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Cinchona feature

Figand acceleration (the so-called Criegee effect) is the important feature of asymmetric dihydroxylation using cinchona ligands.193 In particular, bis-cinchona ligands provide remarkable acceleration (Scheme 48). This enables high turnover rates of the osmium catalysts. [Pg.235]

Scheme 14.9 Main features of cinchona-modified metal systems [35],... Scheme 14.9 Main features of cinchona-modified metal systems [35],...
The main features of the cinchona alkaloid-modified metal system are illustrated in Scheme 14.9. [Pg.512]

A striking feature of the template model is the restriction of the role of the modifier to that of a template, which does not take into account direct binding interactions of the reactant with the modifier. Furthermore, there exists no experimental evidences for the formation of ordered arrays of cinchona molecules on a platinum surface. In 1995, Margitfalvi and Hegedus [235] criticized this model showing that the model is too idealistic and oversimplified. [Pg.513]

Enantioselective hydrogenation of a-ketoesters on cinchona alkaloid-modified Pt/Al203 is an interesting system in heterogeneous catalysis [143-146], The key feature is that on cinchonidine-modified platinum, ethyl pyruvate is selectively hydrogenated to R-ethyl lactate, whereas on einchonine-modified platinum, S-ethyl pyruvate is the dominant product (Figure 16) [143]. [Pg.253]

Figure 16. Main features in enantioselective hydrogenation of a-ketoesters on cinchona alkaloid-modified metal catalysts [ 143]. [Reproduced with permission of Elsevier from Baiker, A. J. Mol. Catal. A 1997,115, 473-493.]... Figure 16. Main features in enantioselective hydrogenation of a-ketoesters on cinchona alkaloid-modified metal catalysts [ 143]. [Reproduced with permission of Elsevier from Baiker, A. J. Mol. Catal. A 1997,115, 473-493.]...
The kinetics of alkylation by benzyl bromide of the Schiff base esters of ammo acids (Ph2C=NCH2CC>2CMe3) in the presence of cinchona salts show features similar to those of enzyme-promoted reactions variable orders, substrate saturation, catalyst inhibition, and non-linear Arrhenius-type plots.125 A tight coordination of the Schiff base substrate by electrostatic interaction with the quaternary N of the cinchona salt provides a favourable chiral environment for asymmetric alkylation. [Pg.318]

Esters 16b,c are used in reactions catalyzed by cinchona alkaloid-based phase-transfer catalysts, since the size of the ester is important for efficient asymmetric induction in these reactions [35], However, the syntheses of esters 16b,c adds considerable cost to any attempt to exploit this chemistry on a commercial basis. Fortunately, it was possible to develop reaction conditions which allowed the readily available and inexpensive substrate 16a to be alkylated with high enantios-electivity using catalyst 33 and sodium hydroxide, as shown in Scheme 8.18 [36]. The key feature of this modified process is the introduction of a re-esterification step following alkylation of the enolate of compound 16a. It appears that under... [Pg.175]

Catalytic asymmetric alkylations of 28 have also been carried out with polymer-bound glycine substrates [43], or in the presence of polymer-supported cinchona alkaloid-derived ammonium salts as immobilized chiral phase-transfer catalysts [44], both of which feature their practical advantages especially for large-scale synthesis. [Pg.133]

By the 1850s and 1860s quinine had reached its apex in the physician s armamentarium. Cinchona spp. were mentioned thirteen times among the primary list of substances in the United States Pharmacopoeia of 1850, twelve times in 1860, and thirteen times again in 1870.71 Cinchona spp. covered nearly fifty pages in the 1858 edition of the United States Dispensatory, and ten pages were spent discussing three different preparations of quinine.72 Never before or since were cinchona and quinine so prominently featured in the pharmaceutical compendia. [Pg.159]

Figure 1. Influence of structural features of the cinchona ligands on binding and reaction rates. Figure 1. Influence of structural features of the cinchona ligands on binding and reaction rates.
Quinine The antimalarial agent quinine is derived from the bark of the cinchona tree along with several other alkaloids and salicylate (aspirin). Many of these agents produce similar toxic features (cinchon-ism) in patients with excessive intake, but only quinine produces blindness. Cinchonism consists of abdominal pain and vomiting, ringing in the ears (tinnitus), and confusion. Visual loss after quinine overdose is due to direct retinal toxicity, although until recently it was believed to be due to spasm of the arterial blood supply to the retina. Treatment is difficult, but limited evidence suggests charcoal hemoperfusion may be beneficial (hemoperfusion is similar to hemodialysis, except in place of a semi-permeable membrane to filter the toxin from the blood, charcoal is used to bind the toxin). [Pg.2366]

As mentioned in the previous section, nowadays, readily available and inexpensive cinchona alkaloids with pseudoenantiomeric forms, such as quinine and quinidine or cinchonine and cinchonidine, are among the most privileged chirality inducers in the area of asymmetric catalysis. The key feature responsible for their successful utility in catalysis is that they possess diverse chiral skeletons and are easily tunable for diverse types of reactions (Figure 1.2). The presence of the 1,2-aminoalcohol subunit containing the highly basic and bulky quinuclidine, which complements the proximal Lewis acidic hydroxyl function, is primarily responsible for their catalytic activity. [Pg.3]

Cinchona alkaloids have characteristic structural features for their diverse conformations and self-association phenomena. Therefore, knowledge of their real structure in solution can provide original information on the chiral inducing and discriminating ability of these alkaloids. [Pg.4]

Another characteristic structural feature of cinchona alkaloids is their multifunctional character and, thus, autoassociation phenomena are possible that could result in the strong dependency of their efficiency on the concentration and temperature [22, 23]. [Pg.7]

Figure 13.2 Structural features of cinchona alkaloid molecules (QN, quinine QD, quinidine CN, cinchonine CD, cinchonidine CPN, cupreine CPD, cupreidine). Figure 13.2 Structural features of cinchona alkaloid molecules (QN, quinine QD, quinidine CN, cinchonine CD, cinchonidine CPN, cupreine CPD, cupreidine).
The development of 4-aminoquinoline antimalarials owes its origin to World War II when the Allies were deprived of the supply of quinine (1) as a consequence of the Japanese occupation of Java, where cinchona was grown widely to produce quinine. To overcome this problem, a synthetic alternative, quinacrine (2) was developed, which was extensively used during the war to treat malaria in man. Neverthless, the need for a more effective and safer antimalarial was pressing. Examination of the structural frame-work of quinine (1) and quinacrine (2) revealed that the common feature in the two drugs is the presence of a 4-substituted quino-... [Pg.393]

One of the most appealing features with the Cinchona alkaloids as chiral ligands, is the pseudoenantiomeric relationship between dihydroquinine (DHQ)... [Pg.680]

Fig. 6. Summary of different features in cinchona alkaloid ligands and their effect on osmium tetroxide binding and ligand acceleration... Fig. 6. Summary of different features in cinchona alkaloid ligands and their effect on osmium tetroxide binding and ligand acceleration...

See other pages where Cinchona feature is mentioned: [Pg.70]    [Pg.109]    [Pg.110]    [Pg.500]    [Pg.6]    [Pg.175]    [Pg.147]    [Pg.49]    [Pg.9]    [Pg.31]    [Pg.63]    [Pg.109]    [Pg.110]    [Pg.10]    [Pg.193]    [Pg.195]    [Pg.328]    [Pg.5446]    [Pg.370]    [Pg.147]    [Pg.133]    [Pg.148]    [Pg.166]    [Pg.325]    [Pg.398]    [Pg.424]    [Pg.434]    [Pg.447]    [Pg.1331]    [Pg.14]   
See also in sourсe #XX -- [ Pg.7 ]




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