Umpolung with Sulfur Dioxide: Carbon-Carbon Cross-Coupling of Electron-Rich 1,3-Dienes and Alkenes; Application to the Enantioselective Synthesis of Long-Chain Polyketide Fragments

At low temperature and in the presence of an acid catalyst, sulfur dioxide undergoes hetero-Diels-Alder additions with 1-oxy-1,3-dienes, giving unstable sultines, which are ionized into zwitterionic species and react as 1-oxyallylic cation intermediates (Umpolung with SO2) with electron-rich alkenes such as allylsilanes or enoxysilanes. The (beta,gamma-unsaturated silyl sulfinates so-obtained can be desulfinylated in situ to generate polyketide fragments containing up to three contiguous stereogenic centers in one-pot operations. This reaction cascade can be applied in two-directional chain elongation approaches for the asymmetric synthesis of long chain polyketide fragments using 1-[(S)- or (R)-1-phenylethoxy]1,3-dienes.