Trifluoroacetic acid 7.3 ml (96 mmol) was added to a solution of 2-(4-ethyl-benzoylamino)-7,8-dihydro-5H-pyrido[4,3-d]pyrimidine-6-carboxylic acid tert-butyl ester 567 mg (1.48 mmol) in CH2Cl2 (10 ml) under nitrogen as inert gas and stirring was performed for 1.5 h at RT. After removal of the solvent, the intermediate (4-ethyl-N-(5,6,7,8-tetrahydro-pyrido[4,3-d]pyrimidin-2-yl)-benzamide trifluoroacetate) was dissolved in CH2Cl2 (10 ml). After the addition of triethylamine 1150 μl (8.20 mmol), the solution was cooled to 0° C. and 3-thiophene sulfonyl chloride 253 mg (1.39 mmol) was added. The reaction mixture was stirred for 1 h at 0° C. and then overnight at RT. After removal of the solvent, purification was performed by means of column chromatography and the product 4-ethyl-N-[6-(thiophene-3-sulfonyl)-5,6,7,8-tetrahydro-pyrido[4,3-d]pyrimidin-2-yl]-benzamide 400 mg (0.93 mmol, 63% of theoretical) was obtained.
