Stereochemical fate of C-26 and C-27 during the conversion of isofucosterol to sitosterol and of 24-methylenecholesterol to campesterol and dihydrobrassicasterol in Oryza sativa cell cultures

Administration of pro- R-methyl- 13C-labeled isofucosterol to cultured cells of Oryza sativa revealed that the pro- R and pro- S methyls at C-25 become the pro- R and pro- S methyls at C-25 of sitosterol, respectively. Similar administration experiments using pro- S-methyl- 13C-labeled 24-methylenecholesterol established that the pro- R and pro- S methyls at C-25 of 24-methylenecholesterol become the pro- R and pro- S methyls of campesterol, and the pro- S and pro- R methyls of dihydrobrassicasterol, respectively. These results are compatible with our recently proposed ‘ syn-S E2′ mechanism’ for double bond isomerization of Δ 24(28) into Δ 24(25).