HPLC-CD on-line coupling in combination with HPLC-NMR and HPLC-MS/MS for the determination of the full absolute stereostructure of new metabolites in plant extracts

The applicability of on-line coupling of gradient eluted nonchiral reversed-phase high-pressure liquid chromatography (HPLC) to circular dichroism (CD) spectroscopy for the stereochemical analysis of crude plant extracts was investigated. A detector measuring simultaneously UV absorbances and CD effects at constant wavelengths in the same detection cell was used. The solvent system water/acetonitrile was found to be suited to give high-quality CD chromatograms as a function of the elution time and even complete characteristic CD spectra in the stop-now mode. The method was applied to the tropical liana Habropetalum dawei (Dioncophyllaceae). First, the extract solution was investigated by the combined application of HPLC-NMR and HPLC-ESI-MS/MS techniques. The constitution and relative configuration of two as yet unknown natural products thus detected, the isoquinoline phylline (5) and the naphthylisoquinoline alkaloid 5'-O-methyldioncopeltine A (6), were characterized. Subsequent HPLC-CD experiments permitted the deduction of the absolute configurations of these new metabolites by empirical analysis of their CD data. Thus, using identical sample solutions and chromatographic conditions (including the same columns and solvent gradient programs), the combination of NMR, MS, and CD data permitted the complete structural elucidation of the two new metabolites in the extract matrix solution, without any isolation and sample purification prior to the coupling experiments.