The 2,4-dihydropyrrolo [3,4-b] indole ( 5 ) was prepared b y the intramolecular 1,3-dipolar cycloaddition of the azidoalkylidenemalonate (1 1 ), followed by 1,3-dipolar cycloreversion of the triazoline intermediates (1 2). Diels-Alder reaction of 2,4-dihydropyrrolo [3,4-b] indole derivative (6) with reactive dienophiles, such as N-phenylmaleimide, benzyne, and dimethyl acetylenedicarboxylate, gave the corresponding cycloadducts (1 5) and (16), (1 7), and (20) respectively. Arenimine (17) could be treated with lithium-ammonia t o afford the dihydrobenzocarbazole (1 8), which was dehydrogenated by D D Q t o give the benzocarbazole (1 9).The novel heterocyclic ring system 2,4-dihydropyrrolo[3,4-b]indole present in compound (l), was first prepared by Welch during the lithium aluminium hydride reduction of 2-benzyl-4-
phenyl-4H-pyrrolo[3,4-b]indol-3(2H)-one.' Closely related ring systems such as 4H-furo[3,4-b)indole (2),2 4H-thieno[3,4blindole (3),3 and 4H-selenolo[3,4-b]indole (4)have also been synthesized recently. These ring systems are of current interest mainly because they are of pharmaceutical i m p~r t a n c e ,~ but additionally they are stable cyclic analogues of indole-2,3quinodimethane (7).4 However, some of the cyclic analogues do not show marked diene c h a r a~t e r . '~~ In particular the 2,4-dihydropyrrolo[3,4-b]indole derivative (I), the only compound prepared by Welch containing this ring system, was not found to undergo Diels-Alder reactions.'
