Extra-annular chirality transfer

The asymmetric alkylation of a carbonyl group is one of the most commonly used chirality transfer reactions. The chirality of a substrate can be transferred to the newly formed asymmetric carbon atom through this process. In surveying chiral enolate systems as a class of nucleophile, three general subdivisions can be made in such asymmetric nucleophilic addition reactions intra-annular, extra-annular, and chelation enforced intra-annular. 

The cases illustrated here are typical examples of extra-annular chirality transfer via the alkylation process (Scheme 2- -9)16 ... 

Scheme 2-9. Extra-annular chirality transfer. Xc stands for the chiral auxiliary.

Scheme 2-10. Two examples of extra-annular chirality transfer.

As a result of the combination of intra- and extra-annular chirality transfer, a productive approach has been developed for the design of chiral enolate systems in which a structurally organized diastereofacial bias is established, as illustrated in the equations in Scheme 2-11. A lithium-coordinated five-membered or six-membered ring fixes the orientation between the inducing asymmetric center and the enolate20 ... 

Hereinafter, the extra- and intra- annular chirality transfer (CT) classification defined by Evans402 will be employed provided the original stereocenter is linked to the enol moiety by one or two points of anchorage, with the possibility of chelate-enforced chirality transfer due to internal chelation with heteroatoms (Figure 4)410. 

FIGURE 4. Intra-, extra-annular and chelate-enforced chirality transfer (CT the symbol 9 represents the chiral information)410... 

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