Characterization of the Dynamic Equilibrium between Closed and Open Forms of the Benzoxaborole Pharmacophore

Benzoxaboroles are a class of five-membered hemiboronic acids that recently attracted significant attention as a new pharmacophore on account of their unique structural and physicochemical properties and their ability to interact selectively with biomolecules. Their structural behavior in water and its effect on their physiological properties remain unclear, especially the question of dynamic hydrolytic equilibrium of the oxaborole ring. Herein, we used NMR spectroscopy, in mixed aqueous-organic solvent, to confirm the strong preference for the closed form of benzoxaborole and its six- and seven-membered homologues over the open boronic acid form. Only with the eight-membered homologue does the cyclic form become unfavorable. Using dynamic VT-NMR studies with designed probe compound 20, we demonstrate that the oxaborole ring undergoes rapid hydrolytic ring closing–opening at ambient temperature at a rate of >100 Hz via a mechanism featuring rate-limiting proton-transfer steps. This knowledge can help provide a better understanding of the behavior of benzoxaboroles in biological systems.