Novel One-Pot Photochemical Synthesis of 3-C-Aroyl-2-aryl-4H-1-benzopyran-4-ones

References

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• In a typical experiment, the substrate, la‐d (0.3 mM) was dissolved in methanol (20 ml) and placed in the photooxygenation compartment of a specially fabricated photooxygenation apparatus equipped with an oxygen inlet/outlet, a water condenser attached to trap and potassium chromate (3% aqueous solution) screen to filter off the possible UV radiation. A catalytic amount of the sensitizer (methylene blue, 5 mg) was added and the mixture was irradiated with tungsten filament lamps (4100 watt), while a stream of oxygen gas was continuously passed through the solution. The progress of the reaction was monitored by t.1.c. on silica gel (BDH, Bombay) and when the reaction was complete, the solvent was removed under reduced pressure and the residue on column chromatography over silica gel gave the products in yields enlisted in Table 1
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• The formation of the product can also be explained by assuming the isomerization of 1, to 3‐benzylbenzopyrones, 3, followed by triplet oxygen oxidation to the products. This mechanism however is untenable because the reaction does not take place in the absence of methylene blue
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• The formation of about 28% of 4‐chlorobenzoic acid, however, suggests that the reaction involves the formation of a dioxetane intermediate which could cleave to the degradation products (Pathway‐b). Since no corresponding acid could be obtained in other cases, it is difficult at this stage to distinguish whether the formation of dioxetane preceeds the formation of the hydroperoxide or vice versa
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