Isolation, Partial Synthesis, and Structure Determination of Sterols with the Four Possible 23,24-Dimethyl-Substituted Side Chains

(23/2,24R)- and (23S,24R)-4α,23,24-trimethyl-5α-cholestan-3β-ol, (23R,24R)- and (23S,24R)-23,24-dimethyl- 5α-cholestan-3β-ol, and (23S,24R)-23,24-dimethylcholest-5-en-3β-ol were isolated from various marine organisms. Methyl substitution at C-23 and C-24 was proved by comparison of their 360-MHz XH NMR spectra with those of the hydrogenation products of dinosterol ((£,24R)-4α,23,24-trimethyl-5α-cholest-23-en-3β-ol) and synthetic 24-epidinosterol and 4-demethyl-5-dehydro-24-epidinosterol. The configuration at C-23 in the natural products was determined by chemical correlation of one of them with peridinosterol ((E,23R,24R)-4α,23,24-trimethyl-5α-cholest-17(20)-en-3β-ol). The configuration at C-23 of sterols with a 23,24(S)-dimethyl-substituted side chain was solved by X-ray analysis of the p-bromobenzoate of (23R,24S)-23,24-dimethyl-5α-cholestan-3β-ol.