Loganin and secologanin derived tryptamine–iridoid alkaloids from Palicourea crocea and Palicourea padifolia (Rubiaceae)

The phytochemical investigation of Palicourea crocea and Palicourea padifolia (Rubiaceae) led to the isolation of a new tryptamine–loganin derived alkaloid with a hexacyclic backbone and five known alkaloids. Their structures were elucidated and a broad screening within several related species revealed their distribution pattern. [Display omitted] •Palicourea crocea and Palicourea padifolia (Rubiaceae) were phytochemically studied.•A tryptamine iridoid alkaloid with a new hexacyclic backbone is described.•Five known alkaloids were additionally isolated.•A broad screening was performed within several related Palicourea species. During comparative analysis on Palicourea species from Costa Rica, two unusual loganin derived tryptamine–iridoid alkaloids were isolated from an accession of Palicourea crocea. Besides the already known brachycerine (2), palicroceaine (1) features a novel hexacyclic backbone. A second provenance, however, yielded strictosidinic acid (3), belonging to the more common secologanin derived tryptamine–iridoid alkaloids, such as those found in Palicourea padifolia. From this species, strictosidine (4), lyaloside (5) and its derivative (E)-O-(6′)-(4″-hydroxy-3″,5″-dimethoxy)-cinnamoyl lyaloside (6) could be isolated. A herbarium specimen-based screening was performed, indicating some degree of regional differentiation in alkaloid content and biosynthetic pathways within the widespread and variable Pal. crocea. It further shows its differentiation from the related strictosidine containing Palicourea croceoides. The occurrence of loganin derived tryptamine–iridoid alkaloids in Pal. crocea, Psychotria brachyceras and Psychotria brachypoda, all putatively unrelated members of the Palicourea s.l. clade, is a noteworthy exception within the genus, otherwise largely characterized by secologanin-derived tryptamine–iridoid alkaloids.