A green protocol for the rapid synthesis of pyrimido-quinoline derivatives

A very simple yet efficient approach has been developed for the synthesis of pyrimido[4,5-b]quinoline derivatives. The three-component condensation of dimedone (1 eq.) with aromatic aldehydes (1 eq.), and 6-amino-1,3-dimethyluracil (1 eq.) by a Brønsted acidic ionic liquid catalyst, namely 1,4-Bis(pyridinium bi-sulfonic acid)benzene dimethylsulfonate (BPBSBDM) produced the mentioned derivatives in short reaction times (8–18 min) with high yields (90–98%). The optimal reaction conditions included the use of 6 mol% catalyst in water solvent at a mild temperature of 50 °C. The two functional groups acidic (sulfonic acid) and weak basic (mesylate) give this catalyst a dual acidic-basic nature, which supports its high performance as shown in the proposed mechanism. The BPBSBDM offered significant reusability as a homogeneous catalyst for four cycles with negligible reduction in catalytic activity. The considerable values of turnover number (TON) and turnover frequency (TOF), simple work-up, the introduction of novel derivatives to the literature (1 k to 1n), easy and cost-effective synthesis of catalyst, the green nature of the reaction, and the comprehensiveness of the protocol in facile production of various derivatives are other attractive aspects of our method.