Two stereoisomers of 4‑hydroxy prenylamine and its hydrochloride salts

Abstract

Crystal structures of two stereoisomers of 4‑hydroxy prenylamine, C24H27NO, and its hydrochloride salt, C24H28NO+·Cl− were determined. Four compounds were characterized by IR, DSC, Hot stage and Hirshfeld surface analysis. Enantiomers (Ia) and (Ib) crystallized in monoclinic space group C2, while (IIa) and (IIb) are chloride salts of (Ia) and (Ib) and crystallized in the monoclinic P21 space group. The absolute configurations of (IIa) and (IIb) are established as R and S unambiguously by the X-ray crystallography method, and based on this, the stereocenters of (Ia) and (Ib) are assigned as R and S, respectively. The N atom of 4‑hydroxy prenylamine of (IIa) and (IIb) is protonated. The bond distances and angles around the N atom show a significant difference between the salt and neutral structures and are further confirmed by IR analysis. It clearly depicts differences in NH stretching between neutral (Ia & Ib) and protonated structures (IIa & IIb). Both (Ia) and (Ib) exhibit anti-periplanar-anti-periplanar-syn-clinal-syn-clinal conformations, while (IIa) and (IIb) have anti-periplanar-anti-periplanr-syn-clinal-anti-periplanar conformations. The R and S isomers have the same melting points. However, the melting points between neutral and salt structures are different. The OH···N hydrogen bond aggregate (Ia) and (Ib) into an infinite chain along a-axis. While in (IIa) and (IIb), the NH···Cl and OH···Cl interactions lead to two-dimensional hydrogen-bonded sheets. The Hirshfeld surface analysis depicts high intermolecular contacts in (Ia) and (Ib), compared to (IIa) and (IIb).