A closer examination of the mechanism for photolytic ring-opening of cyclohexa-2,4-dienones

Abstract

Irradiation of cyclohexa-2,4-dienones in methanol through Pyrex affords cis-ketens, which then react with methanol by 1,2-addition to give βγ:δε-unsaturated esters. This is true even of dienones with alkyl groups in the 2-, 4-, and 6-positions. The cis-geometry is maintained at the βγ double bond of the ester, although cis-ester can isomerise to trans-ester on irradiation through Vycor. In one case, a minor product was the αβ:γδ-unsaturated ester, due to 1,6-addition to the keten. λ_max. in the u.v. spectra of conjugated dienes with alkyl substituents on the central single bond occurs at appreciably shorter wavelength than predicted by Woodward's rules, so this type of spectrum is unreliable in assigning position or geometry to the double bonds in the photolysis products of highly substituted cyclohexa-2,4-dienones. In the present work, structures are based mainly on n.m.r. spectra.