Remarkable stereoselectivity in the hydrolysis of dioxolenium ions and orthoesters fused to anchored six-membered rings

Abstract

Hydrolysis of dioxolenium (acyloxonium) ions fused to anchored six-membered rings gives almost exclusively that hydroxyester in which the ester function is axial (and the hydroxyl group equatorial). With the exception of the orthoformate, a group of related orthoesters reacted similarly. The potential utility of these observations in stereoselective synthesis is suggested by the following examples, (a) With trans-decalin-cis-2,3-diol (21) formation of the mono-benzoate via the orthoester leads to the axial ester (23d) in good yield; this procedure is complementary to reaction with benzoyl chloride and pyridine, which gives the equatorial ester (24d) as the only isolated product, (b) The action of silver acetate and iodine in wet acetic acid (the Woodward–Prevost reaction) on trans-Δ2-octaIin gives the axial acetate–equatorial alcohol (23b) again as the only significant product. The generality of this stereoselectivity is further supported by a number of individual examples drawn from the chemistry of carbohydrates. A rationalization is offered which qualitatively accounts for the observed stereoselectivity and its absence in the hydrolysis of the orthoformate, and which is based on the differences in steric strain among the possible transition states that fulfil the stereoelectronic requirements of dialkoxycarbonium ion formation.