. "Finally, the free boronic acid can be converted to the difluoroboron derivative (???BF2) by treatment with aqueous HF as described by Kinder et al. (supra). [0056] Similarly, different ester groups can be introduced by reacting the free boronic acid with various di-hydroxy compounds (for example, those containing heteroatoms such as S or N) in an inert solvent. [0057] Preparation of H-Ala-boroPro" . . . . . .