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.