Synthesis and Evaluation of 3-(Dihydroxyboryl)benzoic Acids as d,d-Carboxypeptidase R39 Inhibitors

Penicillin binding proteins (PBPs) catalyze steps in the biosynthesis of bacterial cell walls and are the targets for the β-lactam antibiotics. Non-β-lactam based antibiotics that target PBPs are of interest because bacteria have evolved resistance to the β-lactam antibiotics. Boronic acids have been developed as inhibitors of the mechanistically related serine β-lactamases and serine proteases; however, they have not been explored extensively as PBP inhibitors. Here we report aromatic boronic acid inhibitors of the d,d-carboxypeptidase R39 from Actinomadura sp. strain. Analogues of an initially identified inhibitor [3-(dihydroxyboryl)benzoic acid 1, IC₅₀ 400 μM] were prepared via routes involving pinacol boronate esters, which were deprotected via a two-stage procedure involving intermediate trifluorborate salts that were hydrolyzed to provide the free boronic acids. 3-(Dihydroxyboryl)benzoic acid analogues containing an amide substituent in the meta, but not ortho position were up to 17-fold more potent inhibitors of the R39 PBP and displayed some activity against other PBPs. These compounds may be useful for the development of even more potent boronic acid based PBP inhibitors with a broad spectrum of antibacterial activity.