STEREOCONTROLLED FORMATION OF 2 CONTIGUOUS CHIRAL CENTERS BY THIO-CLAISEN REARRANGEMENT OF S-ALLYLATED ALPHA-HYDROXYKETENE DITHIOACETALS

beta-hydroxydithioesters 8-14 were easily doubly deprotonated to afford one single dianion whose cis geometry results from chelation control. These dianions were transformed by S-allylation into (Z)-alpha-hydroxy ketene dithioacetals 20Z-29Z. Thio-Claisen rearrangement of these dienic compounds occurred even at room temperature. It gave the corresponding alpha-allylic beta-hydroxydithioesters 32-41 with a uniformly high level of syn stereoselectivity in excess of 90/10 independently of the ketene geometry as illustrated by the rearrangement of the (E) ketene dithioacetal 20E issued from the S-allyl beta-hydroxydithioester 15 with a syn/anti ratio 93/7. Syn and anti configurations were assigned after transformation of a syn-anti mixture of 32 or 39 into the corresponding esters 44 and 45 and comparison with a rich anti mixture of the same esters prepared according to Frater's alkylation. These assignments were confirmed by a syn selective aldol reaction of 4-pentenedithioates with the appropriate aldehydes. A few C-13 NMR generalization rules allowing syn and anti configuration determination were also put forth. A transition state model is proposed to explain the observed asymmetric induction by the external hydroxy group as a result of both steric and stereoelectronic control.