Other Chiral Phase-Transfer-Catalyzed Reactions

Other Chiral Phase-Transfer-Catalyzed Reactions 7.2.1 

These results were disputed, however, when Dehmlow questioned the reproducibility obtained when using catalysts 1 [12] and 2 [13]. In fact, Dehmlow reported the use of quaternary ammonium salt 3 for the asymmetric hydroxylation reaction, with promising levels of enantioselectivity [8d]. 

R = benzyl, naphthyl methyl, anthra-cenylmethyl, 4-quinolinylmethyl X= Br, Cl 

The asymmetric benzylation of 16 was promoted by phosphonium salt 12 in moderate yield with encouraging levels of enantioselectivity when the catalyst loading was as low as 0.20 mol % (Table 7.1, entry 3). Further, a low temperature improved the enantiomeric excess to 50% ee (entry 5). A low enantiomeric excess obtained using the phosphonium salt 13 (entry 6) suggested a critical role for two mandelamide units in the catalytic efficiency of phosphonium salt 12. Unfortunately, this reaction proved to be highly substrate-sensitive, and other alkylating agents or different ester substituents in 16 afforded low enantioselectivities. 

In continuation with their studies on the synthesis of new analogues of cinchona alkaloids, Dehmlow et al. prepared quaternary ammonium salt 14, which is a 

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Enantioselective aldol reactions also can be used to create arrays of stereogenic centers. Two elegant ot-amino anion approaches have recently been published. Fujie Tanaka and Carlos F. Barbas III of the Scripps Institute, La Jolla, have shown (Org. Lett. 2004,6,3541) that L-proline catalyzes the addition of the aldehyde 6 to other aldehydes with high enantio- and diastereocontroJ. Keiji Maruoka of Kyoto University has developed (J. Am. Chem. Soc. 2004,126,9685) a chiral phase transfer catalyst that mediates the addition of the ester 9 to aldehydes, again with high enantio- and diastcrcocontrol. 

Some other very important events in the historic development of asymmetric organocatalysis appeared between 1980 and the late 1990s, such as the development of the enantioselective alkylation of enolates using cinchona-alkaloid-based quaternary ammonium salts under phase-transfer conditions or the use of chiral Bronsted acids by Inoue or Jacobsen for the asymmetric hydro-cyanation of aldehydes and imines respectively. These initial reports acted as the launching point for a very rich chemistry that was extensively developed in the following years, such as the enantioselective catalysis by H-bonding activation or the asymmetric phase-transfer catalysis. The same would apply to the development of enantioselective versions of the Morita-Baylis-Hillman reaction,to the use of polyamino acids for the epoxidation of enones, also known as the Julia epoxidation or to the chemistry by Denmark in the phosphor-amide-catalyzed aldol reaction. ... 

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