The Dienolate [2,3]-Wittig Rearrangement – Diastereoselective Synthesis of Highly Functionalized Tertiary Alcohols

Abstract

The [2,3]-Wittig rearrangement of dienolates generated from α-allyloxy-substituted α,β- and/or β,γ-unsaturated esters 4/5a–g has been investigated. The rearrangement proceeds with high yield and diastereoselectivity to afford 3-hydroxy-3-alkyloxycarbonyl-substituted 1,5-hexadienes 6a–e. The influence of the reaction conditions and of various substituents on the rearrangement has been studied. A transition state structure is suggested in order to explain the simple diastereoselectivity observed. The starting material for the rearrangement was prepared utilizing an aldol condensation strategy. The α-allyloxy-substituted esters 2a–c were deprotonated and treated with various ketones to chemoselectively afford the alcohols 3a–g. Thionyl chloride mediated elimination provided the desired unsaturated esters 4/5a–g as starting materials for the rearrangement. The difference in reactivity between an enolate and a dienolate is explained on the basis of a DFT quantum chemical calculation of the HOMO/LUMO energy gap.