Asymmetric Hydroxylation of 4‐Substituted Pyrazolones Catalyzed by Natural Cinchona Alkaloids

A natural quinine-catalyzed, efficient and practical asymmetric α-hydroxylation of 4-substituted pyrazolones has been developed, delivering a broad spectrum of pyrazolones bearing an oxygen-attached carbon stereocenter at C-4 in high yields and excellent enantioselectivities. The broad substrate scope, ready availability of the catalyst, ease of operation, and valuable transformation of the product highlight the practical utility of this process.