The C- and D-ring problems of sterol biosynthesis: hydride shift versus carboncarbon bond migration due to conformational changes controlled by counteranion

The first calculations of a C-ring model cation of sterol biosynthesis in the presence of selected counteranions have been performed. [TiCl4OH]− ion significantly stabilizes the perpendicular cation conformer that leads to carboncarbon bond migration and formation of a six-membered ring secondary cation. The [TiF4OH]− ion, however, stabilizes the parallel cation conformer that leads to hydride shift as observed indeed.