Abstract
Photocycloaddition of carbonyl compounds to alkenes (Paterno-Büchi photoreaction) provides a straightforward entry to oxetanes, which are versatile building blocks for organic synthesis. Cycloreversion (CR) of oxetanes can be initiated by photosensitized electron transfer (PET), a possibility that has recently attracted considerable interest in connection with its possible involvement in DNA photorepair. Here it is shown that initial O-C2 bond cleavage of oxetane 1 under PET conditions leads to a distonic 1,4-radical cation, which is trapped by nitriles to give ring expanded oxazine adducts. This is a new reaction, which formally constitutes the creation of a six-membered heterocyclic ring from C=C, C=O and C≡N units. In addition, splitting of the oxetane radical cation, through stepwise cleavage of the C2-C3 and O-C4 bonds, results in a retro-Paterno-Büchi reaction. Theoretical calculations based on UB3LYP/6-31G(d)/PCM level support the proposed mechanism.
