Efficient protocols for the synthesis of enantiopure gamma-amino acids with proteinogenic side chains.

The synthesis of enantiopure gamma-substituted gamma-amino acids with proteinogenic side chains, starting from the corresponding natural alpha-amino acids, was studied. N-Protected amino aldehydes containing various protective groups were prepared from the corresponding amino alcohols by oxidation with NaOCl in the presence of AcNH-TEMPO and directly reacted with methyl, benzyl and tert-butyl phosphoranylidene acetate to produce alpha,beta-unsaturated gamma-amino esters. Simultaneous hydrogenation of the double bond and removal of either the benzyl or benzyloxycarbonyl group led to N- or C-protected gamma-amino acids in high yield. The enantiomeric purity was studied by 1H NMR analysis of Mosher amides and chiral HPLC analysis.