A solution of tert-butyl 4-{1-[(3-ethyl-1,2,4-oxadiazol-5-yl)methyl]-2,4-dioxo-6-phenyl-1,4-dihydrothieno[3,2-d]pyrimidin-3(2H)-yl}piperidine-1-carboxylate (2.79 g; compound B7) in DCM (20 ml) is reacted with a solution of hydrogen chloride in 1,4-dioxane (15 ml, 4.0 M) for 12 h at RT. Subsequently water is added (50 ml) and the pH is adjusted to about pH14 by addition of aqueous sodium hydroxide solution (5 M). The mixture is extracted with DCM (3×100 ml) and the combined organic layers are dried over sodium sulfate. After filtration and evaporation of the solvent a residue is obtained which is recrystallized from DCM to yield the title compound as a solid.
