Catalytic Synthesis of 3-Methyl-1-phenyl-1H-benzo[g]pyrazolo[3,4-b]quinoline-5,10-dione Derivatives Using Cerium Oxide Nanoparticles as Heterogeneous Catalyst in Green Conditions

We have developed a new methodology for the synthesis of 3-methyl-1-phenyl-1H-benzo[g]pyrazolo[3,4-b]quinoline-5,10-dione derivatives in excellent yields. A new green chemistry protocol with the reusability of the nanoparticle as catalyst has been developed for the synthesis of 3-methyl-1-phenyl-1H-benzo[g]pyrazolo[3,4-b]quinoline-5,10-dione derivatives via one-pot reaction of 3-methyl-1-phenyl-1H-pyrazol-5-amine, arylaldehydes, and 2-hydroxynaphthalene-1,4-dione in water as green solvent and using cerium oxide nanoparticles (CONPs) as heterogeneous catalyst. The present methodology affords several advantages such as simple procedure, excellent yields, and short reaction time. The catalyst is inexpensive, stable, easily recycled, and reused for several cycles with consistent activity. 1. Introduction Pyrazolo[3,4-b]quinoline derivatives are used as pharmaceutical agents [1], as inhibitors of oncogenic Ras [2], and as a dopant in the multiplayer OLED fabrication [3]. In the past several decades, three general strategies for the synthesis of pyrazolo[3,4-b]quinolines have been developed: (1) by the Friedlander condensation reaction of 2-aminobenzophenones and pyrazolin-5-ones [4]. Availability of 2-aminobenzophenones limits the range of applicability of this reaction; (2) by cyclization of 4-arylidenepyrazolin-5-ones with anilines [5] or 5-N-arylpyrazoles with aromatic aldehydes [6]. The method is complicated and has a lower yield; (3) by a three-component one-pot reaction of aromatic aldehydes, 5-amino-3-methyl-1-phenylpyrazole, and dimedone under thermal [7] or microwave condition [8]. Nanoparticles have emerged as sustainable alternatives to conventional materials and as robust, high-surface-area heterogeneous catalyst supports. The presence of a catalyst is mainly required by both modern organic syntheses and in fine chemical industries. Thus, the chemical nature and the existing form of the catalyst are of vital importance for the reaction. Recently, a great deal of attention has been focused on satisfying the requirements of environmentally benign and sustainable developments [8]. Consequently, there is a great demand for the discovery and development of novel catalysts with higher catalytic activities, lower prices, good recyclability, and less pollution to the environment in their catalytic systems. Cerium oxide has been extensively used in catalytic converters for automobile exhaust systems as an ultraviolet absorber and as an electrolyte for fuel cells [9–12]. Cerium-oxide based materials are famous for their redox properties, because of