1-Methylirnidazole [1] and 4,5-dimethylthiazole [2] react with aromatic acid chlorides in the presence of triethylamine to form 2-aroyl-substimted derivatives.
We have studied the behavior of 1-methylpyrazoles I-III under these conditions. When heated with benzoyl chloride (IV) or 1-methylpyrazol-5-oyl chloride (V) in acetonitrile and subsequent treatment with triethylamine, compounds I and II give as the sole products the anhydrides of benzoic acid (VI) or 1-methylpyrazole-5-carboxylic acid (VII). Reaction of the dimethylpyrazole III with IV or V also did not lead to acylation of the pyrazole ring but to a mixture of the 5-diaroylmethyl-1methylpyrazoles VIII or IX together with the anhydride VI or VII. ff ,~ + Rcocl --,,.-i + (Rco),2o ~N " Cil3 I'V, V N Ctt(COR) 2 VI, ~1 CH 3 CII 9 m VlIl, IX IV, VI, VIII R=C6I-Is; V, VII, IX R= l.-met..hyl-5-pyrazolyl
According to the PMR spectrum of the product, the reaction of pyrazole III with V also leads to formation of a small amount (--5%) of the monoaroylation product 1,2-bis(1-methylpyrazol-5-yl)ethanone (X).
Acid chloride V (7.2 g, 0.05 mole) was added to a solution of HI (6.8 g, 0.07 mole) in acetonitrile (40 ml), refluxed for 8 h, cooled, and triethylamine (5 g, 0.05 mole) added dropwise. The reaction product was refluxed for 3 h, cooled, the precipitate trite, rod off, and the solvent evaporated. The residue was treated with sodium carbonate (5%, 50 mI) and extracted with chloroform. After distillation of solvent the residue was chromatographed on silica gel (eluent chloroform-ethanol, 20:1) to give anhydride VII (0.7"g, 12%) and (1.1 g, 15%) 5-[bis(1-methylpyrazole-5-carbonyl)methyl]-l-methylpyrazole fiX).