Phase-transfer reagents as C-terminal protecting groups; facile incorporation of free amino acids or peptides into peptide sequences

Phase-transfer reagents (basic, neutral, and acidic) can effect temporary protection of carboxy groups by salt formation in C-terminal free amino acids or peptides during peptide synthesis; the use of acidic or neutral phase-transfer reagents as the C-terminal protecting group will not affect the nucleophilicity of the amino group of the salts thus prepared in an organic solvent.