Stereochemistry acceptable representations

Exercise 6.10 The ground and promoted states of the reactants, R and R are shown in Fig. 6.Ans.8 below using the FO—VB representation, which is the simplest one for making predictions on stereochemistry. The electronic structure of R displays an electron transfer from the nucleophile to the C C bond. A transfer to the a-orbital is not relevant, as this would generate a closed-shell cyclopropane that cannot form a bond with the oxidized nucleophile. It is therefore the a orbital that accepts the transferred electron, thus generating the triplet oo- configuration of the C-C bond. Now, since R and R differ by one-electron shift from Nu to the a orbital of the cation radical, the corresponding... 

The relative ranking of ligands for the description of the stereochemical properties of a molecule is the most utilized and accepted principle throughout stereochemical nomenclature. This is not yet the practice, however, in discussing the stereochemistry in coordination and inorganic chemistry. In the context of coordination and inorganic chemistry, stereochemical information is either presented in the more traditional terminology, or more often by means of a stereospecific structural representation. 

While there is no universally accepted scientific terminology for representation there is general agreement on a certain working nomenclature (refs. 97,98). Thus a-tocopherol, the natural stereoisomer is as stated earlier (RRR)-a-tocopherol, the 2-epimer is 2-epi-a-tocopherol and a mixture of (RRR) and (SRR)-a-tocopherols is 2-ambo-a-tocopherol. Synthetic a-tocopherol prepared from synthetic phytol or isophytol is called all-rac-a-tocopherol, since it is theoretically a mixture of (RRR) and (SSS). The mixture of reduction products of 5,7,8-trimethyltocotrienol is called 4 -ambo-8 -ambo-a-tocopherol since as a natural product, although it has a 2R centre, the stereochemistry from hydrogen addition at the 4- and 8- positions is probably substantially RS. 

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. 

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