Stereocontrolled formation of two adjoining chiral centers by thio-Claisen rearrangement of S-allylated ketenes of α-hydroxydithioacetal

Article abstract of DOI:10.1016/S0040-4020(01)86559-5

β-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)-α-hydroxy ketene dithioacetals 20Z-29Z. Thio-Claisen rearrangement of these dienic compounds occured even at room temperature. It gave the corresponding α-allylic β-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 β-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 ¹³C 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.