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Sodium hydride (340 mg, 60% dispersed in mineral oil) was added in two portions to a solution of 4-chloropyrrolo[2,3-d]pyrimidine (3a) (1.0 g, 6.5 mmol) in anhydrous DMF (15 mL) under the cooling conditions of an ice-salt bath, keeping the reaction temperature at 0 °C. The reaction temperature was controlled not to exceed 10 °C under nitrogen protection with continuous stirring for 1 hour. Subsequently, 2-(trimethylsilyl)ethoxymethyl chloride (SEMCl) (1.4 g, 8.5 mmol) was slowly added via syringe while ensuring that the temperature did not exceed 10 °C. The reaction mixture was gradually warmed to room temperature and stirring was continued overnight. After completion of the reaction, the reaction was quenched with water, extracted with ethyl acetate (EA) and the organic phase was dried over anhydrous sodium sulfate and concentrated. Purification by preparative fast chromatography (eluent: petroleum ether/ethyl acetate = 19:1) afforded 4-chloro-7-((2-(trimethylsilyl)ethoxy)methyl)-7H-pyrrolo[2,3-d]pyrimidine (3b) (1.834 g, oily product) in 97% yield. Mass spectrum (ESI, m/z): 284 [M + H]+.

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