Mechanochemical milling promoted solvent-free imino Diels-Alder reaction catalyzed by FeCl3: diastereoselective synthesis of cis-2,4-diphenyl-1,2,3,4-tetrahydroquinolines

Under mechanochemical ball-milling at room temperature, FeCl3 promoted Diels-Alder cycloaddition of styrene with in situ generated N-aryl aldimines in the absence of any solvent afforded exclusively cis-2,4-diphenyltetrahydroquinolines in good to excellent yields within 90 minutes. The isolation work up just involves washing the resulting reaction mixture with water and recrystallization from EtOH-H2O. The advantages of high diastereoselectivity, short reaction time, free use of organic solvent, low cost, employment of cheap, easily available and nontoxic catalyst, and simple work-up procedure make this protocol a very efficient and green alternative to traditional methods for constructing these kinds of heterocyclic skeletons.