Novel synthesis of substituted pyrrolidines and piperidines via radical addition–ionic cyclization reaction of oxime ethers

We have developed a novel synthetic route to nitrogen-containing heterocycles via radical addition–ionic cyclization reaction. Treatment of oxime ethers carrying the tosyloxy group with Et3B and alkyl iodide in the presence of Lewis acid gave the substituted pyrrolidines and piperidines. The reaction of oxime ethers carrying the methoxycarbonyl group proceeded under the same conditions to give the amino esters, which were easily converted into the corresponding lactams by the treatment with concd HCl. On the other hand, the oxime ether bearing the phenoxycarbonyl group afforded directly alkylated lactams under the radical reaction conditions. The utility of this domino reaction was demonstrated by the synthesis of (±)-bgugaine and the formal synthesis of 5,8-disubstituted indolizidine alkaloids.