Cyclodimerization of acyl-(3-oxo-2-quinoxalinyl)ketenes

Abstract

Two pathways have been described for the stabilization of acyl(imidoyl)ketenes, in which the imidoyl fragment is a part of the 1,2-dihydro-2-quinoxalone heterocyclic system. 3-Oxo-3,4-dihydro-2quinoxalinyl(ethoxycarbonyl)ketene is stabilized by the transfer of the quinoxalone fragment from the amide to the hydroxyimine form with subsequent intramolecular acylation of the hydroxyimine OH group by the ketene fragment [1]. 3-Oxo-4-phenyl-3,4-dihydro-2-quinoxalinyl(ethoxycarbonyl)ketene is stabilized by undergoing a [4+2] cyclodimerization reaction, in which one ketene molecule acts as diene through the imidoylketene fragment, while the other acts as the dienophile by means of the C=C bond of the ketene fragment [2]. The formation of aroyl and heteroyl(3-oxo-4-phenyl-3,4-dihydro-2-quinoxalinyl)ketenes (1a-c) would be expected in the thermal decarbonylation of 3-aroyland 3-heteroyl-5-phenyl-1,2,4,5-tetrahydropyrrolo[1,2-a]quinoxaline-1,2,4-triones (2a-c). However, the type of intramolecular cyclization described above [1] is impossible for ketenes 1a-c and alternatives exist for the participation of both the acylketene and imidoylketene fragments in intermolecular cycloaddition. Maintaining pyrroloquinoxalinetriones 2a-c in decane, which is an inert, aprotic solvent, at 172-173°C for 30-60 min leads to the formation of 4-acyl-3-acyloxy-2-(3-oxo-4-phenyl-3,4-dihydro-2-quinoxalinyl)-6phenyl-5,6-dihydro-1H-pyrido[1,2-a]quinoxaline-1,5-diones 3a-c.