Stereoselective synthesis of tetralins using cationic cyclisations

Tetralins, including the terpene calamenene, were prepared by 6-endo cationic cyclisations, effected by addition of an I(I) reagent to alkenylarenes, followed by reductive deiodination. An activating group on the arene was required for efficient cationic cyclisation. Good diastereoselectivity, relative to a chiral centre in the chain linking the alkene to the arene, was observed, with Z-alkenes giving predominantly 1,4-cis disubstituted tetralins, and E-alkenes giving predominantly 1,4-trans derivatives. Analogous 6-exo cationic cyclisations proved very limited in scope.