Solvent-Controlled Intramolecular [2 + 2] Photocycloadditions of α-Substituted Enones

The regio- and stereoselectivity of intramolecular [2 + 2] photocycloadditions of 2‘-hydroxyenones
are shown to be solvent-dependent. In the presence of aprotic solvents, 2‘-hydroxyenones undergo
photocycloadditions in a manner consistent with the presence of an intramolecular hydrogen bond between
the carbonyl group and the tether's hydroxy functionality. In protic solvents, intermolecular interactions
appear to disrupt the intramolecular hydrogen bond, providing products with complementary diastereoselectivity. If the facial accessibility of the α-tethered olefin is limited, the cycloadditions proceed to give
head-to-tail or head-to-head regioisomers, depending on the nature of the solvent employed.