Large‐scale separation of antipsychotic alkaloids from Rauwolfia tetraphylla L. by pH‐zone‐refining fast centrifugal partition chromatography

pH‐zone‐refining centrifugal partition chromatography was successively applied in the large‐scale separation of close R_f antipsychotic indole alkaloids directly from CHCl₃ fraction of Rauwolfia tetraphylla leaves. Two experiments with increasing mass from 500 mg to 3 g of crude alkaloid extracts ( 1 C) of R. tetraphylla were carried out in normal‐displacement mode using a two‐phase solvent system composed of methyl tert‐butyl ether/ACN/water (4:1:5, v/v/v) where HCl (12 mM) was added to the lower aqueous stationary phase as a retainer and triethylamine (5 mM) to the organic mobile phase as an eluter. The two centrifugal partition chromatography separations afforded a total of 162.6 mg of 10‐methoxytetrahydroalstonine ( 1 ) and 296.5 mg of isoreserpiline ( 2 ) in 97% and 95.5% purity, respectively, along with a 400.9 mg mixture of α‐yohimbine and reserpiline ( 3 and 4 ). Further, this mixture was resolved over medium pressure LC using TLC grade silica gel H (average particle size 10 μm), which afforded 160.4 mg of α‐yohimbine ( 3) and 150.2 mg of reserpiline ( 4) in >95% purities. The purity of the isolated antipsychotic alkaloids was analyzed by high‐performance LC and their structures were characterized on the basis of their 1D, 2D NMR and electrospray ionization‐mass spectroscopic data.