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.