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.