Unusual amino acids .7. Asymmetric synthesis of 3- and 4-pyridylalanines

(Z)-2-N-Acylamino-3-pyridyl-acrylic acids and their esters were prepared by partially known procedures and hydrogenated in the presence of HBF4 to the corresponding optically active 2-N-acetyl-(or benzoyl-)-3-(3- or 4-pyridyl)-alanines or analogous methyl esters with enantiomeric excesses up to 90%. The rhodium complexes of PROPRAPHOS, 6a,b, or of O,N-bis(diphenylphosphino)-2-exo-hydroxy, 3-endo-methylamino-norbor-nane, 6c, as chiral catalysts have been used in the presence ol HBF4 to generate the corresponding pyridinium salts. Deacylation of the recrystallized amino acid derivatives or further recrystallization of the free amino acids yielded enantiomerically pure D- and L-pyridylalanines.