Conformational studies on 1,3-dioxepans. Part III. 2,5-O-methylene-D-mannitol and some related compounds

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Abstract
Examination of the chair(C)–twist-chair(TC) pseudorotational itinerary of 1,6-dideoxy-2,5-O-methylene-D-mannitol indicates that three TC conformations, one having a C2 axis of symmetry and two degenerate conformations, will be predominant contributors to a conformational equilibrium. The 1H n.m.r. spectrum of the ring protons of this 1,3-dioxepan derivative is analysed as an AA′BB′ system to obtain values of coupling constants involving the vicinal protons on C-2, C-3, C-4, and C-5. These values, together with the magnitudes of the coupling constants obtained from analysis of the AA′BB′ system of 1,3,4,6-tetra-O-acetyl-2,5-O-methylene-D-[1,1,6,6-2H4]mannitol, indicate approximately trans relationships between vicinal protons on the substituted 1,3-dioxepan ring. These observations are discussed and shown to be consistent with the conformational properties predicted by conformational analysis. Evidence is also presented which shows that in bicyclic derivatives, where 1,3-dioxolan rings are trans-fused to 1,3-dioxepan rings at C-3 and C-4, the dioxepan ring may possibly assume a twist-boat conformation.

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