Conformational isomers acyclic compounds

We have already seen other meso compounds, although we have not yet called them that. For example, the cis isomer of 1,2-dichlorocyclopentane has two asymmetric carbon atoms, yet it is achiral. Thus it is a meso compound, cis-1,2-Dibromocyciohcxanc is not symmetric in its chair conformation, but it consists of equal amounts of two enantiomeric chair conformations in a rapid equilibrium. We are justified in looking at the molecule in its symmetric flat conformation to show that it is achiral and meso. For acyclic compounds, the Fischer projection helps to show the symmetry of meso compounds. 

The isomer that is the meso compound is the one with a plane of symmetry when an acyclic compound is drawn in its eclipsed conformation (B), or when a cyclic compound is drawn with a planar ring (D and F). 

Diastereoselectivity in compounds without rings is different it is less well controlled because there are many more conformations available to the molecule. But even in acyclic compounds rings can still be important, and some of the best diastereoselectivitles arise when there is a ring formed temporarily in the transition state of the reaction. With or without cyclic transition states, in some cases we have good prospects of predicting which diastereo-isomer will be the major reaction product, or explaining the diastereoselectivity if we already know this. That is the subject of the next chapter. 

The real test of single compound MI/FTIR was to differentiate between p-menthane-based conformational isomers Most of the acyclic and cyclic terpenoids... 

The same stereochemical principles apply to both acyclic and cyclic compounds. With simple cyclic compounds that have little or no conformational mobility, it can even be easier to foUow what is going on. Let us first look at cyclopropane-1,2-dicarboxylic acid. These compounds were considered in Section 3.4.3 as examples of geometric isomers, and cis and trans isomers were recognized. 

The high cis selectivity has been rationalized on the basis of acyclic conformational control27. A model for the attack of an electrophile to an alkene containing an internal nucleophile under kinetic conditions has been developed33 based on the relative affinity of the diastereotopic face of the double bond towards a proton. When an alkoxy group is present, preferential attack of an electrophile on the OR-in-plane-conformer occurs from the face of the double bound syn to the allylic hydrogen. When R = H, the m-diastcrcomcr is formed as the major compound. On the other hand, substrates with a Z double bond, which destabilizes the OR-in-plane-conformer, react on the opposite face, leading to the trans-isomer. 

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