Studies in Phytosterol Biosynthesis: Observations on the Biosynthesis of Fucosterol in the Marine Brown Alga Fucus spiralis

Fucosterol biosynthesised by Fucus spiralis from [2‐ 14 C,(4 R )‐4‐ 3 H 1 ] mevalonic acid has a 3 H/ 14 C atomic ratio of 3/5. Isomerisation of the labelled fucosteryl acetate gave 3β‐acetoxystigmasta‐5,24‐diene with a 3 H/ 14 C atomic ratio of 2/5. This demonstrated that the fucosterol had a 3 H atom located at C‐25 and therefore proves that hydrogen (tritium) migration from C‐24 to C‐25 occurs during alkylation of the phytosterol side chain. Cycloartenol and 24‐methylene cycloartanol also isolated from the Fucus spiralis were shown to be radioactively labelled, there was no evidence of radioactive lanosterol. Tritium was shown to be present at C‐3 of the biosynthesised cycloartenol but was absent from C‐3 of the fucosterol.