Disclosing Polycyclic Heterocycles: Synthesis of Furothienopyran and Pyranothienopyran Derivatives by Palladium Iodide Catalyzed Carbonylative Double Cyclization

Polycyclic heterocyclic derivatives have been synthesized in one step by a carbonylative double cyclization approach starting from readily available starting materials. The method is based on a regioselective 6-endo-dig heterocyclization - cyclocarbonylation sequence catalyzed by the PdI2/KI catalytic system under oxidative conditions, with CO (5-10 atm) as the carbonylating agent and O-2 (from air, 35-70 atm) as the external oxidant and formation of water as benign coproduct. Starting from thiophenecarboxylic acids bearing an omega-hydroxyalkynyl substituent in vicinal position, previously unknown 1H-furo[3,4-b]thieno[3,2-d]pyran-1,5(3H)-diones, 4H-furo[3,4-b]thieno[2,3-d]pyran-4,8(6H)-diones, 3,4-dihydro-1H,6H-pyrano[4,3-b]thieno[3,2-d]pyran-1,6-diones, and 6,7-dihydro-4H,9H-pyrano[4,3-b]thieno[2,3-d]pyran-4,9-diones were obtained in 45-86% yields over 21 examples. The structures of two representative products have been confirmed by X-ray diffraction analysis.