Intramolecular nucleophilic substitution of chlorine in 2-chloro-3-(alkanesulfonylamino) pyridines furnishes N-alkyl-1, 3-dihydroisothiazolo [4, 3-b] pyridine 2, 2-dioxides (pyridosultams, 8), which undergo thermal extrusion of SO2 giving heteroanalogues of aza- ortho-xylylenes 9. These reactive 1-azadienes enter [4+ 2] cycloaddition with dienophiles leading to tetrahydro [1, 5] naphthyridines 11. With an excess of dienophile (N- ...