Synthesis of pyrimido[1,6-a]quinoxalines via intermolecular trapping of thermally generated acyl(quinoxalin-2-yl)ketenes by Schiff bases

Abstract

Acyl(quinoxalin-2-yl)ketenes generated by thermal decarbonylation of 3-acylpyrrolo[1,2-a]quinoxaline-1,2,4(5H)-triones react regioselectively with Schiff bases under solvent-free conditions to form pyrimido[1,6-a]quinoxaline derivatives in good yields.

Acyl(imidoyl)ketenes: Reactive Bidentate Oxa/Aza-Dienes for Organic Synthesis

“…Trapping reagents 31 are added directly to the reaction mixture before heating; the reaction can be performed in a solvent-free mode. The reaction proceeds regioselectively, and no formation of alternative products of cycloaddition at oxa-diene system of ketenes A6 is observed [54]. Such a change in regioselectivity of reaction of acyl(imidoyl)ketenes A6 [54], in comparison with acyl(imidoyl)ketenes A8 [73], can be caused rather by the influence of heterocyclic substituent incorporating imidoyl moiety C=N of ketenes A or to the influence of reaction temperature (it is lower in the case of ketenes A8).…”

“…The reaction proceeds regioselectively, and no formation of alternative products of cycloaddition at oxa-diene system of ketenes A6 is observed [54]. Such a change in regioselectivity of reaction of acyl(imidoyl)ketenes A6 [54], in comparison with acyl(imidoyl)ketenes A8 [73], can be caused rather by the influence of heterocyclic substituent incorporating imidoyl moiety C=N of ketenes A or to the influence of reaction temperature (it is lower in the case of ketenes A8). However, N 4 -substituted acyl(quinoxalin-2-yl)ketenes A6 generated from N 5 -substituted 3-acylpyrroloquinoxalinetriones 2.2 react with carbodiimides 32 to form 1,2-dihydro-3H-pyrimido[1,6-a]quinoxaline-3,5(6H)-diones 36 or 5-(3-oxo-3,4-dihydroquinoxalin-2-yl)-2,3-dihydro-4H-1,3-oxazin-4-ones 37 in dependence on type of acyl substituent COR 2 (Scheme 25) [70].…”

“…N 4 -Unsubstituted acyl(quinoxalin-2-yl)ketenes A3 exist as two tautomers, amide and hydroxyimine. Hydroxyimine form contains OH group prone to react with ketene moiety to form furo [3,2-b]quinoxalines 8 [53][54][55] (Scheme 6). It should be mentioned that there were attempts to trap ketenes A3 by various dienophiles, but all of them were unsuccessful [55].…”

“…Zwitterions D2, formed as a result of intramolecular cyclization of methyleneamino substituted acyl(imidoyl)ketenes A9, react with substituted benzonitriles 27 to afford 4H-pyrazolo [ Interestingly, N 4 -phenyl substituted acyl(quinoxalin-2-yl)ketenes A6 generated from N 5 -phenyl substituted 3-acylpyrroloquinoxalinetriones 2.2 react as aza-dienes with Schiff bases 31 to form 1,2-dihydro-3H-pyrimido[1,6-a]quinoxaline-3,5(6H)-diones 35 as sole products (Scheme 24) [54]. Trapping reagents 31 are added directly to the reaction mixture before heating; the reaction can be performed in a solvent-free mode.…”

A highly selective fluorescent chemosensor for Mg2+ based on a diarylethene with a quinoxaline unit

“…Quinoxaline and its derivatives are important benzopyrazine compounds with aromatic properties. Many functional quinoxaline derivatives are expected to be used as molecular probes, chemical switches and supramolecular receptors , and have practical applications in medical drugs, agricultural drugs, dyes, and other fields . Therefore, the synthesis and research of quinoxaline compounds have attracted much attention.…”