Chair conformations enantiomers

Structures A and A are nonsuperimposable mirror images of each other Thus although as 1 2 dichloro cyclohexane is chiral it is optically inactive when chair-chair interconversion occurs Such interconver Sion IS rapid at room temperature and converts opti cally active A to a racemic mixture of A and A Because A and A are enantiomers interconvertible by a conformational change they are sometimes re ferred to as conformational enantiomers... 

Incorporation of stereogenic centers into cyclic structures produces special stereochemical circumstances. Except in the case of cyclopropane, the lowest-eneigy conformation of the tings is not planar. Most cyclohexane derivatives adopt a chair conformation. For example, the two conformers of cis-l,2-dimethylcyclohexane are both chiral. However, the two conformers are enantiomeric so the conformational change leads to racemization. Because the barrier to this conformational change is low (lOkcal/mol), the two enantiomers arc rapidly interconverted. 

The major constituent of caraway oil is (+)-carvone, and the typical caraway odour is mainly due to this component. On the other hand, the typical minty smell of spearmint oil is due to its major component, (—)-carvone. These enantiomers are unusual in having quite different smells, i.e. they interact with nasal receptors quite differently. The two enantiomeric forms are shown here in their half-chair conformations. 

Fig. 9.5. Stereochemistry of the formation of d-valerolactol (neat, at 38°C < 5% of the free hydroxyaldehyde is present). The top formulas represent the 78 22 mixture of the two chair conformers of one lactol enantiomer, whereas the bottom formulas represent the 78 22 mixture of the two chair conformers of the other lactol enantiomer.

We can draw two nonsuperimposable mirror images of the most stable chair conformation of trans-1,2-dibromocyclohexane with both bromines equatorial. These structures cannot interconvert by ring-flips or other rotations about bonds, however. They are mirror-image isomers enantiomers. 

In contrast to what we have said for the trans-compound, however, we find that these models are interconvertible by flipping one chair conformation into the other. These are conformational enantiomers and hence, except possibly at low temperatures, should interconvert too rapidly for resolution and measurement of optical activity. 

The piperdine (D) ring is in the energetically favored chair conformation, but the C ring of morphine is in a boat conformation, which places the 6a-hydroxyl in the equatorial position (see Ref 298). (+)-Morphine, the enantiomer of the naturally occurring (-)-mor-phine, has been synthesized (see Refs. 283, 299) and is devoid of analgesic activity. 

Examination of the molecular model of A >i -octalin reveals that it can exist in three distinct chair-chair conformations. One modification is a meso form with a plane of symmetry such that one ring is the mirror image of the other it has one twofold axis of symmetry, i.e., a = 2. The other two chair-chair conformations are a dl pair each enantiomer has three twofold axes of symmetry, i.e., v = 4. The chair conformation of Ai> -octalin is a dl pair with no twofold axis of symmetry, i.e., a=l. The chair conformation of chair conformation of trans-Ai.2-octalin is a dl pair with no twofold axis of symmetry, i.e., a=l. These parameters and the differences in conformational energy based on them are collected in Table XXXVIII. 

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