Synthesis of cyclic ethers via bromine assisted epoxide ring expansion

Abstract

Neighbouring group participation by epoxide oxygen in the opening of bromonium ions results in the stereoselective synthesis of cyclic ethers. 9-Oxabicyclo[6.1.0]non-4-ene gives trans,trans-2,6-dibromo-9-oxabicyclo[3.3.1 ]nonane and trans,trans-2,5-dibromo-9-oxabicyclo[4.2.1 ]nonane. Sequential bromination and Bu₃SnH reduction converts 1,2-epoxyhex-5-ene into cis- and trans-2,5-dimethyltetrahydrofuran and 2-methyltetrahydropyran while (+)-cis-limonene oxide is converted into non-chiral cineole.