Influence of electron donating aromatic substituents on the stereochemistry of the products in cycloalkylations of 2-(2-arylethyl)-3,3-dimethyl-1-methylenecyclohexane and related substrates: mechanisms of aromatic cycloalkylations

Abstract

The critical role of the aromatic ring substituents in open chain substrates, in acid-catalysed aromatic cycloalkylations with an unsubstituted and a few isomeric mono- and di-methoxy substituted 2-(2-arylethyl)-3,3-dimethyl-1-methylenecyclohexanes 8a–d,h–j and two isomeric 2-(2-dimethoxyphenylethyl)-1,3,3-trimethylcyclohexanois 5i,j, in determining the distributions of the respective trans- and cis- podocarpa-8,11,13-trienes, has been investigated and the results have been analysed. Olefin or alcohol precursors, having unactivated aromatic rings, proceed with high stereoselectivity via attack of the aromatic ring on a conformationally preferred cyclohexyl cation leading to the respective trans-products, while in substrates with an electron donating aromatic substituent para to the site of electrophilic attack, proceed directly in the exocyclic olefin or through the intervention of cyclohexenyl intermediates forming substantial amounts of the corresponding cis-products, in addition to the trans-products involving cyclohexyl cations.