Freshly prepared 1-propyl-2,4-dioxopyrrolidine was prepared as described in Example 1e from 3-carboethoxy-1-propyl-2,4-dioxopyrrolidine (3.55 g) in acetonitrile (100 ml). The 1-propyl-2,4-dioxopyrrolidine was mixed with methyl 3-amino-2-pentylthiophene-4-carboxylate (2.70 g) in toluene (65 ml). The mixture was refluxed for 2 hours with removal of the water/toluene azeotrope using a Dean-Stark trap. After distilling off the excess toluene, the residue was taken up in ethyl acetate, washed sequentially with saturated aqueous Na2CO3 and brine, then dried (Na2SO4) and concentrated. Chromatographic purification over silica gel afforded the title product (3.92 g, 94%) as a yellow oil; tlc, Rf =0.12, silica gel, ethyl acetate:hexane (1:1).
