Terpenoid ether formation in superacids

Abstract

A number of terpenoid bicyclic ethers have been prepared by cyclisation of suitable precursors in fluorosulphuric acid–sulphur dioxide. The products are formed under a mixture of thermodynamic and kinetic control, and five-, and six-, and seven-membered-ring ethers are obtained. Both primary and secondary alcohols have been cyclised by attack of the hydroxy group on carbocation centres generated by protonation of alkenes or ionisation of tertiary alcohols; tertiary alcohols ionise too readily to provide an ethereal oxygen atom. Reaction of the unsaturated secondary alcohol dihydrocarveol (2) with antimony pentafluoride in sulphur dioxide gives the ether by a reaction which may not involve a carbocation, and hence may be suitable for ether formation from substrates which rearrange readily in acids.