Zinc oxide nanoparticles as efficient catalyst for the synthesis of novel di-spiroindolizidine bisoxindoles in aqueous medium

Developing environmentally benign synthetic protocols such as reaction in water isa major challenge. However, many of the known methods carried out in an aqueous medium are not efficient at large scale and need purification involving volatile solvents. Heterogeneous catalysis by nanoparticles in aqueous medium is a possiblesolution. Therefore, we tested the use of heterogeneous zinc oxide nanoparticles to catalyse the synthesis of di-spiroindolizidine bisoxindoles at room temperature by three-component condensation in an aqueous medium via azomethine ylide-mediated 1,3-dipolar cycloaddition reaction. The products were obtained with regio- and diastereoselectivities, with yields ranging from 83 to 94%, in60min. Absolute structures of products were confirmed by single-crystal X-ray crystallography. A gram-scale reaction along with recyclability experiment of the heterogeneous catalyst was also performed. To establish the sustainability of the reaction presented, green chemistry matrixes were calculated and found to possess ahigh atom economy of96.59% and small E-factor of0.098. The presented method has advantages such as environmental friendly reaction conditions, use of inexpensive heterogeneous zinc oxide nanoparticle as catalyst, broad substrate scope, short reaction times, quantitative yields of complex di-spiroindolizidine bisoxindoles and absence of tedious purification procedure like column chromatography.