A facile and convenient synthesis of 3-alkylamino-5-arylthiophenes with a variety of substituents at C-2 and studies of reaction mechanisms

Thioaroylketene S,N-acetals were treated with active methylene compounds including beta-keto ester, nitromethane, cyanoacetic acid, p-toluenesulfonylacetone, 4-nitrophenylacetic acid, and diethyl (2-oxopropyl)phosphonate in the presence of mercury(II) acetate in CH2Cl2 at room temperature. These reactions gave 3-alkylamino-5-arylthiophenes containing various substituents, which comprised, respectively, alkoxycarbonyl, nitro, cyano, p-toluenesulfonyl, 4-nitrophenyl, and diethylphosphono groups at C-2 in good yields. The reaction of 3-methylamino-3-methylthio-1-phenylthioxopropene with malonic acid or Meldrum's acid under the same conditions gave 3-methylamino-5-phenylthiophene. Similarly, treatment of 3-methylamino-3-methylthio-1-phenylthioxopropene with various enolizable cyclic ketones such as 4-hydroxy-6-methyl-2-pyrone, homophthalic anhydride, 2-hydroxy-1,4-benzoquinone, and 1,3-diethyl-2-thiobarbituric acid gave thieno[3,2-b]pyridin-4-one, thieno[3,2-c]isoquinolin-5-one thieno[3,2-c]benzazepine-1,6-dione, and thieno[3,2-d]pyrimidine-2,4-dione, respectively.