The reaction of dialkyl acetylenedicarboxylates 4 with 1-aryl-2-[(3-arylquinoxalin-2(1H)-ylidene)ethanones 3 in the presence of Ph 3 A C H T U N G T R E N N U N G P leads to dialkyl (2Z)-2-[(E)-1-aryl-2-(3-arylquinoxalin-2-yl)ethenyl]but-2-enedioates 1 in good yields.
Introduction. -The quinoxaline ring system [1] is present in many natural and synthetic products exhibiting antitumor activity [2]. Quinoxalines have also been used for the synthesis of poly(phenylquinoxalines), which possess excellent thermal stabilities, low dielectric constants, high glass-transition temperatures, and good mechanical properties [3]. New living polymerization of 1,2-di(isocyano)arenes via (quinoxalinyl)palladium complexes have been reported [4]. The majority of quinoxaline derivatives are prepared by the reaction of benzene-1,2-diamine or an equivalent reagent with a 1,2dicarbonyl compound [5] [6].
As part of our current studies on the development of new routes in the synthesis of heterocyclic systems [7], we now report a convenient one-pot synthesis of dialkyl
Results and Discussion. -Reaction of benzene-1,2-diamine with the corresponding 1,4-diarylbut-2-yne-1,4-dione 2 gave rise to the 1-aryl-2-[(3-arylquinoxalin-2(1H)-ylidene)ethanones 3. The latter were then reacted with acetylenic esters of type 4 in the presence of Ph 3 A C H T U N G T R E N N U N G P in refluxing toluene to afford the target compounds 1 in good yields (Scheme 1). No products other than 1 were detected by NMR spectroscopy. The structures of 1a -1f were deduced by elemental analysis, MS, IR, 1 H-NMR, and 13 C-NMR spectroscopy.
The 1 H-NMR spectrum of, e.g., 1a exhibited two singlets at d(H) 3.68 and 3.82 due to two MeO groups. The olefinic resonances were observed at d(H) 5.68 and 7.01, along with characteristic multiplets for the aromatic H-atoms. The 1 H-decoupled 13 C-NMR spectrum of 1a showed 24 distinct signals, in agreement with the proposed structure. Unambiguous evidence for the proposed structure of 1a was finally obtained by single-crystal X-ray-diffraction analysis. An ORTEP [8] diagram of 1a is shown in the Figure. For details of the structure determination and refinement, see the Exper. Part. Mechanistically, it is conceivable that the reaction leading to 1 involves the initial formation of a zwitterionic 1 : 1 intermediate 5 of Ph 3 A C H T U N G T R E N N U N G P and the acetylenic compound (Scheme 2) [9 -11]. The intermediate 5 is then protonated by the NH-acidic 3 to afford