Base-Catalyzed Rearrangement of N-(Aryloxy)pyridinium Salts. Effect of a 3-Substituent in the Pyridine Ring upon Orientation. Synthesis of Novel Tricyclic Rings

Article abstract of DOI:10.1021/jo00153a014

The orientation in the base-catalyzed rearrangement of 3-substituted N-(aryloxy)pyridinium tetrafluoroborates (5) to 2- or 6-(2-hydroxyaryl)pyridines has been studied.A 3-methyl group directs exclusively to the 6-position while inductively electron-withdrawing substituents (Cl, Br, I, OMe, CO2Me, COMe) direct mainly, if not exclusively, to C-2.The phenols derived from 3-CO2Me derivates cyclize spontaneously to the substituted pyrido<3,2-d>coumarins (8) in useful yields, and that from the 3-COMe compound gives 10-hydroxy-10-methyl-6-nitropyrido<2,3-d>benzopyran (9).The 3-halo-2-(2-hydroxyaryl)pyridines cyclize to benzofuro<3,2-b>pyridines(10,15) on heating with potassium hydroxide and copper powder.Authentic benzofuro<3,2-b>pyridine (12) is obtained by the Pschorr ring closure of the diazonium salt of 3-(o-aminophenoxy)pyridine (14).The substituent effects are explained mainly on the basis of the inductive effect of the substituent: steric effects play a slight role.A minor side reaction is thought to involve the intermediacy of free radicals.