[reaction: see text] The synthesis of the pyrazino[1,2-a]indole nucleus was achieved by intramolecular cyclization of several 2-carbonyl-1-propargylindoles in the presence of ammonia. The reaction conditions were optimized using microwave heating and a pool of catalysts. Cyclization of 1-alkynylindole-2-carbaldehydes was easily accomplished under standard heating conditions, whereas microwave heating contributed to reduced reaction times and improved overall yields. Moreover, a fine-tuning of the microwave irradiation time made possible the selective synthesis of both pyrazino[1,2-a]indole isomers. TiCl4 proved the catalytic system of choice to achieve pyrazinoindoles in satisfactory yields starting from 1-alkynyl-2-acetylindoles and 1-alkynyl-2-benzoylindole derivatives. Also in these cases, microwave heating contributed to faster reactions and improved yields. The uncatalyzed versus catalyzed reaction mechanism is discussed.
Pyrazine derivatives R 0550 Intramolecular Cyclization of δ-Iminoacetylenes: A New Entry to Pyrazino-[1,2-a]indoles. -Treatment of 1-alkynyl-2-acylindoles with ammonia offers an efficient approach to isomeric pyrazinoindoles. Their ratio and amount are dependent on the conditions used and the reaction time. -(ABBIATI*, G.; ARCADI, A.; BELLINAZZI, A.; BECCALLI, E.; ROSSI, E.; ZANZOLA, S.; J. Org. Chem. 70 (2005) 10, 4088-4095; Ist. Chim. Org., Fac. Farm., Univ. Stud. Milano, I-20133 Milano, Italy; Eng.) -Jannicke 40-163
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