The dienolate [2,3]-Wittig rearrangement - Diastereoselective synthesis ofhighly functionalized tertiary alcohols

The [2,3]-Wittig rearrangement of dienolates generated from alpha-allyloxy- substituted alpha,beta- and/or beta,gamma-unsaturated esters 4/5a-g has bee n investigated. The rearrangement proceeds with high yield and diastereosel ectivity 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 sugg ested in order to explain the simple diastereoselectivity observed. The sta rting material for the rearrangement was prepared utilizing an aldol conden sation strategy. The alpha-allyloxy-substituted esters 2a-c were deprotonat ed and treated with various ketones to chemoselectively afford the alcohols 3a-g. Thionyl chloride mediated elimination provided the desired unsaturat ed eaters 4/5a-g as starting materials for the rearrangement. The differenc e in reactivity between an enolate and a dienolate is explained on the basi s of a DFT quantum chemical calculation of the HOMO/LUMO energy gap.