SELECTIVE DEPROTECTION OF PHENACYL, BENZYL AND METHYL-ESTERS OF N-PROTECTED AMINO-ACIDS AND DIPEPTIDES AND N-PROTECTED AMINO-ACIDS BENZYL ESTER-LINKED TO RESINS WITH BIS(TRIBUTYLTIN) OXIDE

Phenacyl, methyl and benzyl esters of various N-alpha-Boc, N-alpha-Cbz or N,N-dimethylamino protected amino acids and dipeptides, as well as esters of N-alpha-protected amino acids linked to Wang and Pam resins have been efficiently and chemoselectively cleaved by bis(tributyltin ) oxide in aprotic solvents to give the corresponding carboxylic acids in good yields. Moreover, the absence of racemization during the depr otection has been demonstrated. A limitation of the method is the inst ability of the N-epsilon-Fmoc group in the amino acid esters 8 and 10, N-alpha-Fmoc-L-alanine linked to Wang resin 23 and the Cbz protecting groups in N-alpha-Boc-N-epsilon-Cbz-L-lysine benzyl and methyl esters (5 and 7), respectively, and N-alpha-Cbz-L-alanyl-L-alanine methyl es ter 19. In the case of N-alpha-protected dipeptides, there was no evid ence of free amino acid which indicates that the peptide bond is unaff ected.