Relative configuration, acyclic derivatives

Chemical transformation of acyclic diastereomers into cyclic derivatives and subsequent determination of relative configuration is a common and secure method. In general, there are far fewer conformations in cyclic compounds and the stereochemical dependence of NMR parameters is well documented. A few representative examples are given in this section (see also Section A.4.3.). 

Determination of relative configurations in acyclic molecules or acyclic regions of complex molecules258 by NMR methods is a difficult task, since it requires knowledge of the preferred conformation or of conformational equilibria. It is, therefore, common practice to prepare cyclic (very often 5- or 6-membered) derivatives with restricted conformational mobility and then determine the relative configuration by means of appropriate NMR methods. On the basis of the functional groups present in the compound under investigation, it must first be decided which type of cyclic derivative is appropriate. 

Vibrational Raman optical activity of biomolecules, including carbohydrates, has been reviewed (23 refs). l A general procedure for assigning both relative and absolute configuration to multiple stereocentres in acyclic aminopolyols based on application of the exiton chirality method is presented. The method has been applied to the side chain aminopolyol of a natural bacterial-derived terpenoid derivative. ... 

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