Phototransformations of some 3-alkoxy-2-styrylchromones: type II cyclisations of 1,4- and 1,6-biradicals

3-Alkoxy-2-styrylchromones on photo-irradiation with UV-light transform into oxetanopyrananones, pyranopyrones, pyranopyrananones and pyranoalcohols. The products formed have been found to depend upon the structure of alkoxy group (methyl, benzyl and allyls). Alkoxychromones containing a heterocyclic ring (thiophene, furan) in place of phenyl in the styryl group produced only pyranoalcohols as the photoproducts. The photo-conversions have been rationalized through an initial H-abstraction by the Cdouble bond; length as m-dashO group producing a 1,4-biradical. In allyloxy derivatives, cyclisations involve both 1,4- and 1,6-biradicals.