A Diversity‐Oriented Multicomponent Approach to Novel Fused Nitrogen Heterocycles Using 6‐Aminoindazole

Abstract We present a novel, efficient, eco‐friendly sonochemical strategy for the synthesis of fused nitrogen heterocycles via Brønsted acid‐promoted, one‐pot multicomponent reactions (MCRs). This method involves the rapid assembly of 6‐aminoindazole, aryl aldehydes, and 1,3‐cyclodione derivatives (barbituric acid, dimedone, and 1,3‐dimethyl barbituric acid) under mild and identical reaction conditions. The approach delivers a diverse range of highly functionalized heterocyclic frameworks, including 11‐phenyl‐1,6,7,11‐tetrahydro‐8 H ‐pyrazolo[3,4‐ f ]pyrimido[4,5‐ b ]quinoline‐8,10(9 H )‐dione, 8,8‐dimethyl‐11‐phenyl‐1,6,7,8,9,11‐hexahydro‐10 H ‐pyrazolo[3,4‐ a ]acridin‐10‐one, and 7,9‐diphenyl‐1,6,7,9‐tetrahydro‐2′ H ‐spiro[pyrazolo[3,4‐ f ]quinoline‐8,5′‐pyrimidine]‐2′,4′,6′(1′ H ,3′ H )‐trione derivatives. The molecular structures of these compounds were unambiguously confirmed through 1H‐NMR, 13C‐NMR and single‐crystal X‐ray diffraction (SC‐XRD) analysis. Additionally, control experiments provided valuable mechanistic insights into the reaction pathway. This protocol stands out for its efficiency, high yields, and broad functional group tolerance, making it a versatile tool for diversity‐oriented synthesis in modern heterocyclic chemistry.