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

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. Ninhydrin ring system is utilized successfully to develop a new class of fluorophore, namely, 6‐(2‐hydroxyaryl)isoindolo[2,1‐a]quinoxalin‐11(5H)‐ones via base promoted tandem cyclization. The readily prepared 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. The photophysical properties of the newly synthesized compounds were studied which disclose the interesting property of large Stokes shift with all the compounds. [ABSTRACT FROM AUTHOR]