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Chiral hydroxyl-ketones

Various biocatalytic options have been presented for the desymmetrization of meso-diols to chiral hydroxyl-ketones. A particularly facile system is represented by... [Pg.233]

Following their success with chiral ketone-mediated asymmetric epoxidation of unfunctionalized olefins, Zhu et al.113 further extended this chemistry to prochiral enol silyl ethers or prochiral enol esters. As the resultant compounds can easily be converted to the corresponding a-hydroxyl ketones, this method may also be regarded as a kind of a-hydroxylation method for carbonyl substrates. Thus, as shown in Scheme 4-58, the asymmetric epoxidation of enol silyl... [Pg.254]

Combination of achiral enolates vith achiral aldehydes mediated by chiral ligands at the enolate counter-ion opens another route to non-racemic aldol adducts. Again, this concept has been extremely fruitful for boron, tin, titanium, zirconium and other metal enolates. It has, ho vever not been applied very frequently to alkaline and earth alkaline metals. The main, inherent, dra vback in the use of these metals is that the reaction of the corresponding enolate, vhich is not complexed by the chiral ligand, competes vith that of the complexed enolate. Because the former reaction path vay inevitably leads to formation of the racemic product, the chiral ligand must be applied in at least stoichiometric amounts. Thus, any catalytic variant is excluded per se. Among the few approaches based on lithium enolates, early vork revealed that the aldol addition of a variety of lithium enolates in the presence of (S,S)-l,4-(bisdimethylamino)-2,3-dimethoxy butane or (S,S)-1,2,3,4-tetramethoxybutane provides only moderate induced stereoselectivity, typical ee values being 20% [177]. Chelation of the ketone enolate 104 by the chiral lithium amide 103 is more efficient - the j5-hydroxyl ketone syn-105 is obtained in 68% ee and no anti adduct is formed (Eq. (47)) [178]. [Pg.52]

Sulphoxides are well known to activate an a-carbon to lithiation, and, where the sulphoxide group is chiral, chiral induction is possible. These features are exemplified in the reaction of the lithiated chiral sulphoxides (27 R == Me) with nitrile oxides and nitrones to give optically active oximino- and P-hydroxyl-amino-sulphoxides, These intermediates should prove useful in further syntheses of chiral molecules. Similarly the chiral sulphoxide (27 R = H) gives the chiral sulphoxido-ketones (28) after lithiation and reaction with esters... [Pg.258]

Chiral N-sulfonyldiamine ligands are used to create effective chiral bifunctional amidoiridium catalysts for the asymmetric aerobic oxidation of meso- and prochiral diols to give up to >99% ee of hydroxyl ketones and 50%ee oflactones. " These catalysts can be also applied for an efficient oxidative kinetic resolution of racemic secondary alcohols affording R enantiomers with >99% ee and with 46—50% yields. [Pg.122]

A number of ketones, pharmaceutical compounds, alcohols and hydroxy acids have also been resolved on this phase [724,765-767]. A chiral polysiloxane phase with tartramide substituents has been used for the separation of enantiomers capable of hydrogen bonding interactions with the stationary phase, such as enantiomers containing carboxylic, hydroxyl and amine functional groups [768]. [Pg.965]

Mechanistic studies103 revealed that chiral ketone-mediated asymmetric epoxidation of hydroxyl alkenes is highly pH dependent. Lower enantioselectivity is obtained at lower pH values at high pH, epoxidation mediated by chiral ketone out-competes the racemic epoxidation, leading to higher enantioselectivity. (For another mechanistic study on ketone-mediated epoxidation of C=C bonds, see Miaskiewicz and Smith.104)... [Pg.247]

Asymmetric Hydrogenation of Enol Esters. Prochiral ketones represent an important class of substrates. A broadly effective and highly enantioselective method for the asymmetric hydrogenation of ketones can produce many useful chiral alcohols. Alternatively, the asymmetric hydrogenation of enol esters to yield a-hydroxyl compounds provides another route to these important compounds. [Pg.343]

Moreover, Soai et al.53c found that the enantioselective addition of Reformatsky reagents to prochiral ketones proceeds well when N,N-dialkylnorephedine 59 is used as the chiral ligand. When (15, 2R)-59a is used, the //-hydroxyl ester is obtained in 74% ee and 65% yield with ( -configuration predominant. When (lR,25,)-59a is used, the product is obtained in 74% ee and at 47% yield with (R)-configuration prevailing. [Pg.469]

Asymmetric a-hydroxylation of ketones 97 through phase transfer catalysis under alkaline conditions was realized by use of the Merck catalyst 7 (R=4-CF3, X=Br)[721 as well as the chiral azacrown ether 98[731 in conjunction with molecular oxygen, as shown in Scheme 30. The highest enantioselectivity of 79% ee was attained in the a-hydroxylation of the tetralone 100 by use of the Merck cata-... [Pg.139]

Only Cram (36) has published a rationale for the very high (99%) enantiomeric excess achieved in the reaction of methyl vinyl ketone and the hydrindanone in the presence of the chiral crown ether. This mechanism envisions a bimolecular complex comprising the potassium cation and chiral host as one entity and the enolate anion of the hydrindanone as the counterion. Methyl vinyl ketone lies outside this complex. The quinine-catalyzed reaction appears to have a termo-lecular character, since the hydroxyl of the alkaloid probably hydrogen bonds with the methyl vinyl ketone—enhancing its acceptor properties—while the quin-uclidine nitrogen functions as the base forming the hydrindanone—alkaloid ion pair. [Pg.99]

The cycloaddition of aldehydes and ketones with ketene under the influence of quinine or quinidine produce chiral 2-oxetanones [46,47]. Solvolytic cleavage of the oxetanone, derived from chloral, and further solvolysis of the trichloromethyl group leads to (5)- and (R)-malic acids with a 98% ee [46] (the chirality of the product depends on the configuration of the catalyst at C-8 and, unlike other alkaloid-induced reactions, it is apparently independent of the presence of the hydroxyl group). No attempts have been made to catalyse the reaction with chiral ammonium salts. [Pg.529]

Chapter 8 describes the application of hydroxyl nitrile lyases to the synthesis of new chiral cyanohydrins and a-hydroxy acids and includes new approaches to the transformation of difficult aldehyde and ketone substrates using substrate engineering and immobilization techniques. [Pg.417]


See other pages where Chiral hydroxyl-ketones is mentioned: [Pg.47]    [Pg.54]    [Pg.81]    [Pg.51]    [Pg.59]    [Pg.160]    [Pg.2237]    [Pg.352]    [Pg.120]    [Pg.234]    [Pg.21]    [Pg.210]    [Pg.244]    [Pg.76]    [Pg.490]    [Pg.764]    [Pg.238]    [Pg.916]    [Pg.519]    [Pg.61]    [Pg.358]    [Pg.9]    [Pg.251]    [Pg.356]    [Pg.411]    [Pg.209]    [Pg.513]    [Pg.515]    [Pg.109]    [Pg.397]    [Pg.702]    [Pg.542]    [Pg.433]    [Pg.47]   
See also in sourсe #XX -- [ Pg.233 ]




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