Stereochemical preference in the reactions of N-protected L-am...

Stereochemical preference for homochiral or heterochiral couplings was investigated in the reactions of N-protected L-amino acid 1-hydroxybenzotriazole esters with racemic amino acid derivatives. It was found to be dependent on the combination of amino acid residues as the carboxyl and amino components and the protecting groups of the amino acid residues, especially the N-protecting groups. Very high diastereomeric ratios were observed when t-leucine was employed as the carboxyl component and/or proline as the amino component and when the N-protecting group was the phthaloyl group.