Easy and Straightforward Access to Tertiary Pyrazolone Alcohols via Oxidative Cyclization of Cinnamic Acids with Arylhydrazines

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An efficient and direct synthesis of valuable tertiary pyrazolone alcohols has been accomplished via the metal-free oxidative cyclization of cinnamic acids with arylhydrazines. The reaction is regulated by the DBU/TBHP combination and is endowed with a good substrate scope and functional group tolerance. Diverse aryl groups have also been readily introduced in the resulting pyrazolone alcohols by carrying out the reaction in the presence of another aryl source, viz., aryldiazonium tetrafluoroborate, under the same conditions.