Stereospecificite de l'addition de Michael d'enethiolates avec les enones acyliques

Article abstract of DOI:10.1016/S0040-4020(01)80066-1

1,4-Addition reaction of lithiated dithioesters with acyclic β-substituted E enones affords diastereomeric 5-oxo-alkanedithioates.Anti configuration was assigned to yhe major diastereoisomer by chemical correlation.This stereochemistry depends on the configuration of the prochiral partners.The cis/trans ratio of intermediate enethiolates was analyzed by trapping experiments.Two cases of highly selective cis deprotonation of dithioesters were observed.Michael addition with E-enones (3-penten-2-one, chalcone) occurs with high stereospecificity : anti/syn product ratios are identical to the cis/trans enethiolate ratios.A pseudo-cyclic transition state is proposed with substituents of the enone C-C double bond occupying favorable equatorial positions.The 1,4-addition reaction is kineticaly controlled.In contrast to ester enolates, enethiolates allow the use of enones without the need of tertiobutyl or phenyl groups.The observed diastereoselectivity (ratio anti/syn up to 95:5) offers a new means of creation of two vicinal carbon substituted asymmetric centers by carbon-carbon bond formation in the acyclic series.Poor selectivities were however observed when using unsaturated diesters as acceptors.