Macroring contraction methodology : 3. Total syntheses of costunolide and haageanolide using transannular [2,3]-wittig rearrangement of 13-membered diallylic ethers as key reaction

Abstract

A new route to construct the carbon skeleton of germacrane sesquiterpenes is described wherein the 13-membered diallylic ethers 12 and 41 , prepared by intramolecular O-alkylation of the bromo alcohol and C-alkylation of the cyanohydrin ether respectively, undergo [2,3]-Wittig rearrangement to give the ten-membered carbocycles. Diastereoselectivity in [2,3]-Wittig rearrangement is discussed based on the MM2 transition structure model using the ab-initio calculations.