Crystal and Molecular Structure of 12-Epinapelline

Abstract

Previous investigations showed that the conformation of ring A in crystal structures of napelline-type alkaloids depended on the presence or absence of an intramolecular H-bond between an α-oriented C1 hydroxyl and the N atom. Thus, ring A in 12-epilucidusculin had the boat conformation [1]; in acophine it adopted the twist—chair conformation [2]; whereas in 12-acetyl-12-epinapelline it had the ideal chair conformation [3]. These facts generate interest in the structure of 12-epinapelline (1), which was isolated from Aconitum karacolicum Rap. and characterized by IR, PMR, and mass spectral methods [4]. Single crystals were grown from ethanol solution and were elongated transparent prisms. Table 1 lists the principal crystallographic data and the experimental conditions of the x-ray structure analysis. The structure was solved and refined by the usual methods [5]. Figure 1 shows the molecular structure and atomic numbering for 1. 1 with the perhydrophenanthrene C skeleton consists of six main rings. Ring A (C1-C5,C10) adopts an almost ideal chair conformation (within ±0.011 A). Rings B (C5C7,C10,C20) and C (C7-C10,C20) have the 20α-envelope conformation (within ±0.068 and ±0.064 A, respectively) where C20 deviates by 0.887 and 0.840 A, respectively. Six-membered ring D (C8,C9,C11-C14) has an ideal boat conformation (±0.011 A). Five-membered ring E adopts the 14α-envelope conformation (±0.031 A) with C14 deviating (0.758 A) from the plane of the other four atoms. Heterocycle F (C4,C5,C10,C19,C20,N) is a slightly distorted chair (±0.051 A). Rings A and B are cis-fused; C and D, cis. The conformation and fusion of rings in 1 are consistent with those observed in 12-acetyl-12epinapelline. Substituents in the C skeleton are hydroxyls on C1 with the α-equatorial orientation and on C12 and C15 with the β-orientation.