AUTO-PROTONATION DIASTEREOSELECTIVE D'ENOLATES ISSUS DE L'ADDITION-1,4 D'ENETHIOLATES AVEC DES ENONES

Article abstract of DOI:10.1016/S0040-4020(01)85987-1

1,4-Addition reaction of lithiated methyl dithioacetate with alpha,-beta-disubstituted enones affords diastereomeric 5-oxodithioalkanoates.Syn configuration was assigned to the major diastereomer by chemical correlation.High diastereoselectivities were obtained with 2,2,4-trimethyl-4-hexen-3 one and 2-ethyldenecycloalkanones, making this reaction useful for selective synthesis of acyclic or semi-cyclic chains with 1,2-adjacent asymmetric carbons.The stereochemical course is opposite to the one normally observed for the protonation of acyclic diastereotopic enolates (Houk model).Trapping experiments revealed that the species present prior to hydrolysis is not an enolate but an enethiolate, already bearing the syn stereochemistry.These enethiolates arise from the following steps : Michael addition gives and elusive enolate which undergoes a fast "auto-protonation" : transfer of the hydrogen alpha to the thiocarbonyl group towards the enolate moiety.Geometry of the enethiolate double bond is unique and probably cis.An intramolecular concerted auto-protonation mechanism is discussed and a pseudo-cyclic transition state is tentatively assigned.An example of a tandem Michael addition/Claisen rearrangement was achieved by S-allylation of the addition intermediate followed by transposition of the resulting unsaturated ketene dithioacetal at room temperature.Diastereoselectivity for the protonation of 1,3-diastereotopic enolates was also examined.