Palladium iodide – Catalyzed carbonylative double cyclization of (ω-hydroxyalkynyl)thiophenecarboxamides leading to new thiophene-fused heterocycles

The reactivity of (ω-hydroxyalkynyl)thiophenecarboxamides under PdI2/KI-catalyzed oxidative carbonylation conditions has been studied. Under relatively mild conditions [1–5 mol% PdI2, 10–50 mol% KI, in a 2:1 (v/v) HC(OMe)3-MeCN mixture, under 40 bar of a 1:7 mixture CO-air, at 100 °C for 5–8 h], 3-(3-hydroxy-1-ynyl)thiophene-2-carboxamides were smoothly converted into previously unreported (Z)-5-imino-3,5-dihydro-1H-furo[3,4–b]thieno[3,2–d]pyran-1-ones through a carbonylative double cyclization route, involving an initial 6-endo-dig O-cyclization followed by cyclocarbonylation. Under similar conditions, isomeric 4-imino-4,6-dihydro-8H-furo[3,4–b]thieno[2,3–d]pyran-8-ones were obtained from 2-(3-hydroxy-1-ynyl)thiophene-3-carboxamides. The O-cyclization – cyclocarbonylation process could be successfully extended to the higher homologues [3-(4-hydroxy-1-ynyl)thiophene-2-carboxamides, and 2-(4-hydroxy-1-ynyl)thiophene-3-carboxamides], with formation of the corresponding (Z)-6-imino-4,6-dihydro-1H,3H-pyrano[4,3–b]thieno[3,2–d]pyran-1-ones and (Z)-4-imino-6,7-dihydro-4H,9H-pyrano[4,3–b]thieno[2,3–d]pyran-9-ones, respectively. The structures of six representative products have been confirmed by XRD analysis.