Reactivity of ethyl nitrosoacrylate toward pyrrole, indole and pyrrolo[3,2-]carbazole: an experimental and theoretical study.

Nitrosoalkenes react with 8-methyl-1,6-dihydropyrrolo[3,2-]carbazole to give both 2- and 3-alkylated products via hetero-Diels-Alder reaction followed by the cycloadduct ring-opening. Quantum chemical calculations, at DFT level of theory, were carried out to investigate the regioselectivity of the cycloaddition of ethyl nitrosoacrylate with 1,6-dihydropyrrolo[3,2-]carbazoles as well as with pyrrole and indole, allowing a more comprehensive analysis of the reactivity pattern of nitrosoalkenes with five-membered heterocycles. Furthermore, theoretical calculations confirmed that ethyl nitrosoacrylate reacts with these heterocycles via a LUMO-HOMO controlled cycloaddition. The reactivity of one of the oxime-functionalized 1,6-dihydropyrrolo[3,2-]carbazole was explored and a new hexahydropyrido[4',3':4,5]pyrrolo[3,2-]carbazole system was obtained in high yield via a one-pot, two-step procedure.