Quinoxaline synthesis, alkyne oxidations

S. Okumura, Y. Takeda, K. Kiyokawa, S. Minakata, Hypervalent iodine(lll)-induced oxidative [4+2] annulation of o-phenylenediamines and electron-deficient alkynes direct synthesis of quinoxalines from alkyne substrates under metal-free conditions, Chem. Commun. 49 (2013) 9266-9268. 

Another report describes the synthesis of pyrazino[2,3-6]quinoxaline 1,4-dioxides (123) by condensation of furoxano[3,4-Z>]quinoxaline (121) with alkynes and alkenes. The pyrazino[2,3-A]quin-oxaline 1,4-dioxides (123) are also isolated as the final products from the reaction with alkenes and oxidation of the initially formed dihydro derivatives (122) (Scheme 21) <88JHC813>. 

Chandrasekhar S, Reddy NK et al (2010) Oxidation of alkynes using PdClj/CuClj in PEG as a recyclable catalytic system one-pot synthesis of quinoxalines. Tetrahedron Lett 51 3623-3625... 

Cyclic diketones can be the predecessors of quinoxaline macrocycles, obtained in one stage with the oxidation of cyclic alkenes with potassium permanganate (1971JA3303) or in two stages by the cyclotrimerization of cyclic alkynes and subsequent ozonolysis of compounds 18 (1986JOC3257). Along with the formation of diketone 4 the latter reaaion leads, to the unstable hexaketone 19. The reaction of the mixture of compounds 4 and 19 with the DAB leads to macrocycles 20 and 21 with one or three quinoxaline fr pients, respectively, with 10% and 3% yields, calculated in two st es.The synthesis of the macrocycle 20a from the analytically pure diketone 4a was in quantitative yield. 

Y. Xu, X. Wan, Ruthenium-catalyzed oxidation of alkynes to 1,2-diketones under room temperature and one-pot synthesis of quinoxalines. Tetrahedron Lett. 54 (2013) 642-645. 

A practical and high yielding application of DlB-induced oxidative [4+2] annulations of o-phenylenediamines 135 and electron-deficient alkynes 136 was described for the preparation of quinoxaline derivatives 137 (Scheme 30). The oxidative [4+2] annulations between diaminobenzene (4e center) and electron-poor alkynes (2e center) were successfully mediated by 1(111) reagent. Formation of quinoxalines was proposed to involve an initial generation of enamine which was likely to undergo DlB-mediated oxidative [4+2] annulations and aromatization. This metal-free and green protocol is very useful in the synthesis of diverse bioactive quinoxaline scaffolds (Scheme 30) [46]. 

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