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

Using modern NMR techniques, including COSY, HOHAHA, 2D J spectrum, NOESY, HETCOR and HMBC, determination of 13 C T 1 spin−lattice relaxation times, and molecular modeling, permitted the full assignment of all 1 H and 13 C 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 C 18 angular methyl group thereby positioning C 22 to the right of C 20 (H 20 trans oriented to H 17 ) 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.