Studies on the lewis acid mediated ring expansion of cyclic ketones via pinacol-type rearrangement

Abstract

Two new methods for the ring expansion of cyclic ketones have been developed. Vinyl spiroepoxides of cyclobutanones and cyclopentanones readily rearrange to their ring homologated 2-vinylcycloalkanones in the presence of 1.1 equiv. of magnesium (II) triflate and of 1.5 equiv. of zinc(II) bromide respectively. α,β-Epoxysilanes from cyclobutanones have been also found to be rearranged successfully, upon treatment with 1.0 equiv. of magnesium (II) triflate, to produce the ring expanded products, 2-trimethylsilylcyclopentanones. For unsymmetrical ketones, the more highly substituted alkyl group migrates preferentially as many other Pinacol-type rearrangement.