Abstract
A series of seven unusual dimeric naphthylisoquinoline alkaloids was isolated from the leaves of the tropical liana Ancistrocladus ealaensis J. Léonard, named cyclombandakamine A (1), 1-epi-cyclombandakamine A (2), and cyclombandakamines A3–7 (3–7). These alkaloids have a chemically thrilling structural array consisting of a twisted dihydrofuran-cyclohexenone-isochromene system. The 1′″-epimer of 4, cyclombandakamine A1 (8), had previously been discovered in an unidentified Ancistrocladus species related to A. ealaensis. Both lianas produce the potential parent precursor, mbandakamine A (9), but only A. ealaensis synthesizes the corresponding cyclized form, along with a broad series of slightly modified analogs. The challenging isolation required, besides multi-dimensional chromatography, the use of a pentafluorophenyl stationary phase. Featuring up to six stereocenters and two types of chiral axes, their structures were elucidated by means of 1D and 2D NMR, HRESIMS, in combination with oxidative chemical degradation experiments as well as chiroptical (electronic circular dichroism spectroscopy) and quantum chemical calculations. Compared to the ‘open-chain’ parent compound 9, these dimers displayed rather moderate antiplasmodial activities.
Highlights
In recent investigations on the botanically well-described liana Ancistrocladus ealaensis J
The systematic analysis of leaf extracts of A. ealaensis, which had already revealed the presence of three types of unsymmetric dimeric naphthylisoquinoline alkaloids17,18, showed that it was not unlikely to find further singularities
These investigations led to the isolation and purification of cyclombandakamine A [1], its 1-epimer 2, and five related dimeric alkaloids, 3–7, all featuring an unusual condensed cage-like polycyclic backbone (Fig. 1)
Summary
Dieudonné Tshitenge Tshitenge, Torsten Bruhn, Doris Feineis, Virima Mudogo, Marcel Kaiser, Reto Brun5,6 & Gerhard Bringmann. After the isolation of mbandakamines A and B in an as yet botanically undescribed Congolese Ancistrocladus species, our group has intensified the search for new secondary metabolites in Central African taxa, in particular in those found in the region of Mbandaka (Democratic Republic of the Congo)15 In this framework, we have just reported the occurrence of cyclombandakamine A1 [8]16, in the same ‘new’ plant, and postulated that 8 could originate biosynthetically from an ‘open-chain’ N-methylated mbandakamine A precursor. The systematic analysis of leaf extracts of A. ealaensis, which had already revealed the presence of three types of unsymmetric dimeric naphthylisoquinoline alkaloids, showed that it was not unlikely to find further singularities These investigations led to the isolation and purification of cyclombandakamine A [1], its 1-epimer 2, and five related dimeric alkaloids, 3–7, all featuring an unusual condensed cage-like polycyclic backbone (Fig. 1). Their structural elucidation is described, as well as their anti-infective profiles
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