Regioselective 6-endo cyclization of 5-carbomethoxy-5-hexenyl radicals: A convenient synthesis of derivatives of the 1-azabicyclo[2.2.1]heptyl system

[GRAPHICS] Ring closure of the 5-carbomethoxy-5-hexenyl radical is governed largely by the polar effect, and as predicted by frontier molecular orbital considerations, endo cyclization predominates, leading to cyclohexyl rather than cyclopentyl- based products, In cyclization of the corresponding beta-ammonio species 18, stereoelectronic effects do not distinguish between attack of the radical center at C3 or C4, each of which represents a 5-exo ring closure. The radical 18 is found to cyclize with great rapidity and with high stereoselectivity to give bicyclo[2.2.1]heptane products in accordance with expectation based on polar effects; this transformation represents an excellent entry to the physiologically important bicyclic ester 17.