Highly substituted homoallylvinylcyclopropanes by indium-mediated reaction of alpha,beta-unsaturated ketones and aldehydes with allylic halides

Allylindium reagents, prepared from excess allylic halide (Br or I) and indium metal, react with alpha,beta-unsaturated ketones and aldehydes to give, after aerobic acidic workup, homoallyl-substituted vinylcyclopropanes. This process was explored and developed after a chance discovery arising from a side reaction in an attempted Pd-catalysed process. The structure of the cyclopropane arising from the reaction of bis(p-chlorobenzylidine)acetone was confirmed by X-ray crystallography. Whilst bis-alpha,beta-unsaturated ketones give rise to a single homoallylcyclopropane species, alpha,beta-unsaturated ketones and aldehydes give diastereomeric mixtures whose relative stereochemistry were assigned by NOE experiments. Crotylindium reagents react with good to perfect regioselectivity to afford tetrasubstituted cyclopropanes but prenylindium reagents fail to generate the analogous pentasubstituted rings.