Reactivity of Cross-Conjugated Enynones in Cyclocondensations with Hydrazines: Synthesis of Pyrazoles and Pyrazolines

The
cyclocondensation of cross-conjugated enynones, dienynones,
and trienynones (easily available due to low-cost starting compounds)
with arylhydrazines leads to the regioselective synthesis of pyrazole
derivatives (dihetaryl-substituted ethens, buta-1,3-diens, and hexa-1,3,5-triens)
or results in 4,5-dihydro-1H-pyrazoles in good yield.
The reaction path is controlled by the character of the substituent
in enynone: the pyrazoles are obtained from the reaction of substrates
that contain five-membered heteroaromatic substituents with arylhydrazines,
and the 4,5-dihydro-1H-pyrazoles are obtained from
the reaction of 1,5-diphenylpent-1-en-4-yn-3-one with arylhydrazines
consistently. Despite the presence of a substituent, cyclocondensation
of 2-hydrazinylpyridine with all of examined cross-conjugated enynones
leads to the formation of pyrazoles. The reaction does not require
special conditions (temperature, catalyst, inert atmosphere). The
cyclocondensation pathways are determined by the electronic effect
of an electron-rich five-membered heteroaromatic ring in the substrate.
The synthesis allows use of various substituents and functional groups
in enynone and hydrazine. The present method features high yields
and simplicity of the product purification. The obtained pyrazoles
possess fluorescent properties with a quantum yield up to 31%.