Methyl 4-(3-tert.butylphenoxy)-6-dimethylamino-pyrimidin-5-yl-acetate (10.0 g, 29 mmol) is dissolved in a mixture of 1,2-dimethoxyethane (60 ml) and methyl formate (20 ml). Sodium hydride (1.7 g, 80% in oil, 58 mmol) is added in one portion, the temperature being kept at room temperature. After 5 hours the reaction mixture is carefully acidified with hydrochloric acid. After stirring for 1 hour at +5° C. the mixture is neutralized with solid sodium bicarbonat. The intermediate product is extracted with ether and worked up in the usual way. The resulting oil is stirred in dimethylformamide (20 ml) and dimethyl sulfate (4.7 ml, 50 mmol) for 2 hours at room temperature. Dilution with ether washing with brine and chromatography on silicagel (eluant:hexane/ethyl acetate 1:1) gives the methyl α-[4-(3-tert.butylphenoxy)-6-dimethylamino-pyrimidin-5-yl]-β-methoxyacrylate as a single isomer (5.5 g).
