Asymmetric epoxidation chiral quaternary ammonium salts

Hori, K., Tamura, M., Tani, K., Nishiwaki, N., Ariga, M. and Tohda, Y. Asymmetric Epoxidation Catalyzed by Novel Azacrown Ether-type Chiral Quaternary Ammonium Salts under Phase-transfer Catalytic Conditions. Tetrahedron Lett. 2006, 47, 3115-3118. 

ASYMMETRIC EPOXIDATION CATALYZED BY NOVEL AZACROWN ETHER-TYPE CHIRAL QUATERNARY AMMONIUM SALTS UNDER PHASE-TRANSFER CATALYTIC CONDITIONS... 

The azacrown ether-type chiral quaternary ammonium salts as chiral PTCs are easily prepared from BINOL in four steps. Remarkably, Table 6.10 shows that the good efficiency of asymmetric epoxidation of various chalcones can be achieved by adjustment of the length of the carbon chains on the nitrogen atom in the quaternary ammonium salts. 

ASYMMETRIC EPOXIDATION OF (E)-CHALCONE CATALYZED BY THE AZACROWN ETHER-TYPE QUATERNARY AMMONIUM SALT AS CHIRAL PTC... 

Table 6.10 Asymmetric epoxidation of chalcones catalyzed by the azacrown ether-type quaternary ammonium salts as Chiral PTCs (see Figure 6.8).

Maruoka and coworkers designed a new and highly efficient chiral N-spiro-type quaternary ammonium salt (S)-70 with dual functions for the asymmetric epoxidation of various enone substrates (Scheme 5.44) [45]. The exceedingly high asymmetric induction is ascribable to the molecular recognition ability of the catalyst toward enone substrates by virtue of the appropriately aligned hydroxy functionality, as well as the chiral molecular cavity. Indeed, the observed enantioselectivity depends heavily... 

A new C3-symmetric chiral phase-transfer catalyst that offers multipoint inteaction with a nucleophile has been described (Scheme 7.6) [23]. Thus, various quaternary ammonium salts were prepared through the ring opening of optically active epoxides, followed by quaternization of the resulting amines. Asymmetric benzylation of Schiff s base 20 in the presence of catalyst 24—26 yielded (S)-21 with moderate enantioselectivity. As expected, the C3-symmetric catalyst R,R,R)-26a provided... 

Phase-transfer catalysis has been widely been used for asymmetric epoxidation of enones [100]. This catalytic reaction was pioneered by Wynberg et al., who used mainly the chiral and pseudo-enantiomeric quaternary ammonium salts 66 and 67, derived from the cinchona alkaloids quinine and quinidine, respectively [101-105],... 

The Darzens reaction (tandem aldol-intramolecular cyclization sequence reaction) is a powerful complementary approach to epoxidation (see Chapter 5) that can be used for the synthesis of a,P-epoxy carbonyl and a,p-epoxysulfonyl compounds (Scheme 8.32). Currently, all catalytic asymmetric variants of the Darzens reactions are based on chiral phase-transfer catalysis using quaternary ammonium salts as catalysts. 

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. ... 

Galactosylated Mn porphyrins 277 have been prepared as catalysts for asymmetric epoxidation, affording predominantly epoxides of R) configuration from styrenes in up to 23% c.c.125 The apap, oa P and aaaP atropisomos of 277 were isolated. Homochiral C2-symmetric quaternary ammonium salts, e.g. 278, have been prepared from chiral 2,S-imino-hexitols as chiral phase transfer catalysts. However, these were found to give only low asymmetric induction in asymmetric epoxidation and Darzens condensations. 

The asymmetric epoxidation of the chalcone type of substrate has also been accomplished using other types of chiral catalysts [15]. Wynberg was the first to use chiral ammonium salts, and obtained chalcone oxide with 55% ee using alkaline hydrogen peroxide as the stoichiometric oxidant and a quinine-derived quaternary ammonium salt as the chiral phase transfer catalyst [16]. More recently, Lygo... 

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