The C-Ring Problem of Sterol Biosynthesis:  TiCl<sub>4</sub>-Induced Rearrangement into the Anti-Markovnikov Cation Corresponding to the C-Ring

Cation <b>9</b>, generated by the reaction of diol <b>8</b> and BF<sub>3</sub>·Et<sub>2</sub>O, SnCl<sub>4</sub>, Sc(OTf)<sub>3</sub>, FeCl<sub>3</sub>, TiF<sub>4</sub>, or CF<sub>3</sub>SO<sub>3</sub>H, leads to a hydride shift, providing cation <b>11</b>, which corresponds to the initiation of backbone rearrangement. On the other hand, TiCl<sub>4</sub> selectively induces rearrangement to secondary cation<b> 13 </b>by ring expansion, which corresponds to the C-ring formation of sterol biosynthesis. AlCl<sub>3</sub> and ZrCl<sub>4</sub> induce further rearrangement into six-membered ring <i>tert</i>-cation <b>16</b>.