N‐Heterocyclic Carbene–Catalyzed Kinetic Resolution of 3–Nitro–1,2–dihydroquinolines: Asymmetric Synthesis of All C2–, C3– and C4–Functionalized Tetrahydroquinolines

An efficient N-heterocyclic carbene (NHC)-catalyzed kinetic resolution of 2-substituted 3-nitro-1,2-dihydroquinolines (DHQs) allowing the synthesis of densely functionalized enantioenriched tetrahydroquinolines through the resolution at remote stereocenter is described. The Re-face addition of α,β-unsaturated  azolium homoenolate intermediate generated in situ from enal with the more kinetically favored (S)-enantiomer of the racemic 2-substituted 3-nitro-1,2-dihydroquinolines (rac-1), affording the enantiopure tetrahydroquinolines (up to >99:1 dr, >99:1 er) and the opposite (R)-enantiomer of DHQ (ent-1) was recovered (up to >99:1 er).