Cyclisation reactions. Part IV. Stereochemistry of cyclialkylation of 2-(2-arylethyl)-1,3,3-trimethylcyclohexyl cations and their equivalents

Abstract

The stereochemistry of cyclialkylation of 2-(2-arylethyl)-1,3,3-trimethylcyclohexyl cations (I), generated from the corresponding 6-alcohols (VIII) has been studied. Whereas kinetically controlled cyclisation gives a preponderance of the trans-podocarpa-8,11,13-trienes (VI) to the extent of 75%, the thermodynamically controlled reaction favours the cis-isomers (V) almost in a reverse ratio. A qualitative estimate of the relative energies of the two isomers supports the greater stability of the cis. The results explain some anomalous reports in the literature.