Chiral separation of two diastereomeric pairs of enantiomers of novel alkaloid-lignan hybrids from Lobelia chinensis and determination of the tentative absolute configuration

Four novel alkaloid-lignan hybrids, lobechinenoids A-D (1-4), were isolated as a mixture of two diastereomeric pairs of enantiomers from the aerial parts of Lobelia chinensis. These compounds are constituted of the union of a tetrahydroisoquinoline alkaloid and a dihydrobenzofuran neolignan moiety. The structures were determined by detailed analyses of IR, MS and NMR data. These four compounds were characterized as two pairs of enantiomers by on-line chiral high performance liquid chromatography (HPLC)-circular dichroism (CD) analysis. The chiral HPLC separation was accomplished in the normal-phase mode using Chiralpak AD-H, a polysaccharide-derived chiral stationary phase (CSP) and a hexane-ethanol mobile phase. In order to study the chiroptical properties, all of the four single stereoisomers were successfully prepared on an analytical Chiralpak AD-H column and their stereochemical features were determined tentatively based on their CD spectra.