One-Pot Tandem Amidation, Knoevenagel Condensation, and Palladium-Catalyzed Wacker Type Oxidation/C–O Coupling: Synthesis of Chromeno-Annulated Imidazopyridines

A direct one-pot synthesis of chromeno-annulated
imidazo­[1,2-a]­pyridines is achieved by the reaction
of 2-amino-1-(2-ethoxy-2-oxoethyl)­pyridinium
salts with 2-bromoarylaldehydes using Pd­(TFA)2 as a catalyst
and Cu­(OAc)2 as an oxidant. The overall strategy involves
tandem base-mediated amidation and Knoevenagel condensation, followed
by palladium-catalyzed Wacker type oxidation and intramolecular C–O
coupling reaction. The method is simple, tolerates different functional
groups, and gives moderate to good yields of chromeno­[2′,3′:4,5]­imidazo­[1,2-a]­pyridin-12-one derivatives. The developed tandem reaction
was also successfully applied for the synthesis of pyrano-fused imidazo­[1,2-a]­pyridines by using 3-bromo-3-arylacrylaldehydes.