Enol-Ugi Reaction of Hydroxycoumarins: Straightforward Synthesis of Amino Acid Derived Coumarin Enamines

Hydroxycoumarins containing electron-withdrawing groups have been successfully used as acidic components in Ugi-type multicomponent condensations with imines and isocyanides. The reaction takes place smoothly at room temperature, with no need of catalysis, affording 3- and 4-coumarin enamines in a highly convergent manner. Key to this transformation is the conjugate addition-β-elimination rearrangement on the primary adduct, irreversibly leading to the final product.