The Scientific World JournalVolume 2012 (2012), Article ID 954934, 8 pagesdoi:10.1100/2012/954934

Research Article

Photoinduced Synthesis of New Diisochromenochromen-4-ones and Their Antimicrobial Activities

Mohamad Yusuf, Indu Solanki, and Payal Jain

Department of Chemistry, Punjabi University, Punjab, Patiala147002, India

Received 31 October 2011; Accepted 5 December 2011

Academic Editors: R. Dalpozzo and G. Sommen

Copyright © 2012 Mohamad Yusuf 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.

The diisochromenochromen-4-one 3a-3b, 4a-4c, 5a-6a  & 7 have been prepared from the photocyclization reaction of bischromen-4-one 2a-2e. The later compounds are obtained from the O-alkylation of the suitable 3-hydroxy-2-aryl-4H-chromen-4-one 1a-1e with 4,4′-bischloromethyl-diphenyl in dry acetone, anhydrous K₂CO₃, and PTC (Bu₄N⁺I⁻) under refluxing conditions. The structures of compounds 2a-2e, 3a-3b, 4a-4c, 5a-6a  & 7 have been characterized from the rigorous analysis of their IR, ¹H-NMR, ¹³C-NMR, ESI-Mass, and elemental analysis. The antibacterial and antifungal activities of the synthesized products were also evaluated against the Klebsiella pneumoniae, Pseudomonas aeruginosa, Escherichia coli, Staphylococcus aureus, Bacillus subtilis, and Aspergillus janus and Penicillium glabrum, respectively. Some of the tested compounds showed significant activity against the above-said microorganisms.

1. Introduction

Syntheses of six membered heterocyclic compounds have been the subject of major interest for researchers due to their significant biological activities [1–12]. The photochemical reaction of C=O compounds leads to the formation of many exotic carbocyclic and heterocyclic compounds, and these reactions are initiated through the intramolecular H-abstraction by the photoexcited carbonyl group from the and position to give 1, 4 and 1, 5-biradicals which finally collapse to the generation of many unique heterocyclic products [13–20]. 2-Aryl-3-alkoxy-chromen-4-one is such substrates which can undergo H-abstraction to provide pyran derivatives from the cyclisation reaction of 1, 4-biradical and 2-aryl ring [21–23]. The bischromen-4-one is the bichromophoric molecules which are formed by joining two chromen-4-one moieties together through the carbon chain of varying length and structure, and photochemical reaction of these compounds may produce some interesting heterocyclic compounds. Generally, the heterocyclic compounds are obtained via multistage reaction in the presence of specific reagents and reaction conditions. We are aiming hereby to synthesize bispyran derivatives under the influence of light. This aspect has prompted us to investigate the synthesis of diisochromeno-chromen-4-ones from the photochemical reaction of bischromen-4-one 2a-2e built around the diphenyl spacer moiety. The major interest behind this study was to investigate the simple method and antimicrobial evaluations of new diisochromeno-chromen-4-ones.

2. Results and Discussion

The compound 2a-2e required for this study was obtained from the O-alkylation of the suitable 3-hydroxy-chromen-4-one [24–26] with 4,4′-bischloromethyldiphenyl in the presence of anhydrous K₂CO₃ and Bu₄N⁺I⁻ (PTC) in dry acetone (Scheme 1). The reactions carried out in the absence of PTC provided very poor yield of the bischromen-4-ones and also involved very long reaction times. The monoalkoxy chromen-4-one formed in these reactions was removed by using column chromatography (60–120 mesh). The structures of 2a-2e were determined by means of their IR, ¹H-NMR, and ESI-MS spectral data (see the appendix).

Scheme 1

The IR spectra of 2a-2e exhibited strong absorptions in the region of 1640–1650 cm⁻¹ which indicated the presence of conjugated C=O group. The major feature of their ¹H-NMR spectra was the appearance of sharp singlet at 5.11–5.35 which may be assigned to 3-OCH₂ group. The downfield appearance of this proton could be ascribed to their benzylic nature and placement near an electronegative oxygen atom. ¹³C-NMR of these compounds revealed the most significant downfield signal at 176.13–175.01 (C=O group), another signal at 142.12–139.08 due to C-3 due of its direct linkage to oxygen atom, and remaining aromatic carbon atoms were resonating at 161.47–117.97. The most upfield resonance was found placed at 72.07–69.02 due to OCH₂ group.

The photochemical reaction of bischromen-4-ones 2a-2e was carried out under inert atmosphere in dry MeOH and THF (1 : 1) with Pyrex-filtered light from a 125 W Hg arc lamp. The progress of the photoreactions was monitored by TLC, and after about 8–10 hrs most of the starting compound was transformed to new products (Scheme 2). The column chromatographic separation of the reaction mixtures yielded 3a-3b, 4a-4c, 5a, 6a, and 7 in moderate yields. The structure of these products became evident from the comparison of their IR, ¹H-NMR, and ¹³C-NMR spectra with those of starting compound 2a-2e (see the appendix).

Scheme 2

The appearance of one IR absorption band in the carbonyl group region of 1630–1650 cm⁻¹ indicates that these products are obtained involving the both side cyclization on the bischromen-4-one. The ¹H-NMR spectra of 3a and 3b were quite informative which gave broad singlet at 6.81–6.78 (H-1), dd at 5.85–5.80 (H-3), and another dd at 5.30–5.25 (H-4). The noticeable signals were centered at 5.01–5.05 (H-5) and 3.40–3.45 (H-4a) having coupling value of 11.0 Hz which describes their cis disposition at C-4a and C-5. The four-proton multiplet found at 2.95–2.92 may be assigned to H-2a. ¹³C-NMR spectra of these compounds were also very helpful which showed resonances at 178.20–177.82 due to carbonyl group, at 39.00–38.80 and 32.00–31.05 due to C-4a and C-2, respectively. Another signal at 126.53 could be assigned to C-1 which was absent in their starting compounds.

The ¹H-NMR spectrum of 4a-4c was very simple which produced most of the resonances in aromatic region at 8.30–6.95, and a significant singlet at 5.85–5.78 (2H) may be allotted to H-5. ¹³C-NMR of these compounds showed the suitable signals due to aromatic carbon in the region of 156.68–114.58, and the signals at 72.93–71.74 could be resulted by C-5.

Similarly the major features of ¹H-NMR spectra of 5a-6a were the signals due to H-3a, H-11b, &H-4 which clearly resonating at 3.70–3.67 (2H, ddd), 5.11–5.06 (2H, d), and 5.48-5.45 (2H, d), respectively. The coupling value of  Hz and  Hz describes the cis relationship both between H-3a & H-11b and H-3a & H-4. In the ¹³C-NMR spectra, suitable resonances were found to be placed at 178.20–177.62, 83.30–82.10, 47.20–47.12 and 34.50–33.48 which may be very well represented by C=O, C-4, C-3a, and C-11b, respectively. The various spectral data also fully confirmed the structural features of the compound 7.

Mechanistically, the phototransformation of bischromen-4-ones 2a-2c may be occurring through the H-abstraction by the photoexcited C=O group from the 3-benzyloxy group to give 1,4-biradical which undergoes cyclization with the 2-aryl ring to provide 1,7-biradical. The later may suffer [1,7]-H shift to give 3a-3b while oxidation of the biradical produces 4a-4c (Scheme 3). Similarly, the formation of the products 5a-6a and 7 from the photocyclization of bischromon-4-ones 2c-2d may also be described.

Scheme 3: Mechanism of the photocyclizations of bischromon-4-ones 2a-2c.

3. Antimicrobial Activity

The antimicrobial activity of synthesized compounds was screened in vitro against selected pathogens which include Staphylococcus aureus (MTCC 96), Bacillus subtilis (MTCC 441), Escherichia coli (MTCC 443), Pseudomonas aeruginosa (MTCC 424), and Klebsiella pneumoniae (MTCC 3384), and fungus strains were Aspergillus janus (2751) and Penicillium glabrum (4951). All the compounds were also screened for MIC by using serial tube dilution method [27] at concentration 3.12, 6.25, 12.5, 25, 50, and 100 μg/mL against the above-said microorganisms, and observed minimum inhibitory concentration (MIC-μg/mL) values are given in Tables 1 and 2. Compounds 2a, 2d, and 2e showed significant activity against Escherichia coli and Klebsiella pneumoniae whereas 2b and 2c showed significant activity against Pseudomonas aeruginosa, Staphylococcus aureus, and Pencilluim glabrum. The photoproducts 3a, 3b, 4a, 5a, and 6a also exhibited significant activity against Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, Staphylococcus aureus, and Pencilluim glabrum, respectively. It is evident from the above study that 2-phenyl/thienyl/furanyl-bischromen-4-ones seem to be better antimicrobial agents than 2-tolyl/anisyl-bischromen-4-one derivatives. The importance of this work lies in the possibility that newly prepared compounds (2a, 2b, 2c, 2d, 3a, 3b, 4a, 5a, and 6a) might be more efficacious derivatives against the above-said bacterial and fungal strains.

Table 1: In vitro antimicrobial MIC (μg/mL) of compounds 2a-2b.

Compound No.
Gram-negative bacteria
Gram-positive bacteria
Fungi

Escherichia coli
Klebsiella pneumonia
Pseudomonas aeruginosa
Staphylococcus aureus
Bacillus subtilis
Aspergillus janus
Penicillium glabrum

2a
12.5
12.5
50
25
25
50
25

2b
50
25
12.5
25
12.5
25
50

2c
25
25
12.5
12.5
25
25
12.5

2d
12.5
12.5
25
25
50
12.5
12.5

2e
12.5
12.5
12.5
25
25
25
12.5

Amoxicillin
3.12
3.12
3.12
3.12
3.12

Fluconozole
—
—
—
—
—
3.12
3.12

Table 2: In vitro antimicrobial MIC (μg/mL) of photoinduced compounds 3a-3b, 4a-4c, 5a-6a & 7.

Compound No.
Gram-negative bacteria
Gram-positive bacteria
Fungi

Eschericha coli
Klebsiella pneumonia
Pseudomonas aeruginosa
Staphylococcus aureus
Bacillius subtilis
Aspergillus janus
Penicillium glabrum

3a
12.5
12.5
50
12.5
50
25
12.5

3b
25
25
25
25
12.5
12.5
12.5

4a
12.5
50
12.5
12.5
12.5
12.5
12.5

4b
50
25
25
50
25
50
25

4c
25
12.5
25
25
25
12.5
50

5a
12.5
25
12.5
25
25
25
50

6a
50
12.5
25
12.5
12.5
12.5
12.5

7
12.5
25
50
25
25
25
12.5

Amoxicillin
3.12
3.12
3.12
3.12
3.12

Fluconozole
—
—
—
—
—
3.12
3.12

4. Conclusion

This study provides the photochemical method for the preparation of new diisochromenochromen-4-one linked through the diphenyl moiety. The products have been obtained without using any specific and toxic reagent. The antimicrobial analysis of the prepared compounds has also been carried out and the importance of this work lies in the possibility that newly synthesized compounds (2a, 2b, 2c, 2d, 3a, 3b, 4a, 5a, and 6a) might be more efficacious derivatives against the tested bacterial and fungal strains. The investigations regarding the more biological studies of these bischromen-4-ones could be helpful in designing the potent antimicrobial agents.

Appendix

A. Experimental

A.1. Synthesis of 3,3′-(Biphenyl-4,4′-Diylbis(Methylene))Bis(Oxy)Bis(2-Phenyl-4H-Chromen-4-One) 2a

The bischromenone 2a was synthesized by reacting compound 1a (2.00 g, 0.0089 mole) with 4,4′-bischloromethyl-diphenyl (1.12 g, 0.0044 mole) in presence of anhyd. K₂CO₃ and Bu₄N⁺I⁻ as PTC in dry acetone. Progress of reaction was monitored by TLC. After the completion of reaction, reaction mixture was turned into colorless mass which was poured into iced HCl to obtain a crude product which was recrystallized with CHCl₃ : MeOH to yield pure bischromone 2a.

Colorless solid; yield 32%; m.p.: 180–182°C; IR (KBr) cm⁻¹: 1640 (C=O), 1616 (C=C); ¹H-NMR (400 MHz, DMSO-d₆): 8.32 (2H, dd, 1.6, 8.0 Hz, H-5), 7.99 (4H, dd, 1.5, 5.8 Hz, H-2′, 6′), 7.69 (2H, m, H-7), 7.53 (2H, d, 8.3 Hz, H-6), 7.46 (12H, m, H-8, 4′, 2′′, 3′′, 5′′, 6′′), 7.34 (4H, d, 8.3 Hz, H-3′, 5′), 5.16 (4H, s, OCH₂); ¹³C-NMR (DMSO-d₆): 176.04 (C-4), 155.23 (C-2), 153.02 (C-8a), 139.08 (C-3), 139.00 (C-4′′), 136.04 (C-1′′), 135.82 (C-7), 130.30 (C-1′), 128.51 (C-3′, 5′), 128.10 (C-3′′, 5′′), 127.9 (C-2′, 4′, 6′), 127.6 (C-2′′, 6′′), 125.71 (C-4a), 124.21 (C-5), 123.42 (C-6), 113.64 (C-8), 72.02 (OCH₂); MS (ESI): m/z 655; Anal. Calc. For C₄₄H₃₀O₆: C, 80.73; H, 4.58; found C, 80.69; H, 4.57%.

A.2. Synthesis of 3,3′-(Biphenyl-4,4′-Diylbis(Methylene))Bis(Oxy)Bis(2-p-Tolyl-4H-Chromen-4-One) 2b

The bischromenone 2b was obtained from the reaction of compound 1b (2.00 g, 0.0079 mol) with 4,4′-bischloromethyl-diphenyl (0.99 g, 0.0039 mole) under similar condition as described above for 2a.

Colorless solid; yield 24%; m.p.: 208–210°C; IR (KBr) cm⁻¹: 1642 (C=O), 1611 (C=C); ¹H-NMR (400 MHz, DMSO-d₆): 8.30 (2H, dd, 1.4, 7.9 Hz, H-5), 7.93 (4H, d, 8.3 Hz, H-2′,6′), 7.66 Hz (2H, ddd, 0.9, 1.6, 8.6 Hz, H-7), 7.51 (2H, d, 8.4 Hz, H-6), 7.45 (2H, d, 8.2 Hz, H-8), 7.40 (8H, m, H-2′′, 3′′, 5′′, 6′′), 7.27 (4H, d, 8.2 Hz, H-3′, 5′), 5.12 (4H, s, 3-OCH₂), 2.44 (3H, s, 4′-CH₃); ¹³C-NMR (DMSO-d₆): 175.16 (C-4), 156.78 (C-8a), 155.32 (C-2), 141.13 (C-3), 140.61 (C-4′), 139.62 (C-4′′), 135.81 (C-1′′), 133.40 (C-7), 129.32 (C-3′, 5′), 129.08 (C-1′), 128.80 (C-3′′, 5′′), 128.18 (C-2′′, 6′′), 126.92 (C-2′, 6′), 125.81 (C-5), 124.69 (C-6), 124.24 (C-4a), 118.06 (C-8), 73.71 (OCH₂), 21.61 (4′-CH₃); MS (ESI): m/z 683; Anal. Calc. For C₄₆H₃₄O₆: C, 80.93; H, 4.98%; found C, 80.86; H, 4.97%.

A.3. Synthesis of 3,3′-(Biphenyl-4,4′-Diylbis(Methylene))Bis(Oxy)Bis(2-(4-Methoxyphenyl)-4H-Chroman-4-One) 2c

The bischromenone 2c was synthesized from the reaction of compound 1c (2.00 g, 0.0074 mole) with 4,4′-bischloromethyl-diphenyl (0.93 g, 0.0037 mole) under similar condition as described above for 2a.

Colorless solid; yield 36%; m.p.: 194–196°C; IR(KBr) cm⁻¹: 1638 (C=O), 1603 (C=C); 1H-NMR (400 MHz, DMSO-d₆): 8.20 (2H, dd, 1.4, 7.9 Hz, H-5), 8.04 (4H, dd, 1.8, 7.0 Hz, H-2′,6′), 7.74 (2H, ddd, 1.0, 2.0, 7.6 Hz, H-7), 7.60 (2H, d, 8.3 Hz, H-6), 7.51 (4H, d, 8.3 Hz, H-3′′, 5′′), 7.43 (6H, m, H-8, 2′′, 6′′), 7.00 (4H, dd, 1.8, 7.0 Hz, H-3′, 5′), 5.11 (4H, s, 3-OCH₂), 3.88 (6H, s, 4′-OCH₃); 13C-NMR (DMSO-d₆): 175.07 (C-4), 161.47 (C-8a), 156.61 (C-4′), 155.23 (C-2), 140.59 (C-1′′), 139.18 (C-3), 135.82 (C-7), 133.32 (C-5), 130.62 (C-3′′, 5′′), 129.36 (C-2′′, 6′′), 126.90 (C-2′, 6′), 125.79 (C-4a), 124.66 (C-6), 124.21 (C-4′′), 123.37 (C-1′), 117.97 (C-3′, 5′), 113.75 (C-8), 73.67 (OCH₂), 55.45 (4′-OCH₃); MS (ESI): m/z ; Anal. Calc. For C₄₆H₃₄O₄: C, 77.31%; H, 4.76%; found C, 77.54; H, 4.77%.

A.4. Synthesis of 3,3′-(Biphenyl-4,4′-Diylbis(Methylene))Bis(Oxy)Bis(2-(Thiophen-2-yl)-4H-Chromen-4-One 2d

The bischromenone 2d was synthesized from the reaction of compound 1d (2.00 g, 0.0087 mole) with 4,4′-bischloromethyl-diphenyl (1.10 g, 0.0043 mole) under similar condition as described above for 2a.

Colorless solid; yield 33%; m.p.: 225–227°C; IR (KBr) cm⁻¹: 1641 (C=O), 1603 (C=C); ¹H-NMR (400 MHz, DMSO-d₆): 8.22 (2H, dd, 1.7, 8.0 Hz, H-5), 7.96 (2H, d, 3.7 Hz, H-3′), 7.72 (2H, ddd, 1.2, 2.4, 7.8 Hz, H-7), 7.65 (2H, d, 4.8 Hz, H-5′), 7.62 (4H, d, 8.0 Hz, H-3′′, 5′′), 7.54 (6H, m, H-8, 2′′, 6′′), 7.48 (2H, d, 8.0 Hz, H-6), 7.20 (2H, dd, 3.7, 4.8 Hz, H-4′), 5.35 (4H, s, 3-OCH₂), ¹³C-NMR (DMSO-d₆): 176.13 (C-4), 157.21 (C-8a), 142.12 (C-3), 140.14 (C-1′′), 136.93 (C-2), 135.52 (C-4′′), 135.3 (C-7), 130.61 (C-5), 130.53 (C-5′), 129.08 (C-2′), 128.80 (C-3′′, 5′′), 128.21 (C-4′), 128.18 (C-2′′, 6′′), 127.74 (C-3′), 124.00 (C-4a), 123.24 (C-6), 117.6 (C-8), 69.62 (OCH₂); MS (ESI): m/z 667; Anal. Calc. For C₄₀H₂₆O₃S; C, 72.07, H, 3.90, S, 9.61; found C, 72.13, H, 3.91, S, 9.63%.

A.5. Synthesis of 3,3′-(Biphenyl-4,4′-Diylbis(Methylene))Bis(Oxy)Bis(2-(Furan-2-yl)-4H-Chromen-4-One 2e

The bischromenone 2e was synthesized from the reaction of compound 1c (2.00 g, 0.0082 mole) with 4,4′-bischloromethyl-diphenyl (1.02 g, 0.0041 mole) under similar condition as described above for 2a.

Colorless solid; yield 25%; m.p.: 248–250°C; IR (KBr) cm⁻¹: 1640 (C=O), 1605 (C=C); ¹H-NMR (400 MHz, DMSO-d₆): 8.20 (2H, dd, 1.5, 8.1 Hz, H-5), 7.81 (2H, d, 1.2 Hz, H-3′), 7.75 (2H, ddd, 1.5, 2.4, 7.8 Hz, H-7), 7.62 (10H, m, H-8, 2′′, 6′′, 3′′, 5′′), 7.45 (2H, t, 8.0 Hz, H-6), 7.34 (2H, d, 3.8 Hz, H-5′), 6.66 (1H, dd, 3.6 Hz, H-4′), 5.32 (4H, s, 3-OCH₂); 13C-NMR (DMSO-d₆): 176.01 (C-4), 157.16 (C-8a), 141.92 (C-3), 140.16 (C-1′′), 138.91 (C-2), 135.23 (C-4′′), 135.11 (C-7), 130.59 (C-5), 130.49 (C-5′), 129.12 (C-2′), 128.81 (C-3′′, 5′′), 128.23 (C-4′), 128.10 (C-2′′, 6′′), 127.72 (C-3′), 124.02 (C-4a), 123.12 (C-6), 117.43 (C-8), 69.63 (OCH₂); MS (ESI): m/z 635; Anal. Calc. For C₄₀H₂₆O₈: C, 75.70, H, 4.10; found C, 75.47, H, 4.08%.

A.6. Synthesis of (4aS,5R)-5-(4′-((4aR,5S)-7-Oxo-2,4a,5,7-Tetrahydroisochromeno [4,3-b]Chromen-5-yl)Biphenyl-4-yl)-4a,5-Dihydroisochromeno [4,3-b]Chromen-7(2H)-One 3a

A deoxygenated solution of bischromone 2a (100 mg, 0.00015 mole) was carried out in dry IPA and dry THF with light from a 125 Hg arc lamp under nitrogen atmosphere in a pyrex vessel for 5 hours. The progress of reaction was monitored by TLC. The solvent was distilled out under reduced pressure, and the resulting photolysate thus obtained was subjected to extensive column chromatography (100–200 mesh), packed in petroleum ether-benzene (1 : 1). Elution of column with benzene-EtOAc (3 : 1) furnished starting compound 2a (14%, IR, co-tlc and mmp) and two new compounds 3a and 4a which were further recrystallized using MeOH.

Pale yellow solid; yield 22%; m.p.: 222–224°C; IR(KBr) cm⁻¹: 1648 (C=O); ¹H-NMR (400 MHz, DMSO-d₆): 8.30 (2H, dd, 1.6, 8.2 Hz, H-7), 7.67 (2H, m, H-9), 7.63 (4H, dd, 1.0, 8.5 Hz, H-3′′, 5′′), 7.60 (2H, m, H-10), 7.58 (4H, dd, 1.0, 8.6 Hz, H-2′′, 6′′), 7.55 (2H, d, 8.1 Hz, H-8), 6.78 (2H, brs, H-1), 5.85 (2H, dd, 1.2, 10.0 Hz, H-3), 5.30 (1H, dd, 2.0, 10.3 Hz, H-4), 5.01 (2H, d, 11.0 Hz, H-5), 3.40 (2H, dd, 2.0, 11.0 Hz, H-4a), 2.95 (4H, m, H-2a); 13C-NMR (DMSO-d₆): 177.82 (C-6), 155.52 (C-10a), 142.80 (C-6a), 141.50 (C-11b), 138.70 (C-4′′), 136.43 (C-1′′), 134.82 (C-9), 129.82 (C-4), 128.10 (C-3′′, 5′′), 127.62 (C-2′′, 6′′), 126.53 (C-1), 125.78 (C-7), 126.02 (C-3), 122.80 (C-8), 122.72 (C-11a), 117.02 (C-10), 82.5 (C-5), 38.02 (C-4a), 32.00 (C-2a); MS (ESI): m/z 655; Anal. Calc. For C₄₄H₃₀O₆: C, 80.73; H, 4.58%; C, 80.97; H, 4.57%.

A.7. 5,5′-(Biphenyl-4,4′-Diyl)Diisochromeno [4,3-b]Chromen-7(5H)-One 4a

Brown solid; Yield 24%; m.p.: 218–220°C; IR(KBr)cm⁻¹: 1652 (C=O), ¹H-NMR (400 MHz, DMSO-d₆): 8.30 (2H, dd, 1.6, 8.0 Hz, H-7), 7.98 (2H, dd, 2.4, 8.9 Hz, H-1), 7.64 (2H, m, H-9), 7.62 (4H, dd, 1.0, 8.5 Hz, H-3′′, 5′′), 7.59 (2H, m, H-10), 7.57 (4H, dd, 1.0, 8.4 Hz, H-2′′, 6′′), 7.58 (2H, d, 8.0 Hz, H-8), 7.48 (4H, m, H-2, 3), 7.10 (2H, dd, 2.4, 8.5 Hz, H-4), 5.85 (2H, s, H-5); ¹³C-NMR (DMSO-d₆): 180.02 (C-7), 156.62 (C-12a), 156.21 (C-11a), 139.00 (C-4′′), 136.04 (C-1′′), 134.82 (C-10), 128.32 (C-3′′, 5′′), 128.23 (C-1), 128.12 (C-3), 127.93 (C-8), 127.6 (C-2′′, 6′′), 127.19 (C-4a), 124.40 (C-4), 123.02 (C-2), 122.98 (C-9), 121.67 (C-6a), 113.68 (C-11), 72.93 (C-5); MS (ESI): m/z 648; Anal Calc. for C₄₄H₂₆O₆: C, 81.23; H, 4.00; found C, 81.26; H, 4.16%.

A.8. Synthesis of (4aS,5R)-3-Methyl-5-(4′-((4aR,5S)-1-Methyl-7-Oxo-2,4a,5,7-Tetrahydroisochromeno [4,3-b]Chromen-5-yl)biphenyl-4-yl)-4a,Dihydroisochromeno-[4,3-b]Chromen-7(2H)-One 3b

The photoirradiation of 2b was carried out under similar condition as described above for 2a. The resulting reaction mixture upon chromatographic workup yielded two new photoproducts 3b and 4b.

Light yellow solid; yield 32%; m.p.: 168-170°C; IR(KBr)cm⁻¹: 1638 (C=O); ¹H-NMR (400 MHz, DMSO-d₆): 8.28 (2H, dd, 1.4, 8.1 Hz, H-7), 7.65 (2H, ddd, 1.0, 2.6, 8.5 Hz, H-9), 7.57 (2H, d, 8.0 Hz, H-8), 7.54 (2H, m, H-10) 7.53 (8H, m, H-2′′, 3′′, 5′′, 6′′), 6.81 (2H, brs, H-1), 5.25 (1H, dd, 1.0, 1.9 Hz, H-4), 5.05 (2H, d, 11.0 Hz, H-5), 3.45 (2H, dd, 1.9, 11.0 Hz, H-4a), 2.92 (4H, m, H-2a), 2.42 (3H, s, 4′-CH₃); ¹³C-NMR (DMSO-d₆): 177.20 (C-6), 157.20 (C-11), 143.50 (C-11b), 139.70 (C-4′′), 139.52 (C-1′′), 135.20 (C-9), 129.90 (C-4), 128.07 (C-3′′, 5′′), 127.62 (C-2′′, 6′′), 126.53 (C-1), 125.78 (C-7), 125.10 (C-3), 123.40 (C-8), 122.68 (C-11a), 118.72 (C-10), 81.5 (C-5), 39.02 (C-4a), 20.98 (3-CH₃); MS (ESI): m/z 683; Anal Calc. For C, 80.93; H, 4.98; found C, 80.89; H, 4.96%.

A.9. 5,5′-(Biphenyl-4,4′-Diyl)Bis(3-Methylisochromeno [4,3-b]Chromen-7(5H)-One) 4b

Light brown solid; yield 38%; m.p.: 179–181°C; IR(KBr)cm⁻¹: 1638 (C=O); ¹H-NMR (400 MHz, DMSO-d₆): 8.26 (2H, dd, 1.3, 8.2 Hz, H-7), 7.91 (2H, d, 8.9 Hz, H-1), 7.63 (2H, ddd, 1.0, 2.5, 8.4 Hz, H-9), 7.55 (2H, d, 8.0 Hz, H-8),7.50 (2H, m, H-10), 7.48 (8 H, m, H-2′′, 3′′, 5′′, 6′′), 7.08 (4H, m, H-2, 4), 5.82 (2H, s, H-5), 2.43 (3H, s, 4′-CH₃); ¹³C-NMR (DMSO-d₆): 180.62 (C-7), 156.68 (C-12a), 156.24 (C-11a), 139.04 (C-4′′), 136.01 (C-1′′), 134.78 (C-10), 128.62 (C-3′′, 5′′), 128.23 (C-1), 128.01 (C-3), 127.98 (C-8), 127.62 (C-2′′, 6′′), 127.01 (C-4a), 124.42 (C-4), 123.08 (C-2), 122.95 (C-9), 121.58 (C-6a), 114.52 (C-11), 72.73 (C-5), 20.98 (3-CH₃); MS (ESI): m/z 679; Anal Calc. for C₄₆H₃₀O₆: C, 81.41; H, 4.42; found C, 81.39; H, 4.43%.

A.10. Synthesis of 5,5′-(Biphenyl-4,4′-Diyl)Bis(3-Methoxyisochromeno [4,3-b]Chromen-7(5H)-One 4c

The deoxygenated THF solution of 2c was photolysed under similar conditions as employed for 2a and the chromatographic separation of photolysate furnished starting compound 2a and compound 4c.

Yellow solid; yield 30%; m.p.: 240-242°C; IR(KBr)cm⁻¹: 1642 (C=O); ¹H-NMR (400 MHz, DMSO-d₆): 8.22 (2H, dd, 1.4, 7.8 Hz, H-7), 7.86 (2H, d, 8.7 Hz, H-1), 7.72 (2H, ddd, 1.0, 2.1, 8.3 Hz, H-9), 7.59 (2H, d, 8.2 Hz, H-8), 7.52 (8H, m, H-2′′, 3′′, 5′′, 6′′), 7.49 (2H, m, H-10), 6.95 (2H, m, H-2, 4), 5.78 (2H, s, H-5), 3.84 (6H, s, 4′-OCH ₃); 13C-NMR (DMSO-d₆): 182.02 (C-6), 156.04 (C-11, 11a), 139.04 (C-4′′), 136.07 (C-1′′), 128.4 (C-3′′, 5′′), 128.13 (C-1), 127.82 (C-2), 127.52 (C-2′′, 4′′), 125.80 (C-3), 124.32 (C-4), 123.52 (C-2), 114.58 (C-11), 71.74 (C-5); MS (ESI): m/z 711; Anal Calc. for C₄₆H₃₀O₈: C, 77.75; H, 4.22; found C, 77.98; H, 4.20%.

A.11. Synthesis of 5a

The photolysis of 2d under similar conditions as used for 2a provided starting bischromone 2a and two new products 5a and 7.

Light yellow solid; yield 32%; m.p.: 218-220°C; IR(KBr)cm⁻¹: 1658 (C=O); ¹H-NMR (400 MHz, DMSO-d₆): 8.20 (2H, dd, 1.6, 8.0 Hz, H-7), 7.70 (2H, ddd, 1.0, 2.3, 7.9 Hz, H-9), 7.62 (4H, d, 8.0 Hz, H-3′′, 5′′), 7.55 (6H, m, H-10, 2′′, 6′′), 7.52 (2H, d, 8.1 Hz, H-8), 6.40 (2H, dd, 1.0, 5.8 Hz, H-2), 5.45 (2H, d, 10.0 Hz, H-4), 5.26 (2H, dd, 3.5, 5.8 Hz, H-3), 5.10 (2H, d, 8.5 Hz, H-11b), 3.70 (2H, ddd, 3.5, 8.5, 10.0 Hz, H-3a); ¹³C-NMR (DMSO-d₆): 178.20 (C-6), 156.92 (C-10a), 138.08 (C-4′′), 137.01 (C-1′′), 134.92 (C-9), 130.61 (C-11a), 129.62 (C-7), 128.52 (C-3′′, 5′′), 128.00 (C-2′′, 6′′), 127.7 (C-2), 124.61 (C-3), 123.42 (C-8), 122.80 (C-6a), 116.08 (C-10), 82.08 (C-4), 47.18 (C-3a), 33.52 (C-11b); MS (ESI): m/z 667; Anal Calc. for C₄₀H₂₆O₆S₂: C, 72.07; H, 3.90, S, 9.61; found C, 72.09; H, 3.87; S, 9.59%.

A.12. Synthesis of 7

Pale yellow solid; yield 21%; m.p.: 197–199°C; IR(KBr)cm⁻¹: 1650 (C=O); ¹H-NMR (400 MHz, DMSO-d₆): 8.18 (2H, dd, 1.5, 8.1 Hz, H-7), 7.72 (2H, ddd, 0.9, 2.1, 8.2 Hz, H-9), 7.62 (2H, d, 6.0 Hz, H-2), 7.59 (4H, d, 8.2 Hz, H-3′′, 5′′), 7.54 (6H, m, H-10, 2′′, 6′′), 7.48 (2H, d, 8.3 Hz, H-8), 6.92 (2H, d, 6.0 Hz, H-3), 5.72 (2H, s, H-4); ¹³C-NMR (DMSO-d₆): 178.92 (C-6), 155.58 (C-10), 141.18 (C-3), 140.92 (C-4), 139.72 (C-4′′), 137.41 (C-1′′), 135.02 (C-9), 134.82 (C-2), 132.18 (C-11b), 131.12 (C-11a), 128.12 (C-3′′, 5′′), 127.42 (C-2′′, 6′′), 124.82 (C-7), 123.08 (C-3), 122.82 (C-8), 115.98 (C-10), 73.58 (C-4); MS (ESI): m/z 663; Anal Calc. for C₄₀H₂₂O₆S: C, 72.50; H, 3.32; S, 9.66; found C, 72.48; H, 3.30; S, 9.64%.

A.13. Synthesis of 6a

The bischromone 2e provided starting compound 2e and one new photoproduct 6a in THF under similar condition as used earlier for 2a.

Light yellow solid; yield 28%; m.p.: 251–252°C; IR(KBr)cm⁻¹: 1649 (C=O); ¹H-NMR (400 MHz, DMSO-d₆): 8.20 (2H, dd, 1.5, 8.1 Hz, H-7), 7.74 (2H, ddd, 1.4, 2.3, 7.9 Hz, H-9), 7.58 (10 H, m, H-10, 2′′, 3′′, 5′′, 6′′), 7.48 (2H, d, 8.0 Hz, H-8), 6.43 (2H, brs, H-2), 5.48 (2H, d, 11.0 Hz, H-4), 5.20 (2H, d, 1.0, 2.7 Hz, H-3), 5.06 (2H, d, 8.2 Hz, H-11b), 3.67 (2H, ddd, 1.0, 8.8, 11.0 Hz, H-3a); ¹³C-NMR (DMSO-d₆): 177.68 (C-6), 156.88 (C-10a), 136.92 (C-4′′), 136.01 (C-1′′), 134.82 (C-9), 130.52 (C-11a), 129.58 (C-7), 128.48 (C-3′′, 5′′), 128.11 (C-2′′, 6′′), 127.68 (C-2), 124.58 (C-3), 123.38 (C-8), 122.72 (C-6a), 116.12 (C-10), 82.10 (C-4), 47.12 (C-3a), 33.48 (C-11b); MS (ESI): m/z 635; Anal Calc. for C₄₀H₂₆O₈: C, 75.70; H, 4.10; found C, 75.92; H, 4.11%.

Acknowledgment

Authors are highly thankful to CSIR, New Delhi for providing the grant for this research work in the form of CSIR, JRF.

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