Ethyl ethoxyacetate → 2-Butylsulfanyl-6-ethoxy-8,8a-dihydro-4ahpyrido[2,3-d]pyrimidin-7-one

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4-Amino-2-butylsulfanyl-4,5-dihydro-pyrimidine-5-carbaldehyde (3 g, 14.2 mmol, furnished by scaleup) and ethyl ethoxyacetate (2.34 g, 2.4 ml, 17.75 mmol) were stirred in 80 ml toluene at 0°-5° C. under nitrogen. Potassium t-butoxide (1.75 g, 15.6 mmol) was added gradually. The mixture was stirred to ambient temperature, and then to 65° C. for 48 hours. An additional 20 ml of toluene and 2.4 ml ethyl ethoxyacetate were added and the reaction was maintained at 65° C. over the weekend. The reaction mixture was concentrated in vacuo and triturated with EtOAc to remove the remaining starting aldehyde. The remaining solid was further triturated with chloroform to remove additional impurities. 3.66 g of 2-Butylsulfanyl-6-ethoxy-8,8a-dihydro-4aHpyrido[2,3-d]pyrimidin-7-one (1A) of a purity greater than 80% as judged by MS/HPLC and NMR was collected.

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