Suppression of epimerization was suggested to be due to CuCl2 induced ring opening of oxazolone intermediates. The amino acid tri-tert-butoxysilyl esters proved stable as the hydrochloride salts and the corresponding peptide silyl esters could be cleaved effectively with 5% trifluoroacetic acid in dichloromethane without loss of acid labile tert-butyl ether functionality.