Synthesis of 6H-indolo[2,3-b][1,6]naphthyridines and related compounds as the 5-aza analogues of ellipticine alkaloids

Treatment of 2-(1-alkynyl)phenyl isocyanates 6 with the iminophosphorane 14 produced in situ the benzoenynyl carbodiimides 15. Thermolysis of 15 under refluxing p-xylene furnished the 6H-indolo[2,3-b][1,6]naphthyridines 5, which could be regarded as the 5-aza analogues of ellipticine alkaloids. Similarly,condensation of 6 with the iminophosphorane 20 led to the formation of the 6H-indolo[2,3-b][1,5]naphthyridines 25 as the major isomer and the 10H- indolo[2,3-b][1,7]-naphthyridines 26 as the minor isomer. The indolonaphthyridines 32, 33, and 34 having a methoxyl substituent were likewise synthesized. Treatment of the diisocyanate 43 with 2 equiv of the iminophosphorane 7 furnished 45 having two indoloquinoline units incorporated in a seven-fused-ring system.