Synthesis of 1,3-diol derivatives from sterically overcrowded oxiranes. Ring-opening reactions of 1-t-butyl-1,2-epoxycyclohexane.

Abstract

The acid-catalysed ring-opening reactions of 1-t-butyl-1,2-epoxycyclohexane (1)(methanolysis, hydrolysis, and trichloroacetolysis in non-protic solvents) lead to very complex mixtures. In these reactions, in addition to the usual 1,2-primary addition products, and to the non-addition products in which the t-butyl skeleton is still present, considerable amounts of other products are formed. These compounds include 1,3-secondary addition products and other rearranged non-addition products which arise by rearrangement of the original skeleton of (1), by methyl group migration; the aldehyde (20) which is lacking both the t-butyl group and the cyclohexane skeleton is also obtained. However, the opening reactions of (1) in acid media are highly regioselective, most of the reaction products arising from C–O breaking on the tertiary carbon. The structures and the configurations of all the reaction products have been well established by a study of their i.r. and n.m.r. data; however, in some cases the structures and the configurations were confirmed either through unequivocal syntheses and/or chemical correlations. The stereoselectivity of the trichloroacetolysis reactions of (1) is not completely anti, even if the amounts of 1,2 adducts formed are somewhat small, and the syn/anti ratio increases with the polarity of the solvent. The results obtained were rationalized through a mechanism analogous to that previously proposed for 2-aryl- and 2-ethynyl-oxiranes in which different kinds of carbenium ion species are involved.