Conformational stereoisomer

The staggered and eclipsed forms of ethane are conformational stereoisomers (conformational isomers, conformers) because they have the same molecular formulas and sequences of bonded elements but different spatial arrangements due to rotations around single bonds. (Actually there are an infinite number of conformational isomers (also called conformations) because there are an infinite number of degrees of rotation around the bond, but normally one only needs to be concerned with energy minima and maxima.)... 

The cyclohexyl group is commonly found in many medicinal agents, and is also susceptible to mixed-function oxitlti-tion (alicyclic hydroxylation)." Enzymatic introduction of a hydroxyl gmup into a monosubstituted cyclohexane ring generally occurs at C-3 or C-4 and can lead to cis and trims conformational stereoisomers, as shown in the diagrammed. scheme. 

Conformational isomers (conformers) Stereoisomers at potential energy minima (local or global) having identical constitution and configuration, which differ only in torsion angles. 

Molecules with a single stereocenter as the only source of configurational stereoisomerism can exist as one of two enantiomers no configurational diastereomers are possible. Conformational stereoisomers, on the other hand, are most commonly diastereomeric. Molecules with two dissimilar stereocenters as the source of stereoisomerism can exist as one of... 

The potential importance of rotational dynamics about the aryl C-N bond in asymmetric Mizoroki-Heck cyclizations of o-iodoacrylanilides was highhghted in a recent study by Curran and coworkers [68]. Because of the presence of two ortho substituents, racemic ( )-acryl o-iodoanilide 117 could be resolved by preparative high-performance liquid chromatography, and the enantiomers were shown to have a barrier for interconversion of 26 kcalmol" (Scheme 16.30). Although these conformational stereoisomers would racemize readily at the elevated temperatures typically used in asymmetric Mizoroki-Heck... 

Conformational stereoisomers are related to one another by bond rotations. 

Therefore, although the two structures represent enantiomers, they cannot be separated because they rapidly interconvert even at low temperature. They simply represent different conformations of the same compound. Therefore, structures (c) and (d) are not configurational stereoisomers they are conformational stereoisomers (see Section 4.9A). 

As we shall see later, there are some compounds whose conformational stereoisomers can be isolated in enantiomeric forms. Isomers of this type are called atropisomers (Section 5.18). 

Conformational stereoisomers (Section 4.9A) Stereoisomers differing in space only due to rotations about single (o ) bonds. 

Draw additional (conformational) stereoisomers of methylcyclohexane. (Hint Use molecular models in conjunction with Figure 4-8.)... 

Another reminder is to watch out for the term stereoisomer used by itself In many places and in many textbooks this term is used in place of the term configurational stereoisomer. (Recall that configurational stereoisomers are the E/Z, cis/trans, R/S, D/L, +/- enantiomers, diastereomers, etc. that we have been working with in this ChemActivity.) The strictly correct use of the term stereoisomer includes all stereoisomers. That is, both configurational stereoisomers and conformational stereoisomers (conformers—covered in ChemActivity 6A). 

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