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Reduction of 5-(bromomethy1)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. l o 5 s-' at 25 "C. Condensation of the dimethylamine enamine of 4methylcyclohexanone with acrylaldehyde and subsequent treatment with methyl iodide and base gave 6-methylocta hydro-Ibenzopyran-2-one.The number of useful syntheses of bi-and poly-cyclic structures which involve free radical cyclisation is steadily growing. lP7 In a transannular cyclisation of a radical of type (1) the side chain must normally be at least two carbon atoms long (n > 1) for significant bicyclisation to O C C U ~. ~' ~' ~ Exceptions to this rule, i.e. 0""bicyclisation with a one-carbon side chain, can, however, be found when the ring conformation is favourable. In particular, if the C, side chain (CR,' group) occupies an axial or quasi-axial site then the radical centre may be able to approach from above the plane of the double bond. This is the stereoelectronically preferred orientation from which cyclisation is favoured. In the cyclohex-3-enylmethyl case (2) the side chain can occupy the quasi-axial site in the half-chair conformation, but bicyclisation was not observed in the parent radical (2; R = H)" or in derivatives.8 Models indicated that transannular cyclisation might occur more readily for seven-and eight-membered rings, and this proved to be the case." This paper describes our studies of the bicyclisation of cycloheptenylmethyl and cyclooctenylmethyl radicals and a study of the conformational preferences of the former. Results and DiscussionSynthesis of BicycloC3.2. lloctane and Bicyclo[4.2.l]nonane.-Cyclohept-4-enylmethyl bromide (4) was obtained by reduction of cyclohept-4-enecarboxylic acid (3) and conversion of the alcohol into the bromide via the mesyl ester (Scheme 1).

Format ion of Bicyclo[3.2.1 Iocta ne, Bicycle[ 4.2.1 1 nona ne, and Bicycle[ 3.3.1 ] nonane by Transannular Radical Cyclisations