Sceletium alkaloids. Part 7. Structure and absolute stereochemistry of (–)-mesembrane and 3′-methoxy-4′-O-methyljoubertiamine, two minor bases from S. namaquense L. Bolus: X-ray analysis of (–)mesembrane hydrochloride monohydrate

Abstract

From S. namaquense L. Bolus, we have isolated the known base tortuosamine (7) and two new alkaloids, 3′methoxy-4′-O-methyljoubertiamine (6) and (–)-mesembrane (2) the structures of which have been established by spectroscopic and X-ray methods. Previous suggestions concerning the absolute configuration of alkaloids of the 3a-aryl-cis-octahydroindole ring system are confirmed by an X-ray analysis of (–)-mesembrane hydrochloride monohydrate which crystallizes in the monoclinic system, space group P2₁, with a= 14.50(1), b= 8.02(1), c= 7.57(1)Å, β= 93.4(1)°, Z= 2. The crystal structure was solved by direct methods and refined by least-squares calculations to R 0.055 over 1 158 reflections from diffractometer measurements. The absolute configuration was established by the anomalous dispersion effect. The c.d. spectrum of (–)-mesembrine, considered as an axial β-aminoketone, is shown to exhibit normal octant behaviour.