Steric and electronic effects on the photochemical reactivity of oxime acetates of β,γ-unsaturated aldehydes

Abstract

A general synthesis of 5-substituted N-acetoxy 3,3-dimethyl-1-azapenta-1,4-dienes starting from 2-(1,3-dithian-2-yl)-2-methylpropanal is described. The influence of 5-phenyl, 4-phenyl, 5-cyclohexyl, 5-tert-butoxycarbonyl and 5,5-dicyclohexyl substitution on the outcome of the photochemical reactions of the oxime acetates of β, γ-unsaturated aldehydes has been studied with a view to proving or disproving the operation of a deactivating free rotor in the aza-di-π-methane rearrangement. The results obtained show that if the radical formed at C-5 can be stabilized by conjugation with an aryl group or by certain types of disubstitution then the aza-di-π-methane rearrangement takes place successfully. In any other situation the reaction fails. These results clearly show that the free rotor effect is not responsible for the failure of C-5 monosubstituted 1-aza-1,4-dienes to undergo the aza-di-π-methane rearrangement.