Potentially macrocyclic peptidyl boronic acids as chymotrypsin inhibitors

The possibility of forming a peptide boronate adduct in a serine protease active site that mimics the first tetrahedral intermediate in the peptide hydrolysis mechanism was explored with the complex boronic acid analogs 7, 8-OH, and 8-NH2. In these structures, the P-1 and P-2 residues and the P-1'-P-3' residues are connected through the P-2 and P-1' side chains, to encourage formation of the diester or amide-ester adducts via macrocyclization. These inhibitors were assembled from suitably protected derivatives of 2,4-diaminobutanoic acid or 2,4-diaminopentanoic acid (11), borophenylalanine (12), aspartic acid, malic acid or the substituted malic acid analog 13, and Leu-Arg dipeptide. Stereoselective syntheses were developed for the (S,S)-2,4-diaminopentanoate 11 and for the (S,S)-beta-isobutylmalate 13 derivatives. The complex peptidyl boronates 7 (K-i = 26 nM) and 8-OH (68 nM) are potent inhibitors of alpha-chymotrypsin; however, the affinity of 7 is neither time- nor pH-dependent, and it is only moderately greater than that found for comparison compounds like 8-H (114 nM), 9 (356 nM), and 10 (219 nM) that cannot cyclize or form a diester adduct.