Asymmetric synthesis of (S)-(+)-carnitine and analogs

A general asymmetric route to enantiomerically pure (S)-(+)-carnitine and analogs has been investigated that involves monoaddition of organometallic reagents to the lactone carbonyl group of (5R,6S)-4-(benzyloxycarbonyl)-5,6-diphenyl-2,3,5,6-tetrahydro-4H-1,4-oxazin-2-one and Lewis acid promoted stereoselective allylation of the resulting hemiacetals. The diastereomerically pure allyl oxazines thus obtained were readily converted into enantiomerically pure (S)-(+)-carnitine and two substituted analogs.