Chlorotrimethylsilane → 1-(1,5-dimethyl-1H-pyrazol-3-yl)ethanone

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Ethyl 1,5-dimethylpyrazole-3-carboxylate (3.59 g, 21.4 mM) was dissolved in dry tetrahydrofuran (70 ml) and cooled to −100° C. whilst under an atmosphere of argon. Trimethylsilylchloride (13.5 ml, 107 mM) was added by rapid dropwise addition. Immediately after methyllithium (77.7 ml of 1.1M solution in diethylether, 85.5 mM) was added dropwise in such a way that the internal temperature never exceeded −85° C. After complete addition the heterogeneous reaction was allowed to warm to room temperature. Most of the solvent was removed by evaporation in vacuo before treating with ethanol (6 ml) followed by water (6 ml). After vigorously stirring for 5 minutes the mixture was diluted with ethyl acetate and water. The pH was adjusted to 7 by treating with saturated sodium bicarbonate solution. The organic phase was separated and washed with brine before drying (MgSO4). Purification was accomplished by chromatography on silica gel (12×4.25 cm) loading in dichloromethane and eluting with 40% ethylacetate in hexane followed by 60% ethyl acetate in hexane. Removal of the solvents gave the title compound as a coloured oil which solidified on standing at room temperature (1.42 g): υmax (CH2Cl2) 1679, 1551, 1448, and 1373 cm−1; δH (CDCl3) 2.30 (3H, s), 2.54 (3H, s), 3.84 (3H, s), 6.53 (1H, d, J 0.55 Hz); E.I. m/e 138 (95%), NH3DCI m/e 139 (100%).

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