The Conformation of Cycloartenol Investigated by NMR and Molecular Mechanics

Abstract

AbstractCycloartenol (4), a natural plant sterol, was shown to be an effective membrane reinforcer; this was attributed to its conformation. We now present a conformational analysis of 4 by molecular modeling and NMR. Molecular modeling suggests that two conformations I and II coexist, differing mainly at the level of ring C, and of nearly equal energy, I and II each having ring A and B in a chair and half‐chair conformation, respectively, with ring C 1,3‐diplanar in I (solid‐state structure as determined by X‐ray crystallography) and in chair conformation in II. A complete assignment of the 1H‐ and 13C‐NMR spectra of 4 and the entire coupling network in rings A and B is determined by various modern NMR techniques. The conformation of rings A and B thus determined is in agreement with conformations I and II. Low‐temperature NMR experiments show a fast equilibrium between two conformations, presumably I and II. It is concluded, therefore, that the cyclopropane ring of 4 produces a flexibility at the level of ring C which may be important for the membrane properties.