STEREOSPECIFIC THIO-CLAISEN REARRANGEMENT OF S-CROTYLIC ALPHA-HYDROXYKETENE DITHIOACETALS - CREATION OF 3 CONTIGUOUS STEREOGENIC CENTERS

All four diastereoisomeric S-crotylic alpha-hydroxy ketene dithioaceta ls (ZE', ZZ', EE' and EZ') were prepared uniquivocally from S-methyl o r S-crotyl (Z or E) beta-hydroxy dithioesters by a tandem cis-deproton ation with LDA and S-alkylation. These dithioacetals underwent, in a r efluxing cyclohexane solution, an easy thio-Claisen rearrangement into dithioesters, containing three contiguous chiral centres. The rearran gement is stereospecific. Furthermore each of the four system led to t he formation of a different major diastereoisomer, thus making all of the four possible isomers (anti-anti, syn-syn, anti-syn and syn-anti) accessible. A relationship between the main component configuration an d the starting dithioacetal geometry has been ruled out. The observed stereospecificity originates from two independant stereocontrols, an i nternal and an external one. The former is in aggreement with the clas sical internal control obtained with a [3.3] sigmatropic shift. The la tter is a result of an asymmetric induction but surprisingly, is depen dent on the S-crotylic double bond geometry. All the results were rati onalised by transition state models and the configurations proven by c hemical correlations: transformation into known esters and Swern oxida tion.