Water Extract of Onion Catalyzed Tandem CN and CC Bond Formation via Enaminone-Michael Addition-Intramolecular Cyclization: An Efficient Green Synthesis of Multi-Substituted Pyrrole Derivatives and Their Antioxidant Activity

An efficient tandem reaction method catalyzed by aqueous onion extract comprising enaminone formation followed by Michael addition-cyclization is disclosed by 1,3-diketone 1, primary amine 2, and β-nitroalkene 3. In this work, diverse structures of enaminones and β-nitroalkenes 3 were involved to produce various multi-substituted pyrroles 6 in excellent yields (up to 97%). The structure of compound 6d was confirmed by single-crystal X-ray diffraction. This method has been applicable for a broad range of substrates and good functional group tolerance. Further, this proposed methodology has added advantages such as a simple, short reaction time, easy workup procedure, and environmentally benign manner. The in vitro antioxidant activity studies of the produced compounds, 6a–6y, were examined. When compared to 6e–6g, 6l, 6m, 6q, 6r, 6t, and 6u, the compounds such as 6h, 6i, 6j, 6k, and 6x have shown excellent IC₅₀ value with standard (1.80 × 10⁻⁴ M). In addition, compound 6h has demonstrated exceptional IC₅₀ value (1.58 × 10⁻⁴ M) among the synthesized compounds.