Phototransformations of some 3-alkoxy-6-chloro-2-{(E)-1′-methyl-2′-phenylvinyl}chromones: A study of type-II and triene bichromophoric system

Irradiation of a methanolic solution of 3-alkoxy-6-chloro-2-{(E)-1′-methyl-2′-phenylvinyl}-4-oxo-4H-1-benzopyrans yielded 12H-benzo[a]xanthen-12-one, pyranopyrones, oxetanopyrananones and 7-chloro-3-methyl-2-phenyl-1,4-dioxa-cyclopenta-[b]naphthalene-9-one depending upon the structure of 3-alkoxy group (methoxy, benzyloxy, and allyloxy). The methyl group on the styryl moiety has been found to have a profound effect on the product formation due to the variation of energy gap between the π–π* excited state of triene system as compared to the n–π* state of the CO group.