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

Abstract

This paper describes the characterization of certain dioxolenium salts in which the heterocyclic ring is fused (via positions4 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 a n 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.

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

“…1) and noted their prefer-and finally includes a rationalization of the ence for forming the diaxial product (2) rather observed stereoselectivity. Some of these results than the diequatorial isomer (3) when allowed to have been outlined already in a preliminary react with halide anions (1). A number of nucleo-communication (2).…”

Regiocontrol in the Oxidative Radical Fragmentation of Benzilidene Acetals and Its Mechanistic Implications

“…Overall, taken together with the earlier results of Hannessian, the evidence suggests that the NBS-mediated fragmentation proceeds through path C outlined above terminating through an ion-pair recombination involving a formal antarafacial 1,3-bromide shift. The S N 2-like opening is also in accord with earlier studies reported by King in the trapping of similar cations. We conclude that in the presence of NBS, the rate of chain transfer from radical 19 with NBS to give the benzyl bromide 20 is faster than direct fragmentation, whereas in the absence of NBS, the direct radical fragmentation pathway is operative allowing trapping of open-chain radicals .…”