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Abstract
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 originated from asymmetric induction with a noteworthy stereoelectronic effect of the hydroxyl group. Configurations were assigned by a spectroscopic method using previously reported 13 C 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 follwed by a thio-Claisen rearrangement.
Published Version
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