Thermal and Photochemical Oxetane Formation. A Contribution to the Synthesis of Branched-Chain Aldonolactone

Oxetane synthesis through the Paternò-Büchi reaction

The Paternò-Büchi reaction is a photochemical reaction between a carbonyl compound and an alkene to give the corresponding oxetane. In this review the mechanism of the reaction is discussed. On this basis the described use in the reaction with electron rich alkenes (enolethers, enol esters, enol silyl ethers, enanines, heterocyclic compounds has been reported. The stereochemical behavior of the reaction is particularly stressed. We pointed out the reported applications of this reaction to the synthesis of naturally occuring compounds.

Rapid Regio- and Diastereoselective Paternò-Büchi Reaction of Alkyl Phenylglyoxylates

Triplet excited states of alkyl phenylglyoxylates react rapidly (k = 9.4 x 10(9) M(-)(1) s(-)(1)) with electron rich alkenes forming oxetanes with high regio and stereoselectivity. The well-known intramolecular gamma-hydrogen abstraction (Norrish type II) cannot compete. When less electron-rich alkenes are used, the Norrish type II reaction becomes competitive. Intramolecular Paternò-Büchi reactions predominate in those alkyl phenylglyoxylates containing properly situated electron-rich alkene groups. The regioselectivity of this reaction is explained by the stability of the intermediate 1,4-biradical. The appropriate conformation at the instant of intersystem crossing determines the stereoselectivity of the products. The priority of the Paternò-Büchi over the Norrish type II reaction is understood by considering the conformation of phenylglyoxylate esters.