Enolates enantioselective

Besides stereoselective alkylations of glycine-derived enolates, enantioselective construction of chiral quaternary carbon centers from a-amino acids is one of the most challenging topics in current organic synthesis , since nonproteinogenic a,a-disubstituted amino acids often show a remarkable influence on the conformation of peptides. Moreover, they can act as enzyme inhibitors or as building blocks for the synthesis of a wide range of natural products . 

The most common nucleophiles employed in the allylic alkylation are soft species such as malonate esters. However, Trost and Schroeder have discovered that high ees can be achieved in the addition of cyclic lithium enolates using ligand (10.54) with one equivalent of trimethyltin chloride, which may act to soften the nucleophilic species by transmetaUation to the tin enolate. Enantioselective additions of nonstabilised ketone enolates can also be achieved using an alternate palladium-catalysed decarboxylation protocol. In this approach an allyl 3-ketoester (10.72) or allyl vinyl carbonate (10.73) undergoes decarboxylation in the presence of Pd(0) to... 

A chiral auxiliary is a chiral molecule attached to the starting material of the reaction dia-stereoselective reactions of compounds from the chiral pool are likewise controlled by chirality in the starting material, and we call this type of stereocontrol substrate control. But is it also possible for enantioselective reactions to be controlled by chiral reagents. For example, a typical achiral base will just remove a proton from a substrate, but an enantiomerically pure chiral base can select one of two enantiotopic protons and form a product enantioselectively. The product of course has to be chiral, so we can t use a chiral base to make planar enolates enantioselectively, for example, but we can a chiral base to make chiral organolithiums. 

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