RAPID REGIOSELECTIVE AND DIASTEREOSELECTIVE PATERNO-BUCHI REACTION OFALKYL 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 wi th high regio and stereoselectivity. The well-known intramolecular gam ma-hydrogen abstraction (Norrish type II) cannot compete. When less el ectron-rich alkenes are used, the Norrish type II reaction becomes com petitive. Intramolecular Paterno-Buchi reactions predominate in those alkyl phenylglyoxylates containing properly situated electron-rich alk ene groups. The regioselectivity of this reaction is explained by the stability of the intermediate 1,4-biradical. The appropriate conformat ion at the instant of intersystem crossing determines the stereoselect ivity of the products. The priority of the Paterno-Buchi over the Norr ish type II reaction is understood by considering the conformation of phenylglyoxylate esters.