ortho-hydroxyalkylation of aminopyridines: A novel approach to heterocycles

Reaction of 6-substituted N-alkyl-3-aminopyridines with aldehydes in the pr esence of dichlorophenylborane gives selectively 2-hydroxyalkyl-3-aminopyri dines. Dehydration of the latter under pyrolytic or Lewis acid conditions g enerates a quinone methide imine intermediate which can undergo electrocycl ic or [4+2] cycloaddition reactions to give naphthyridines, dihydronaphthyr idines or tetrahydronaphthyridines. Palladium-catalyzed cyclization of the 2-(1-hydroxyallyl)-3-aminopyridines gives the corresponding 4-azaindoles.