Stereochemistry of bimolecular nucleophilic opening of a dioxolenium ring fused to an anchored cyclohexane system

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Abstract
This paper describes the characterization of certain dioxolenium salts in which the heterocyclic ring is fused (via positions-4 and -5) to an anchored cyclohexane ring, and the steric course of their bimolecular cleavage with nucleophilic reagents. It was found that diaxial opening is normally favored strongly over diequatorial; this is accounted for in terms of differences in angle strain in the transition states. The presence of an axial (angular) methyl group gave a preference for diequatorial opening; on the other hand an analogous investigation of the diaxial: diequatorial opening ratio with epoxides showed the stereochemistry of the reaction to be virtually insensitive to an angular methyl group. These observations are interpreted in terms of the different non-bonded interactions arising from the different geometrical arrangements of epoxides and dioxolenium ions. The variation in the diaxial: diequatorial opening ratio is shown to be of significance in the interpretation of rates of diaxial → diequatorial rearrangement.

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