Diastereomers conformational

Problem 9.17 Give your reasons for selecting the isomers of dimethylcyclohexane shown in Figs. 9-8 to 9-12 that exist as (a) a pair of configurational enantiomers, each of which exists in one conformation (ft) a pair of conformational diastereomers (c) a pair of configurational enantiomers, each of which exists as a pair of conformational diastereomers (d) a single conformation (c) a pair of conformational enantiomers. ... 

Other groups would behave similarly, with the axial isomer being higher in energy (less stable) than the equatorial isomer because of 1,3 diaxial interactions. These two isomers are conformational isomers because they are interconvertible by rotations about C-C single bonds, but they are also called conformational diastereomers since they have different physical properties and are nonsuperim-posable, non-mirror images. 

Chiral centers, more than one, lljf Chiral stereomer, 69 Cholesterol, chirality in, 81 Cinnamaldehyde, 328 Cis-trans interconversion, 111 Cis-trans isomerism, in cyclic compounds, 163 Claisen condensation, 394 rearrangement, 439 Cleavage, oxidative, 117 Clemmensen reduction, 219, 311 Coenzyme A, 354 Collins reagent, 264 Collision frequency, 39 Configuration, 72 relative, 76 Conformation, 51 Conformational diastereomers, 78 enantiomers, 78 stereomers, 78... 

Butane exists as three conformational isomers, one anti and two gauche (Sec. 3.5). The gauche conformers, II and III, are mirror images of each other, and hence are (conformational) enantiomers. Conformers I and II (or I and III) are not mirror images of each other, and hence are (conformational) diastereomers. 

For convenience, we laid down (Sec. 4.20) the following ground rule for discussions and problems in this book unless specifically indicated otherwise, the terms stereoisomers," ""enantiomers," and ""diastereomers" will refer only to configurational isomers, including geometric isomers, and will exclude conformational isomers. The latter will be referred to as conformational isomers," con-formers, conformational enantiomers, and conformational diastereomers. ... 

To summarize, then, 1,2-dimelhyIcycIohexane exists as a pair of (configurational) diastereomers the cis- and trans-iaomers. The m-isomer exists as a pair of conformational enantiomers. The irans-homcr exists as a pair of configurational enantiomers, each of which in turn exists as two conformational diastereomers (axial -axial and equatorial-equatorial). 

The structures 27 and 28 are conformational diastereomers. In principle, they can interconvert by rotation about single bonds. 

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