Synthesis of Angular Pyrrolocoumarins

The new pyrrolocoumarin 3 was synthesized in two steps from 7-amino-4-methylcoumarin by selective o-chloroacetylation at position 8 and subsequent cyclization (the Sugasawa route to indoles). Regioselective inverse electron demand Diels-Alder reaction of 3 with dimethyl 1,2,4,5-tetrazine-3,6-dicarboxylate or 3,6-bis(trifluoromethyl)-1,2,4,5-tetrazine then gave the angular pyridazinepyrrolocoumarins 4 and 5, respectively, in good yield.