Base Promoted Tandem Cyclization ofo‐Phenylenediamine with Ninhydrin‐phenol Adducts: An Unprecedented Route to Phenol Appended Isoindolo[2,1‐a]quinoxaline Fluorophore

Abstract

Synthesis of fluorescent molecules has drawn considerable attention due to their potential applications in life and material sciences. Here we report a metal‐free approach to a new class of fluorophore, namely, 6‐(2‐hydroxyaryl)isoindolo[2,1‐a]quinoxalin‐11(5H)‐ones exploiting ninhydrin ring system via base promoted tandem cyclization. The readily obtainable phenolic adducts of ninhydrin undergoes smooth reaction with o‐phenylenediamine in refluxing ethylene glycol/Et3N via breaking of C−C bond, N‐insertion, followed by heterocyclization to furnish the fully unsaturated isoindoloquinoxaline framework. Structures of the products are in good agreement with their IR, 1H NMR, 13C NMR, and mass spectral data. The photophysical properties of the synthesized compounds were studied by UV‐vis absorption and fluorescence spectroscopic techniques under ambient conditions, and the results reveal an interesting property of large Stokes shift.

Recent advances in the transition-metal-free synthesis of quinoxalines

“…A metal-free tandem cyclization approach to access other fused quinoxalines, namely isoindolo[2,1- a ]quinoxalines 127 has been demonstrated by Das et al , in 2019 ( Scheme 37 ). 117 To find out the optimum condition, the reaction of ninhydrin 126 bearing phenolic adducts, with o -phenylenediamine 35 was initially performed in the presence of different bases including Et 3 N, piperazine, pyridine, Et 2 NH, and solvent system like ethylene glycol, water, MeOH, EtOH, glycerol. From these experiments, it was concluded that the utilization of Et 3 N in ethylene glycol provides easy access to the products within a very short reaction time.…”

“…Consequently, the intramolecular cyclization of (Int-30), followed by elimination of water leads to the final products 127 ( Scheme 37 ). 117 …”

Transition‐Metal‐Free Synthesis of 1‐Indanone Skeleton: A Brief Update

“…Donepezil (AChE inhibitor) is an approved drug for the treatment of Alzheimer's disease [8] . Moreover, substituted 1‐indanones are also served as indispensable intermediates for the synthesis of diverse organic scaffolds including heterocycles [9–13] . There are so many reports where indanone analogues are successfully employed as chiral ligands in asymmetric synthesis [14] and metal‐ligands for the preparation of metal complexes [15] .…”

Recent applications of ninhydrin in multicomponent reactions

“…26 Recently, we employed monoarylated ninhydrin-adducts to develop a new uorophore system. 27,28 Some indanone-based uorophores were also explored to act as a receptor for specic metal ions. 29,30 Ninhydrin is basically an indanone class compound, and indanone core structures have been found in numerous natural products (Fig.…”