A Novel Synthesis of 1 , 2 , 3-Benzotriazinones from 2-( o-Aminophenyl ) oxazolines

1,2,3-Benzotriazinones are compounds widely investigated due to their interesting biological and chemical properties. These heterocyclic compounds have been studied as anaesthetic [1], anti-inflammatory [2], anticancer [3, 4], and antitumoral [5, 6] agents. In organic synthesis, triazinones are used as an activating moiety in coupling agents for the preparation of peptides and amino acids [7–9]. As a result of their biological and synthetic importance, there is still interest in the development of methods for the synthesis of compounds which contain the 1,2,3-triazinone moiety. In general, 1,2,3-benzotriazinones are prepared via intramolecular cyclization through diazonium ion condensation with an adjacent nucleophilic function. Using this strategy, 1,2,3-triazinones have been obtained from aminobenzamides [10–12], aminonitriles [13, 14], and triazenes [15–17]. Other workers have reported the synthesis of 1,2,3-benzotriazinones by the oxidation of indazol-3-amines with hydrogen peroxide in the presence of sodium carbonate, though in low yields, and by the cyclization of 2-azido-N-4-toluoylbenzamide in moderate yields [18]. In this report, we describe facile access to the title compounds from chiral and nonchiral 2-(o-aminophenyl)oxazolines. Since racemization is not possible during the reaction, this method allows the preparation of enantiopure 1,2,3benzotriazinones in excellent yields.


Introduction
1,2,3-Benzotriazinones are compounds widely investigated due to their interesting biological and chemical properties.These heterocyclic compounds have been studied as anaesthetic [1], anti-inflammatory [2], anticancer [3,4], and antitumoral [5,6] agents.In organic synthesis, triazinones are used as an activating moiety in coupling agents for the preparation of peptides and amino acids [7][8][9].As a result of their biological and synthetic importance, there is still interest in the development of methods for the synthesis of compounds which contain the 1,2,3-triazinone moiety.
In this report, we describe facile access to the title compounds from chiral and nonchiral 2-(o-aminophenyl)oxazolines.Since racemization is not possible during the reaction, this method allows the preparation of enantiopure 1,2,3benzotriazinones in excellent yields.

Results and Discussion
The first step for the synthesis of 1,2,3-benzotriazinones was the preparation of 2-(o-aminophenyl)oxazolines 3a-3c (see (1)).These oxazolines were prepared in high yields (58-80%) by cyclization of anthranilonitrile with ethanolamines under basic conditions in a mixture of glycerol and ethylene glycol, following the methodology reported by Gómez and coworkers [19].In the literature, it has been reported that oxazolines 3a and 3b were obtained as oils, whereas in our work both were isolated as solids; we believe this fact is due to problems in the purification process in the previous report.Nevertheless, NMR and IR spectroscopic and mass spectrometry data are consistent with those previously reported [20].
It is assumed that the reaction takes place by the intermediacy of the diazonium ion 5 formed from 2-(oaminophenyl)oxazoline (3) using a large excess of isoamyl nitrite in order to ensure formation of the diazonium salt (Scheme 1).The electron withdrawing effect of the diazonium group in 5 could promote oxazolinyl ring hydrolysis, taking water from the alcoholic media, and producing the 2carbamoylbenzenediazonium 6.The 1,2,3-triazinone moiety in 4 is then achieved by ring closure between the amide and the adjacent diazonium group in 6.Similar mechanistic assumptions were proposed by Colomer and Moyano in the synthesis of this kind of compound from aminonitriles [13,21].
Infrared spectroscopy of compounds 4a-4c showed a single band at ca. (]) 3430 cm −1 due to the O-H stretching; the bands at ca. (]) 1687 cm −1 and at ca. (]) 1657 cm −1 correspond to the N=N and the C=O stretching, respectively. 1H NMR spectroscopy showed the following: aromatic hydrogen from the 1,2,3-benzotriazinone moiety appeared as an ABCD system with the characteristic two doublets and two triplets between 8.29 and 7.87 ppm, slightly shifted to lower field relative to the 2-(o-aminophenyl)oxazolines; aliphatic protons appeared between 6.39 and 0.90 ppm.For compound 4b, the phenyl protons are at () 7.43 ppm. 13C NMR spectroscopy showed carbonyl carbon at ca. () 156.5 ppm; aromatic carbon appeared in the range 146.1-120.3ppm and aliphatic carbon in the range 64.2-10.8ppm.The proton of hydroxyl group in the proposed structure was not identified by 1 H NMR, but the carbinol moiety was inferred from the chemical shifts on 13 C NMR data.Since compounds 4a-4c are novel, high-resolution mass spectrometry was run with no significant difference between calculated and obtained values (less than ±0.003 amu).In general, the molecular formula assignment and the spectroscopic characterization were consistent with their structures.
In order to corroborate the presence of the hydroxyl group, the acetyl derivative 7a was obtained by reaction of 4a with acetic anhydride in pyridine (see (2)).NMR spectra for the acetyl derivative 7a turned out to be similar to 4a, but with new signals for the acetyl moiety at  H 1.95 and  C 21.5 for the methyl group and  C 171.9 for the ester carbonyl.
Suitable crystals for X-ray diffraction studies of 1,2,3benzotriazinone 7a were grown by slow vapor diffusion of hexane into a saturated solution in ethyl acetate.Compound 7a crystallized in the triclinic system with the space group Pī.Molecular structure of 1,2,3-benzotriazinone 7a is shown in Figure 1, which confirms the presence of the 1,2,3-triazinone system and indirectly corroborates the hydroxyl group in 1,2,3-benzotriazinones 4a-4c.
Of interest to pharmaceutical applications, Reingruber et al. have suggested that the coplanar structure in 1,2,3benzotriazinones could have DNA-intercalating abilities such as those displayed by some anticancer agents [17].
In summary, we have presented an efficient method for the synthesis of 1,2,3-benzotriazinones from 2-(o-aminophenyl)oxazolines in high yields.distillation.Infrared spectra (FTIR) were recorded on a Perkin Elmer FT-IR 1600 spectrophotometer.Nuclear magnetic resonance 1 H (at 200 MHz) and 13 C (at 50 MHz) spectra were recorded on a Varian Mercury 200 MHz spectrometer in (CD 3 ) 2 CO with TMS as internal standard.Melting points were obtained on an Electrothermal 88629 apparatus.Optical rotations were determined using an Autopol III polarimeter.EIMS were recorded on an Agilent Technologies 5975C mass spectrometer.The ESI-HRMS data were performed at High-Resolution Mass Spectrometry Facility, UC Riverside.

General Procedure for the Synthesis of 2-(o-Aminophenyl)Oxazoline (3).
Anthranilonitrile (1 equiv.),potassium carbonate (0.1 equiv.), and ethanolamine (1.7 equiv.)were placed in a Schlenk flask, followed by a solution of glycerol in dry ethylene glycol (5 : 9) (3.3 mL of solution per mmol of anthranilonitrile).The resulting mixture was stirred at 105 ∘ C under argon until the disappearance of the nitrile (followed by TLC, hexane/ethyl acetate, 3 : 1).The mixture was cooled to room temperature and then poured over crushed ice.The resulting white solid was filtered and then dissolved in dichloromethane.The organic solution was washed with water, dried over anhydrous Na 2 SO 4 , and filtered, and the solvent was evaporated under reduced pressure to give a crude product that was purified by flash chromatography on neutral alumina (hexane/ethyl acetate, 20 : 1) to give the pure product.

X-Ray Data Collection and Refinement.
The details of the structure determination are given in Table S1, atomic coordinates are given in Table S2, and bond lengths ( Å) and angles ( ∘ ) are given in Table S3 in Supporting Information.Crystallographic data for the structural analyses have been deposited with the Cambridge Crystallographic Data Centre CCDC 903418.Further details of the crystal structure investigation are available free of charge via https://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033; e-mail: deposit@ccdc.cam.ac.uk).