Studies on cyclitols—XVI : Conformational analysis of substituted cyclopentanes, cyclopentenes and cyclopentene oxides

Abstract

PMR spectroscopy has been used to study the conformations at room temperature of several classes of cyclopentanoid compounds, including substituted cyclopentenes, cyclopentene oxides, tri- and tetrasubstituted cyclopentanes. The substituents are either halogens or -OR, where R is H, Ac, Bz or Me. Unlike the cyclohexanoid compounds, for most of which only the two chair forms need be considered, the cyclopentanoid compounds generally have several different conformers contributing significantly to the conformational equilibrium. The cyclopentane ring is so flexible that conformations with less than maximum puckering have little additional strain, and there appears to be considerable pseudolibration about the energy well representing a particular fully puckered conformation. The series whose conformations are most restricted is the cyclopentene oxides. Regardless of substituents, an endo-conformation is preferred, except for the cis-4-bromo-1,2-oxide which exists as an equal mixture of endo and exo conformers; all the other oxides seem to exist in fully puckered endo conformations. Unlike the cyclohexanes, the cyclopentanes easily tolerate 1,3- syn diaxial substituents, and 1,2- trans substituents are generally disposed diaxially rather than diequatorially; for example the tribenzoate esters of (1,3/2)-cyclopentane-triol and (1,3/2)-cyclopentenetriol contain 70–80° of all-axial conformers. The O-isopropylidene derivatives prefer conformations in which the average puckering is such that the fused dimethyldioxolane-cyclopentane ring system approximates an overall endo- or boat shape. Coupling constants involving oxirane protons ofcyclopentene oxides are much smaller than corresponding coupling constants observed in other ring systems: oxirane-oxirane, 2·7 ± 0·3 Hz; oxirane- cis vicinal, 1·4 Hz; and oxirane- trans vicinal 0·6 Hz. The bases for all these effects are discussed.