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 was treated with BF3·Et2O, SnCl4, TiF4, Sc(OTf)3, FeCl3, or TfOH, spirocyclic ether was formed as the sole product via a tert-cationic intermediate through 1,2-hydride shift. However, the treatment with TiCl4 afforded six-membered ring products , , , , , and via the ring expansion into the unstable six-membered ring secondary cation . Occurrence of both α and β chloride and is distinctive evidence of the existence of secondary cation , ruling out the idea of the concerted mechanism. Molecular mechanics calculations of the naked cation elucidated two possible conformers, parallel (five membered ring and cationic plane) that is favorable for the hydride shift generating and perpendicular leading to C–C bond migration to . 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 , it destabilizes the parallel conformer significantly, and thus, the C–C bond migration to becomes the only possible pass. On the other hand, [TiF4OH]−, [BF3OH]−, and [OTf]− stabilize parallel conformer and the hydride shift to 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 to be perpendicular leading to six-membered C-ring secondary cation . The parallel conformation of the cation 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.