Convenient Formation of Six- to Nine-Membered Carbocyclic Rings by 2-Pyridyl Radical Cyclization: A Generalized Synthesis of Pyridine-Fused Linear Tricyclic Systems

Abstract

Intramolecular cyclization of 2-pyridyl radicals derived from 2-bromopyridin-3-yl substituted methylenecycloalkanes 19-22, vinylcycloalkanols 23 and 24, allylcycloalkanols 26-28, and butenylcycloalkanols 32-34 proceed via 6-, 7-, 8-, and 9-endo-trig pathways, giving six-, seven-, eight-, and nine-membered ring annulated pyridines respectively. However, the vinyl cyclopentanol 25 produced only the 6-exo-cyclized product 46, while the substituted allylcyclohexanol 29 and the butenylcyclohexanol 31 failed to cyclize, producing only the double bond isomerized debrominated olefins. Thus the method provides a useful entry to diverse classes of fused pyridines, with the ring size of the cycloalkane and the presence of additional substituents in the ring playing an important role in determining the outcome of the reactions.