SYNTHESIS OF NITROGEN BRIDGEHEAD BICYCLIC HETEROCYCLES VIA RING-CLOSURE OF BETA-AMMONIO 5-HEXENYL RADICALS

The 2(3-methylenepiperidinyl)ethyl radical (6) displays considerable r eluctance to ring-closure under conditions which its carbocyclic analo g, the 2-(3-methylenecyclohexyl)ethyl radical (2), cyclizes essentiall y completely. Molecular mechanics calculations suggest that the increa sed activation barrier associated with ring-closure of 6 is the result of a higher than expected transition state energy. A study of the beh avior of beta-ammonio-substituted 5-hexenyl radicals, such as the 3,3- dimethyl-3-azonia-5-hexenyl radical (22), reveals that cyclization occ urs readily. Treatment of 1-(2-(phenylselenyl)ethyl)-3-methylenepiperi dinium iodide (20) with tributyltin hydride in tert-amyl alcohol yield s the bridgehead nitrogen bicyclic heterocycle, 1,5-dimethyl-1-azoniab icyclo-[3.2.1]octane iodide (26), in excellent yield and without conta mination, thus providing an attractive synthetic route to this hithert o unknown heterocyclic system.