A photometric screening method for dimeric naphthylisoquinoline alkaloids and complete on-line structural elucidation of a dimer in crude plant extracts by the LC-MS/LC-NMR/LC-CD triad
Bringmann G; Wolfarth M; Rischer H
An efficient evaluation procedure for the chemical screening and on-line structural elucidation of dimeric naphthylisoquinoline alkaloids has been developed. The method is based on the lead tetraacetate oxidation of the central binaphthalene core of the alkaloids. UV spectra of the extracts after addition of the oxidant show in the presence of naphthylisoquinoline dimers the appearance of a characteristic long-wavelength absorption indicative of dinaphthoquinones. The efficiency and relevance of the method has been demonstrated in the discovery of a constitutionally and configurationally new dimeric naphthylisoquinoline alkaloid named ancistrogriffithine A (4) from the previously uninvestigated Asian liana Ancistrocladus griffithii. After verification of this screening result by LC-ESI-MS/MS the constitution and the relative configuration of the compound were elucidated on line by LC-NMR and LC-CD on the extract. Using an LC-NMR-WET-ROESY experiment it was possible for the first time to determine the relative axial configuration of a natural biaryl compound on line by observing long-range ROE interactions. Finally an oxidative degradation right on the extract delivered the absolute configuration of 4 without isolation of the alkaloid. Ancistrogriffithine A is the as yet only dimeric naphthylisoquinoline from an Asian Ancistrocladaceae plant. Reprinted by permission of the publisher.