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

•A group of novel alkaloid-lignan hybrids were isolated from Lobelia chinensis simultaneously.•The compounds were determined to be two diastereomeric pairs of enantiomers by chiral HPLC–CD.•All the four stereoisomers were prepared by chiral HPLC.•CD spectra were used for the determination of the absolute configurations.

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.