Facile and Chemically Sustainable One-Pot Synthesis of a Wide Array of Fused O- and N-Heterocycles Catalyzed by Trisodium Citrate Dihydrate under Ambient Conditions

A simple, straightforward, and highly efficient multicomponent one-pot synthesis of pharmaceutically interesting diverse functionalized O- and N-heterocyclic scaffolds, such as spiro[indolin-2-one-3,4-pyrans] (4aa-4ah), spiro[indolin-2-one-3,4-pyran]-annulated (4ba-4bd, 4ba-4bb, 4ca-4cf, 4da-4dd, 4ea-4ee), and spiro[acenaphthylen-1-one-3,4-pyran]-annulated (7ba, 7ba, 7ca, 7da, 7ea) derivatives, pyrano[2,3-c]pyrazoles (4fa-4fz, 4ha-4hf), spiro[indolin-2-one-3,4-pyrano[2,3-c]pyrazoles] (4ga-4gh, 4ia-4ic), and spiro[acenaphthylen-1-one-3,4-pyrano[2,3-c]pyrazoles] (7fa-7fd), has been developed based on commercially available trisodium citrate dihydrate as an inexpensive and eco-friendly catalyst for the tandem Knoevenagel-cyclocondensation of isatin/aldehyde/acenaphthylene-1,2-dione, malononitrile, and various C-H activated acids in aqueous ethanol at room temperature. The salient features of this protocol are mild reaction conditions, excellent yields, applicability for the synthesis of a large number of diverse heterocycles, high atom economy, eco-friendliness, easy isolation of products, no column chromatographic separation, and the reusability of reaction media. It is noteworthy to mention that the present method offers a general way to synthesize various O- and N-heterocyclic scaffolds following a unified approach, which could replace a huge number of alternative methods reported so far.