Cyclisation of 5-bromomethyl-cycloheptene and -cyclo-octene: a new route to bicyclo[3.2.1]octanes and bicyclo[4.2.1]nonanes

Abstract

Reduction of 5-(bromomethyl)cycloheptene with tributyltin hydride gave bicyclo[3.2.1]octane; similar reduction of 5-(bromomethyl)cyclo-octene gave bicyclo[4.2.1]nonane together with some bicyclo[3.3.1]nonane. The cyclohept-4-enylmethyl radical intermediate exists as a rapidly equilibrating mixture of conformers, including the axial boat form from which cyclisation occurs. The rates of the two main cyclisation reactions are ca. 10⁵ s^–1 at 25 °C. Condensation of the dimethylamine enamine of 4-methylcyclohexanone with acrylaldehyde and subsequent treatment with methyl iodide and base gave 6-methyloctahydro-1-benzopyran-2-one.