9β,19-Cyclosterol Analysis by ¹H and ¹³C NMR, Crystallographic Observations, and Molecular Mechanics Calculations

Abstract

Using modern NMR techniques, including COSY, HOHAHA, 2D J spectrum, NOESY, HETCOR and HMBC, determination of 13C T1 spin−lattice relaxation times, and molecular modeling, permitted the full assignment of all 1H and 13C chemical shifts and geminal and vicinal coupling constants of cycloartenol. Stereochemical assignments related to the solution conformation of cycloartenol established unambiguously the nucleus assumes a flat structure and the side chain orients preferentially into a “right-handed” conformation, i.e., 20α-H atom is in front and opposing (in a 1,3-diaxial relationship with) the C18 angular methyl group thereby positioning C22 to the right of C20 (H20 trans oriented to H17) in the usual view of the molecule. The crystal structures of two naturally occurring 9β,19-cyclopropylcycloartenol metabolites, 4-normethylcycloartenol and 4,4-dinormethylcycloartenol, have also been determined and found to possess a three-dimensional shape similar to that of cycloartenol. The biosynthetic implications of these findings are discussed in relation to the cyclization of squalene oxide to cycloartenol.