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Catalytic Asymmetric Synthesis


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generated, the enolate/catalyst ion‐pair could be matched with a number of other classes of electrophiles, with Michael acceptors being an early example (Scheme 4.7). In 1998, Corey reported the use of an anthracene‐functionalized cinchoninium catalyst for the enantioselective addition of a diphenyl glycine ester to acrylates, and later acrylonitrile derivatives [31, 32]. A few years later in 2002, Shibasaki reported the use of a tartrate‐derived chiral bis‐ammonium catalyst for the same class of reactions [15]. In the same year, Arai and Nishida reported that an N‐spirocyclic ammonium catalyst was also a competent catalyst for this reaction, albeit affording moderate enantioselectivity [33]. In recent years, Maruoka reported the use of bifunctional chiral ammonium salts to catalyze the enantioselective addition of nucleophiles to Michael acceptors such as nitroolefins[34–36] and maleimides[37, 38] under neutral conditions. The successful implementation of this base‐free phase‐transfer strategy hinged on the use of water‐rich solvent mixtures proposed to allow formation of the key reactive catalyst‐substrate ion‐pair. This strategy was later extended for successful asymmetric aldol, chlorination, and sulfenylation reactions [39, 40].

Schematic illustration of atropselective enolate O-alkylation. Schematic illustration of enantioselective addition of glycine imines to Michael acceptors. Schematic illustration of enantioselective epoxidation of chalcones. Schematic illustration of enantioselective aziridination of alpha,beta-unsaturated esters.

       4.2.1.3. Addition to Carbonyls and Imines

Schematic illustration of enantioselective aldol reaction using glycine imines.

      Source: Based on [51].

      Asymmetric phase‐transfer catalysis has also been exploited successfully for a number of other reactions involving the addition of an enolate to a carbonyl or imine electrophile. For example, the addition of α‐halo carbonyl compounds to aldehydes or ketones (Darzens reaction) to yield epoxides was first achieved using chiral phase‐transfer catalysis in 1998 by Arai and Shioiri and has since been further developed [57, 58]. Another example is the addition of cyanide nucleophiles to imine derivatives (Strecker reaction) leading to chiral α‐amino nitriles, which has been extensively studied; the first