Identification of 27-nor-(24R)-24-methylcholesta-5,22-dien-3β-ol and brassicasterol as the major sterols of the marine dinoflagellateGymnodinium simplex.

The major 4α-monomethyl sterol of the dinoflagellateGymnodinium simplex was identified as (24S)-4α,24- dimethylcholestan-3β-ol. The major 4-demethyl sterols were characterized as (24R)-24-methylcholesta-5,22-dien-3β-ol (brassicasterol) and 27-nor-(24R)-24-methylcholesta-5,22-dien-3β-ol. The latter sterol has the opposite configuration at C-24 to that assigned to occelasterol, which has the same basic structure and has previously been reported as a constituent of the sterols of a marine worm. 24-Nor-cholesta-5,22-dien-3β-ol was also identified along with several other trace sterols. The co-occurrence of 27-nor-(24R)-cholesta-5,22-dien-3β-ol together with 24-nor-cholesta-5,22-dien-3β-ol and brassicasterol provides new evidence for the biosynthetic origins of the two former nor-sterols. It is suggested that they may be produced de novo by a route involving nor-isoprenoid pyrophosphates and nor-squalene as intermediates, rather than as bacterial degradation products of brassicasterol (or related sterols) as previously suggested in the literature.