Abstract
Alkyl substituents on Cβ of the CC double bond hinder both photodimerization of endocyclic oxa‐enones (1–4) and their photocycloaddition to olefins, and they cause a loss of regiospecificity in dimerization in unpolar solvents. The oxa‐enone 3 which like the bicyclic compounds 4 fails to dimerize at all, is shown to incorporate deuterium at the tertiary isopropyl carbon atom in CD3OD solution as a consequence of a bimolecularly initiated photoenolization.