Acid-catalyzed isomerization of fucosterol and Δ⁵-avenasterol

This work shows that fucosterol, Δ⁵-avenasterol, and similar ethylidene-side chain sterols can undergo acid-catalyzed isomerization to give a mixture of five isomers. Four isomers formed from fucosterol were analyzed, using gas chromatography-mass spectrometry, and were characterized as Δ⁵-avenasterol, two Δ^5,23-stigmastadienols, and Δ(5,24(25))- stigmastadienol. When the unsaponifiables fraction from oat oil was subjected to acid hydrolysis, the two Δ^5,23-stigmastadienol isomers and Δ(5,24(25))-stigmastadienol were detected while fucosterol coeluted with sitosterol. Interisomerization of ethylidene-side chain sterols represents a limitation to the use of the acid hydrolysis method in the determination of sterols in food and other plant materials rich in these sterols, e.g., oat lipids.