Novel Synthesis of 5,6-Dihydro-4<i>H</i>-thieno[3,2-<i>b</i>]pyrrol-5-ones via the Rhodium(II)-Mediated Wolff Rearrangement of 3-(Thieno-2-yl)-3-oxo-2-diazopropanoates

Treatment of thioaryolketene S,N-acetals 12 with Hg(OAc)2 followed by addition of 2-diazo-3-trimethylsilyloxy-3-butenoic acid alkyl esters 15 in CH2Cl2 at room temperature gave 3-(3-alkylamino-5-arylthieno-2-yl)-3-oxo-2-diazopropanoates 16 in good yields. Subsequent reactions of 16 with a catalytic amount of Rh2(OAc)4·2H2O in benzene at reflux afforded a mixture of 5,6-dihydro-4H-thieno[3,2-b]pyrrol-5-ones 18 and the corresponding enols 19 in excellent yields.