To a solution of tert-butyl 4-[6-(3,4-difluorophenyl)-1-[(3-ethyl-1,2,4-oxadiazol-5-yl)methyl]-2,4-dioxo-1,4-dihydrothieno[3,2-d]pyrimidin-3(2H)-yl]piperidine-1-carboxylate (400 mg, compound B67) in 1,4-dioxane (7 ml) is added a solution of hydrogen chloride in 1,4-dioxane (1.7 ml, 4.0 M). The reaction mixture is stirred for 3 h at 50° C. and subsequently for 12 h at RT. In order to complete the reaction an additional amount of a solution of hydrogen chloride in 1,4-dioxane (0.9 ml, 4.0 M) is added and the reaction mixture is stirred for 1 h at 65° C. Afterwards all volatiles are removed under reduced pressure to afford the title compound as a solid.
