RhI-catalysed formation of two conformational diastereoisomers due to a cis-cyclohexane-1,2-diol moiety. X-Ray molecular structure of two stereoisomers of 4′-isopropenyl-7,9-dioxabicyclo[4.3.0]nonane-8-spirocyclohexan-3′-yl p-bromobenzoate

Abstract

It is well known that cis-cyclohexane-1,2-diol and its mirror image are not superposable, but that these isomers are rapidly interconvertible by flipping from one chair conformation to the other. That is they exist as a pair of conformational enantiomers. Two aldehydes, 2-[8-(4-methylpent-3-enyl)-7,9-dioxabicyclononan-8-yl]ethanals, with the cis-cyclohexane-1,2-dioxy function at C(3) are cyclized by a Rh^I(Wilkinson) complex to give two conformational diastereoisomers due to the cyclohexane-1,2-diol moiety. Each conformational diastereoisomer of the title compound's parent alcohol yielded trans-3-hydroxy-4-isopropenylcyclohexanone and cis-cyclohexane-1,2-diol after deprotection with 5% aq. AcOH. Unambiguous stereochemistry of two of the parent alcohols was determined by X-ray crystallographic analysis of their p-bromobenzoates. The stereochemistry of the 1,3-dioxolane ring, including C(1) and C(2) of the cis-cyclohexane-1,2-diol and C(1′) of the trans-3-hydroxy-4-isopropenylcyclohexanone, was C(1′)-axial-O-axial-C(1) and C(1′)–equatorial–O–equatorial–C(2) for the two title compounds, and C(1′)-axial-O-equatorial-C(1) and C(1′)–equatorial-O–axial–C (2) for the other pair of isomers.