Stereochemistry preferred representations

Unambiguous, accurate graphical representation of the stereochemistry of chemical structures is essential. To this end, a task group in the lUPAC Chemical Nomenclature and Structure Representation Division prepared a document that specifies preferred, acceptable, and not acceptable graphical representations for a wide variety of structures. In Figure 2.2, representation 3a is considered acceptable, but 3b is preferred. However, structure 3f (Figure 2.3) is considered not acceptable. 

These chains are depicted in their Fischer projections rather than as conformational representations for the specific reason that the planar zigzag orientation of the chain is clearly favored only in chains having the arabino stereochemistry ( ) For the other configurations, the conformational preference is for non-extended conformations that may be conformational mixtures separated by low energy-barriers. It is clearly naive to depict exact molecular orientations for putative transition states in such reactions. Nevertheless, the model depicted here for interpreting the course of the reaction, which is in accord with the general model proposed by Trost (10) for diastereofacial selectivity in additions to alkenes having an adjacent asymmetric center, has predictive utility in these reactions. 

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