A green and simple approach was developed for the regioselective synthesis of structurally diverse chromenopyrrole frameworks from 3-formylchromones, active methylenes, and α-azido ketones using piperidine as a catalyst in the aqueous medium through a tandem one-pot multicomponent reaction. Further, the synthesized pyrrole framework was successfully converted into biologically significant 6-azaindole derivatives in a simple synthetic transformation. An exciting feature of this synthetic protocol is that the reaction mechanism and formation of the products depend on the nature of the active methylene used. This approach has several advantages such as a transition-metal-free catalyst, a short reaction time, easy separation, an excellent yield, practically simple execution, high regioselectivity, very good atom economy, low E-factor, and no requirement of toxic solvents and chromatographic purification.

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Great attempts have been done for the development of novel antiviral compounds against SAR-CoV-2 to end this pandemic situation and save human society. Herewith, we have synthesized 3-substituted indole/2-substituted pyrrole 1,2dihydropyridine and azaxanthone scaffolds using simple, commercially available starting materials in a one-pot, green, and regioselective manner. Further, the regioselectivity of product formation was confirmed by various studies such as controlled experiments, density functional theory (DFT), Mulliken atomic charge, and electrostatic potential (ESP) surface. In addition, 3substituted indole 1,2-dihydropyridine was successfully converted into a biologically enriched pharmacophore scaffold, viz., indolylimidazopyridinylbenzofuran scaffold, in excellent yield. Moreover, the synthesized 3-substituted indole 1,2-dihydropyridine/ 2-substituted pyrroles were analyzed in docking studies for anti-SARS-CoV-2 properties against their main protease (M pro ) and anti-Delta plus properties against their protein of the Delta plus K417N mutant. Further, the drug-likeness prediction was analyzed by the Lipinski rule and other pharmacokinetic properties like absorption, distribution, metabolism, excretion, and toxicity using preADMET prediction. Interestingly, the docking results show that out of 20 synthesized compounds, 5 of them for M pro of SAR-CoV-2 and 9 of them for 7NX7 spike glycoprotein's A chain of Delta plus K417N show greater binding affinity when compared with remdesivir that is the first to receive FDA approval and is currently used as a potent drug for the treatment of COVID-19. These results suggest that indole/pyrrole substituted 1,2-dihydropyridine derivatives are capable of combating SARS-CoV-2 and its Delta plus mutant.

A novel strategy was developed for the synthesis of five different N-fused 1,4-dihydropyridine (1,4 DHP) scaffolds such as imidazopyridine, pyridopyrimidine, benzoimidazopyridine, thiazolopyridine and benzopyridooxazine derivatives by coupling nitroketene S,S-acetal, various nitrogen containing dinucleophiles, malanonitrile and substituted salicylaldehydes/aldehydes in the presence of InCl 3 as catalyst in water-EtOH solvent mixture under reflux condition. Furthermore, mechanism for the formation of 1,4-DHPs was explored through experimental and DFT calculations. DFT studies reveal that the reaction went through lower energy triheterocyclic intermediate than the higher energy chromenoimidazopyridine imtermediate. The attractive features of this protocol include short reaction time, easy separation of the product without chromatographic purification, simple execution with excellent yield and possibility to synthesize structurally diverse 1,4-dihydropyridine derivatives through greener approach.

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