Alkylation extraannular chirality transfer

Extraannular chirality transfer has been observed for alkylations of acyclic ketone enolates having chiral (3-carbon atoms (Scheme 24). In these reactions, methylation occurs anti to the (3-dimethylphe-nylsilyl and (3-isopropyl groups with good to excellent diastereoselectivity. Variation in the size of the... 

In recent years, investigations of the diastereoselectivity and enantioselectivity of alkylations of metal enolates of carboxylic acid derivatives have become one of the most active areas of research in synthetic organic chemistry. Intraannular, extraannular and chelate-enforced intraannular chirality transfer may be involved in determining the stereochemistry of these alkylations. 

Extraannular and chelate-enforced intraannular chirality transfer in enolate alkylations of carboxylic acid derivatives may occur in cases where (i) the chiral center, e.g. a 3-carbon atom, is present and remains in the carboxylic portion of the molecule or (ii) the substrate contains a chiral auxiliary, e.g. the alcohol portion of an ester or the amine portion of an amide, which is removed after the alkylation to generate a chiral carboxylic acid. Alkylations of the former type will be discussed in this section. Alkylations of systems containing chiral auxiliaries are described in Section 1.1.6.4. 

Alkylations of acyclic enolates containing a collection of chiral auxiliary groups have been used successfully for the asymmetric synthesis of carboxylic acids. The chiral, nonracemic substrates that have been used include amides, imides, esters, imine derivatives of glycinates and acyl derivatives of chiral transition metals. In these systems either extraannular or chelate-enforced intraannular chirality transfer may control the sense of the alkylation step. 

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