STEREOCHIMIE DE LA DEPROTONATION ET DE LA REACTION D'ALDOLISATION DES DITHIOESTERS.

Article abstract of DOI:10.1016/S0040-4020(01)96370-7

Cis lithium thioenolates are preferentially formed by deprotonation of dithiopropanoates with lithium diisopropylamide in tetrahydrofuran at -78deg C.The cis selectivity observed, is improved by increasing the alkylthio group size or by the ability of this group to chelate the lithium cation.When more bulkier bases such as lithium 2,2,6,6-tetramethylpiperidide or lithium hexamethyldisilazane are used, the selectivity is lowered.This lowering of selectivity is suppressed when the deprotonation is performed in presence of 12-crown-4.Addition of hexamethyl phosphoramide to the base does not invert the selectivity as it was reported for the deprotonation of esters ; a rather better cis selectivity is reached in the case of methoxymethyl dithiopropanoate.These original results are well understood in terms of an open transition state model.Preformed lithium thioenolates are reacted with a variety of aldehydes and affored stereospecifically syn aldols.The influence of hexamethylphosphoramide and reaction time is also examined.