Side chain modification of sterols in marine invertebrates

Biosynthesis of branched and highly alkylated sterols in marine invertebrates has been the subject of speculation. Tris- or quadruple-methylation would produce branched sterols in Porifera. Epi-codisterol (C-24-Epi-9B) can be assumed to be a precursor of 25(26)-dehydroaplysterol (20) which is biomethylated again to give 

The evidence that the Cjv-nor-sterol, occelasterol (85), has the same 24a-config-uration as 24a-methylcholesta-5,22-dien-3i8-ol (7B) which was identified in a scallop, indicates the demethylation pathway from Cjv-sterol to 27-nor-sterol. The C26-sterol (83) may be formed from C27-nor-sterol (85) (Fig. 14). Since C26-sterol is widely distributed in marine invertebrates, it seems likely that this sterol may be produced in a rather early stage of the marine food chain. 

---

Side chain modification of sterols in marine invertebrates... 

---