Arbolodinines A−C, Biologically-active Aspidofractinine-Aspidofractinine, Aspidofractinine-Strychnan, and Kopsine-Strychnan Bisindole Alkaloids from Kopsia arborea

ABSTRACT Three new bisindole alkaloids, arbolodinines A−C, comprising aspidofractinine-aspidofractinine, aspidofractinine-strychnan, and kopsine-strychnan alkaloids, respectively, were isolated from the stem bark extract of the Malayan Kopsia arborea. The structures of the alkaloids were elucidated based on analysis of the spectroscopic data while the absolute configurations were established based on experimental and calculated ECD data. Arbolodinine A is characterized by the union of two aspidofractinine halves with rare and unusual branching from C-5' of an iminium/carboxylate zwitterion of one aspidofractinine half to the olefinic C-14 of the other aspidofractinine half, while arbolodinines B and C represent first examples of aspidofractinine-strychnan and kopsine-strychnan bisindoles, respectively. Arbolodinine B displayed pronounced cytotoxic effects against an array of human cancer cell lines, including KB, vincristine-resistant KB, PC-3, HCT 116, HT-29, MDA-MB-231, MCF7 and A549 cells with IC50 values ranging from 1.3 to 9.6 μM.