Fucosterol 24,28-epoxide (1) is a key intermediate in the conversion of sitosterol to cholesterol in phytophagous insects. e have investigated the stereochemical fate of the diastereotopic C-26 and C-27 methyl groups of the epoxide during an enzymatic conversion into desmosterol (2). Incubation of the chirally labeled fucosterol (24R,28R)-epoxides, having ^<13>C at pro-S- (6) or pro-R- (7) methyl group with a cell free enzyme system obtained from guts of the silkworm, Bombyx mori, and ^<13>C-NMR analysis of the product have revealed that the conversion is stereospecific with the C-25 prochiral center and the pro-S and pro-R methyl groups of (1) turn into (Z)- and (E)-methyl of (2). The similar transformation can be achieved in a chemical manner; treatment of fucosterol epoxide 3-benzoate with BF_3 etherate affords three products, desmosterol benzoate, a ketone and an aldehyde. The mechanism of this chemical reaction was also investigated from the stereochemical point. The results suggested that: 1) the formation of desmosterol benzoate involves a carbonium ion intermediate, 2) formation of the ketone (involves migration of hydrogen) takes place with the retention of the stereochemistry at the migration terminus (C-24), and 3) formation of the aldehyde (involves migration of methyl group) proceeds with the inversion of stereochemistry at the migration terminus. Another example, which follows the aforementioned stereo-chemical course, of trisubstituted epoxide-carbonyl rearrangement is also provided.
