The C-Ring Problem of Sterol Biosynthesis:  TiCl4-Induced Rearrangement into the Anti-Markovnikov Cation Corresponding to the C-Ring

Cation 9, generated by the reaction of diol 8 and BF3·Et2O, SnCl4, Sc(OTf)3, FeCl3, TiF4, or CF3SO3H, leads to a hydride shift, providing cation 11, which corresponds to the initiation of backbone rearrangement. On the other hand, TiCl4 selectively induces rearrangement to secondary cation 13 by ring expansion, which corresponds to the C-ring formation of sterol biosynthesis. AlCl3 and ZrCl4 induce further rearrangement into six-membered ring tert-cation 16.