On the mechanism for the phototransformation of 3-alkoxy-2-(2′-furyl)-4-oxo-4H-1-benzopyrans

Photoirradiation of 3-alkoxy-6-chloro-2-(2′-furyl)-4-oxo-4H-1-benzopyrans 3 led to the formation of methyl 8-chloro-10-oxo-2-phenyl-2,3,4,10-tetrahydropyrano[3,2-b][1]benzopyran-3-ylacetate 4. The reaction proceeds through the formation of 8-chloro-4-phenyl-3a,4,6,11b-tetrahydrofuro-[2′,3′:4,5]pyrano[3,2-b]benzopyran-6-one 5a, which subsequently undergoes a ring contraction–ring expansion mechanism to give the cyclopropanecarbaldehyde 8 followed by its rearrangement to ketene 10via the carbene 9 to furnish ester 4. The various intermediates have been isolated and identified, and their stereochemistry was established from their 1H NMR spectra.