Chiral N-Oxides as Catalysts

An asymmetric Mannich reaction was recently successfully achieved by means of different types of catalyst, metal- and organocatalysts [20, 21]. With the latter the reaction can be performed asymmetrically by use of L-proline and related compounds as chiral organocatalyst [22-35]. A key advantage of the proline-catalyzed route is that unmodified ketones are used as donors, which is synthetically highly attractive. In contrast, many other asymmetric catalytic methods require preformed enolate equivalents as nucleophile. 

In addition, chiral Schiff base catalysts, which were developed previously for the Strecker reaction, were also found to be suitable catalysts for the Mannich reaction starting from imines and enolates [36, 37]. Very recently, further efficient organocatalysts for the Mannich reaction, such as chiral pyrrolidinyltetrazole and chiral binaphthyl phosphoric acids, have been reported [38]. 

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