Three-Step Synthesis of Ethyl Canthinone-3-carboxylates from Ethyl 4-Bromo-6-methoxy-1,5-naphthyridine-3-carboxylate via a Pd-Catalyzed Suzuki-Miyaura Coupling and a Cu-Catalyzed Amidation Reaction

Ethyl canthin-6-one-1-carboxylate (1b) and nine analogues 1c-k were prepared from readily prepared ethyl 4-bromo-6-methoxy-1,5-naphthyridine-3-carboxylate (2b) via a three-step non-classical approach that focused on construction of the central pyrrole (ring B) using Pd-catalyzed Suzuki-Miyaura coupling followed by Cu-catalyzed C-N coupling. Furthermore, treatment of the ethyl canthinone-1-carboxylate 1b with NaOH in DCM/MeOH (9:1) gave the canthin-6-one-1-carboxylic acid (6) in high yield. All compounds are fully characterized.