3.3- Dimethyl-5-ethynylpyrazole

After having determined the nature of the side reaction it became clear that in order to obtain the desired ethynylpyrazole 11 the reaction between the ketone and PCI5 would have to be performed at low temperature. Indeed, the reaction was carried out in CH2CI2 at room temperature and a mixture of chlorides 7 and 8 was obtained. Dehydrochlorination of this mixture gave 66% of 3,5-dimethyl-4-ethynylpyrazole (11). Thus, by varying the conditions it is possible to carry out the reaction of the ketone with phosphorus pentachloride selectively in any of the above-mentioned directions. 

Ring-methylated 1-ethynylpyrazoles were similarly obtained as minor produets in the pyrolysis of 3,5-dimethyl- and a mixture of 3- and 5-methyl- 1-propynoylpyrazoles. Pyrolysis of the 3-methyl derivative gave only pyrazolo[l,5-a]pyridin-5-ol... 

The series of l,3-dimethyl-5-ethynylpyrazoles, including the functionally substituted ones, was obtained at lower temperature (100-105° C, 10 wt % of powdered KOH, 1.5-2 mm Hg) (86TH1). 

The effect of substituents in the ring on the mobility of a methyne proton can be followed from the series of 5-substituted l-methyl-4-ethynylpyrazoles and 1,3-dimethyl-4-ethynylpyrazoles. In both structural series, the acidity of compounds increases with a change in the character of substituents in the following order CH3 < H < CH2NH2 < Cl. 

Dipolar addition of 2-diazopropane to diacetylene in EtaO at -25°C to give 3,3-dimethyl-5-ethynylpyrazole (83) (42% yield) and at 0°C to give dipyrazole 84 (60% yield) has been described (83TL1775). 

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