Crystal and molecular structure of eserine (physostigmine)

Abstract

The crystal structure of the alkaloid eserine, a constituent of the Calabar bean, Physostigma venenosum Balf., has been determined from diffractometer data. Crystals are orthorhombic, space group P2₁2₁2₁, a= 1458(1), b= 1435(1), c= 727·1(6) pm, Z= 4. The structure was solved by multi-solution direct methods and refined by block-diagonal least squares to R 0·06 (1521 significant reflections) and R 0·10 (all 2531 observations). These results confirm those obtained by use of the nuclear Overhauser effect, and show that the solid-state conformation, at least of the three fused rings, persists in solution. The two pyrrolidine rings are cis-fused. That fused to the benzene ring is almost flat, and the other is in the C₂ half-chair conformation with a two-fold symmetry axis passing through C(8) and the mid-point of the C(6)–C(10) bond.