Lanthanide Triflate Catalyzed Hetero-annulation Route to Tetrahydroquinolines

Significance

Described here is a lanthanide Lewis acid catalyzed synthesis of simple and fused tetrahydroquinolines from aniline ortho-­alkylidene malonates. The reaction appears to proceed by a redox process involving a 1,5-hydride shift followed by an intramolecular ring closure although mechanistic evidence was not provided. A β-keto ester derived starting material (one example; R^³ ∂ R⁴) gave the product as a mixture of diastereomers (64:36). Diketone alkylidenes worked efficiently but with longer reaction times while the single case of a dinitrile alkylidene (R^³ = R⁴ = CN) failed to give any product.