Hydroxy-Group Directivity in the Regioselective and Diastereoselective [2+2] Photocycloaddition (Paternò-Büchi Reaction) of Aromatic Carbonyl Compounds to Chiral and Achiral Allylic Substrates: The Preparation of Oxetanes with up to Three Stereogenic Centers as Synthetic Building Blocks

Abstract

A set of achiral and chiral allylic alcohols and derivatives 4 is regio- and diastereoselectively converted to the oxetanes 5-7 by the Paternò-Büchi photocycloaddition with the carbonyl partners benzophenone (1), acetophenone (2) and benzaldehyde (3). Chiral allylic alcohols with 1,3-allylic strain yield the oxetanes in high threo diastereoselectivity, which was increased by higher steric interactions. The synergistic interplay between hydrogen bonding and 1,3-allylic strain in the CO-bond-forming step is made responsible for this stereoselectivity, as proven by solvent (methanol versus benzene) effects and masking of the allylic hydroxy group. For the prochiral carbonyl partners 2 and 3, an excellent cis diastereoselectivity is found for the substituents on the oxetane ring. This is rationalized in terms of the preferred orthogonal alignment of the 2p orbitals at the radical sites in the intermediary triplet 1,4-diradical for the intersystem-crossing step (ISC). The present highly regio- and diastereoselective photocycloaddition provides an attractive preparative route to sidechain-functionalized oxetanes with up to three stereogenic centers.