Biosyntheses of phytosterols such as dammaranoid, lupanoid, oleananoid, and tirucallanoid are explained by the cyclization of oxidosqualene via 6/6 bicyclic cation, 6/6/5 tricyclic tertiary cation (pre-C ring cation), 6/6/6 tricyclic secondary cation (C-ring cation), and 6/6/6/5 tetra-cyclic tertiary cation In the animal kingdom, steroids are also constructed through the corresponding boat-form B-ring intermediates. Transformation of pre-C ring cation to secondary 6/6/6-cation involves ring expansion of a tertiary cation into a six-membered ring secondary cation, namely, an anti-Markovnikov cation. We chose 3-cyclopentyl-1,3-butanediondiol as the model compound and investigated the chemical behavior of the generated tert-cation. Although, BF3�EEt2O, SnCl4, Sc(OTf)3, FeCl3, TiF4 or CF3SO3H leads a hydride shift to give 5/5 spirocyclic ether, TiCl4 selectively induces rearrangement to six-membered ring secondary cation by the ring expansion that corresponds to the C-ring formation of sterol biosynthesis, and afforded 6/5 cis ether, chlorination products. Thus, the biological C-ring formation will be able to achieve as a chemical reaction.