Application of O-sulfonic acid poly(4-vinylpyrrolidonium) chloride as an efficient polymer-supported catalyst in the rapid synthesis of 2H-indazolo[2,1-b]phthalazine-triones and Biginelli-like products

In this work, a series of 2 H -indazolo[2,1-b]phthalazine-triones and Biginelli-like products were efficiently synthesized for the first time using O-Sulfonic acid poly(4-vinylpyrrolidonium) chloride ([PVP-SO 3 H]Cl) as an eco-friendly polymer-supported catalyst. At first, the catalyst preparation process was optimized based on the chlorosulfonic acid amount and the sulfonation time to obtain a catalyst with the maximum acidity content. The maximum acidity content for [PVP-SO 3 H]Cl was obtained to be 3.31 mmol g −1 . Then, the optimized [PVP-SO 3 H]Cl was used as a catalyst for the synthesis of a series of 2 H -indazolo[2,1-b]phthalazine-triones and Biginelli-like products. It was observed that only 10 mg of the catalyst was enough to catalyze these reactions under mild conditions, and the corresponding products were obtained in high to excellent yields. Moreover, the catalyst could be repeatedly used for at least five times without a noticeable decrease in its catalytic activity.