Ring-Expansion Reaction of Isatins with Ethyl Diazoacetate Catalyzed by Dirhodium(II)/DBU Metal-Organic System: En Route to Viridicatin Alkaloids

We present here the NHC‐dirhodium(II)/DBU‐catalyzed ring expansion reaction of isatins with ethyl diazoacetate. This new one‐pot protocol yields the ethyl 3‐hydroxy‐2(1H)‐oxoquinoline‐4‐carboxylate core, regioselectively and in good to excellent yields. A DFT mechanistic study indicates metallocarbene formation between the 3‐hydroxyindole‐diazo intermediate and the dirhodium(II) complex to be the rate‐limiting step of the reaction. The synthesized ethyl 3‐hydroxy‐2(1H)‐oxoquinoline‐4‐carboxylate core could be readily converted to viridicatin alkaloids, in yields up to 80 % by a microwave‐assisted Suzuki–Miyaura cross coupling of the 3‐hydroxy‐4‐bromoquinolin‐2(1H)‐one with arylboronic acid. Herein is presented an efficient method for catalytic ring expansion of isatins with ethyl diazoacetate that selectively yields ethyl 3‐hydroxy‐2(1H)‐oxoquinoline‐4‐carboxylates; these are characterized as containing a useful core for the synthesis of viridicatin alkaloids.