Three-step isolation of a new pregnane from Cynanchum marnieranum Rauh (Apocynaceae)

Abstract

While there is a rich bibliography concerning the phytochemistry of Asian leaf-bearing Cynanchum species, quite limited work has been done about leafless stem succulent Cynanchum species endemic to Madagascar [1]. Hence, we decided to investigate several Cynanchum species, focusing on Cynanchum marnieranum Rauh from the South of Madagascar. The ethanol extract from the dried stems was submitted to liquid/liquid partition followed by solid phase extraction (SPE) of the non-polar phase. The most interesting fraction was further separated by centrifugal partition chromatography (CPC), choosing the quaternary solvent system cyclohexane: ethyl acetate: methanol: water (6:5:6:5). This led to the isolation of a new pregnane glycoside, which was identified by combination of HR-ESI-TOF-MS and 1D and 2D NMR data as 12,20 dibenzoyl-(20S) – pregn-6-ene-3β,5α,8β,12β,14β,17β,20β -heptol 3-O-β-D-oleandropyranosyl (1→4)-β-D-cymaropyranoside, named marnieranoside A (figure 1). The novelty of this structure is based on the double substitution with benzoyl moieties fixed on a Δ6,7 pregnane structure which occurs for the first time in the genus Cynanchum. Indeed, only Δ5,6 pregnanes have been reported from Cynanchum species until now [2]. This CPC fractionation led also to other Δ6,7 pregnane glycosides according to the 1 H NMR data, which could indicate a specific Δ6,7 pregnane biosynthetic pathway in Cynanchum marnieranum.