Rhodium-Catalyzed Cascade Reactions of Indoles with 4-Hydroxy-2-alkynoates for the Synthesis of Indole-Fused Polyheterocycles

Herein, an efficient and regioselective Rh(III)-catalyzed [4+2] annulation/lactonization cascade of indoles with 4-hydroxy-2-alkynoates at room temperature to access the furo[3 ',4 ':4,5]pyrimido[1,6-a]indole-1,5(3H,4H)-diones is described. This method features mild reaction conditions, operational simplicity, excellent regioselectivity, broad substrate scope with good functional group tolerance, and good to excellent yields.