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(E)2' mechanism' for double bond isomerization of Δ(24(28)) into Δ(24(25)). (C) 2000 Elsevier Science Ltd.