Article ID 436397, 20 pages

http://dx.doi.org/10.1155/2013/436397

Research Article

Synthesis Characterization and Antibacterial, Antifungal Activity of N-(Benzyl Carbamoyl or Carbamothioyl)-2-hydroxy Substituted Benzamide and 2-Benzyl Amino-Substituted Benzoxazines

Tyson Belz, Saleh Ihmaid, Jasim Al-Rawi, and Steve Petrovski

School of Pharmacy and Applied Science, La Trobe University, P.O. Box 199, Bendigo, UIC 3552, Australia

Received 17 June 2013; Revised 8 August 2013; Accepted 11 August 2013

Academic Editor: Armando Rossello

Copyright © 2013 Tyson Belz et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

New N-(benzyl carbamothioyl)-2-hydroxy substituted benzamides 13, 20, and 21 were synthesized using sodium bicarbonate and benzyl amine with 2-thioxo-substituted-1,3-benzoxazines 6, 10a, b, 11c, and 12a–n. The 2-thioxo-substituted-1,3-oxazines 6, 10a-b, 11d 12a–n, and 26 were converted to the corresponding 2-methylthio-substituted-1,3-oxazines 14a–l and 24 which were then converted to 2-benzyl amino-substituted-benzoxazines 15a–i by refluxing with benzylamine. Products 15a, b, e, f, and g were also synthesized by boiling the corresponding N-(benzyl carbamothioyl)-2-hydroxy substituted benzamides 13a, b, f, l, and m in acetic acid. 2-Oxo-substituted-1,3-benzoxazines 22 and 25 were prepared by treating the corresponding 2-methylthio-substituted-1,3-oxazines 14 and 24 with dilute HCl. The N-(benzyl carbamoyl)-2-hydroxy substituted benzamide 23 was synthesized from the reaction of 2-oxo-substituted-1,3-benzoxazine 22 with benzylamine. The new products were characterized using IR, ¹H, and ¹³C NMR in addition to microanalysis. Selected compounds were tested in vitro for antibacterial and antifungi activity and the most active compounds were found to be the 4-(substituted-benzylamino)-2-hydroxy benzoic acids 9a and d (M. chlorophenolicum, MIC 50 and 25 µgm L⁻¹, resp.), N1, N3-bis (benzyl carbamothioyl)-4,6-dihydroxy-substituted phthalamides 20a and 20c (B. subtilis MIC 12.5, 50 µgm L⁻¹, resp.) and 21 (M. chlorophenolicum, MIC 50 µgm L⁻¹).

1. Introduction

The search for new antibacterial compounds is a challenging task as bacteria are continuously developing resistance to antimicrobial compounds; however, infections due to such bacterial strains are infrequent although potentially fatal [1–3]. This ongoing problem has resulted in the search for newer, more effective antibacterial compounds [1–3].

Urea, thiourea 3 (X=O or S), and benzo-1,3-oxazine compounds 5 and 6 (Scheme 1) have been shown to possess antibacterial and antifungal properties [4–11]. The benzyl thiourea analogue 3 has been reported to show activity against Gram-positive bacteria [12].

Scheme 1: Previous synthesis of urea or thiourea 3 (X=O or S) and 2-amino benzo1,3-oxazine 5.

The N-benzoyl-2-hydroxybenzamides [13] are important pharmacophores for antibacterial activity in which the 2-hydroxy group (hydrogen bonding donor) contributes to the activity, the imide linker (preferred) or urea linker retains activity and free NH is required for high activity.

The Topliss method [14] was used in the optimization of salicylic acid derivatives for potential use as antibacterial agents. The employment and analysis of physicochemical parameters and molecular electronic surfaces which highlight the electronic, lipophilic, and steric features may be useful guidelines in the continuous search for new, more effective 3-amino-salicylic acid analogs. The synthesis of the urea or thiourea product (3, Scheme 1) was previously achieved from the reaction of benzoylisocyanate or benzoylisothiocyanate 1 (X=O or S, resp.) with amines 2 [15–17].

The substitution R on the aromatic ring could be alkyl, alkoxy, –OC=OCH₃ in positions 2, 3, 4, 5, and 6, and the R₁-N-R₂in product 3 could be aliphatic, aromatic, or cyclic amine substitutions. Limitation associated with this method is that R, the substitution on the aromatic ring, cannot be –OH or RN–, which is desirable for antimicrobial activity, particularly the hydroxy group [18].

Furthermore, the synthesis of 2-amino-substituted-1,3- benzoxazine 5 was achieved through the reaction of the corresponding amine 2 with ethyl 2-cyanobenzoate 4 [19–21]. However, again the limitation in this method is that the R in product 5 cannot be –OH or –NH₂.

The reaction of substituted 2-thio-1,3-benzoxazine-4-one 6 (Scheme 1) with primary and secondary amines showed considerable interest in the literature.

The secondary amine 2 (dimethyl, diethyl, and cyclic amines) reaction with 2-thioxo-2,3-substituted 1,3-benzoxazin-4-one 6 gave only 2-amino-substituted-1,3-benzoxazine 5 [19, 20, 22, 23]. Reactions with primary amines lead to the opening of the oxazine ring and produce thiourea type analogues (compound 3 R = 2-OH and R₁NR₂ = –HN-alkyl) while with NH₂ CH₂Ph gave 3 R = 2-OH and R₁NR₂ = NHCH₂Ph mixed with 2-(benzyl amino)-4H-benz[e]-1,3-oxazin-4-one (5 R = H and R₁NR₂ = –NHCH₂Ph [19, 20]. The above reaction produced low yields or a mixture of products 13a and 15a (Scheme 2) which was difficult to separate.

Scheme 2: Synthesis of 7-N-substituted-1,3-oxazines 10-11 starting from 2-hydroxy-substituted benzoic acids 8 and 9. Reaction conditions: (i) compound a (Ac)₂O, b PhCH=O, c 2-hydroxy-C₆H₄-CH=O, and d 3-ethoxy-2-hydroxy-C₆H₃-CH=O, (ii) Ph₃P(SCN)₂ in CH₂Cl₂, (iii) NaBH₄, and (iv) Ph₃P(SCN)₂ in CH₂Cl₂.

In this work, we developed simple general procedures for the synthesize of N-(benzyl carbamothioyl)-2-hydroxybenzamides 13, 20, and 21, N-(benzyl carbamoyl)-2-hydroxy-substituted-benzamides 23, and 2-(benzyl amino)-substituted-1,3-benzoxazin-4-one 15 and 26. The antibacterial and antifungal activity was evaluated for a number of these products with the intension of producing novel products that can be used to eliminate problematic bacteria in the environmental and medical settings. These compounds could potentially result in novel antibiotic.

2. Results and Discussion

2.1. Chemistry

2.1.1. Synthesis of Substituted 2-Hydroxy Aromatic Carboxylic Acids

5-Substituted-4,6-dihydroxybenzene-1,3-dicarboxylic acids 16a–c, and 2,3-dihydroxybenzene-1,4-dicarboxylic acid 17 were prepared by carboxylation of 2-substituted-1,3-hydroxy-benzene and 1,2-dihydroxy-benzene, respectively, according to the previously reported method [24]. Compound 7 was selectively acetylated in the presence of NaOH and acetic anhydride with the reaction maintained at pH 6-7 in accordance with the reported procedure [25–27] to give the acid 8a an excellent yield (82%) (Scheme 2).

Allowing compound 7 to react with substituted benzaldehyde gave the corresponding Schiff base 8b-c (Scheme 2) [28–33]. Reduction of Schiff base 8b-c with sodium borohydride gave 4-(substituted-benzylamino)-2-hydroxybenzoic acids 9a–c with high yield (73–85%). The structures of the prepared acids were confirmed using mp, IR, ¹H, and ¹³CNMR which was in good agreement with the previously reported physical and spectroscopy data [26–28].

2.1.2. Synthesis of Substituted 2-Thio-substituted-benzoxazines

The 2-thio-substituted-1,3-benzoxazines 6, 10, 11, 12 (a R = 8-CH₃, b R = 8-Ph, c R = 6-Br, d R = 6-OCH₂CH₃, e R = 7-OCH₂CH₃, f R = 8-OCH₂CH₃, g R = 6-OCH₃, h R = 7-OCH₃, i R = 8-OCH₃, j R = 6,8-I, k R = 7-OH, l R = 7-OH,-8-CH₃, and m R = 8-OH), 18, and 19 were synthesized from the reaction of the substituted 2-hydroxy benzoic acid with freshly prepared Ph₃P(SCN)₂ (Schemes 2 and 3) following the reported conditions [22, 34] with some modifications (see Experimental). It is worth noting that the reaction of 4-benzylideneamino-2-hydroxybenzoic acid 8d formed the corresponding 7-(benzylidene)amino-2-thio-1,3-benzoxazines which were hydrolysed during the isolation of the product and gave 7-amino-2-thio-1,3-benzoxazines 10b. However, the reaction of 4-amino-2-hydroxybenzoic acid 7 with the freshly prepared Ph₃P(SCN)₂ failed to produce the expected product 10b however gave a complex, unidentifiable molecule containing a triphenylphosphene group attached to the oxazine product.

Scheme 3: Synthesis of N-(benzyl carbamothioyl)-2-hydroxy-substituted benzamide 13, substituted 2-benzylamino-1,3-benzoxazines 15.

7-(Substituted benzylamino)-2-thioxo-2,3-dihydro-4H-1,3-benzoxazin-4-one 11a–c were synthesized from the reaction of 4-(substituted-benzylamino)-2-hydroxybenzoic acids 9a–c with the freshly prepared Ph₃P(SCN)₂ [22, 32] (Scheme 2). The structures of the new products 11a–c were confirmed using ¹H, and ¹³CNMR and microanalysis.

Similarly, the substituted-2-thioxo-1,3-benzoxazin-4-one 6 and 12b–n prepared using a previously reported [22, 23, 33] method gave products with identical physical and spectroscopic data.

The benz-bis-(1,3-oxazine) 18a–c and 19 were prepared from the reaction of dihydroxy-dicarboxybenzoic acids 16a–c and 17 with the freshly prepared Ph₃P(SCN)₂ [22, 32] according to the previously reported method [34] for the synthesis of 18c with some modification to improve the yield.

The structures of the new dibenzoxines products 18a.b and 19 were confirmed using ¹H and ¹³CNMR and microanalysis (Scheme 4).

Scheme 4: Synthesis of substituted-N,N-bis(benzyl carbamothioyl)-dihydroxy-iso and tetra phthalamides 20a, c, and 21. Reaction conditions: (i) Ph₃P(SCN)₂ in CH₂Cl₂ and (ii) NaHCO₃, PhCH₂NH₂.

2.1.3. Synthesis of the 2-Methylthio-substituted-1,3-oxazines

2-Methylthio-substituted-1,3-oxazines 14a–i (a R = H, b R = 8-CH₃, c R = 8-Ph, d R = 7-OCH₃, e R = 7-OCH₂CH₃, f R = 7-OH, g R = 7-OH,-8-CH₃, h R = AcNH, and I R = NH₂) and 24 were prepared by the reaction of CH₃I in the presence of NaHCO₃ with substituted-1,3-oxazines 6, 10a,b, 11c, and 12 (a R = 8-CH₃, b R = 8-Ph, e R = 7-OCH₂CH₃, h R = 7-OCH₃, k R = 7-OH, and l R = 7-OH,-8-CH₃) according to the previously reported procedure [35]. Physical properties, IR, ¹H NMR, and ¹³C NMR, collected for products 14a–I were found to be identical to the reported data [35]. The structures of new product, 14 h, i, and 24 were confirmed with analysis of the IR, ¹H, and ¹³C NMR data.

Compounds 14a–i and 24 were used in the synthesis of compounds 15a–I and 26 with no further purification.

2.1.4. Synthesis of Benzyl Thiourea 13a–q, 20a, c, and 21

Initially, the equimolar reaction of compound 6 with benzylamine in dioxane was repeated according to the earlier reported method [19, 20], in which the mixture that was refluxed for 4 hours gave 13a and was a 46% yield (Scheme 3).

However, when the reaction was carried out using solvent free conditions, excess benzylamine was added directly to powered 2-thio-1,3-benzoxazine 6 and the mixture was left at room temperature for 2 days, the benzyl thiourea product 13a had decreased yield (19%), and none of the cyclic analogue 15a could be isolated. When 2-thio-1,3-benzoxazine 6 was allowed to react with excess benzylamine (4-fold) and the mixture was then heated to reflux in dioxane for 2 hours, the open product 13a was again isolated in 18% yield.

To overcome this low yield and possible mixture formation, the reaction procedure was modified in which the benzoxazines 6, 10a-b, 11d, and 12b–m were mixed with NaHCO₃ and suspended in 1 : 1 mixture of methanol, water and the mixture was then heated to 40°C for a few minutes. Excess (1.5-fold) benzyl amine was added dropwise at room temperature and left stirring for 4 hours. Products 13a–q was isolated according to the general procedure B (see Experimental) and the yield were moderate to high (69–87%) (Scheme 3).

With slight modification to the procedure B, by altering the ratio of benzylamine to starting material (3 : 1) and the reaction time to 16 hours, the bis-oxazines 18a,c and 19 were found to react in a similar fashion to give substituted bis(benzyl carbamothioyl) analogues 20a,c (yield 63, 49%) and 21 (yield 63%) (Scheme 4).

The previously prepared N-(benzyl carbamothioyl)-2-hydroxybenzamide 13a was characterized by comparison of its physical data (mp, IR,¹H and ¹³C NMR spectra) with values found in the literature [12, 19, 20]. The structures of new benzyl thiourea compounds 13b–q, 20a,c, and 21 were confirmed using IR, ¹H NMR and ¹³C NMR spectroscopy and microanalysis. The ¹H NMR and IR spectra also showed a high correlation with the previously prepared benzyl thiourea 13a [19, 20]. In the ¹H NMR spectra, the CH₂, H-5′ of the benzyl amine in compounds 13a–q, 20a,c, and 21 appeared as a doublet at ~δ 4.9 ppm and the 4′-NH, appeared as a triplet at ~δ 11.0 ppm in all cases. Assignment of the carbon-13 chemical shifts was made using the previous reported chemical shifts of 13a [19, 20]. The ¹H and ¹³C NMR spectra of the parent 2-thioxo-2H-benz[e]-1,3-oxazin-4(3H)-one 12 were also used to aid with structural identification. The simulated ¹H and ¹³C NMR spectra using ChemDraw V12 ultra were also used as references to aid the analysis of the observed ¹H and ¹³C NMR spectra of the new products.

2.1.5. Synthesis of 2-Benzyl amino-(substituted)-benz-1,3-oxazines 15

As mentioned earlier the reaction of the 2-thio-1,3-benzoxazine 6 with a primary amine did not consistently give the cyclic product 13a [19, 20].

The reaction of 2-methylthio-(substituted)-1,3-benzoxazines 14 (a R = H, b R = 8-CH₃, c R = 8-Ph, d R = 7-OCH₃, e R = 7-OCH₂CH₃, f R = 7-OH, g R = 7-OH,-8-CH₃, and h R = AcNH-) and 24 with excess (5-fold) benzyl amine according to the general procedure C gave 2-benzyl amino-substituted-1,3-benzoxazines 15a–h and 26 with moderate to high yields 62–83% (Schemes 3 and 5).

Scheme 5: Synthesis of 2-dione-1,3-benzoxazines 22a–I and 25a,b from the 2-methylthio-1,3-benzoxazines 14a,b, d–g, and 24a-b.

The reaction of benzylamine with 2-methylthio-benzoxazine took place with no trace of the thiourea analogue 13.

Following their successful synthesis, many of the N-(benzyl carbamothioyl)-2-hydroxybenzamides 13a, b, f, and g were then cyclised by refluxing in acetic acid for 2 hours according to the general procedure D (Scheme 3) and gave the corresponding 2-benzylamino-1,3-benzoxazines 15a, b, f, and g fair to good yields.

Previously prepared 2-benzyl amino-1,3-benzoxazine 15a was characterized by comparison of the physical data (mp, IR, and ¹H and ¹³C NMR spectra) with that found in the literature [19, 20]. The structures of new 2-benzyl amino-1,3-benzoxazine compounds 15b–g were confirmed using IR, ¹H NMR, and ¹³C NMR spectroscopy and microanalysis. In the ¹H spectra the CH₂ of the benzyl amine appears as a doublet at ~δ 4.5 ppm. The previously analysed ¹H and ¹³C NMR spectra of the parent 2-methylthio-1,3-benzoxazines 14 [35] were used to aid in the analysis of the new products 15b–g s.

2.1.6. Synthesis of 2-Oxo-substituted-benzoxazines 22a–h and 25

2-Methylthio-substituted-benzoxazines 14a, b, and d–g were allowed to react with 10% HCl according to the general procedure E and gave 2-dione-1,3-benzoxazines 22a–h with good to excellent yield (Scheme 5); however, product 22h was produce if 40% HCl was used in the hydrolysis of 14h. Similarly, the 2,8-dioxo product 25 was prepared from the hydrochloric acid hydrolysis of the corresponding 2,8-dimethylthio-analogue 24 (Scheme 5).

2.1.7. Synthesis of Substituted-N-(benzyl carbamoyl)-2-hydroxybenzamides 23a–g

The synthesis of the substituted-N-(benzyl carbamoyl)-2-hydroxybenzamides 23a–g was achieved using the reaction of the relevant benzoxazine-2, 4-di-one 22a–h with excess benzylamine in dioxane with refluxed according to the general procedure F.

Structure Elucidation of Substituted-N-(benzyl carbamoyl)-2-hydroxybenzamides 23a–g. The structures of the newly prepared substituted urea compounds 23a–g were confirmed using IR and ¹H and ¹³C NMR spectroscopy and microanalysis. The ¹H NMR and IR spectra supported the proposed structures and showed some correlation with the previously prepared benzyl thiourea 13a–q, 20a, c, and 21. In the ¹H spectra, the methylene CH₂ (H-11 of compounds 23a–g) of the benzyl amine appears to shift up field as a doublet at ~δ 4.3 ppm. The NH (H-10 of compounds 23a–g) appears as a broad triplet at ~δ 8.5 ppm. The ¹H and ¹³C NMR spectra of the parent 2-oxo-1,3-benzoxazines 22 and 25.

2.2. Biological Testing

2.2.1. Broth Dilution Susceptibility Testing

In this study, some of the newly prepared compounds were tested and showed antimicrobial activity against 8 different bacterial strains and 4 cultures of fungi. The bacterial species investigated were P. aeruginosa, B. subtilis, S. aureus, A. baumannii, E. coli, S. agalactiae M. smegmatis, and M. chlorophenolicum. The antifungal evaluation was determined against A. niger, A. corymbifera, R. oryzae, and A. alternata. The minimal inhibitory concentrations (MICs) and minimal fungicidal concentrations (MFCs), defined as the lowest concentration of drug that inhibits the growth of bacteria or fungi in the inoculums’, were determined using the broth dilution methods. The compounds which demonstrated MIC and MFC values less than 300 and 200 μg mL⁻¹, respectively, are listed in Tables 1 and 2. According to [36], antimicrobial agents are effective on a range of bacterial species at low concentrations, that is, <128 μg mL⁻¹. Therefore, we conducted our MIC experiments using concentrations as high as 300 μg mL⁻¹. We also selected a range of bacterial and fungal species to test our newly synthesised compounds. Some of the species are potentially pathogenic to humans and animals and others are problematic in an environmental setting. Furthermore, the bacterial species selected have different cell wall compositions, that is, some are Gram-negative and some are Gram-positive strains. Some antimicrobial agents inhibit bacteria by interacting with components of the cell wall that are absent in Gram-negative bacteria [37]; therefore, the selection of strains was carefully selected with the possibility that an inhibitory compound would also hint to its mechanism of action. Based on the results obtained it is clear that the Gram-negative strains, that is, P. aeruginosa, E. coli, and A. baumannii were least affected by the compounds and when inhibition was observed it was at high levels 200 μg mL⁻¹ or higher. Interestingly, compound 9d seemed to have a more dramatic effect Gram-positive strains with the exception of M. smegmatis. Despite the effects that some of the compounds had on the bacterial strains, it appears that these compounds are not so effective when tested on the four fungal cultures chosen with the exception of 9d on A. corymbifera.

Table 1: Broth dilution susceptibility MIC values for inhibition growth of bacteria (μgmL⁻¹).

Bacteria

#
P. aeruginosa
B. subtilis
S. aureus
A. baumannii
E. coli
S. agalactiae
M. smegmatis
M. chloro

9c
200
200
>200
200
200
n/a
>200
50

9d
200
100
25
200
200
n/a
>200
25

13c
>300
>300
>300
>300
>300
>300
n/a
n/a

13d
200
>300
>300
200
>300
300
n/a
n/a

13e
>300
>300
300
>300
>300
>300
n/a
n/a

13g
>300
>300
>300
>300
>300
>300
n/a
n/a

13j
>300
>300
>300
>300
>300
200
n/a
n/a

13l
>300
>300
300
>300
>300
>300
n/a
n/a

13m
200
200
300
>300
>300
>300
n/a
n/a

13n
300
>300
300
300
>300
300
n/a
n/a

20a
>200
12.5
>200
>200
>200
n/a
>200
>200

20c
>200
25
>200
>200
>200
n/a
>200
>200

21
>200
25
200
>200
>200
n/a
100
50

22g
200
>200
>200
200
>200
n/a
>200
200

Table 2: Broth dilution susceptibility MFC values for inhibition growth of fungi (μgmL⁻¹).

Fungi

#
R. oryzae
A. niger
A. corymbifera
A. alternata

9c
200
>200
>200
>200

9d
200
>200
100
>200

22g
>200
>200
>200
>200

20a
>200
>200
>200
>200

20c
>200
>200
>200
>200

21
>200
>200
200
200

Based on the results obtained for each of the newly synthesised compounds, it is evident that compound 9d has more potent effect when compared to the others. This reveals that despite the fact that most of the compounds do not seem to have a noteworthy effect on the strains, compound 9d is of interest and further investigation is required.

2.2.2. Disc Diffusion Susceptibility Testing

Disc diffusion susceptibility testing was performed on compounds with poor solubility in broth dilution susceptibility testing. The preliminary antimicrobial testing was achieved using the standard agar disk diffusion methods. Compounds that inhibited certain bacteria or fungi are summarized in (Tables 3 and 4).

Table 3: Results showing Disc diffusion susceptibility for the synthesised compounds.

Bacteria

#
P. aeruginosa
B. subtilis
S. aureus
A. baumannii
E. coli
S. agalactiae
M. smegmatis
M. chloro

8c
S (2 mm)
R
R
R
R
n/a
S (2 mm)
R

8d
S (2 mm)
S (3 mm)
R
R
R
n/a
S (2 mm)
S (2 mm)

9c
S (2 mm)
S (2 mm)
R
R
R
n/a
S (2 mm)
S (2 mm)

9d
S (2 mm)
S (4 mm)
R
R
R
n/a
S (2 mm)
S (2 mm)

13b
R
S (6 mm)
S (5 mm)
R
R
S (5 mm)
n/a
n/a

13k
R
S (2 mm)
S (5 mm)
S (2 mm)
R
S (5 mm)
n/a
n/a

13f
R
S (2 mm)
S (5 mm)
S (2 mm)
R
S (5 mm)
n/a
n/a

15d
R
S (2 mm)
S (5 mm)
S (2 mm)
R
S (5 mm)
n/a
n/a

15e
R
R
R
R
R
R
n/a
n/a

13o
R
R
R
R
R
—
R
R

13p
R
R
R
R
R
n/a
S (2 mm)
R

13q
R
R
R
R
R
n/a
R
R

20a
R
S (2 mm)
R
R
R
n/a
R
R

20c
R
S (2 mm)
R
R
R
n/a
S (3 mm)
S (4 mm)

21
R
S (4 mm)
R
R
R
n/a
R
R

22h
R
R
R
R
R
n/a
S (3 mm)
S (3 mm)

22i
R
R
R
R
R
n/a
S (2 mm)
R

23g
R
R
R
R
R
n/a
R
R

Table 4: Disc diffusion susceptibility for the synthesised compounds.

Fungi

#
R. oryzae
A. niger
A. corymbifera
A. alternata

8c
R
R
R
R

8d
S (2 mm)
S (3 mm)
S (5 mm)
R

9c
R
R
R
R

9d
R
R
R
R

13p
S (2 mm)
R
R
R

13q
S (2 mm)
S (2 mm)
S (3 mm)
R

21c
S (2 mm)
S (2 mm)
S (3 mm)
R

20a
R
R
R
R

20c
R
R
R
R

22h
R
R
R
R

22i
R
R
R
R

25b
R
R
R
R

The concentrations of the prepared compounds were 10−4 μg mL⁻¹ (see Experimental). The control data is used to determine if the bacterial strains are resistant (R) or sensitive (S) to the prepared compounds tested. The disc diffusion assay was used as a preliminary guide for all compounds and used in correspondence with the broth dilution method for determining MIC/MFC values. This method is particularly useful when MIC/MFC values are unable to be determined using the broth dilution method due to the compounds insolubility. The insoluble compounds zones of inhibition therefore can be determined in millimeters relative to the control and used as a rough guide. Since the zone of inhibition of clearance may be affected by other parameters, such as, the nutrient agar depth of the plate and solvent used, the results shown using this method therefore should be used as a guide. MIC/MFC values are determined using the broth dilution method. All compounds that showed clearance zones are listed in Tables 3 and 4 and were tested in duplicate with the average given. Any zone of inhibition that was noted around the disc was considered sensitive and the zone of clearly was noted. These results are more useful for compounds that were difficult to dissolve, but equally, these results can indicate resistance if the compound does not diffuse through the agar properly.

Based on the results obtained in Section 2.2.1, it is clear that compound 9d is of interest. Based on the results obtained in Table 3, compound 9d has an inhibitor effect on M. smegmatis but in the MIC study had no effect. This could indicate solubility problems with the compound when in solution; however, this is only speculative; further studies are required to reveal the cause. In addition, compound 9d seems to have no inhibitory effect on S. aureus but in the MIC studies had a dramatic affect. This difference in result is unusual but clearly indicates that different methods could reveal different results and therefore it is important to perform both methods prior to further investigation on their inhibitory effects.

In the MIC studies, we used 300 μg mL⁻¹ as the highest cut-off level. If a compound has an inhibitory effect on any strain that is greater than this level, then this should be revealed in the disc diffusion assay. However, further investigation is required as some of these compounds are dissolved in DMSO and when applied to bacterial cultures can come out of solution. The disc diffusion assays seem to indicate some sensitivity to fungal cultures despite the fact that they were undetectable in the MIC studies.

2.3. The Structure Activity Relationships of the Tested Compounds (Broth Dilution)

The results in Tables 1 and 2 show that the 4-(benzylamino)-2-hydroxybenzoic acid derivative 9d showed the broadest range of activity of the compounds tested, exhibiting activity against the Gram-positive and Gram-negative bacteria and also M. chlorophenolicum. Furthermore, compound 9d showed to be more active than others against S. aureus with an MIC value of 25 μg/mL. Compounds 20a,c and 21 (bis-thiourea products) were found to be particularly active towards Gram-positive B. subtilis at MIC values of 12.5, 25, and 25 μg mL⁻¹. In addition, compound 21 also showed activity towards M. smegmatis (MIC 50 μg mL⁻¹). Other synthesized compounds which showed an inhibitory effect were 13n which had an inhibitory effect on four bacterial species at 300 μg mL⁻¹ and 13d and 13m which had an inhibitory effect on two of the bacterial species at concentration 200 μg mL⁻¹. Interestingly, E. coli was not inhibited by any of the compounds.

2.4. The Chemical Compounds Activity and Structural Relationships of the Antimicrobial Assay Results (From Disk Diffusion Assay)

In the presence of a compound, a zone of clearing was greater than the control which was indicative that the strain was sensitive to the compound, whereas a zone of clearing equal to the control indicated resistance. The results reveal that none of the compounds had an inhibitory effect on E. coli at concentration 10⁴ μg mL⁻¹ (Table 3). The B. subtilis bacterial species tested showed inhibitory effects to most of the compounds tested, for example, 13d inhibited S. aureus most strongly and compounds 13k, 13f, and 13d inhibited growth of A. baumannii, B. subtilis, and S. agalactiae (Table 3). Some bacterial species that were sensitive to a compound showed similar sized zones of inhibition. One example was 13k which exhibited activity of 2 mm for A. baumannii and B. subtilis and 5 mm for S. aureus and S. agalactiae. The same applies to compounds 8d, 9c, and 9d which had shown a 2 mm clearance zone against P. aeruginosa M. smegmatis M. chlorophenolicum. Similarly to the broth dilution results, compounds 21 and 9d were found to be active against three fungi species, R. oryzae, A. niger, and A. corymbifera, with clearance zones 2–5 mm, respectively.

All the compounds tested showed a 2–4 mm zone of clearing for most of the susceptible species with the exception of compound 13b which had a larger 6 mm zone of inhibition. This larger zone indicates a hypersensitive effect on the bacterial species; however, it is specific for the compound and species. Because the mechanism of action of the compound is unknown, it is difficult to explain the reason for the hypersensitive effect. One possible explanation is that B. subtilis encodes a protein that can transport compound 13b into the cell and this has a more toxic effect than those working from outside the cell. A similar phenomenon has been shown with bacterial mercury resistance where the presence of a mercury import protein displays a larger zone of clearing in a disc diffusion assay [38].

The data obtained revealed patterns of inhibition, especially those conducted with the disc diffusion assay. This suggests that a similar mechanism of action could be involved in the inhibition of growth.

3. Conclusion

In conclusion, we have prepared seven new compounds of 2-benzylamino-substituted-1,3-benzoxazines, nineteen new N-(benzyl carbamothioyl)-substituted-benzamide and have evaluated some for their activity against bacteria and fungi. It appears that N-(benzyl carbamothioyl)-substituted-benzamide has shown antibacterial activity such as 20a, 20c, 21, 13d, 13m, and 13n.

We are in the process of synthesising new substituted products by replacing the benzyl group of N-(benzyl carbamothioyl) by 6-aminopenicillanic acid and test their bacteria activity.

4. Experimental

4.1. Chemistry

Infrared spectra were obtained using a Perkin Elmer FT-IR 1720x spectrometer. ¹H NMR and ¹³C NMR spectra were obtained using a Bruker AC 200 NMR spectrometer at 200 and 50 MHz, respectively. All ¹H NMR and ¹³C NMR spectral results are recorded as chemical shifts (δ) relative to the internal TMS for proton and 77.0 ppm in CDCl₃ solvent and 39.4 ppm in DMSO-d₆ solvent for ¹³C NMR. Microanalysis was performed by Chemical and Micro analytical Services (CMAS), Australia. Melting point determinations were carried out using a Stuart Scientific (SMP3) melting point apparatus and all melting points are uncorrected.

4.1.1. Starting Materials

The stating reagents benzyl amine, sodium hydrogen carbonate, methyl iodide-amino-2-hydroxybenzoic acid, and dry 1,4-dioxane were purchased from Aldrich Chemical Company and were used as received.

4.1.2. Synthesis of 4-(Acetyl amino)-2-hydroxybenzoic Acid 8a

According to the previously reported method [29, 30, 37], product 8a was prepared from the reaction of 4-amino-2-hydroxybenzoic acid 7 and acetic anhydride and recrystallised from 1,4-dioxane, 82% yield, mp 221–224°C (lit [37] and mp 235°C). The physical and spectroscopic data is consistent with the literature values [27, 35].

4.1.3. Synthesis of 4-Substituted ((Benzylidene)amino)-2-hydroxybenzoic Acid Intermediates 8b–e

According to the previously reported method [29, 30], intermediates 8b–e were prepared from the appropriate substituted benzaldehyde and 4-amino-2-hydroxy-benzoic acid 7.

Products 8b, c, and e were not identified and used immediately in the synthesis of compound 9a, b, and d.

(Z)-4-((3-Ethoxy-2-hydroxybenzylidene)amino)-2-hydroxybenzoic Acid 8d. 3-Ethoxy-2-hydroxybenzaldehyde (1.66 g, 0.01 mol) was allowed to react with 4-amino-2-hydroxybenzoic acid 7 (1.53 g, 0.01 mol) for 1 hour according to the reported procedure [29, 30] and gave solid which recrystallised from methanol to give 8d 2.95 g, 98% as red crystals, mp 185–188°C decomp. (KBr)/cm⁻¹ 1655 (C=O), 1622, 1600 (C=N): ¹HNMR (200 MHz, 300 K, d₆-acetone) δ 8.93 (s, 1H, H-8), 8.02 (d, 1H, J H6,H5 = 8.8 Hz, H-6), 7.33 (dd, 1H, J H15,H14 = 7.8 Hz, J H15,H13 = 1.6 Hz, H-15), 7.24 (dd, 1H, J H13,H14 = 8.0 Hz, J H13,H15 = 1.6 Hz, H-13), 6.91–7.09 (m, 3H, H-3,H-5, and H-14), 4.24 (q, 2H, JH16,H17 = 7.0 Hz, H-16), 1.41 (t, 3H, JH17,H16 = 7.0 Hz, H-17). Product 8d was used immediately in the synthesis of 9d.

4.1.4. Synthesis of 4-Substituted-(benzylamino)-2-hydroxybenzoic Acids 9a–d

General Procedure A. In slight modification to a previous reported method, [28] the appropriate 4-substituted ((benzylidene) amino)-hydroxybenzoic acids 8b-c reduced using sodium borohydride (2 equiv).

4-(Benzylamino)-2-hydroxybenzoic Acid 9a. 2-Hydroxy-4- benzoic acid 8b (2.41 g, 10 mmol) was allowed to react with sodium borohydride (0.76 g, 20 mmol) according to general procedure A. The resulting solid was recrystallised from methanol/water to give 9a (1.76 g, 73%), mp 122–125°C. (KBr)/cm⁻¹ 3500–3200 br (OH), 3024, 2569 (NH), 1632(C=O); ¹HNMR (200 MHz, 340 K d₆-DMSO) δ 11.34 (bs, 1H, OH of COOH exchangeable with D₂O), 7.5 (d, 1H, J H6,H5 = 8.6 Hz, H-6), 7.21–7.47 (m, 6H, 5 x CH, 8-NH exchangeable with D₂O), 6.22 (dd, 1H, J H5,H6 = 8.6 Hz, J H5,H8 = 2.0 Hz, H-5), 6.03 (d, 1H, J H3,H5 = 2.0 Hz, H-3), 4.38 (s, 2H, H-9). 3.32 (2-OH under the water envelope); ¹³C NMR (50 MHz, 340 K, d₆-DMSO) δ 171.7 (C-7), 163.2 (C-2), 154.8 (C-4), 139.1 (C-10), 130.1 (C-6), 128.1, 126.9, 126.6 (C-12, C-11 and C-13), 105.3 (C-5), 100.4 (C-1), 96.8 (C-3), 45.8 (C-9). The resulting product 9a was not stable and was used immediately in the genral procedure B.

2-Hydroxy-4-((2-hydroxybenzyl)amino)benzoic Acid 9b. (E)-2-Hydroxy-4-((2-hydroxybenzylidene)amino)benzoic acid 8c (2.6 g, 10 mmol) was allowed to react with sodium borohydride (0.76 g, 20 mmol) according to general procedure A. The resulting solid was recrystallised from methanol/water to give 9b as white crystals (2.20 g, 85%), mp 184–186°C decomp. (KBr)/cm⁻¹ 3500–3200 br (OH), 1614 (C=O); ¹HNMR (200 MHz, 300 K d₆-DMSO) δ 11.45 (bs, 1H, 8-NH or 11-OH exchangeable with D₂O), 9.61 (bs, 1H, 11-OH or 8-NH exchangeable with D₂O) 7.44 (d, 1H, J H6,H5 = 8.8 Hz, H-6), 7.1–6.7 (m, 6H, H-13, H-15, H-14, H-12 and 11-OH), 6.28 (dd, 1H, J H5,H6 = 8.8 Hz, J H5,H3 = 1.1 Hz, H-5), 5.92 (d, 1H, J H3,H5 = 1.1 Hz, H-3); ¹³C NMR (50 MHz, 350 K, d₆-DMSO) δ 171.9 (C-7), 163.5 (C-2), 155.2/155.0 (C-11/C-4), 131.2 (C-6), 128.5 (C-13), 127.9 (C-14), 125.0 (C-10), 119.1 (C-15), 115.3 (C-12), 105.6 (C-5), 100.6 (C-1), 97.0 (C-3), 41.1 (C-8). The resulting product 9b was used immediately in the general procedure B.

4-((3-Ethoxy-2-hydroxybenzyl)amino)-2-hydroxybenzoic Acid 9c. (E)-4-((3-Ethoxy-2-hydroxybenzylidene)amino)-2-hydroxybenzoic acid 8d (10 mmol, 3.0 g) was allowed to react with sodium borohydride (20 mmol, 0.76 g) according to general procedure A. The resulting solid was collected and recrystallised from methanol/water to give 9d (2.66 g, 81%) as white crystals, mp 159–161°C decomp. (KBr)/cm⁻¹ 3500–3200 br (OH, NH absorption under the OH envelope), 1625 (C=O); ¹HNMR (200 MHz, 300 K d₆-DMSO) δ 7.38 (d, 1H, J H6,H5 = 8.6 Hz, H-6), 6.85–6.65 (m, 3H, H-13, H-14 and H-15), 6.35 (bs, 4H, 3 x OH and NH), 6.17 (dd, 1H, J H5,H6 = 8.6 Hz, J H5,H3 = 1.8 Hz, H-5), 5.87 (d, 1H, J H3,H5 = 1.8 Hz, H-3), 4.23 (s, 2H, CH₂NH, H-9), 4.05 (q, 2H, J H16,H17 = 6.8 Hz, H-16), 1.35 (t, 3H, J H17,H16 = 6.8 Hz, H-17); ¹³C NMR (50 MHz, 300 K, d₆-DMSO) δ 172.2 (C-7), 163.6 (C-2), 155.3 (C-4), 146.6 (C-12), 144.1 (C-11), 131.4 (C-6), 125.7 (C-10), 120.3 (C-14), 119.2 (C-15), 111.8 (C-13), 105.8 (C-1), 100.2 (C-5), 96.7 (C-3), 64.4 (C-16), 40.8 (C-9), 15.0 (C-17); Anal. Calcd. For C₁₆H₁₇NO₅: C, 63.36; H, 4.62; N, 5.65. Found: C, 63.51; H, 4.48; N, 5.66.

4.1.5. Synthesis of Substituted-dihydroxy-di-carboxylic Acids 16 and 17

According to the previously reported general procedures [23–26], the appropriate substituted phenol was used in the synthesis substituted-dihydroxy-di-carboxylic acids 16 and 17.

4.1.6. Synthesis of 7-N-Substituted-amino-1,3-oxazines 10a, b, and 11a, b, and d

General Procedure B. The substituted-2-hydroxy benzoic acid was allowed to react with the freshly prepared Ph₃P(SCN)₂ according to previously reported general procedure [22, 34].

N-(4-Oxo-2-thioxo-3,4-dihydro-2H-benz[e][1,3]oxazin-7-yl)acetamide 10a. Slightly modified to the previously reported general procedure B [22, 34], 4-(acetyl amino)-2-hydroxybenzoic acid 8a (1.56 g, 8 mmol) was allowed to react with freshly prepared Ph₃P(SCN)₂ (10 mmol) at room temperature for 2 hours then under reflux for 16 hours. At the completion of the reaction, the PbBr₂ filter cake was washed by acetic acid (150 mL) to extract the desired product. The acetic acid filtrate was evaporated and minimal toluene was added to dissolve any oil with the product. The crude solid was filtered and recrystallised from ethanol to give 10a (1.22 g, 65%) as light red crystals, mp 285–287°C decomp. (KBr)/cm⁻¹ 3290, 3183 (9-NH), 3072, 2923 (3-NH), 1704 (C=O), 1188 (C=S); ¹HNMR (200 MHz, 390 K, d₆-DMSO) δ 13.38 (bs, 1H, 9-NH), 10.56 (s, 1H, 3-NH), 7.86 (d, 1H, J H5,H6 = 8.6 Hz, H-5), 7.77 (d, 1H, J H8,H6 = 1.8 Hz, H-8), 7.44 (dd, 1H, J H6,H5= 8.6 Hz, J H6,H8 = 1.8 Hz, H-6), 2.11 (s, 3H, 11-CH₃); ¹³C NMR (50 MHz, 330 K, d₆-DMSO) δ 181.9 (C-2), 169.2 (C-10), 156.7 (C-8a), 146.0 (C-7), 127.3 (C-5), 116.3 (C-6), 109.6 (C-4a), 104.2 (C-8), 24.0 (C-11) 155.9 (C-4); Anal. Calcd. For C₁₀H₈N₂O₃S: C, 50.84; H, 3.41; N, 11.86 C. Found: C, 50.69; H, 3.53; N, 11.86.

Synthesis of 7-Amino-2-thioxo-2H-benz[e][1,3]oxazin-4(3H)-one 10b. A suspension of (E)-4-((3-ethoxy-2-hydroxybenzylidene)amino)-2-hydroxybenzoic acid 8d (1.2 g, 4 mmol) in dry DCM (20 mL) was added to a mixture of freshly prepared Ph₃P(SCN)₂ (10 mmol) according to the general procedure B. The resulting solid was isolated upon evaporation of the DCM filtrate 10b (0.74 g, 91% crude yield). The solid was recrystallised from methanol, mp 250–253°C decomp. (KBr) cm⁻¹ 3443, 3328, (NH₂), 3059, 2916 (NH), 1749 (C=O), 1679, 1616 (C=C), 1205 (C=S); ¹HNMR (200 MHz, 370 K, d₆-DMSO) δ 13.04 (s, 1H, 3-NH), 7.58 (d, 1H, J H5,H6 = 8.6 Hz, H-5), 6.65 (s, 2H, 7-NH₂), 6.60 (dd, 1H, J H6,H5 = 8.6 Hz, J H6,H8 =1.8 Hz, H-6), 6.37 (d, 1H, J H8,H6 = 1.8 Hz); ¹³C NMR (50 MHz, 330 K, d₆-DMSO) δ 182.4 (C-2), 157.3, 156.9, 156.5 (C-4, C-8a, C-7), 128.1 (C-5), 112.7 (C-6), 102.3 (C-4a), 96.8 (C-8); Anal. Calcd. For C₈H₆N₂O₂S: C, 49.47; H, 3.11; N, 14.42. Found: C, 49.51; H, 3.20; N, 14.36.

7-(Benzylamino)-2-thioxo-2H-benz[e][1,3]oxazin-4(3H)-one 11a. In slight modification to previously reported general procedure B, 4-(benzylamino)-2-hydroxybenzoic acid 9a (0.94 g, 4 mmol) was allowed to react with the freshly prepared Ph₃P(SCN)₂ (10 mmol) at room temperature for 2 hours then under reflux for 16 hours. The resulting crude solids (0.94 g, 82%) were filtered, collected, and recrystallised from toluene to give 11a (0.88 g, 77%) as yellow crystals, mp 210–212°C decomp. (KBr)/cm⁻¹ 3301 (9-NH) 3068, 2926 (3-NH), 1689 (C=O), 1197 (C=S); ¹HNMR (200 MHz, 340 K, d₆-DMSO) δ 12.90 (bs, 1H, 3-NH exchangeable with D₂O), 7.61 (d, 1H, J H5,H6 = 8.8 Hz, H-5), 7.37–7.26 (m, 6H, Ar and 9-NH exchangeable with D₂O), 6.73 (dd, 1H, J H6,H5 = 8.8 Hz, J H6,H8 = 2.0 Hz, H-6), 6.42 (d, 1H, J H8,H6 = 2.0 Hz, H-8), 4.41 (d, 2H, J H10,H9 = 5.9 Hz, H-10); ¹³C NMR (50 MHz, 340 K, d₆-DMSO) δ 182.1 (C-2), 157.2, 156.5 (C-4, C-8a), 155.2 (C-7), 138.2 (C-11), (128.2, 127.3, 127.0, 126.8), (C-13, C-12, C-14 and C-5), 111.9 (C-6), 102.5 (C-4a), 95.1 (C-8), 45.8 (C-10); Anal. Calcd. For C₁₅H₁₂N₂O₂S: C, 59.99; H, 4.03; N, 9.33. Found: C, 59.83; H, 4.14; N, 9.45.

7-((2-Hydroxybenzyl)amino)-2-thioxo-2H-benz[e][1,3]oxazin-4(3H)-one 11b. In slight modification to previously reported general procedure B, 2-hydroxy-4-((2-hydroxybenzyl)amino)benzoic acid 9b (1.07 g, 4 mmol) was allowed to react with the freshly prepared Ph₃P(SCN)₂ (10 mmol) at room temperature for 2 hours then under reflux for 16 hours. The resulting crude solid recrystallised from acetonitrile to give 11b (0.55 g, 46%) as yellow crystals, mp 173–175°C decomp. (KBr)/cm⁻¹ 3500–3000 br (OH), 3310 (9-NH), 2926 (3-NH), 1684 (C=O), 1192 (C=S); ¹HNMR (200 MHz, 340 K, d₆-DMSO) δ 12.86 (bs, 1H, 3-NH), 9.50 (bs, 1H, 12-OH), 7.59 (d, 1H, J H5,H6 = 8.6 Hz, H-5), 7.41 (bs, 1H, 9-NH), 7.05–7.20 (m, 2H, H-14, H-16), 6.69–6.88 (m, 3H, H-13, H-15 and H-6), 6.42 (d, 1H, J H8,H6 = 2.0 Hz, H-8), 4.32 (s, 2H, H-10); ¹³C NMR (50 MHz, 340 K, d₆-DMSO) δ 182.2 (C-2), 157.3, 156.7, 155.4, 154.9 (C-4, C-8a, C-12 and C-7), 128.5, 127.9, 127.3 (C-14, C-16 and C-5) 123.9 (C-11), 118.8, 115.1 C-15, C-13), 111.9 (C-6), 102.2 (C-4a), 94.9 (C-8), 40.9 (C-10); Anal. Calcd. For C₁₅H₁₂N₂O₃: C, 59.99; H, 4.03; N, 9.33 Found: C, 59.83; H, 4.14; N, 9.45.

7-((3-Ethoxy-2-hydroxybenzyl)amino)-2-thioxo-2H-benz[e][1,3]oxazin-4(3H)-one 11c. In slight modification to previously reported general procedure [22, 34], 4-((3-ethoxy-2-hydroxybenzyl)amino)-2-hydroxybenzoic acid 9d (1.21 g, 4 mmol) was allowed to react with the freshly prepared Ph₃P(SCN)₂ (10 mmol) heated to room temperature for 2 hours then under reflux for 16 hours. The resulting solids (1.94 g) were recrystallised from acetic acid/water to give 11c (0.95 g, 68%) as yellow crystals, mp 227–229°C decomp. (KBr)/cm⁻¹ 3500–3200 br (OH), 3496 (7-NH), 3301 (3-NH), 1686 (C=O), 1620 (C=C), 1196 (C=S); ¹HNMR (200 MHz, 390 K, d₆-DMSO) δ 12.86 (bs, 1H, 3-NH), 8.44 (s, 1H, 12-OH exchangeable with D₂O), 7.59 (d, 1H, J H5,H6 = 8.4 Hz, H-5), 7.42 (t, 1H, 9-NH) 6.89–6.40 (m, 4H, Ar and H-6), 6.41 (d, 1H, J H8,H6 = 1.8 Hz, H-8), 4.33 (d, 2H, 10-CH₂), 4.07 (q, 2H, J H17,H18 = 7.0 Hz, 17-CH₂), 1.36 (t, 3H, J H18,H17 = 7.0 Hz, 18-CH₃); ¹³C NMR (50 MHz, 390 K, d₆-DMSO) δ 181.8 (C-2), 156.9, 156.1, 155.2 (C-4, C-8a, C-7), 146.2, 144.2 (C-13, C-12), 126.9, 124.4 (C-15, C-11), 120.2, 118.4 (C-5, C-16), 112.2, 111.5 (C-14, C-6), 102.1 (C-4a), 94.8 (C-8), 64.2 (C-17), 40.7 (C-10), 14.0 (C-18); Anal. Calcd. For C₁₅H₁₂N₂O₂S: C, 59.29; H, 4.68. Found: C, 59.07; H, 4.70.

4.1.7. Synthesis of Dithioxo-benz-bis-(1,3-oxazine)-diones 18a–c and 19

2,8-Dithioxo-2,3,7,8-tetrahydrobenzo[1,2-e: 5,4-e ′ ]bis([1,3]oxazine)-4,6-dione 18a. In slight modification to the general procedure B, 4,6-dihydroxyisophthalic acid 16a (0.79 g, 4 mmol) was allowed to react with the freshly prepared Ph₃P(SCN)₂ (10 mmol) heated to room temperature for 3 hours then under reflux for 5 hours. At the completion of the reaction, the reaction mixture was filtered and the PbBr₂ filter cake washed with THF (100 mL) to extract product 18a. Both THF and DCM filtrates were evaporated to dryness and minimal toluene added to remove any oil which may be present. The crude solid was recrystallised using dioxane/chloroform to give 18a (0.63 g, 56%) as yellow crystals, mp >300°C decomp. (KBr)/cm⁻¹ 3104, 3031, 2939, 2856 (3 and 7-NH), 1698 (C=O), 1152 (C=S); ¹HNMR (200 MHz, 350 K d₆-DMSO) δ 13.65 (bs, 2H, 2 x NH), 8.27 (bs, 1H, H-5), 7.69 (bs, 1H, H-10); ¹³C NMR (50 MHz, d₆-DMSO) δ 181.1 (C-2,8), 159.0 (C-4,6), 156.1 (C-9a,10a), 126.5 (C-5), 113.8 (C-4a,5a), 104.1 (C-10); Anal. Calcd. For C₁₀H₄N₄O₄S₂: C, 42.85; H, 1.44; N, 9.99. Found: C, 42.71; H, 1.48; N, 10.05.

10-Hydroxy-2,8-dithioxo-2,3,7,8-tetrahydrobenzo[1,2-e: 5,4-e ′ ]bis([ 1,3]oxazine)-4,6-dione 18b. In slight modification to general procedure B, 4,5,6-trihydroxyisophthalic acid 16b (0.86 g, 4 mmol) was allowed to react with the freshly prepared Ph₃P(SCN)₂ (10 mmol) at room temperature for 3 hours and then under reflux for 5 hours. At the completion of the reaction, the mixture was filtered and the PbBr₂ cake washed with 100 mL 1,4-dioxane. Both dioxane and DCM filtrates were evaporated to dryness under reduced pressure and minimal toluene was added to remove any oil which may be present. The resulting solid was recrystallised from 1, 4 dioxane/chloroform to give 18b (0.55 g, 46%) as yellow crystals, mp 286–289°C decomp. (KBr)/cm⁻¹ 3300–3000 br (OH), 3103, 3047, 2938 (NH), 1698 (C=O), 1227 (C=S); ¹H NMR (200 MHz, 390 K d₆-DMSO) δ 13.69 (bs, 2H, 2 x NH), 11.64 (bs, 1H, OH), 7.80 (bs, 1H, H-5); ¹³CNMR (50 MHz, 390 K d₆-DMSO) δ 181.0 (C-2,8), 156.8 (C-4,6), 148.6 (C-9a,10a), 131.9 (C-10), 113.8 (C-5), 113.8 (C-4a,5a); Anal. Calcd. For C₁₀H₄N₄O₄S₂: C, 42.85; H, 1.44; N, 9.99. Found: C, 42.71; H,1.48; N, 10.05.

10-Methyl-2, 8-dithioxo-2, 3, 7, 8-tetrahydrobenzo[1,2-e: 5,4-e ′ ]bis([1 ,3]oxazine)-4,6-dione 18c. In slight modification to general procedure B, 4,6-dihydroxy-5-methylisophthalic acid 16c (1.7 g, 8 mmol) was allowed to react with freshly prepared Ph₃P(SCN)₂ [22, 34] (10 mmol) at room temperature for 3 hours then under reflux for 5 hours. At the completion of the reaction, the reaction mixture was filtered and the PbBr₂ cake washed with approx 100 mL THF and filtered. Both THF and DCM filtrates were evaporated to dryness under reduced pressure and minimal toluene was added to remove any oil, which may be present. The solid which remained was then recrystallised using THF to give product 18c (1.18 g, 50%). The physical and spectroscopic data is consistent with the literature values [22].

2,9-Dithioxo-2,3,8,9-tetrahydrobenzo[1,2-e:4,3-e ′ ]bis([1,3]oxazine)-4,7-dione 19. In slight modification to the general procedure B [22, 34], 2,3-dihydroxyterephthalic acid 17 (0.86 g, 4 mmol) was allowed to react with freshly prepared Ph₃P(SCN)₂ (10 mmol) at room temperature for 3 hours then under reflux for 5 hours. At the completion of the reaction, the reaction mixture was filtered and the PbBr₂ cake was washed with THF (100 mL). Both THF and DCM filtrates were evaporated to dryness under reduced pressure and minimal toluene was added to remove any oil, which may be present. The remaining solid is recrystallised from ethyl acetate to give 19 (0.54 g, 50%) as yellow crystals, mp > 300°C decomp. (KBr)/cm⁻¹ 3079, 2904, 2864 (NH), 1718 (C=O), 1252 (C=S); ¹H NMR (200 MHz, 390 K d₆-DMSO) δ 13.90 (bs, 2H, 2 x NH), 7.89 (s, 2H, H-5 & H-6); ¹³C NMR (50 MHz, 390 K (d₆-DMSO) δ 181.1 (C-2,9), 157.0 (C-4,7), 142.7 (C-10a,10b), 122.5 (C-5,6), 121.3 (C-4a,6a); Anal. Calcd. For C₁₀H₄N₂O₄S₂: C, 42.85; H, 1.44; N, 9.99. Found: C, 42.74; H, 1.50; N, 9.93.

4.1.8. Synthesis of Benzyl Thiourea 13a–q

General Procedure C. The appropriate 2-thio-1,3-benzoxazines (1.7 mmole) 6, 10a,b, 11c, and 12a–m were suspended in a mixture of sodium bicarbonate (1 gm) and water (5 mL)/methanol (5 mL) with stirring, then the reaction mixture was warm to 40°C for few minutes then benzyl amine (4.25 mmol) was added dropwise, directly from the a pipette, left stirring at room temperature for 4 hours. At the completion of the reaction, the mixture was evaporated to dryness under reduced pressure and the pH was adjusted to 5-6 by using conc. HCl. The resulting solid was collected by vacuum filtration and washed with minimal water and recrystallized from an appropriate solvent.

N-(Benzyl carbamothioyl)-2-hydroxybenzamide 13a. 2-Thioxo-2H-benz[e]-1,3-oxazin-4(3H)-one 12a was allowed to react with benzyl amine according to general procedure C. The crude solid was collected and recrystallized from toluene to give 13a (75% yield), mp 176-177°C (Lit. [19, 20] 179°C). (KBr)/cm⁻¹ 3301 (N–H), 3200–2700 (O–H), 1661 (C=O), 1605 (C=S); ¹H NMR (d₆-DMSO) δ 12.42 (bs, 1H, H-2′), 11.41 (s, 1H, O–H), 11.00 (t, 1H, J = 5.4 Hz, H-4′), 7.70–7.00 (m, 9H, ArH), 4.9 (d, 2H, J = 5.2 Hz, H-5′); ¹³C NMR (d₆-DMSO) δ 179.6 (C-3′), 168.3 (C-1′), 160.7 (C-2), 136.3 (C-6′), 135.9 (C-4), 129.0 (C-6), 128.1 (C-7′), 127.9 (C-8′), 127.5 (C-9′), 120.1 (C-5), 118.8 (C-3), 113.3 (C-1), 50.0 (C-5′).

N-(Benzyl carbamothioyl)-2-hydroxy-3-methylbenzamide 13b. 8-Methyl-2-thioxo-2H-benz[e]-1,3-oxazin-4(3H)-one 12b was allowed to react with benzyl amine according to general procedure C. The crude solid was collected and recrystallized from toluene to give 13b (79% yield), mp 220–223°C. (KBr)/cm⁻¹ 3062, 2883 (N–H), 1696 (C=O), 1610 (C=S); ¹H NMR (d₆-DMSO) δ 11.32 (bs, 1H, H-2′), 10.73 (s, 1H, O–H), 8.80 (t, 1H, J = 5.4 Hz, H-4′), 7.82 (d, 1H, J = 7.5 Hz, H-6), 7.40–7.20 (m, ArH, H-4), 6.8 (t, 1H, J = 7.5 Hz, H-5), 4.43 (d, 2H, J = 6.0 Hz, H-5′), 2.20 (s, 3H, 8-CH₃); ¹³C NMR (d₆-DMSO) δ 169.9 (C-3′), 157.7 (C-1′), 152.9 (C-2), 137.5 (C-6′), 136.0 (C-4), 128.4 (C-7′), 127.3 (C-8′), 127.0 (C-9′), 126.9 (C-3), 126.5 (C-6), 119.0 (C-5), 115.1 (C-1), 42.9 (C-5′), 15.9 (CH₃); Anal. Calcd. For C₁₆H₁₆N₂O₃: C, 67.59; H, 5.67; N, 9.85. Found: C, 67.10; H, 5.38; N, 9.25.

N-(Benzyl carbamothioyl)-2-hydroxy-[1,1 ′ -biphenyl]-3-carboxamide 13c. 8-Phenyl-2-thioxo-2H-benz[e]-1,3-oxazin-4(3H)-one 12c was allowed to react with benzyl amine according to general procedure C. The crude solid was collected and recrystallized from toluene to give 13c (80% yield), mp 250–253°C. (KBr)/cm⁻¹ 3062, 2883 (N–H), 1686 (C=O), 1595 (C=S); ¹H NMR (d₆-DMSO) δ 11.30 (s, 1H, 2′-N–H), 10.70 (s, 1H, O–H), 8.80 (t, 1H, J = 5.4 Hz, 4′-N–H), 7.90 (d, 1H, J = 7.6 Hz, H-6), 7.80 (d, 1H, J = 7.6 Hz, H-4), 7.60 (dd, 1H, J H8,H10 = 1.6 Hz, J H8,H9 = 8.0 Hz, H-8), 7.50 (m, 4H, H-5/H-9/H-10), 7.30–7.20 (m, ArH, H-4), 4.90 (d, 2H, J = 6.0 Hz, H-5′); ¹³C NMR (d₆-DMSO) δ 169.4 (C-3′), 157.7 (C-1′), 155.3 (C-2), 137.2 (C-6′), 134.2 (C-4), 129.5 (C-6), 128.4 (C-7′), 127.3 (C-8′), 127.0 (C-9′), 126.3 (C-3), 120.1 (C-1), 118.9 (C-5), 50.1 (C-5′); Anal. Calcd. For C₁₅H₁₄N₂O₂S: C, 62.92; H, 4.93; N, 9.78. Found: C, 63.03; H, 4.88; N, 9.80.

N-(Benzyl carbamothioyl)-5-bromo-2-hydroxybenzamide 13d. 6-Bromo-2-thioxo-2H-benz[e]-1,3-oxazin-4(3H)-one 12d was allowed to react with benzyl amine according to general procedure C. The crude solid was collected and recrystallized from toluene to give 13d (82% yield), mp 185°C. (KBr)/cm⁻¹ 3296, 3073 (N–H), 1678 (C=O), 1601 (C=S); ¹H NMR (d₆-DMSO) δ 12.42 (bs, 1H, H-2′), 11.41 (s, 1H, O–H), 11.00 (t, 1H, J = 5.4 Hz, H-4′), 7.90 (d, 1H, J = 2.2 Hz, H-6), 7.60 (dd, 1H, J H4,H6 = 2.2 Hz, J H4,H3 = 7.5 Hz, H-4), 7.40–7.30 (m, 5H, ArH), 7.00 (d, 1H, J = 7.5 Hz, H-3), 4.90 (d, 2H, J = 5.5 Hz, H-5′); ¹³C NMR (d₆-DMSO) δ 179.4 (C-3′), 163.3 (C-1′), 155.9 (C-2), 137.4 (C-4), 137.3 (C-6′), 132.9 (C-6), 128.5 (C-7′), 127.6 (C-8′), 127.3 (C-9′), 119.7 (C-1), 118.8 (C-3), 111.1 (C-5), 48.3 (C-5′); Anal. Calcd. For C₁₅H₁₃BrN₂O₂S: C, 49.33; H, 3.59; N, 7.67. Found: C, 49.39; H, 3.68; N, 7.93.

N-(Benzyl carbamothioyl)-5-ethoxy-2-hydroxybenzamide 13e. 6-Ethoxy-2-thioxo-2H-benz[e]-1,3-oxazin-4(3H)-one 12e was allowed to react with benzyl amine according to general procedure C. The crude solid was collected and recrystallized from toluene to give 13e (87% yield), mp 210°C. (KBr)/cm⁻¹ 3300, 3051 (N–H), 1665 (C=O), 1630 (C=S); ¹H NMR (d₆-DMSO) δ 12.00 (bs, 1H, H-2′), 11.32 (s, 1H, O–H), 11.12 (t, 1H, J = 5.4 Hz, H-4′), 7.50 (s, 1H, H-6), 7.30–7.10 (m, 7H, ArH/H-3/H-4), 4.80 (d, 2H, J = 5.5 Hz, H-5′), 4.10–4.00 (q, 2H, J = 6.7 Hz, O–CH₂), 1.30–1.20 (t, 3H, J = 6.7 Hz, CH₃); ¹³C NMR (d₆-DMSO) δ 179.8 (C-3′), 164.3 (C-1′), 164.0 (C-2), 158.3 (C-5), 137.4 (C-6′), 128.5 (C-7′), 127.6 (C-8′), 127.3 (C-9′), 120.2 (C-1), 107.9 (C-3), 107.7 (C-4), 101.7 (C-6), 63.6 (O–CH₂), 48.2 (C-5′), 14.5 (CH₃); Anal. Calcd. For C₁₇H₁₈N₂O₃S: C, 61.80; H, 5.49; N, 8.48. Found: C, 61.90; H, 5.55; N, 8.61.

N-(Benzyl carbamothioyl)-4-ethoxy-2-hydroxybenzamide 13f. 7-Ethoxy-2-thioxo-2H-benz[e]-1,3-oxazin-4(3H)-one 12f was allowed to react with benzyl amine according to general procedure C. The crude solid was collected and recrystallized from ethyl acetate to give 13f (73% yield), mp 215–218°C. (KBr)/cm⁻¹ 3307, 3071 (N–H), 1655 (C=O), 1506 (C=S); ¹H NMR (d₆-DMSO) δ 11.91 (bs, 1H, H-2′), 11.00 (s, 1H, O–H), 10.90 (t, 1H, J = 5.6 Hz, H-4′), 7.80 (d, 1H, J = 8.9 Hz, H-6), 7.30–7.20 (m, 5H, ArH), 6.60 (d, 1H, J = 8.9 Hz, H-5), 6.40 (sd, 1H, J = 1.5 Hz, H-3), 4.80 (d, 2H, J = 5.5 Hz, H-5′), 4.10–4.00 (q, 2H, J = 6.7 Hz, O–CH₂), 1.30–1.20 (t, 3H, J = 6.7 Hz, CH₃); ¹³C NMR (d₆-DMSO) δ 179.8 (C-3′), 164.3 (C-1′), 164.0 (C-4), 158.3 (C-2), 137.4 (C-6′), 130.9 (C-6), 128.5 (C-7′), 127.6 (C-8′), 127.3 (C-9′), 109.2 (C-5), 107.9 (C-1), 101.7 (C-3), 63.6 (O–CH₂), 48.2 (C-5′), 14.5 (CH₃); Anal. Calcd. For C₁₇H₁₈N₂O₃S C, 61.80; H, 5.49; N, 8.48. Found: C, 62.02; H, 5.55; N, 8.51.

N-(Benzyl carbamothioyl)-3-ethoxy-2-hydroxybenzamide 13g. 8-Ethoxy-2-thioxo-2H-benz[e]-1,3-oxazin-4(3H)-one 12g was allowed to react with benzyl amine according to general procedure C. The crude solid was collected and recrystallized from toluene to give 13g (82% yield), mp 188–190°C. (KBr)/cm⁻¹ 3310, 3080 (N–H), 1656 (C=O), 1525 (C=S); ¹H NMR (d₆-DMSO) δ 11.80 (s, 1H, 2′-N–H), 11.60 (s, 1H, O–H), 11.00 (t, 1H, J = 5.4 Hz, 4′-N–H), 7.30–7.20 (m, 6H, ArH/H-5), 7.10 (d, 1H, J = 7.5 Hz, H-6), 6.90 (d, 1H, J = 7.5 Hz, H-4), 4.80 (d, 2H, J = 5.5 Hz, H-5′), 4.00–3.90 (q, 2H, J = 6.7 Hz, O–CH₂), 1.30–1.20 (t, 3H, J = 6.7 Hz, CH₃); ¹³C NMR (d₆-DMSO) δ 179.5 (C-3′), 164.3 (C-1′), 151.8 (C-3), 150.5 (C-2), 137.3 (C-6′), 128.5 (C-7′), 127.6 (C-8′), 127.3 (C-9′), 123.0 (C-5), 118.5 (C-6), 116.5 (C-1), 114.2 (C-4), 63.5 (O–CH₂), 48.3 (C-5′), 14.6 (CH₃); Anal. Calcd. For C₁₇H₁₈N₂O₃S: C, 61.80; H, 5.49; N, 8.48. Found: C, 61.96; H, 5.61; N, 8.54.

N-(Benzyl carbamothioyl)-2-hydroxy-5-methoxybenzamide 13h. 6-Methoxy-2-thioxo-2H-benz[e]-1,3-oxazin-4(3H)-one 12h was allowed to react with benzyl amine according to general procedure C. The crude solid was collected and recrystallized from ethyl acetate to give 13h (69% yield), mp 140°C. (KBr)/cm⁻¹ 3313, 3081 (N–H), 1667 (C=O), 1610 (C=C), 1516 (C=S); ¹H NMR (d₆-DMSO) δ 12.40 (s, 1H, 2′-N–H), 11.30 (s, 1H, O–H), 11.20 (t, 1H, J = 5.4 Hz, 4′-N–H), 7.50–7.30 (m, 8H, ArH/H-3/H-4/H-6), 3.80 (s, 3H, O–CH₃); ¹³C NMR (d₆-DMSO) δ 179.9 (C-3′), 165.4 (C-1′), 151.5 (C-5), 149.0 (C-2), 137.4 (C-6′), 128.5 (C-7′), 127.6 (C-8′), 127.3 (C-9′), 121.8 (C-3), 117.9 (C-4), 116.4 (C-1), 56.2 (O–CH₃), 48.1 (C-5′); Anal. Calcd. For C₁₆H₁₆N₂O₃S: C, 60.74; H, 5.10; N, 8.85. Found: C, 60.82; H, 5.18; N, 8.93.

N-(Benzyl carbamothioyl)-2-hydroxy-4-methoxybenzamide 13i. 7-Methoxy-2-thioxo-2H-benz[e]-1,3-oxazin-4(3H)-one 12i was allowed to react with benzyl amine according to general procedure C. The crude solid was collected and recrystallized from ethyl acetate to give 13i (87% yield), mp 210°C. (KBr)/cm⁻¹ 3313, 3081 (N–H), 1667 (C=O), 1610 (C=C), 1520 (C=S); ¹H NMR (d₆-DMSO) δ 12.40 (s, 1H, 2′-N–H), 11.30 (s, 1H, O–H), 11.20 (t, 1H, J = 5.4 Hz, 4′-N–H), 7.70 (d, 1H, J = 7.5 Hz, H-6), 7.50–7.30 (m, 5H, ArH), 6.70 (m, 2H, H-3/H-5), 3.80 (s, 3H, O–CH₃); ¹³C NMR (d₆-DMSO) δ 179.9 (C-3′), 165.4 (C-1′), 160.5 (C-4), 159.0 (C-2), 137.4 (C-6′), 128.5 (C-7′), 127.6 (C-8′), 127.3 (C-9′), 126.8 (C-6), 117.9 (C-5), 116.4 (C-1), 106.0 (C-3), 56.2 (O–CH₃), 48.1 (C-5′); Anal. Calcd. For C₁₆H₁₆N₂O₃S: C, 60.74; H, 5.10; N, 8.85. Found: C, 60.72; H, 5.20; N, 8.90.

N-(Benzyl carbamothioyl)-2-hydroxy-3-methoxybenzamide 13j. 8-Methoxy-2-thioxo-2H-benz[e]-1,3-oxazin-4(3H)-one 12j was allowed to react with benzyl amine according to general procedure C. The crude solid was collected and recrystallized from toluene to give 13j (82% yield), mp 190–193°C. (KBr)/cm⁻¹ 3313, 3081 (N–H), 1667 (C=O), 1610 (C=C), 1518 (C=S); ¹H NMR (d₆-DMSO) δ 12.40 (s, 1H, 2′N–H), 11.30 (s, 1H, O–H), 11.20 (t, 1H, J = 5.4 Hz, 4′-N–H), 7.50-7.20 (m, 8H, ArH/H-4/H-5/H-6), 3.80 (s, 3H, O–CH₃); ¹³C NMR (d₆-DMSO) δ 179.9 (C-3′), 165.4 (C-1′), 149.5 (C-3), 148.0 (C-2), 137.4 (C-6′), 128.5 (C-7′), 127.6 (C-8′), 127.3 (C-9′), 121.8 (C-6), 117.9 (C-5), 116.4 (C-1), 116.0 (C-4), 56.2 (O–CH₃), 48.1 (C-5′); Anal. Calcd. For C₁₆H₁₆N₂O₃S: C, 60.74; H, 5.10; N, 8.85. Found: C, 60.82; H, 5.18; N, 8.93.

N-(Benzyl carbamothioyl)-2-hydroxy-3,5-diiodobenzamide 13k. 6,8-diiodo-2-thioxo-2H-benz[e]-1,3-oxazin-4(3H)-one 12k was allowed to react with benzylamine according to general procedure C. The solid was collected and recrystallized from toluene to give 13k (75% yield), mp 173–175°C. (KBr)/cm⁻¹ 3264, 3025 (N–H), 1633 (C=O), 1576 (C=C), 1545 (C=S); ¹H NMR (d₆-DMSO) δ 11.90 (s, 1H, 2′-N–H), 10.90 (t, 1H, J = 5.4 Hz, 4′-N–H), 8.20 (d, 1H, J = 2.2 Hz, H-6), 8.00 (d, 1H, J = 2.2 Hz, H-4), 7.40–7.30 (m, 5H, ArH), 4.90 (d, 2H, J = 5.5 Hz, H-5′), ¹³C NMR (d₆-DMSO) δ 179.8 (C-3′), 165.6 (C-1′), 157.0 (C-2), 149.7 (C-4), 137.9 (C-6′), 137.3 (C-6), 128.5 (C-7′), 127.7 (C-8′), 127.3 (C-9′), 121.1 (C-1), 91.75 (C-5), 81.9 (C-3), 48.2 (C-5′); Anal. Calcd. For C₁₅H₁₂I₂N₂O₂S: C, 33.48; H, 2.25; N, 5.21. Found: C, 33.43; H, 2.66; N, 5.78.

N-(Benzyl carbamothioyl)-2,4-dihydroxybenzamide 13l. 7-Hydroxy-2-thioxo-2H-benz[e]-1,3-oxazin-4(3H)-one 12l was allowed to react with benzylamine according to general procedure C. The solid was collected and recrystallized from toluene to give 13l (87% yield), mp190°C. (KBr)/cm⁻¹ 3200–2700 (O–H), 3118, 3033 (N–H), 1665 (C=O), 1626 (C=C), 1554 (C=S); ¹H NMR (d₆-DMSO) δ 12.10 (bs, 1H, O–H), 11.30 (s, 1H, N–H), 11.20 (t, 1H, J = 5.4 Hz, N–H), 10.50 (O–H), 7.80 (d, 1H, J = 7.6 Hz, H-6), 7.30 (m, 5H, ArH), 6.40 (m, 2H, H-3/H-5), 4.80 (d, 2H, J = 5.7 Hz, H-5′); ¹³C NMR (d₆-DMSO) δ 180.0 (C-3′), 164.7 (C-1′), 163.9 (C-4), 158.5 (C-2), 137.5 (C-6′), 133.3 (C-6), 128.7 (C-7′), 127.7 (C-8′), 127.5 (C-9′), 109.2 (C-5), 107.9 (C-1), 102.9 (C-3), 48.2 (C-5′); Anal. Calcd. For C₁₅H₁₄N₂O₃S: C, 59.59; H, 4,67; N, 9.27. Found: C, 59.88; H, 4.68; N, 9.59.

N-(Benzyl carbamothioyl)-2,4-dihydroxy-3-methylbenzamide 13m. 7-Hydroxy-8-methyl-2-thioxo-2H-benz[e]-1,3-oxazin-4(3H)-one 12m was allowed to react with benzylamine according to general procedure C. The solid was collected and recrystallized from ethanol to give 13m (78% yield), mp 193–195°C. (KBr) 3381, 3225 (O–H), 3020–2800 (N–H), 1639 (C=O), 1613 (C=S); ¹H NMR (d₆-DMSO) δ 11.60 (bs, 1H, 2-OH), 11.10 (t, 1H, J = 5.6 Hz, 4′-NH), 10.40 (bs, 1H, 2′-NH), 7.70 (d, 1H, J H6,H5 = 8.8 Hz; H-6), 7.40–7.30 (m, 6H/H-7′/H-8′/H-9′/4-OH), 6.50 (d, 1H, J H5,H6 = 8.8 Hz; H-5), 4.90 (d, 2H, J = 5.6 Hz, H-5′), 2.00 (s, 3H, 3-CH₃); ¹³C NMR (d₆-DMSO) δ 179.9 (C-3′), 166.1 (C-1′), 161.5 (C-4), 157.2 (C-2), 137.3 (C-6′), 129.1 (C-7′), 128.4 (C-8′), 127.5 (C-9′), 127.2 (C-6, C-3), 111.8 (C-1), 108.3 (C-5), 48.1 (C-5′), 8.6 (3-CH₃). Anal. Calcd. For C₁₆H₁₆N₂O₃S C, 60.74; H, 5.10; N, 8.85. Found: C, 60.60; H, 5.08; N, 8.80; %.

N-(Benzyl carbamothioyl)-2,3-dihydroxybenzamide 13n. 8-Hydroxy-2-thioxo-2H-benz[e]-1,3-oxazin-4(3H)-one 12n was allowed to react with benzyl amine according to general procedure C. The crude solid was collected and recrystallized from ethanol to give 13n (72% yield), mp 172°C. (KBr)/cm⁻¹ 3411, 2942 br (O–H), 3118, 3033 (N–H), 1661 (C=O), 1626 (C=C), 1553 (C=S); ¹H NMR (d₆-DMSO) δ 11.61 (s, 1H, 2′-N–H), 11.22 (t, 1H, J = 5.6 Hz, 4′-N–H), 10.53 (O–H), 7.30 (m, 7H, ArH/H-6), 7.00 (d, 1H, J = 7.6 Hz, H-4), 6.70 (t, 1H, J = 7.9 Hz, H-5), 4.80 (d, 2H, J = 5.7 Hz, H-5′); ¹³C NMR (d₆-DMSO) δ 179.7 (C-3′), 165.0 (C-1′), 146.6 (C-2), 146.3 (C-3), 137.3 (C-6′), 128.5 (C-7′), 127.6 (C-8′), 127.3 (C-9′), 120.5 (C-5), 119.4 (C-4), 119.2 (C-5), 116.8 (C-1), 48.2 (C-5′); Anal. Calcd. For C₁₅H₁₄N₂O₃S: C, 59.59; H, 4.67; N, 9.27. Found: C, 60.08; H, 5.06; N, 9.59.

4-Amino-N-(benzyl carbamothioyl)-2-hydroxybenzamide 13o. 7-Amino-2-thioxo-2H-benz[e][1,3]oxazin-4(3H)-one 10b was allowed to react with benzylamine following general procedure C. The resulting solid was collected and recrystallised from methanol/water to give 13o (0.35 g, 78%) as off white solid, mp 290–293°C decomp. (KBr)/cm⁻¹ 3500–3200 (OH), 3442, 3332 (NH), 1750, 1677 (C=O), 1388 (C=S);. ¹HNMR (200 MHz, 300 K, d₆-DMSO) δ 11.54 (s, 1H, 8-NH exchangeable with D₂O), 11.33–11.28 (bm, 2H, 10-NH, 2-OH exchangeable with D₂O), 7.60 (d, 1H, J H6,H5 = 8.6 Hz, H-6), 7.33–7.29 (m, 7H, Ar, H-11, H-12, H-13 and 4-NH₂ exchangeable with D₂O), 6.21 (dd, 1H, J H5,H6&H5H3 = 8.6 Hz, J H5,H3 = 1.6 Hz, H-3), (d, 1H, J H3,H5 = 1.6 Hz, H-3), 4.84 (d, 2H, J H11,H10 = 5.7 Hz, H-11); ¹³C NMR (50 MHz, 300 K, d₆-DMSO) δ 180.3 (C-9), 165.2 (C-7), 158.6 (C-2), 155.1 (C-4), 137.7 (C-12), 133.2 (C-6), 128.9 (C-14), 127.8, 127.7 (C-13, C-15), 108.2 (C-5), 104.3 (C-1), 99.7 (C-3), 48.4 (C-11); Anal. Calcd. For C₁₅H₁₅N₃O₂S: C, 59.78; H, 5.02; N, 13.94. Found: C, 59.55; H, 5.08; N, 13.72.

4-Acetamido-N-(benzyl carbamothioyl)-2-hydroxybenzamide 13p. N-(4-Oxo-2-thioxo-3, 4-dihydro-2H-benz[e][1,3]oxazin-7-yl) acetamide 10a was allowed to react with benzylamine following general procedure C. The resulting solid was collected and recrystallised from ethanol to give 13p (0.27 g, 47%) as off white crystals. mp 263–266°C. (KBr)/cm⁻¹ 3500–3200 (OH), 3292, 3113, (NH), 1656 (C=O), 1374 (C=S); ¹HNMR (200 MHz, 300 K, d₆-DMSO) δ 12.09 (s, 1H, 16-NH), 11.40 (s, 1H, 8-NH), 11.18 (t, 1H, J H10,H11 = 4.6 Hz, 10-NH), 10.26 (s, 1H, 2-OH), 7.84 (d, 1H, J H6,H5 = 7.8 Hz H-6), 7.64 (s, 1H, H-3), 7.37–7.30 (m, 5H, Ar, H-13, H-14, H-15), 7.02 (d, 1H, J H5,H6 = 7.8 Hz, H-5), 4.85 (d, 2H, J H11,H10 = 4.8 Hz, H-11), 2.07 (s, 3H, CH₃); ¹³C NMR (50 MHz, 300 K, d₆-DMSO) δ 179.7 (C-9), 169.1 (C-7), 164.2 (C-17), 157.4 (C-2), 145.3 (C-4), 137.3 (C-12), 131.9 (C-6), 128.5, 127.6, 127.3 (C-13, C-14, C-15), 111.0, 110.8 (C-1, C-5), 105.9 (C-3), 48.1 (C-11), 24.2 (C-18); Anal. Calcd. For C₁₇H₁₇N₃O₃S: C, 59.46; H, 4.97; N, 12.24. Found: C, 59.20; H, 5.07; N, 12.02.

N-(Benzyl carbamothioyl)-4-((3-ethoxy-2-hydroxybenzyl)amino)-2-hydroxybenzamide 13q. 3-Ethoxy-2-hydroxy-1,3-benzoxazine 11c was allowed to react with benzylamine according to the general procedure C. The resulting solid was collected and recrystallised from acetonitrile to give 13q (0.59 g, 77%) as yellow crystals, mp 201–204°C. (KBr)/cm⁻¹ 3500–3200 (OH), 3496, 3395, 3256 (NH), 1686, 1646 (C=O), 1342 (C=S); ¹HNMR (200 MHz, 300 K, d₆-DMSO) δ 11.26 (8-NH), 8.33 (m, 1H, 2-OH), 7.63 (d, 1H, J H6,H5 = 9.0 Hz, H-6), 7.66–7.28 (m, 5H, Ar H-13, H-14, H-15), 7.1 (t, 1H, J H16,H17 = 5.1 Hz, H-16), 6.92–6.66 (m, 5H, H-21, H-22, H-25, 10-NH and 19-OH exchangeable with D₂O), 6.30 (dd, 1H, J H5,H6&H5,H3 = 9.0 Hz, J H5,H3 = 1.6 Hz, H-5), 6.14 (d, 1H, J H3,H5 = 1.6 Hz, H-3), 4.85 (d, 2H, J H11,H10 = 5.1 Hz, H-11), 4.25 (d, 2H, J H17,H16 = 5.3 Hz, H-17), 4.08–4.01 (m, 3H, J H24,H25 = 7.0 Hz, H-24 and 16-NH), 1.45 (t, 3H, J H25,H24 = 7.0 Hz, H-25); ¹³C NMR (50 MHz, 340 K, d₆-DMSO) δ 180.1 (C-9), 164.8 (C-7), 158.2 (C-2), 154.7 (C-4), 146.3, 144.1 (C-20, C-19), 137.2 (C-12), 132.1 (C-6), 128.2, 127.3, 127.0 (C-14, C-13, C-15), 125.1 (C-18), 119.9, 118.5 (C-21, C-23), 111.8 (C-22), 106.1, 103.6 (C-1, C-5), 97.2 (C-3), 64.1 (C-24), 47.9 (C-17), 40.8 (C-11), 14.4 (C-25); Anal. Calcd. For C₂₄H₂₅N₃O₄S·H₂O: C, 61.39; H, 5.80; N, 8.95. Found: C, 61.59; H, 5.57; N, 9.38.

4.1.9. N1,N3-Bis(benzyl carbamothioyl)-4,6-dihydroxy-lisophthalamide 20a

In slight modification to the general procedure C, 2, 8-dithioxo-2, 3, 7, 8-tetrahydrobenzo[1,2-e: 5,4-e′]bis([1,3]oxazine)-4,6-dione 18a (1 mmol, 0.28 g) was allowed to react with benzylamine (3 mmol. 0.32 g) and sodium hydrogen carbonate solution (1 g in 12 mL methanol and 2.4 mL water) for 16 hour. The resulting solid was collected and recrystallised from ethanol to give 20a (0.31 g, 63%) as off white crystals.mp 276–279°C decomp. (KBr)/cm⁻¹ 3313, (NH), 1672 (C=O), 1328 (C=S), ¹HNMR (200 MHz, 340 K, d₆-DMSO) δ 11.25 (bs, 2H, 2 x NH), 11.06 (t, 2H, J H10,H11 = 5.7 Hz, H-10,18), 8.60 (s, 1H, H-6), 7.38–7.29 (10H, 2 x Ar), 6.63 (s, 1H, H-3), 4.87 (d, 4H, J H11,H10 = 5.7 Hz, H-11,20), 3.3 (OH under the water envelope); ¹³C NMR (50 MHz, 340 K, d₆-DMSO) δ 179.5 (C-9,17), 163.3 (C-7,15), 161.5 (C-2,4), 137.0, 136.6 (C-11, 20 and C-6), 128.2, 127.3, 127.0 (C-13, 22, C-12, 21 and C-14, 25), 110.3 (C-1,15), 103.7 (C-3), 48.0 (C-11,19); Anal. Calcd. For C₂₄H₂₂N₄O₄S₂: C, 58.28; H, 4.48; N, 11.33. Found: C, 58.37; H, 4.67; N, 11.22.

4.1.10. N1,N3-Bis(benzyl carbamothioyl)-4,6-dihydroxy-5-methylisophthalamide 20c

In slight modification to general procedure C, 10-methyl-2,8-bis(methylthio)benzo[1,2-e: 5,4-e′]bis([1,3]oxazine)-4,6-dione 18c (1 mmol, 0.32 g) was allowed to react with benzylamine (3 mmol. 0.32 g), Sodium hydrogen carbonate solution (1 g in 12 mL methanol and 2.4 mL water) for 16 h. The resulting solid was collected and recrystallised from ethanol to give 20c (0.25 g, 49%) as off white crystals. mp 227–229°C decomp. (KBr)/cm⁻¹ 3500–3200 (OH), 3414, 3257 (NH), 1651 (C=O), 1321 (C=S); ¹HNMR (200 MHz, 340 K, d₆-DMSO) δ 11.51 (bs, 2H, 2 x NH), 10.96 (s, 2H, J H10-H11 = 5.7 Hz, H-10,19), 8.48 (s, 1H, H-6), 7.39–7.28 (m, 10H, 2 x Ar), 4.90 (d, 2H, J H9,H8 = 5.7 Hz, H-11, 20), 2.10 (s, 3H, H-3), 3.3 (OH under the water envelope); ¹³C NMR (50 MHz, 340 K, d₆-DMSO) δ 179.6 (C-9, 18), 166.3 (C-7,16), 161.3 (C-2, 4), 136.9 (C-12, 21), 131.2 (C-4), 128.1 (C-14, 23), 127.3, 127.0 (C-13, 22 and C-15, 24), 113.2 (C-1,5), 108.9 (C-3), 48.1 (C-11, 20), 8.4 (C-1, 5); Anal. Calcd. For C₂₅H₂₅N₄O₄S·H₂O: C, 57.02; H, 4.98; N, 10.64. Found: C, 57.32; H, 4.48; N, 10.85.

4.1.11. N1,N4-Bis(benzyl carbamothioyl)-2,3-dihydroxyterephthalamide 21

In slight modification to the general procedure C, 2,9-dithioxo-2,3,8,9-tetrahydrobenzo[1,2-e:4,3-e′]bis([1,3]oxazine)-4,7-dione 19 (1 mmol, 0.28 g) was allowed to react with benzylamine (3 mmol. 0.32 g), sodium hydrogen carbonate solution (1 g in 12 mL methanol and 2.4 mL water) for 16 hour. The resulting solid was collected and recrystallised using ethanol to give 21 (0.31 g, 63%) as off white crystals mp 195–198°C. (KBr)/cm⁻¹ 3500–3200 (OH), 3401, 3248 (NH), 1671, 1649 (C=O), 1338 (C=S); ¹HNMR (200 MHz, 300 K, d₆-DMSO) δ 11.88 (bs, 2 x NH, H-8,17), 11.07 (t, 1H, J H10,H11 = 5.9 Hz, H-10,19), 8.56 (bs, 2H, 2,3-OH), 7.39–7.28 (m, 12H, 2 x Ar and 2 x CH, H-13, H-14, H-15 and H-5,6), 4.88 (d, 4H, J H11,H10 = 5.9 Hz, H-11,20); ¹³C NMR (50 MHz, 340 K, d₆-DMSO) δ 179.6 (C-9,18), 164.9 (C-7,16), 147.9 (C-2,3), 136.9 (C-12, 21), 128.2 (C-14,23), 127.3 (C-13,22), 127.0 (C-15,24), 120.9 (C-1,4), 119.2 (C-5,6); Anal. Calcd. For C₂₄H₂₂N₄O₄S₂·H₂O: C, 56.23; H, 4.72; N, 11.33. Found: C, 55.79; H, 4.78; N, 10.93.

4.1.12. Synthesis N-(2-(Methylthio)-4-oxo-4H-benz[e][1,3]oxazin-7-yl)acetamide 14h

N-(4-Oxo-2-thioxo-3,4-dihydro-2H-benz[e][1,3]oxazin-7-yl)acetamide 10a (0.59 g, 2.5 mmol) was allowed to react with methyl iodide following the previously reported [35]. The resulting beige solid 14h (0.61 g, 97%) is collected and used without further purification but can be crystallised from ethanol, mp 268–269°C (KBr)/cm⁻¹ 3278, 3142, 3110 (NH), 3062 (CH Ar), 2929 (CH Aliphatic), 1760, 1709, 1671 (C=O), 1615 (C=N), 1554 (C=C); ¹HNMR (200 MHz, 300 K d₆-DMSO) δ 10.60 (s, 1H, 9-NH), 7.88 (b, 1H, H-8), 7.87 (d, 1H, J H5,H6 = 8.6 Hz, H-5), 7.45 (d, 1H, J H6,H5 = 8.6 Hz, H-6), 2.58 (s, 3H, 11-CH₃), 2.12 (s, 3H, 3′-CH₃); ¹³C NMR (50 MHz, 300 k, d₆-DMSO) δ 172.8 (C-2), 169.6 (C-10), 162.1 (C-4), 155.8 (C-8a), 145.2 (C-7), 128.0 (C-5), 117.5 (C-6), 112.1 (C-4a), 104.4 (C-8); Anal. Calcd. For C₁₅H₁₂N₂O₂S: C, 59.99; H, 4.03; N, 9.33. Found: C, 59.83; H, 4.14; N, 9.45.

4.1.13. Synthesis of 2-Benzyl amino-1,3-benzoxazines 15a–h

General Procedure D. The appropriate 2-methylthio-1,3-benzoxazine 14a–h (2.5 mmol) was suspended in dry 1,4-dioxane (10 mL) in a 50 mL round-bottomed flask. Benzyl amine (12.5 mmol) was then added dropwise, directly from the pipette, with stirring, and then the reaction mixture was heated to reflux for 4 hours. At the completion of the reaction, the reaction mixture was evaporated to dryness under reduced pressure and triturated with minimal diethyl ether. The resulting solid product 14 was collected by vacuum filtration and recrystallized from an appropriate solvent.

General Procedure E. N-(Benzyl carbamothioyl)-substituted-2-hydroxy-benzamides 13a, b, e, f, and g (0.5 mmol) were suspended in acetic acid (3 mL) in a 25 mL round-bottomed flask. The reaction mixture was heated to reflux for 2 hours then; the acetic acid was evaporated off under reduced pressure. The oily reaction mixture was triturated with minimal diethyl ether and the resulting solid products 15a, b, e, f, and g were collected by vacuum filtration and recrystallized from an appropriate solvent.

Products 15a, b, e, f, and g prepared in this procedure gave identical mp, IR, ¹H NMR and ¹³C NMR to the analogues prepared from compound 14 with comparable yields (Scheme 3).

2-(Benzyl amino)-4H-benz[e]-1,3-oxazin-4-one 15a. 2-(Methylthio)-4H-benz[e]-1,3-oxazin-4-one 14a was allowed to react with benzyl amine according to general procedure D. The crude solid was collected and recrystallized from ethanol to give 15a (75% yield), mp 210°C. (KBr)/cm⁻¹ 3065, 2872 (N–H), 1681 (C=O), 1635 (C=C), 1460 (C=N); ¹H NMR (d₆-DMSO) δ 8.70 (bs, 1H, N–H), 7.90 (d, 1H, J = 7.5 Hz, H-5), 7.70 (t, 1H, J = 7.5 Hz, H-6), 7.60–7.30 (m, 7H, ArH, H-7, H-8), 4.50 (s, 2H, H-9); ¹³C NMR (d₆-DMSO) δ 165.4 (C-4), 154.9 (C-2), 151.6 (C-8a), 137.6 (C-1′), 135.2 (C-7), 127.7 (C-2′), 126.8 (C-4′), 126.4 (C-3′), 124.3 (C-5), 124.7 (C-6), 124.5 (C-8), 117.0 (C-4a), 43.8 (C-9); Anal. Calcd. For C₁₅H₁₂N₂O₂: C, 71.42; H, 4.79; N, 11.10. Found: C, 71.65; H, 4.95; N, 11.25.

2-(Benzyl amino)-8-methyl-4H-benz[e]-1,3-oxazin-4-one 15b. 8-Methyl-2-(methylthio)-4H-benz[e]-1,3-oxazin-4-one 14b was allowed to react with benzyl amine according to general procedure D. The crude solid was collected and recrystallized from ethyl acetate to give 15b (72% yield), mp 257–258°C. (KBr)/cm⁻¹ 3062, 2883 (N–H), 1678 (C=O), 1639 (C=C), 1482 (C=N); ¹H NMR (d₆-DMSO) δ 8.80 (bs, 1H, N–H), 7.70 (d, 1H, J = 7.5 Hz, H-5), 7.50 (d, 1H, J = 7.5 Hz, H-7), 7.40–7.20 (m, ArH/H-6), 4.50 (s, 2H, H-9) 2.30 (s, 3H, 8-CH₃); ¹³C NMR (d₆-DMSO) δ 165.4 (C-4), 157.9 (C-2), 151.6 (C-8a), 137.6 (C-1′), 134.2 (C-7), 127.7 (C-2′), 126.8 (C-4′), 126.4 (C-3′), 124.3 (C-5), 124.7 (C-6), 124.5 (C-8), 117.0 (C-4a), 43.8 (C-9), 13.5 (CH₃); Anal. Calcd. For C₁₆H₁₄N₂O₂: C, 72.16; H, 5.30; N, 10.52. Found: C, 72.10; H, 5.38; N, 10.25.

2-(Benzyl amino)-8-phenyl-4H-benz[e]-1,3-oxazin-4-one 15c. 8-Phenyl-2-(methylthio)-4H-benz[e]-1,3-oxazin-4-one 14c was allowed to react with benzyl amine according to general procedure D. The crude solid was collected and recrystallized from ethanol to give 15c (65% yield), mp 215°C. (KBr)/cm⁻¹ 3040, 2860 (N–H), 1679 (C=O), 1635 (C=C), 1489 (C=N); ¹H NMR (d₆-DMSO) δ 9.20 (bs, 1H, N–H), 7.90 (d, 1H, J = 7.5 Hz, H-5), 7.70–7.20 (m, 11H, ArH/ArH/H-7), 7.90 (d, 1H, J = 3.0 Hz, H-6), 4.50 (s, 2H, H-9); ¹³C NMR (d₆-DMSO) δ 165.1 (C-4), 157.9 (C-2), 151.2 (C-8a), 137.6 (C-1′), 134.7 (C-7), 134.1 (C-5), 129.2–125.8 (C-2′, C-4′ C-3′, C-8 C-9, C-10 C-11, C-12), 124.8 (C-6), 117.7 (C-4a), 43.9 (C-9); Anal. Calcd. For C₂₁H₁₆N₂O₂ 0.5H₂O: C, 76.81; H, 4.91; N, 8.53. Found: C, 74.31; H, 4.99; N, 8.46.

2-(Benzyl amino)-7-methoxy-4H-1,3-benzoxazin-4-one 15d. 7-Methoxy-2-(methylthio)-4H-benz[e]-1,3-oxazin-4-one 14d was allowed to react with benzylamine according to general procedure D. The crude solid was collected and recrystallised from toluene to give 15d (83% yield), mp 234–236°C. (KBr)/cm⁻¹ 3069–2891 (N–H), 1678 (C=O), 1619 (C=C), 1499 (C=N); ¹H NMR (d₆-DMSO) δ 9.00 (bs, 1H, N–H), 7.80 (d, 1H, J = 8.6 Hz, H-5), 7.30 (m, 5H, ArH), 6.90 (dd, 1H, J H6,H8 = 2.4 Hz, J H6,H5 = 8.6 Hz, H-6), 6.70 (d, 1H, J = 2.2 Hz, H-8), 4.50 (s, 2H, H-9), 3.90 (s, 3H, 7-OCH₃); ¹³C NMR (d₆-DMSO) δ 164.7 (C-4), 163.3 (C-7), 157.9 (C-2), 154.5 (C-8a), 137.6 (C-1′), 127.7 (C-5), 127.6 (C-2′), 126.7 (C-4′), 126.4 (C-3′), 112.2 (C-6), 110.6 (C-4a), 99.3 (C-8), 55.4 (OCH₃), 43.7 (C-9); Anal. Calcd. For C₁₆H₁₄N₂O₃: C, 68.07; H, 5.00; N, 9.92. Found: C, 67.86; H, 4.89; N, 10.01.

2-(Benzyl amino)-7-ethoxy-4H-1,3-benzoxazin-4-one 15e. 7-Ethoxy-2-(methylthio)-4H-benz[e]-1,3-oxazin-4-one 14e was allowed to react with benzylamine according to general procedure D. The crude solid was collected and recrystallized from toluene to give 15e (80% yield), mp 214–216°C. (KBr)/cm⁻¹ 3069–2827 (N–H), 1673 (C=O), 1600 (C=C), 1466 (C=N); ¹H NMR (d₆-DMSO) δ 9.00 (bs, 1H, N–H), 7.80 (d, 1H, J = 8.6 Hz, H-5), 7.30 (m, 5H, ArH), 6.90 (dd, 1H, J H6,H8 = 2.4 Hz, J H6,H5 = 8.6 Hz, H-6), 6.70 (d, 1H, J = 2.2 Hz, H-8), 4.50 (s, 2H, H-9), 4.20 (q, 2H, J = 5.6, CH₂–O), 3.90 (s, 3H, 7-OCH₃), 1.30 (t, 3H, J = 5.6, CH₃); ¹³C NMR (d₆-DMSO) δ 164.4 (C-4), 163.1 (C-7), 157.9 (C-2), 154.9 (C-8a), 137.8 (C-1′), 127.6 (C-5), 127.5 (C-2′), 126.7 (C-4′), 126.4 (C-3′), 112.6 (C-6), 110.3 (C-4a), 99.8 (C-8), 63.6 (CH₂–O), 43.7 (C-9), 13.5 (CH₃); Anal. Calcd. For C₁₇H₁₆N₂O₃: C, 68.91; H, 5.44; N, 9.45. Found: C, 69.11; H, 5.76; N, 9.28.

2-(Benzyl amino)-7-hydroxy-4H-1,3-benzoxazin-4-one 15f. 7-Hydroxy-2-(methylthio)-4H-benz[e]-1,3-oxazin-4-one 14f was allowed to react with benzyl amine according to general procedure D. The crude solid was collected and recrystallized from ethanol to give 15f (77% yield), mp decomp 262°C. (KBr)/cm⁻¹ 3300–3058 (O–H), 3058–2851 (N–H), 1667s (C=O), 1607 m (C=C), 1543s (C=N); ¹H NMR (d₆-DMSO) δ 8.70 (bs, 1H, N–H), 7.50 (d, 1H, J = 8.6 Hz, H-5), 7.30–7.20 (m, 6H, ArH/7-OH), 6.50 (d, 1H, J = 8.6 Hz, H-6), 6.30 (s, 1H, H-8), 4.50 (s, 2H, H-9); ¹³C NMR (d₆-DMSO) δ 169.6 (C-4), 166.1 (C-7), 158.1 (C-2), 155.8 (C-8a), 138.5 (C-1′), 128.3 (C-2′), 127.7 (C-4′), 127.2 (C-3′), 127.0 (C-5), 116.1 (C-6), 104.9 (C-4a), 100.6 (C-8), 43.7 (C-9); Anal. Calcd. For C₁₅H₁₂N₂O₃: C, 67.16; H, 4.51; N, 10.44. Found: C, 67.14; H, 4.34; N, 10.49.

2-(Benzyl amino)-7-hydroxy-8-methyl-4H-1,3-benzoxazin-4-one 15g. 7-Hydroxy-8-methyl-2-(methylthio)-4H-benz[e]-1,3-oxazin-4-one 14g was allowed to react with benzylamine according to general procedure D. The crude solid was collected and recrystallised from ethanol to give 15g (74% yield), mp 245–247°C. (KBr)/cm⁻¹ 3300–2860 (O–H), 3058–2851 (N–H), 1678 (C=O), 1608 m (C=C), 1549s (C=N); ¹H NMR (d₆-DMSO) δ 8.70 (bs, 1H, N–H), 7.60 (d, 1H, J = 8.6 Hz, H-5), 7.30–7.20 (m, 6H, H-2′/H-3′/H-4′/7-OH), 6.80 (d, 1H, J = 8.6 Hz, H-6), 4.50 (s, 2H, H-9); ¹³C NMR (d₆-DMSO) δ 165.2 (C-4), 159.6 (C-7), 157.8 (C-2), 152.5 (C-8a), 137.8 (C-1′), 127.6 (C-2′), 126.7 (C-4′), 126.4 (C-3′), 124.2 (C-5), 112.0 (C-6), 109.4 (C-8), 108.1 (C-4a), 43.7 (C-9), 6.8 (8-CH₃); Anal. Calcd. For C₁₆H₁₄N₂O₃: C, 68.07; H, 5.00; N, 9.92. Found: C, 67.86; H, 5.28; N, 9.68.

N-(2-(Benzylamino)-4-oxo-4H-benz[e][1,3]oxazin-7-yl)acetamide 15h. In modification to the general procedure D, N-(2-(methylthio)-4-oxo-4H-benz[e][1,3]oxazin-7-yl)acetamide 14i (0.26 g 1 mmol) was allowed to react with benzylamine (0.2 mL, 1 mmol) for 4 hours. The resulting solid was collected and recrystallised from acetonitrile to give 15 h (0.1, 47%) as white crystals, mp 250–253°C. decomp. (KBr)/cm⁻¹ 3284, 3234, 3215 (NH), 1669 (C=O); ¹HNMR (200 MHz, 390 K, d₆-DMSO) δ 10.28 (s, 1H, H-15), 8.86 (t, 1H, J H9,H10 = 4.7 Hz, H-9), 7.81–7.78 (m, 2H, H-8 and H-5), 7.37–7.26 (m, 6H, Ar, H-12–14 and H-6), 4.52 (d, 2H, J H10,H9 = 4.7 Hz, H-10), 2.10 (s, 3H, 17-CH₃); ¹³C NMR (50 MHz, 390 K, d₆-DMSO) δ 169.3 (C-16), 165.4 (C-4), 158.4 (C-8a), 154.2 (C-2), 144.3 (C-7), 138.2 (C-11), 128.4 (C-5), 127.5/127.3/127.2 (C-12/C-13/C-14), 115.7 (C-6), 112.1 (C-4a), 104.2 (C-8), 44.0 (C-10), 24.3 (C-17); Anal. Calcd. For C₁₇H₁₅N₃O₃: C, 66.01; H, 4.89; N, 13.58. Found: C, 65.93; H, 5.02; N, 13.52.

4.1.14. -methyl-2,8-bis(methylthio)benzo[1,2-e:5,4-e′]bis([1,3]oxazine)-4,6-dione 24

Following the previously reported [35], 10-methyl-2, 8-dithioxo-2, 3, 7, 8-tetrahydrobenzo[1,2-e:5,4-e′]bis([1,3]oxazine)-4,6-dione 18c (0.74 g, 2.5 mmol) was allowed to react with methyl iodide (5.4 mL, 86.8 mmol) and NaHCO₃ (3.0 g, 32 mmol) for 2 hrs. The resulting yellow solid 24 (0.78 g, 98%) is collected and used without further purification. mp > 300°C decomp. (KBr)/cm⁻¹ 1623 (C=O), 1548 (C=N); ¹H NMR (200 MHz, 390 K, d₆-DMSO) δ 8.27 (s, 1H, H-5), 2.64 (s, 6H, H-3′and H-9′), 2.33 (s, 3H, H-10′). ¹³C NMR (50 MHz, 390 K, d₆-DMSO) δ 173.0 (C-2,8), 160.6 (C-4,6), 155.3 (C-6a, 9a), 124.0 (C-5) 123.6 (C-10), 115.2 (C-4a, 5a), 13.4 (C-3′, 9′), 6.7 (C-10′).

4.1.15. 2,8-Bis(benzylamino)-10-methylbenzo[1,2-e:5,4-e′]bis([1,3]oxazine)-4,6-dione 26

In slight modification to the general procedure D,10-methyl-2,8-bis(methylthio) benzo[1,2-e:5,4-e′]bis([1,3]oxazine)-4,6-dione 24 (0.29 g, 1 mmol) was allowed to react with benzyl amine 0.4 mL, (2 mmol) for 16 hours. The resulting solid was collected and recrystallised from DMSO/water to give 26 (0.25 g, 56%) as an off white solid, mp 285–288°C decomp. (KBr)/cm⁻¹ 3394, 3202, 3029 (NH), 1681, 1635 (C=O); ¹HNMR (200 MHz, 340 K, d₆-DMSO) δ 8.90 (bs, 2H, H-11,17), 8.27 (s, 1H, H-5), 7.43–7.24 (m, 10H 2 x Ar, H-14–16 and H-20–22), 4.58 (s, 4H, H-12,18), 2.31 (s, 3H, H-10′); ¹³C NMR (50 MHz, 340 K, d₆-DMSO) δ 164.0 (C-4), 164.0 (C-9a,10a), 157.6 (C-2), 137.2 (C-13,19), 127.7, 126.9, 126.6, 122.5 (C-15,21, C-14,20 and C-16,22), 113.9 (C-10), 115.5 (C-4a,5a), 44.0 (C-12,17), 6.6 (C-10′); Anal. Calcd. For C₂₅H₂₀N₄O₄·2H₂O: C, 63.02; H, 5.08; N, 11.76. Found: C, 63.22; H, 4.99; N, 11.96.

4.1.16. Synthesis of Substituted-1,3-benzoxazine-diones 22a–h and 25

General Procedure F. The appropriate substituted methylthio-1,3-benzoxazine 2.5 mmol was hydrolysed with 10 mL hydrochloric acid (10%) at 80°C for 4 hours. At the completion of the reaction, the reaction mixture was washed with R.O water, filtered, and recrystallised from an appropriate solvent.

Products 22a–h were used in the synthesis of products 23a–h with no further purification.

2H-Benz[e]-1,3-oxazin-2,4(3H)-dione 22a. 2-(Methylthio)-4H-benz[e]-1,3-oxazin-4-one 14a was allowed to react with hydrochloric acid (10%) according to general procedure F. The crude solid was collected and recrystallised from ethanol to give 22a (75% yield), mp 228°C. (lit. [19, 20] 229-230°C). (KBr)/cm⁻¹ 3179, 2877 (N–H), 1771 (C=O), 1690 (C=O), 1610 (C=C); ¹H NMR (200 MHz, d₆-DMSO) δ 7.90 (d, 1H, J = 7.5 Hz, H-5), 7.80 (t, 1H, J = 7.5 Hz, H-6), 7.40 (m, 2H, H-7/H-8); ¹³C NMR (50 MHz, d₆-DMSO) δ 161.6 (C-4), 153.7 (C-2), 147.6 (C-8a), 136.2 (C-7), 126.9 (C-5), 125.2 (C-6), 116.5 (C-8), 114.6 (C-4a).

8-Methyl-2H-benz[e]-1,3-oxazin-2,4(3H)-dione 22b. 8-Methyl-2-(methylthio)-4H-benz[e]-1,3-oxazin-4-one 14b was allowed to react with hydrochloric acid (10%) according to general procedure F. The crude solid was collected and recrystallised from ethanol to give 22b (85% yield), mp 210°C (lit. [19, 20] 210–212°C). (KBr)/cm⁻¹ 3221, 2845 (N–H), 1746 (C=O), 1717 (C=O), 1614 (C=C); ¹H NMR (200 MHz, d₆-DMSO) δ 8.80 (bs, 1H, N–H), 7.70 (d, 1H, J = 7.5 Hz, H-5), 7.60 (d, 1H, J = 7.5 Hz, H-7), 7.30–7.20 (t, 1H, J = 7.1 Hz, H-6), 2.30 (s, 3H, 8-CH₃); ¹³C NMR (50 MHz, d₆-DMSO) δ 161.6 (C-4), 151.9 (C-2), 147.3 (C-8a), 136.9 (C-7), 125.5 (C-8), 124.6 (C-5), 124.4 (C-6), 114.4 (C-4a), 13.8 (CH₃).

7-Methoxy-2H-benz[e]-1,3-oxazin-2,4(3H)-dione 22c. 7-Methoxy-2-(methylthio)-4H-benz[e]-1,3-oxazin-4-one 14d was allowed to react with hydrochloric acid (10%) according to general procedure F. The crude solid was collected and recrystallised from ethyl acetate to give 22c (78% yield), mp 213°C. (KBr)/cm⁻¹ 3203, 2930 (N–H), 1771 (C=O), 1723 (C=O), 1620 (C=C); ¹H NMR (200 MHz, d₆-DMSO) δ 12.10 (s, 1H, N–H), 7.90 (d, 1H, J = 8.3 Hz, 5-H), 7.10 (s, 1H, 8-H), 7.00 (d, 1H, J = 8.3 Hz, 6-H), 3.0 (s, 3H, 7-OCH₃); ¹³C NMR (50 MHz, d₆-DMSO) δ 160.6 (C-4), 157.9 (C-2), 155.4 (C-7), 151.6 (C-8a), 128.2 (C-5), 113.2 (C-6), 107.1 (C-4a), 100.8 (C-8), 56.5 (7-OCH₃); Anal. Calcd. For C₉H₇NO₄: C, 55.96; H, 3.65; N, 7.25. Found: C, 55.75; H, 3.45; N, 7.55.

7-Ethoxy-2H-benz[e]-1,3-oxazin-2,4(3H)-dione 22d. 7-Ethoxy-2-(methylthio)-4H-benz[e]-1,3-oxazin-4-one 14e was allowed to react with hydrochloric acid (10%) according to general procedure F. The crude solid was collected and recrystallised from ethyl acetate to give 22e (75% yield), mp 225–227°C. (KBr)/cm⁻¹ 3157–2862 (N–H), 1771 (C=O), 1698 (C=O), 1620 (C=C); ¹H NMR (200 MHz, d₆-DMSO) δ 11.90 (bs, 1H, N–H), 7.80 (d, 1H, J = 8.6 Hz, H-5), 6.90 (m, 2H, H-6/H-8), 4.10 (q, 2H, J = 6.8 Hz, O–CH₂), 1.30 (t, 3H, J = 6.8 Hz, CH₃); ¹³C NMR (50 MHz, d₆-DMSO) δ 164.6 (C-4), 160.9 (C-2), 155.4 (C-7), 147.6 (C-8a), 128.2 (C-5), 113.2 (C-6), 107.1 (C-4a), 100.8 (C-8), 64.4 (CH₂–O), 14.3 (CH₃); Anal. Calcd. For C₁₀H₉NO₄: C, 57.97; H, 4.38; N, 6.76. Found: C, 57.86; H, 4.45; N, 6.55.

7-Hydroxy-2H-benz[e]-1,3-oxazin-2,4(3H)-dione 22e. 7-Hydroxy-2-(methylthio)-4H-benz[e]-1,3-oxazin-4-one 14f was allowed to react with hydrochloric acid (10%) according to general procedure F. The crude solid was collected and recrystallised from ethanol to give 22e (65% yield), mp 245°C (Lit [39] 310). (KBr)/cm⁻¹ 3200–2700 (O–H), 3078, 2929 (N–H), 1780 (C=O), 1688 (C=O), 1616 (C=C); ¹H NMR (200 MHz, d₆-DMSO) δ 11.80 (s, 1H, N–H), 11.00 (s, 1H, 7-O–H), 7.70 (d, 1H, J = 8.6 Hz, H-5), 6.80 (d, 1H, J = 8.6 Hz, H-6), 6.60 (s, 1H, H-8); ¹³C NMR (50 MHz, d₆-DMSO) δ 164.6 (C-4), 160.9 (C-2), 155.4 (C-7), 147.6 (C-8a), 128.6 (C-5), 113.8 (C-6), 105.9 (C-4a), 101.8 (C-8); Anal. Calcd. For C₈H₅NO₄: C, 53.64; H, 2.81; N, 7.82. Found: C, 53.75; H, 2.45; N, 7.55.

7-Hydroxy-8-methyl-2H-benz[e]-1,3-oxazin-2,4(3H)-dione 22f. 7-Hydroxy-8-methyl-2-(methylthio)-4H-benz[e]-1,3-oxazin-4-one 14g was allowed to react with hydrochloric acid (10%) according to general procedure F. The crude solid was collected and recrystallised from ethanol to give 22f (70% yield), mp 250°C decomp. (KBr)/cm⁻¹ 3250, 2900 (O–H), 3188, 2956 (N–H), 1771 (C=O), 1713 (C=O), 1619 (C=C); ¹H NMR (200 MHz, d₆-DMSO) δ 11.70 (s, 1H, N–H), 10.10 (s, 1H, 7-O–H), 7.60 (d, 1H, J = 8.4 Hz, H-5), 6.80 (d, 1H, J = 8.4 Hz, H-6), 2.10 (s, 3H, 8-CH₃); ¹³C NMR (50 MHz, d₆-DMSO) δ 162.4 (C-4), 161.5 (C-2), 153.4 (C-7), 147.9 (C-8a), 125.4 (C-5), 112.6 (C-6), 110.8 (C-4a), 106.0 (C-8), 8.1 (8-CH₃); Anal. Calcd. For C₉H₇NO₄: C, 55.96; H, 3.65; N, 7.25. Found: C, 55.75; H, 3.45; N, 7.55.

N-(2,4-Dioxo-3,4-dihydro-2H-benz[e][1,3]oxazin-7-yl)acetamide 22g. In slight modification to the general procedure F, N-(2-(methylthio)-4-oxo-4H-benz[e][1,3]oxazin-7-yl) acetamide 14i (0.63 g, 2.5 mmol) was allowed to reflux with water (10 mL) for 2 hours. The resulting crude solid was filtered, and recrystallised from methanol to give 22 g (0.52 g, 95%) as light grey solid, mp 284–287°C (Lit [39]. 310–14° decomp.). (KBr)/cm⁻¹ 3350, 3050 (NH), 1758, 1704 (C=O); ¹HNMR (200 MHz, 340 K, d₆-DMSO) δ 11.80 (bs, 1H, 3-NH), 10.57 (s, 1H, 9-NH), 7.83 (d, 1H, J H5,H6 = 8.6 Hz, H-5), 7.72 (d, 1H, J H8,H6 = 1.6 Hz, H-8), 7.45 (dd, 1H, J H6,H5 = 8.6 Hz, J H6,H8 = 1.6 Hz, H-6), 2.11 (s, 3H, 11-CH₃); ¹³C NMR (50 MHz, 340 K, d₆-DMSO) δ 169.0 (C-10), 160.3 (C-4), 154.2 (C-2), 147.1, 145.7 (C-8a, C-7), 127.3 (C-5), 115.3 (C-6), 108.6 (C-4a), 104.6 (C-8), 23.5 (C-11). Anal. Calcd. For C₁₀H₈N₂O₄: C, 54.55; H, 3.66; N, 12.72. Found: C, 54.49; H, 3.77; N, 12.69.

7-Amino-2H-benz[e][1,3]oxazine-2,4(3H)-dione 22h. In modification to the general procedure F, N-(2-(methylthio)-4-oxo-4H-benzo[e][1,3]oxazin-7-yl) acetamide 14h (0.63 g, 2.5 mmol) was allowed to react in the presence of hydrochloric acid (15 mL, 40%) for 4 hours. At the completion of the reaction, the reaction mixture was neutralized by NaHCO₃, filtered and recrystallised from ethanol to give 22h (0.34 g, 79%), mp 284–287°C decomp. (KBr)/cm⁻¹ 3479, 3372 (NH), 2843 (NH), 1752, 1708 (C=O); ¹HNMR (200 MHz, 340 K, d₆-DMSO) δ 11.49 (s, 1H, 3-NH), 7.54 (d, 1H, J H5,H6 = 8.4 Hz, H-5), 6.52 (m, 3H, H-6 and 7-NH₂), 6.31 (s, 1H, H-8).¹³C NMR (50 MHz, 340 K, d₆-DMSO) δ 160.8 (C-4), 156.0, 155.6 (C-2, C-7), 147.9 (C-8a), 127.8 (C-5), 111.4 (C-6), 101.5 (C-4a), 97.3 (C-8). Compound 22h was used for the synthesis of compound 23g with no further purification.

10-Methyl-2H,6H-[1,3]oxazino[5,6-g][1,3]benzoxazine-2,4,6,8(3H,7H)-tetrone 25. 10-Methyl-2,8-bis(methylthio)benzo[1,2-e:5,4-e′]bis([1,3]oxazine)-4,6-dione 24 (0.74 g, 2.5 mmol) was allowed to react with hydrochloric acid (10%) for 4 hours according to the general procedure F. The resulting solid was recrystallised from DMF to give 25 (0.55 g, 85%) as off white crystals. mp > 300°C decomp. (KBr)/cm⁻¹ 3183, 3119, 3049 (NH), 1784, 1714 (C=O), 1616 (C=C); ¹HNMR (200 MHz, 340 K, d₆-DMSO) δ 12.18 (bs, 2H, 3,7-NH), 8.26 (s, 1H, H-5), 2.31 (s, 3H, H-10′); ¹³C NMR (50 MHz, 340 K, d₆-DMSO) δ 159.7 (C-4,6), 155.4 (C-2,8), 145.9 (C-9a,10a), 123.3 (C-5), 113.0 (C-10), 111.3 (C-4a,5a), 7.3 (C-10′); Anal. Calcd. For C₁₁H₆N₂O₆: C, 50.39; H, 2.31; N, 10.68. Found: C, 50.54; H, 2.42; N, 10.74.

4.1.17. Synthesis of N-(Benzyl carbamoyl)-2-hydroxy-substituted-benzamide 23a–g

General Procedure G. The appropriate 2-dione-1,3-benzoxazines 22a–b and 22d–i (2.5 mmol) were suspended in dry 1,4-dioxane (10 mL) in a 50 mL round-bottomed flask. Benzyl amine (12.5 mmol) was then added dropwise, directly from the pipette, with stirring, and then the reaction mixture was heated to reflux for 4 hours. At the completion of the reaction, it evaporated to dryness under reduced pressure and triturated with minimal diethyl ether. The resulting solid was collected by vacuum filtration and recrystallised from an appropriate solvent.

N-(Benzyl carbamoyl)-2-hydroxybenzamide 23a. 2H-Benz[e]-1,3-oxazin-2,4(3H)-dione 22a was allowed to react with benzyl amine according to general procedure F to give 23a which was recrystallised from ethanol (70% yield), mp211–213°C. (KBr)/cm⁻¹ 3340–2940 (O–H), 3230–3161 (N–H), 1687 (C=O), 1648 (C=O); ¹H NMR (200 MHz, d₆-DMSO) δ 11.75 (bs, 1H, H-2′), 10.40 (bs, 1H, 1-OH), 9.05 (t, 1H, J = 5.6 Hz, 4′-NH), 7.90 (d, 1H, J = 7.7 Hz, H-3), 7.50 (m, 1H, H-5), 7.30–7.20 (m, 5H, ArH), 7.05–6.95 (m, 2H, H-4/H-6), 4.40 (d, 2H, J = 5.2 Hz, H-5′); ¹³C NMR (50 MHz, d₆-DMSO) δ 166.3 (C-1′), 158.7 (C-2), 153.1 (C-3′), 137.2 (C-6′), 133.7 (C-4), 130.8 (C-6), 128.4 (C-8′), 127.3 (C-7′), 127.0 (C-9′), 119.9 (C-1), 117.2 (C-5/C-3), 42.8 (C-5′); Anal. Calcd. For C₁₅H₁₄N₂O₃: C, 66.66; H, 5.22; N, 10.36. Found: C, 66.74; H, 5.36; N, 10.29.

N-(Benzyl carbamoyl)-2-hydroxy-3-methylbenzamide 23b. 8-Methyl-2H-benz[e]-1,3-oxazin-2,4(3H)-dione 22b was allowed to react with benzyl amine according to general procedure G to give 23b which was recrystallised from ethanol (65% yield), mp 220–223°C. (KBr)/cm⁻¹ 3337–2946 (O–H), 3249 (N–H), 1696 (C=O), 1642 (C=O); ¹H NMR (200 MHz, d₆-DMSO) δ 11.30 (bs, 1H, H-2′), 10.70 (bs, 1H, 1-OH), 8.90 (t, 1H, J = 5.6 Hz, 4′-NH), 7.80 (d, 1H, J = 7.5 Hz, H-6), 7.40–7.20 (m, 6H, ArH and H-4), 6.80 (t, 1H, J = 7.5 Hz, H-5), 7.00–6.90 (m, 2H, H-4 and H-6), 4.60 (d, 2H, J = 5.2 Hz, H-5′), 2.20 (s, 3H, CH₃); ¹³C NMR (50 MHz, d₆-DMSO) δ 167.9 (C-1′), 159.7 (C-2), 154.9 (C-3′), 137.2 (C-6′), 135.0 (C-4), 128.4 (C-8′), 127.3 (C-7′), 127.0 (C-9′), 126.3 (C-3), 125.5 (C-6), 119.0 (C-5), 115.1 (C-1), 42.9 (C-5′), 15.9 (CH₃); Anal. Calcd. For C₁₆H₁₆N₂O₃: C, 67.59; H, 5.67; N, 9.85. Found: C, 67.64; H, 5.54; N, 9.87.

N-(Benzyl carbamoyl)-4-ethoxy-2-hydroxybenzamide 23c. 7-Ethoxy-2H-benz[e]-1,3-oxazin-2,4(3H)-dione 22d was allowed to react with benzyl amine according to general procedure F to give 23c which was recrystallised from ethanol (70% yield), mp 204–206°C. (KBr)/cm⁻¹ 3333–2876 (O–H), 3225–3156 (N–H), 1692 (C=O), 1650 (C=O); ¹H NMR (200 MHz, d₆-DMSO) δ 11.90 (bs, 1H, H-2′), 10.20 (bs, 1H, 2-OH), 8.00 (t, 1H, J = 5.4 Hz, 4′-NH), 7.90 (d, 1H, J = 8.9 Hz, H-6), 7.30 (m, 5H, ArH), 6.60 (dd, 1H, = 2.6 Hz, = 8.9 Hz, H-5), 6.50 (d, 1H, J = 2.2 Hz, H-3), 4.40 (d, 2H, J = 6.0 Hz, H-5′), 4.10 (q, 2H, J =6.8 Hz, CH₂–O), 1.30 (t, 3H, J = 6.8 Hz, CH₃); ¹³C NMR (50 MHz, d₆-DMSO) δ 166.1 (C-1′), 160.7 (C-4), 158.9 (C-2), 153.2 (C-3′), 137.2 (C-6′), 132.4 (C-6), 128.4 (C-8′), 127.3 (C-7′), 127.0 (C-9′), 110.5 (C-5), 109.4 (C-1), 101.7 (C-3), 64.6 (CH₂–O), 42.8 (C-5′), 14.5 (CH₃); Anal. Calcd. For C₁₇H₁₈N₂O₄: C, 64.96; H, 5.77; N, 8.91. Found: C, 65.03; H, 5.82; N, 8.84.

N-(Benzyl carbamoyl)-2,4-dihydroxybenzamide 23d. 7-Hydroxy-2H-benz[e]-1,3-oxazin-2,4(3H)-dione 22e was allowed to react with benzyl amine according to general procedure G and gave 23d which was recrystallised from ethanol (75% yield), mp 220°C. (KBr)/cm⁻¹ 3200–2700 (O–H), 3118, 3033 (N–H), 1692 (C=O), 1665 (C=O); ¹H NMR (200 MHz, d₆-DMSO) δ 9.00 (t, 1H, J = 5.6 Hz, 4′-N–H), 7.70 (d, 1H, J = 7.6 Hz, H-6), 7.30 (bm, 6H, ArH, 2′-OH), 6.30 (s, 1H, H-3), 6.2 (d, 1H, J = 7.6 Hz, H-5), 5.60 (m, 2H, 4-OH and 2′-NH) 4.40 (d, 2H, J = 6.0 Hz, H-5′); ¹³C NMR (50 MHz, d₆-DMSO) δ 163.5 (C-1′), 161.9 (C-4), 158.5 (C-2), 153.2 (C-3′), 138.5 (C-6′), 135.3 (C-6), 128.7 (C-8′), 127.7 (C-7′), 127.5 (C-9′), 109.2 (C-1), 102.9 (C-5), 107.9 (C-3), 48.2 (C-5′); Anal. Calcd. For C₁₅H₁₄N₂O₄: C, 59.59; H, 4,67; N, 9.27. Found: C, 59.88; H, 4.68; N, 9.59.

N-(Benzyl carbamoyl)-2,4-dihydroxy-3-methylbenzamide 23e. 7-Hydroxy-8-methyl-2H-benz[e]-1,3-oxazin-2,4(3H)-dione 22f was allowed to react with benzylamine according to general procedure G to give 23g which was recrystallised from ethyl acetate (65% yield), mp 230°C. (KBr)/cm⁻¹ 3381, 3225 (O–H), 3020–2800 (N–H), 1687 (C=O), 1639 (C=O); ¹H NMR (200 MHz, d₆-DMSO) δ 11.00 (bs, 1H, 2′-NH), 9.80 (bs, 1H, 2-OH), 8.5 (t, 1H, J = 5.6 Hz, 4′-NH), 7.5 (d, 1H, J = 8.8 Hz; H-5), 7.4 (bm, 5H, ArH), 6.9 (d, 1H, J = 8.8 Hz; H-6), 4.50 (d, 2H, J = 6.0 Hz, H-5′), 2.00 (s, 3H, CH₃); ¹³C NMR (50 MHz, d₆-DMSO) δ 165.2 (C-1′), 162.9 (C-4), 159.3 (C-2), 157.2 (C-3′), 137.5 (C-6′), 128.7 (C-8′), 127.7 (C-7′), 127.5 (C-9′), 127.3 (C-6), 112.2 (C-1), 110.9 (C-3), 107.9 (C-5), 44.2 (C-5′), 8.6 (3-CH₃); Anal. Calcd. For C₁₆H₁₆N₂O₄: C, 63.99; H, 5.37; N, 9.33. Found: C, 63.88; H, 5.68; N, 9.59.

4-Acetamido-2-hydroxy-N-(phenylcarbamoyl)benzamide 23f. N-(2,4-Dioxo-3,4-dihydro-2H-benz[e][1,3]oxazin-7-yl) acetamide 22 g was allowed to react with benzyl amine according to general procedure G. The resulting solid was filtered and recrystallised from ethanol to give 23f (0.25 g, 76%) mp 257–260°C. (KBr)/cm⁻¹ 3500–3200 (OH), 3281, 3122 (NH), 1694, 1664 (C=O), 1613, 1559 (C=C); ¹H NMR (200 MHz, 340 K, d₆-DMSO) δ 11.63 (bs, 1H, 10-NH), 10.24, 10.06 (15-NH, 2-OH), (s, 1H, J H10,H11 = 5.4 Hz, 10-NH), 7.84 (d, 1H, J H6,H5 = 8.8 Hz, H-6), 7.58 (d, 1H, J H3,H5 = 1.8 Hz, H-3), 7.38–7.24 (m, 5H, ArH, H-13, H-14, H-15), 7.03 (dd, J H5,H6 = 8.8 Hz, J H5,H3 = 1.8 Hz, H-3), 4.45 (d, 2H, J H11,H10 = 5.9 Hz, H-11), 2.07 (s, 3H, H-17); ¹³C NMR (50 MHz, 300 K, d₆-DMSO) δ 168.7 (C-16), 165.7 (C-7), 157.5 (C-2), 152.9 (C-9), 144.6 (C-4), 138.8 (C-12), 131.1 (C-6), 128.1, 126.9, 126.7 (C-14, C-13, C-15), 111.2 (C-1), 110.5 (C-5), 106.1 (C-3), 42.6 (C-11), 23.9 (C-17); Anal. Calcd. For C₁₇H₁₇N₃O₄: C, 62.38; H, 5.23; N, 12.84. Found: C, 62.44; H, 5.28; N, 12.90.

4-Amino-N-(benzyl carbamoyl)-2-hydroxybenzamide 23g. 7-Amino-2H-benz[e][1,3]oxazine-2,4(3H)-dione 22h was allowed to react with benzylamine for 16 h in slight modification to general procedure G. The resulting solid was filtered and recrystallised from toluene to give 23g (0.15 g, 54%) as light brown solid, mp 208–211°C decomp. (KBr) cm⁻¹ 3500–3200 (OH), 3489, 3390, 3330 (NH), 1686 1645 (C=O); ¹H NMR (200 MHz, 340 K, d₆-DMSO) δ 10.18 (bs, 1H, 2-OH), 9.00 (t, 1H, J H10,H11 = 5.1 Hz 10-NH), 7.65 (d, 1H, J H6,H5 = 8.6 Hz, H-6), 7.34–7.27 (m, 6H, Ar, H-13, H-14, H-15 and 8-NH exchangeable with D₂O), 6.19–6.11 (m, 2H, H-5 and H-3), 5.88 (bs, 2H, 4-NH₂), 4.42 (d, 2H, J H11,H10 = 5.1 Hz, H-11); ¹³C NMR (50 MHz, 300 K, d₆-DMSO) δ 166.6 (C-7), 159.4 (C-2), 154.8, 153.3 (C-4,C-9), 139.0 (C-12), 131.7 (C-6), 128.0 (C-14), 126.9, 126.6 (C-15, C-12), 106.6 (C-5), 105.5 (C-1), 99.2 (C-3), 42.5 (C-11); Anal. Calcd. For C₁₅H₁₅N₃O₃: C, 63.15; H, 5.30; N, 14.73. Found: C, 62.78; H, 4.90; N, 14.30.

4.2. Antibacterial Assays

4.2.1. Determination of Minimal Inhibitory Concentrations (MICs) of Novel Compounds

Minimal inhibitory concentrations (MICs) were determined on 7 different bacterial and 4 fungi cultures. The bacteria include Escherichia coli (FB5), Acinetobacter baumannii (ATCC19606), Pseudomonas aeruginosa (PAO9503), Staphylococcus aureus (FB13), Bacillus subtilis (PAO9503), Streptococcus agalactiae (FB31) Mycobacterium smegmatis (CON 21), and Mycobacterium chlorophenolicum (CON 24). The fungi include Aspergillus niger, Rhizopus oryzae, Absidia corymbifera, and Alternaria alternata. Each one of the different strains of bacteria were cultured into 10 mL of nutrient broth (NB) or malt extract broth for fungi and incubated overnight at 37°C except for Mycobacterium smegmatis at 30°C for 24 hours and Mycobacterium chlorophenolicum for 4 days. All fungal cultures were grown at 25°C for up to seven days except for Alternaria alternate, 3 days 35°C. The grown cultures were diluted (1/10 in NB or Malt extract broth) and incubated for a further 2 hours (to reach exponential phase) and then used in the MIC assay. Compound stock solutions of 10⁴ μg/mL were made up in DMSO ensuring that a maximum of 30 μL is used, otherwise inhibitory effects will be shown on some bacterial/fungal cultures). In sterile microcentrifuge tubes, varying amounts of exponential phase culture (0.1 mL) were added and NB (0.9 mL) to make up a total volume of 1 mL. Each culture was then incubated at 37°C overnight. Control tubes were made using DMSO without the addition of compound. At the completion of the incubation, the microcentrifuge tubes containing culture were vortexed and compared to their respective controls (without compound). Compounds which displayed an absence of turbidity lower than 50 μg/mL were subject to further dilutions, while if there is growth (or turbidity) at a particular concentration then the value is recorded as the MIC. The dilution series was carried out in factors of 2 as recommended (i.e., at 200, 100, 50, 25, 12.5, 6.25, and 3.125 μg mL⁻¹ resp.). The MIC was determined by the absence of turbidity at the lowest concentration.

4.2.2. Agar Disk Diffusion Method

Compounds that were insoluble in DMSO or the NB were evaluated for their antimicrobial activity by agar diffusion assays. The surface of an NA or Malt agar plate was flood-inoculated with an overnight NB or malt broth culture of a particular culture adjusted to 10⁸ CFU/mL (10⁸ colony forming units per millimeter). Each disk contained a specific culture and sterile 12.7-mm paper disk (oxide) was placed onto the dry surface for each compound. The insoluble compound was resuspended in DMSO and a 20 μL aliquot was impregnated onto the surface of a sterile paper disc including 20 μL of DMSO control. The diameter of the zone of inhibition was measured after incubation for 18 hr and compared to the control zone (DMSO).

Conflict of Interests

The authors do not have any conflict of interests.

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