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Five-Membered 2,3-Dioxo Heterocycles: L. Synthesis and Thermolysis of 3-Aroyl- and 3-Hetaroyl-5-phenyl-1,2,4,5-tetrahydropyrrolo[1,2-a]quinoxalin-1,2,4-triones

“…The 10-H proton is deshielded due to interaction with the C 1 =O carbonyl group, and its signal is displaced downfield (δ 9.05-9.20 ppm). The spectral parameters of compounds IIIa-IIIk are similar to structurally related dimers derived from ketenes B and C; the structure of the latter was proved by X-ray analysis [15,16].…”

“…Presumably, ketenes D undergo [4 + 2]-cyclodimerization through the imidoylketene fragment in a way similar to that reported in [15,16], provided that no other partner is present. Primary [4 + 2]-cycloadducts E are stabilized via [1,3]-C-O migration of the acyl group, which is typical of ketenes like B and C [15,16].…”

“…Primary [4 + 2]-cycloadducts E are stabilized via [1,3]-C-O migration of the acyl group, which is typical of ketenes like B and C [15,16]. The described reaction may be regarded as a convenient and regioselective method for building up difficultly accessible functionalized pyrido[1,2-a]-quinoxaline system.…”

“…Thermal decarbonylation of five-membered dioxo heterocycles [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] underlies a convenient method for generation of functionally substituted heterocumulenes, including acyl(imidoyl)ketenes. In continuation of our studies in the field of generation and stabilization of acyl(imidoyl)ketenes in which the imidoyl fragment is a part of a heterocyclic system, we examined methods of synthesis and thermal transformations of compounds belonging to a new class of five-membered dioxo heterocycles.…”

“…Ethoxycarbonyl(3-oxo-3,4-dihydroquinoxalin-2-yl)ketene A stabilizes via transformation of the quinoxalinone fragment from the amide to hydroxyimino form, followed by intramolecular acylation of the hydroxyimino group by the ketene moiety [14]. Aroyl(2-oxo-3,4-dihydro-2H-1,4-benzoxazin-2-yl)ketenes B and aroyl(3-arylquinoxalin-2-yl)ketenes C undergo [4 + 2]-cyclodimerization where one ketene molecule acts as diene (imidoylketene fragment), and the other, as dienophile 1084 (ketene C=C bond) [15,16]; next follows thermal [1,3]-migration of the aroyl group in the primary cycloadduct. Intramolecular cyclization like that described in [14] is impossible (for structural reasons) for aroyland hetaroyl(3-oxo-4-phenyl-3,4-dihydroquinoxalin-2-yl)ketenes D which are expected to be formed as a result of thermal decarbonylation of 3-aroyl-and 3-hetaroyl-5-phenyl-1,2,4,5-tetrahydropyrrolo[1,2-a]-quinoxaline-1,2,4-triones IIa-IIk (Scheme 1).…”