Evidence for the Origin of the Ethyl Group of β-Sitosterol

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

“…When doubly labelled fucosteryl acetate (3H/14C atomic ratio = 3.2315) was isomerised by this method the resulting 3B-acetoxy-stigmasta-5,24-diene had a 3H/I4C atomic ratio of 2.2015. The loss of tritium therefore demonstrates the location of one tritium atom a t C-25 of the fucosteryl acetate and proves the migration of the hydrogen (or tritium in this case) from C-24 to C-25 during the biosynthetic alkylation reaction as predicted by theory [5]. This hydrogen migration has also been demonstrated to occur in the C-24 alkylation reaction involved in ergosterol biosynthesis by yeast [26] and poriferasterol biosynthesis in Ochromonas malhamensis [27].…”

“…The transmethylation mechanism proposed [5] for the introduction of the alkyl group a t C-24 of phytosterols requires a hydrogen migration from C-24 to C-25. The demonstration of this hydrogen migration has been made possible in the present work by the availability of [4R-4-3Hl] mevalonic aH/14C atomic ratio very close to 315.…”

Microbial Biosynthesis of B12-Like Compounds

“…This model is con sistent with the fact that only 2 of the 3 carbon bridges are methyl a ted and that only one of the methyl groups is at the position of a propionic side chain. Since C-methylation at double bonds has since been shown as well (61,117,130; for a recent review see 165a ) this kind of model remains a possible but not a necessary prerequisite for C-methylation.…”

The Mechanism of Alkylation at C-24 During the C29Sterol Biosynthesis in Cucurbita maxima Seedlings

“…The alkylation mechanism first proposed (scheme I), postulated that methionine acted as methyl group donor with a A24-sterol (1) as substrate [3].…”