Guanidine Alkaloids and Pictet−Spengler Adducts from Black Cohosh (Cimicifuga racemosa)

As an extension of work on the recently discovered nitrogenous metabolites from Cimicifuga/Actaea species, three new guanidine alkaloids have been isolated and characterized from C. racemosa (syn. A. racemosa) roots. Of these, cyclo-cimipronidine (1) and cimipronidine methyl ester (2) are congeners of cimipronidine (3), whereas dopargine (5) is a derivative of dopamine. By employing NMR- and MS-guided chemodiversity profiling of a polar serotonergic (5-HT(7)) fraction, the guanidine alkaloids were initially detected in a clinical extract of black cohosh and were isolated along with a congener of salsolinol 4, 5, and 3-hydroxytyrosol 3-O-glucoside (7). The structures of 1, 2, and 5 were confirmed by 1D and 2D NMR spectroscopy as well as LC-MS and HRMS spectroscopy. A plausible biosynthetic relationship may be inferred between the homoproline-analogue cimipronidines and the dopamine-derived Cimicifuga alkaloids. These strongly basic and frequently zwitterionic nitrogenous metabolites contribute considerable chemical diversity to the polar serotonergic fraction of black cohosh.