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The sterol components of the Rhodophyceae, Porphyridium cruentum and Porphyridium aerugineum have been investigated. The 4α‐methyl sterol fraction of P. cruentum contained two novel sterols, 4α‐methyl‐5α‐cholesta‐8,22‐dien‐3β‐ol and 4α,24ζ‐dimethyl‐5α‐cholesta‐8,22‐dien‐3β‐ol, together with 4α‐methyl‐5α‐cholest‐8‐en‐3β‐ol, 4α,24ζ‐dimethyl‐5α‐cholest‐8‐en‐3β‐ol and obtusifoliol. It is particularly significant that the two new sterols are the first reported 4‐methyl sterols to contain δ22 unsaturation. Broken cell preparations of both organisms cyclized squalene‐2,3‐oxide exclusively to cycloartenol. A possible biosynthetic pathway based on the structures of all the sterols isolated from P. cruentum is proposed, which allows for the introduction of δ22 unsaturation at the 4α‐methyl level.
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Beastall et al. (1974) studied this question.
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