Brønsted-Acid-Promoted Diaza-Nazarov Cyclization to Access Tetrasubstituted Pyrazoles.

We have developed a new diaza-Nazarov cyclization for the synthesis of densely substituted pyrazoles. Treatment of <i>N</i>-acylazo substrates with 1-1.5 equiv of trifluoroacetic acid (TFA) as a Brønsted acid promoter at 23 °C afforded hydroxypyrazole products in good to excellent yields (up to 99%). Reactions of substrates with secondary alkyl groups at the β position of the α,β-unsaturated carbonyl moiety gave minor amounts of dihydropyridazinone compounds as side products, proposed to form via an intriguing 6π electrocyclization.