Chemotaxonomy of the pantropical genus Merremia (Convolvulaceae) based on the distribution of tropane alkaloids

A comprehensive GC–MS analysis of 18 spp. revealed altogether 74 tropanes and 13 pyrrolidines, respectively. Four aromatic 3α-acyloxytropanes (merresectines A–D) of chemotaxonomic relevance as well as 3α, 6β-di-(4-methoxybenzoyloxy)tropane (merredissine) have been isolated and structurally elucidated. The occurrence and distribution of tropane and biogenetically related pyrrolidine alkaloids in 18 Merremia species of paleo-, neo-, and pantropical occurrence have been studied. The extensive GC–MS study included members of almost all sections of the genus and has been carried out with epigeal vegetative parts as well as with roots. It comprises altogether 74 tropanes and 13 pyrrolidines including nicotine. Along with datumetine known already from a solanaceous species, the study led to the isolation (from M. dissecta and M. guerichii, respectively) and structure elucidation (spectral data) of four novel 3α-acyloxytropanes, merresectines A–D: 3α-(4-methoxybenzoyloxy)nortropane (A), 3α-kurameroyloxytropane (B), 3α-nervogenoyloxytropane (C), 3α-[4-(β- d-glucopyranosyloxy)-3-methoxy-5-(3-methyl-2-butenyl)benzoyloxy]tropane (β- d-glucoside of D). Moreover, the novel 3α,6β-di-(4-methoxybenzoyloxy)tropane (merredissine) has been isolated from M. dissecta and structurally elucidated. In addition the structures of datumetine and merresectine A could be confirmed by synthesis. Spectral data for two known 3α-acyloxytropanes (merresectine E β- d-glucoside, 4′-dihydroconsabatine) and one known 3β-acyloxytropane (concneorine) are documented for the first time. The structures of three further merresectines (F-H) have been determined by mass spectrometry. Furthermore, the linkage (2′,3- and 2′,4-, respectively) of two position isomer N-methylpyrrolidinylhygrines was proven by synthesis. The results of the study contribute to the solution of infrageneric taxonomic problems. Whereas all species yield pyrrolidine alkaloids without suitably differentiating results the diverging occurrence of tropane alkaloids leads to three groups of sections: (1) taxa free of tropanes, (2) taxa with simple tropanes, and (3) taxa with merresectines in addition to simple tropanes.