Enantioselective Synthesis of α-Amino Acid Derivatives via the Stereoselective Alkylation of a Homochiral Glycine Enolate Synthon

Article abstract of DOI:10.1021/jo00277a032

A new synthetic method for the enantioselective preparation of α-amino acid derivatives is presented.The key step involves the diastereoselective alkylation of the new chiral glycine enolate synthons 7 and 8 providing alkylation adducts with de of > 97.6percent in good yields (73-90percent).The reactivities of the enolates of 7 and 8 were extraordinarily sensitive to the metal counterion and solvent.Experimental conditions are described to maintain high diastereoselectivities in the alkylation step for electrophiles varying from highly reactive (α-haloacetate esters) to less reactive (n-butyl iodide).The alkylation diastereoselectivities were established to be under kinetic control by equilibration experiments on selected alkylation products.A model is presented which hinges on an A(1,3) interaction between the termini of the N4-acyl protecting group and the C5-phenyl group of 7 and 8 which in turn dictates the ?-facial selectivity of the enolate.The model successfully accounts for the observed results and is corroborated by the conformation of an alkylation adduct as revealed by a single-crystal X-ray determination.A simple one-pot, three-step deprotection procedure provides the desired α-amino acid as the ethyl ester hydrochloride salts (60-80percent overall yield) with no attending racemization as determined by conversion of the amino acid esters to the corresponding (+)- or (-)-Mosher amides.