Aryl Radical Endo Cyclization of Enamidines. Selective Preparation of Trans and Cis Fused Octahydrobenzo[f]quinolines.

Abstract

Aryl radicals from N-protected 6-[2-(2-halophenyl)ethyl]-1,2,3,4-tetrahydropyridines and 6-[3-(2-halophenyl)propyl]-1,2,3,4-tetrahydropyridines undergo intramolecular cyclization onto the enamide/enamidine double bond by 6-endo and 7-endo closure, respectively. In the 6-endo cyclization the trans/cis ratio of the formed N-protected octahydrobenzo[f]quinoline can be controlled, and selective synthesis of either the trans or the cis isomer can be achieved with triphenyltin hydride and tris(trimethylsilyl)silicon hydride, respectively. In the 7-endo cyclization to N-protected octahydro-1H-benzo[3,4]cyclohepta[1,2-b]pyridine, the trans fused isomer predominates, although the selectivity is low. The oxidized cyclization products, with a restored enamide/enamidine double bond, are formed at low concentrations of tris(trimethylsilyl)silicon hydride.