Diastereoselective alkylation, glycine Schiff bases

Diastereoselective Alkylation of Glycine Schiff Base with Optically Enriched Alkyl Halides... 

Diastereoselective preparation of a-alkyl-a-amino acids is also possible using chiral Schiff base nickel(II) complexes of a-amino acids as Michael donors. The synthetic route to glutamic acid derivatives consists of the addition of the nickel(II) complex of the imine derived from (.S )-,V-[2-(phenylcarbonyl)phenyl]-l-benzyl-2-pyrrolidinecarboxamide and glycine to various activated olefins, i.e., 2-propenal, 3-phenyl-2-propenal and a,(f-unsaturated esters93- A... 

Alkylation of the Schiff base of glycine with 2-hydroxy-3-pinanone proceeds in an extremely diastereoselective manner. Thus, fluoro-functionalization on the a-carbon of the Schiff base followed by hydrolysis provides fluorinated a-amino acids in a highly enantiomerically enriched form. 2-Fluoroethylation and 2-fluoroallylation of 157 (see Scheme 9.35) and bromodifluoromethylation of 161 (see Scheme 9.36) give the desired adducts 158 and 162, respectively, with excellent diastereoselectivities. Lithium enolate dimer 160 has been proposed as a reactive intermediate for the stereocontrolled alkylation [61]. The adducts 158 and 162 were transformed to 4-fluoro-2-amino acids (>96% ee) 159 [61] and 3,3-difluoroglutamine 164 [62], respectively. 

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