In order to investigate the mechanism of the five-membered ring enlargement in carbocation intermediates in steroid biosynthesis, the rearrangement processes in a model carbocation, the 2-cyclopentyl-2-propyl cation 5, were followed by means of 1H and 13C NMR spectroscopy. Carbocations 5A and 5B were prepared in SbF5–SO2ClF–SO2F2 from the corresponding alcohol precursors. At about −100 °C a ring enlargement process takes place to give the 1,2-dimethylcyclohexyl cation 7. Quantum chemical calculations of model carbocation structures 5A, 5B, 6 and 7 were carried out at the HF/6–31G(d) and B3LYP/6–31G(d) levels of theory.
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ek et al. (2000) studied this question.
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