New Types of Reactivity of α,β-Unsaturated N,N-Dimethylhydrazones: Chemodivergent Diastereoselective Synthesis of Functionalized Tetrahydroquinolines and Hexahydropyrrolo[3,2-b]indoles

The indium trichloride‐catalyzed reaction between aromatic imines and α,β‐unsaturated N,N‐dimethylhydrazones in acetonitrile afforded 1,2,3,4‐tetrahydroquinolines bearing a hydrazone function at C4 through a one‐pot diastereoselective domino process that involves the formation of two CC bonds and the controlled generation of two stereocenters, one of which is quaternary. This reaction constitutes the first example of an α,β‐unsaturated dimethylhydrazone that behaves as a dienophile in a hetero Diels–Alder reaction. The related reaction between anilines, aromatic aldehydes, and methacrolein dimethylhydrazone in CHCl3 with BF3⋅Et2O as catalyst afforded polysubstituted 1,2,3,3a,4,8b‐hexahydropyrrolo[3,2‐b]indoles as major products through a fully diastereoselective ABB′C four‐component domino process that generates two cycles, three stereocenters, two CC bonds, and two CN bonds in a single operation. Playing dominos: The reaction between anilines, aldehydes, and α,β‐unsaturated N,N‐dimethylhydrazones afforded C4‐functionalized 1,2,3,4‐tetrahydroquinolines or 1,2,3,3a,4,8b‐hexahydropyrrolo[3,2‐b]indoles through domino mechanisms, depending on the solvent and catalyst employed (see scheme).