Experimental and theoretical approaches into the C- and D-ring problems of sterol biosynthesis. Hydride shift versus C-C bond migration due to cation conformational changes controlled by the counteranion

The C- and D-ring problems of sterol biosynthesis, how an enzyme overcomes the Markovnikov wall, were investigated by using a model compound from an experimental as well as theoretical standpoint. When model diol 20 was treated with BF₃·Et₂O, SnCl₄, TiF₄, Sc(OTf)₃, FeCl₃, or TfOH, spirocyclic ether 21 was formed as the sole product via a tert-cationic intermediate 16 through 1,2-hydride shift. However, the treatment with TiCl₄ afforded six-membered ring products 22, 23, 24, 25, 26, and 27 via the ring expansion into the unstable six-membered ring secondary cation 17. Occurrence of both α and β chloride 23 and 24 is distinctive evidence of the existence of secondary cation 17, ruling out the idea of the concerted mechanism. Molecular mechanics calculations of the naked cation 15 elucidated two possible conformers, parallel 15-I (five membered ring and cationic plane) that is favorable for the hydride shift generating 16 and perpendicular 15-II leading to C-C bond migration to 17. The first ab initio calculation of the cation conformation in the presence of counteranions such as [TiCl₄OH]⁻, [TiF₄OH]⁻, [BF₃OH]⁻, and [OTf]⁻ entirely supported our experimental results. Although the counteranion [TiCl₄OH]⁻ stabilizes perpendicular cation 15-II, it destabilizes the parallel conformer 15-I significantly, and thus, the C-C bond migration to 17 becomes the only possible pass. On the other hand, [TiF₄OH]⁻, [BF₃OH]⁻, and [OTf]⁻ stabilize parallel conformer 15-I and the hydride shift to 16 becomes the only possible pass. The relative location or distance of the counteranion from the cation should be the biggest factor to control the stability and, thus, the conformation of the cation. Our results indicate that the carboxylate anions in the enzyme cavity enable to control the conformation of pre-C-ring cationic intermediate 3 to be perpendicular leading to six-membered C-ring secondary cation 4. The parallel conformation of the cation 5 could lead to hydride shift to give tirucallanoids or lanostanoids. Therefore, this result is the first example that overcame the big Markovnikov wall experimentally and theoretically at least to our knowledge.