The C-Ring Problem of Sterol Biosynthesis: TiCl_4-Induced Rearrangement into the Anti-Markovnikov Cation Corresponding to the C-Ring

摘要

Cation 9, generated by the reaction of diol 8 and BF_3·Et_2O, SnCl_4, Sc(OTf)_3, FeCl_3, TiF_4, or CF_3SO_3H, leads to a hydride shift, providing cation 11, which corresponds to the initiation of backbone rearrangement. On the other hand, TiCl_4 selectively induces rearrangement to secondary cation 13 by ring expansion, which corresponds to the C-ring formation of sterol biosynthesis. AlCl_3 and ZrCl-4 induce further rearrangement into six-membered ring tert-cation 16.