Enantiotopic allylic termini

Differentiation of the enantiotopic allylic termini of symmetric intermediates. 

Substrates with two identical substituents at one allyl terminus (a nonstereogenic center) are also interesting candidates for Hgand-controUed allylations. For these reactions, either chiral or achiral substrates can be used. When achiral substrates such as A are utihzed, the chiral catalyst can differentiate between the two enantiotopic faces of the aUcene (Scheme 12.23). On the other hand, when substrates such as B are used as starting materials, the initially formed it-allyl complex has to isomerize rapidly to make sure that the chiral information of the substrate completely gets lost during the reaction and that the stereochemical outcome of the alkylation is only controlled by the chiral ligand. 

The only examples of this class of substrates, that have been studied, are geminal dicarboxylates 44 which are readily prepared from aldehydes [44,45]. Enantioselective displacement of one of the enantiotopic carboxylate groups by the chiral catalyst leads to the chiral allyl complex 45 (Scheme 18). This intermediate which is of the same type as 18 (Scheme 12) has been shown to undergo regioselective nucleophihc addition at the acetoxy-bearing terminus with inversion of configuration (see Sect. 9.5). 

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