Quantitative Cascade Condensations betweeno-Phenylenediamines and 1,2-Dicarbonyl Compounds without Production of Wastes

Abstract

o-Phenylenediamines 1 underwent a series of cascade condensations with 1,2-dicarbonyl compounds to afford quantitative yields (eight cases) of heterocycles in solid-state syntheses that avoided waste formation. The products were produced in pure form and did not require purifying workup. The components were ball-milled in stoichiometric ratio, or in exceptional cases they were melted together and heated in the absence of solvents (some of them giving quantitative yields). Benzils and 2-hydroxy-1,4-naphthoquinone afforded quinoxaline derivatives 3 and 5, 2-oxoglutaric acid gave a 3-oxodihydroquinoxaline 7, and oxalic acid afforded the dihydroquinoxaline-2,3-dione 9. This last condensed with 1a in the melt, to afford a mixture of bis(benzimidazolyl) 10 and fluoflavin 11. Alloxane hydrate provided a 100% yield of the 3-oxodihydroquinoxaline-2-carbonylureas 15/16 at room temperature. Parabanic acid required a melt reaction providing a 78% yield of 3-oxodihydroquinoxalinyl-2-urea 22 and side products. Despite numerous reaction steps, most of these uncatalyzed stoichiometric reactions proceeded quantitatively in the solid state to give only one product (plus water), with unsurpassed atom economy. If catalysis with HCl was tried, the results were inferior. If melt reactions were required it appeared to be advantageous to have the products crystallize directly at the reaction temperature. The synthetic results have been interpreted mechanistically and compared to some similar solution reactions that do not exhibit the benefits of the solid-state techniques. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)