[Benzo(f)furo(3,4-c)(2,7)naphthyridines from furo(3,4-b)pyridines]

The pyridines 5, obtained by dehydrogenation of the 4-(2-nitrophenyl)-1,4-dihydropyridine lactones 4, give the tetracycles 6 and 7 by reduction of the nitro group with zinc/ammonium chloride followed by ring closure. Irradiation of the compounds 4 with UV-A light leads to the 4-(2-nitrosophenyl)pyridines 8. During the reaction of 8 with conc. hydrochloric acid the nitroso group cyclizes with the 5-acyl group yielding the chloro substituted tetracycles 9 and 10. The hydroxamic acids 6 and 9 are deoxygenated by phosphorus trichloride to yield the lactames 11, while O-acetylation with acetic anhydride affords 12. Phosphorus trichloride reduces the N-oxides 7 and 10 to the bases 13. Using the reaction with acetic anhydride the acetates 14 are formed by Boekelheide-transformation. Alcaline saponification of 14 gives the alcohols 15.