Synthesis and structural confirmation of calibagenin and saxosterol

Abstract Calibagenin and saxosterol are cholesterol-based plant steroids isolated from Calibanus hookerii and Narthecium ossifragum, respectively. To date, the configurations of their 16- and 22-hydroxy groups have not yet been determined. In this study, all the four 16,22-stereoisomers were chemically synthesized. The 1H and 13C NMR spectra were fully assigned using 2D NMR techniques, and the structures were determined unambiguously using X-ray crystallography. The H-18 and H-22 signals in the NMR spectra of the products are diagnostic for determining the configuration of the 16- and 22-hydroxy groups. A comparison of the NMR, [α]D, and mp data of the four isomers with those of natural calibagenin and saxosterol confirmed that the configurations of the 16- and 22-hydroxy groups of the former and the later are identical to 16β, 22S and 16β,22R, respectively.