Biosynthetic Studies of Marine Lipids. 31. Evidence for a Protonated Cyclopropyl Intermediate in the Biosynthesis of 24-Propylidenecholesterol

The biosynthesis of 24-propylidenecholesterol (11-N) was determined by the use of cell-free extracts from the Chrysophyte alga Chrysoderma mucosa. The biosynthetic sequence was shown to proceed via desmosterol (28-N), 24-methylenecholesterol (29-N), and isofucosterol (30-N) to 24-propylidenecholesterol (11-N). 24-Vinylcholesterol (13-N) was neither a substrate nor a product of the S-adenosylmethionine (SAM)-methyltransferase, nor was it detected in cultures grown in the presence of azasterol inhibitors. Chemical degradation of 24-propylidenecholesterol (11-N) from enzymatic [3H]SAM methylation of isofucosterol (30-N) provided evidence for the protonated cyclopropane intermediate (27-N). The mechanism involving such an intermediate is consistent with the structures and stereochemical assignments of trace sterols found in this alga. Evidence for the intermediacy of protonated cyclopropanes in the SAM sterol methyl-transfer reactions leading to isofucosterol (30-N), fucosterol (31-N), and 24-methylenecholesterol (29-N) could not be found.