"In another embodiment the ??-amino acid residue is bonded to the heteroatom terminus (e.g., to an --O--, --S--, or --NR'-- group) of the chain and is of the formula ##STR9## wherein R4 is as defined above and R5 is hydrogen or a blocking group such as organocarbonyl (e.g., acetyl) as defined above." . . . . .