Conformational evaluation and detailed ¹H and ¹³C NMR assignments of flavoxate, a urinary tract antispasmodic agent

¹H and ¹³C NMR chemical shift assignments for the urinary tract antispasmodic flavoxate (1) and flavoxate hydrochloride (2) were obtained from one- and two-dimensional measurements. A Monte Carlo random search using molecular mechanics, followed by geometry optimization of each minimum energy structure employing DFT calculations at the B3LYP/6-31G* level, and a Boltzmann analysis of the total energies, provided accurate molecular models which describe the conformational behavior of flavoxate (1). The electron density surfaces for the global minimum and the second minimum conformers 1a and 1b of this L-type Ca²⁺ channel inhibitor were calculated. The presence of both conformers in solution was demonstrated in full agreement with 2D NOESY data and NOE difference spectroscopy.