Vilsmeier-Haack formylation of 2-(1H-pyrazol-1-yl)ethanol and its methyl derivatives

Abstract

It is known that 2-(3,5-dimethyl-1H-pyrazol-1-yl) ethanol does not undergo the Vilsmeier-Haack formylation [1]; the reaction results was the replacement of the hydroxyl group with the chlorine atom. Apparently, this is caused by the decreased nucleophilicity of the pyrazole ring due to the quaternization of the pyridine nitrogen atom. This assumption is supported by the fact that the direct Vilsmeier-Haack formylation of 1-(2-chloroethyl)-3,5-dimethylpyrazole affords the target product in 70% yield [2], and the reaction does not occur in the presence of hydrochloric acid.In this work we found that 2-(1H-pyrazol-1-yl)-ethanol I, 2-(3-methyl-1H-pyrazol-1-yl)ethanol II, and 2-(5-methyl-1H-pyrazol-1-yl)ethanol III underwent no formylation under the Vilsmeier-Haack reaction conditions, but were transformed into 1-(2-chloroethyl)pyrazoles IV-VI (Scheme 1).The Vilsmeier-Haack formylation of compounds I-III and VII [4, 5] required their protection. In particular, prior acetylation of the hydroxyl group allowed formylation of the pyrazole ring at position 4 [1, 4]. Acylation of compounds I-III and VII with vinyl acetate in the presence of a catalytic amount of copper acetate afforded 2-(1H-pyrazol-1-yl)ethyl acetate VIII and its methyl derivatives IX-XI, which in turn were easily subject to formylation via the VilsmeierHaack reaction to form 2-(4-formyl-1H-pyrazol-1-yl)-ethyl acetate XII and its methyl derivatives XIII-XV. Hydrolysis of the latter yielded the corresponding aldehydes XVI-XIX (Scheme 2).In addition, 1-(2-chloroethyl)pyrazole-4-carbaldehydes XX-XXIII were isolated from the reaction mixture with yields of 7-10% (Scheme 3).The starting compounds I-III, VII-XI were obtained as described elsewhere [4,5].1-(2-Chloroethyl)pyrazole (IV). 30 g (0.2 mol) of phosphorus oxychloride was added to a mixture of 11.2 g of (0.1 mol) of 2-(1H-pyrazol-1-yl)ethanol I and 95.0 g (0.6 mol) of DMF upon stirring at 90°C (maintaining the temperature below 120°C). The reac-

Pyrazole-carboxaldehydes as versatile precursors for different pyrazole-substituted heterocyclic systems

“…Subsequent hydrolysis of 18 afforded 19 which underwent chlorination to give 16 upon treatment of Vilsmeier reagent (Scheme 6) . 87,88 Scheme 5. Synthesis of 3,5-dimethyl-1H-pyrazole-4-carboxaldehyde 13.…”