Asymmetric Induction in the Thio-Claisen Rearrangement. Creation of three Contiguous Stereogenic Centres from α-Hydroxy Ketene Dithioacetals

Article abstract of DOI:10.1016/S0040-4020(01)92192-1

Syn β-hydroxy γ-methyl dithioesters were deprotonated with two equivalents of LDA.A subsequent S-alkylation of the resulting dianions yielded α-hydroxy S-allyl ketene dithioacetals.A single isomer was observed, arising from a selective cis deprotonation.These dithioacetals underwent a rapid and highly diastereoselective thio-Claisen rearrangement at an ambient temperature.Syn-syn α-allyl β-hydroxy γ-methyl dithioesters were mainly formed.The syn-syn/anti-syn ratios ranged from 85 : 15 to 99 : 1.The selectivity observed originates from asymmetric induction with a noteworthy stereoelectronic effect of the hydroxyl group.Configurations were assigned by a spectroscopic method using previously reported 13C NMR rules and then confirmed by a syn-syn diastereoselective aldol reaction.This novel and efficient method for the creation of frameworks with three contiguous stereogenic centres, involves two diastereoselective steps: an aldol reaction followed by a thio-Claisen rearrangement.