Structural and Chemical Characterization of Kopsiyunnanines O1, O2, and P: Monoterpenoid Bisindole Alkaloids from Yunnan Kopsia arborea

Two previously undescribed monoterpenoid bisindole alkaloids, kopsiyunnanines O1 and P, were isolated from Kopsia arborea (Apocynaceae) and structurally characterized. These two bisindoles are composed of eburnamine- and rhazidigenine-related monomeric units and differ in substituents attached to C-7′ of the rhazidigenine-related unit. Furthermore, kopsiyunnanine O2 was identified as an interconvertible isomer of kopsiyunnanine O1. The reversible interconversion between kopsiyunnanines O1 and O2 was influenced by the solvent and the addition of acid.