Crystal structures of racemic and enantiopure synephrine correlated with physicochemical properties from IR spectroscopy and thermal analysis

Abstract

The proto-alkaloid synephrine {SYN; systematic name: 4-[1-hydroxy-2-(methylamino)ethyl]phenol}, C9H13NO2, is found to crystallize as a neutral molecule in the racemate and as a zwitterion in the pure enantiomer, in which the phenolic H atom has been transferred to the amino group. In the racemate crystal, an enantiomeric pair on an inversion centre is weakly linked by alcoholic O—H and N—H groups into an R 2 2(10) ring. The trigonal pyramidal amino group is also linked to the phenolic and alcoholic groups to form a C(6) chain. In the enantiopure crystal, the deprotonated phenolic O atom is involved in trifurcated hydrogen bonding to two quaternary ammonium groups and an alcoholic O—H group to form a fused R 2 4(11) ring and a C(7) chain. From the results of the crystal structure analysis, thermal analyses and DFT calculations validated from FT–IR spectra, a different tautomer was found in the racemic molecule (RS-SYN) versus the enantiopure molecule (R-SYN).