NMR studies of buspirone (an anxiolytic drug) analogues

Conformations of piperazine rings in 8-{4-4-(2-pyrimidyl)-1-piperazinyl]butyl}-8-azaspiro4.5]-decane-7,9-dione (buspir-one — 1) and its two analogues 8-{4-4-(2-quinolinyl)-1-piperazinyl]butyl}-8-azaspiro4.5]-decane-7,9-dione (kaspar — 2) and 4,4-dimethyl-1-{4-4-(2-quinolinyl)-1-piperazinyl]butyl}-2,6-piperidinedione (mesmar — 3) (Fig. 1) have been studied with the aid of ¹H NMR and ¹³C NMR spectra. For free bases the two bands corresponding to piperazine hydrogen atoms in the spectra broaden considerably with a decrease in temperature to divide into four separate bands, indicating the presence of a dynamic exchange process. A similar dynamic process, but for higher temperatures, was observed for buspirone (1), kaspar (2) and mesmar (3) hydrochlorides. Proton and carbon atom resonance lines have been assigned with the aid of 2D COSY and 2D HETCOR two-dimensional spectra.