Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Stereoselective control chiral catalysts

Dipolar cydoadditions are one of the most useful synthetic methods to make stereochemically defined five-membered heterocydes. Although a variety of dia-stereoselective 1,3-dipolar cydoadditions have been well developed, enantioselec-tive versions are still limited [29]. Nitrones are important 1,3-dipoles that have been the target of catalyzed enantioselective reactions [66]. Three different approaches to catalyzed enantioselective reactions have been taken (1) activation of electron-defident alkenes by a chiral Lewis acid [23-26, 32-34, 67], (2) activation of nitrones in the reaction with ketene acetals [30, 31], and (3) coordination of both nitrones and allylic alcohols on a chiral catalyst [20]. Among these approaches, the dipole/HOMO-controlled reactions of electron-deficient alkenes are especially promising because a variety of combinations between chiral Lewis acids and electron-deficient alkenes have been well investigated in the study of catalyzed enantioselective Diels-Alder reactions. Enantioselectivities in catalyzed nitrone cydoadditions sometimes exceed 90% ee, but the efficiency of catalytic loading remains insufficient. [Pg.268]

Scheme 2.25 shows some examples of additions of enolate equivalents. A range of Lewis acid catalysts has been used in addition to TiCl4 and SnCl4. Entry 1 shows uses of a lanthanide catalyst. Entry 2 employs LiC104 as the catalyst. The reaction in Entry 3 includes a chiral auxiliary that controls the stereoselectivity the chiral auxiliary is released by a cyclization using (V-methylhydroxylamine. Entries 4 and 5 use the triphenylmethyl cation as a catalyst and Entries 6 and 7 use trimethylsilyl triflate and an enantioselective catalyst, respectively. [Pg.193]

As described hitherto, diastereoselectivity is controlled by the stereogenic center present in the starting material (intramolecular chiral induction). If these chiral substrates are hydrogenated with a chiral catalyst, which exerts chiral induction intermolecularly, then the hydrogenation stereoselectivity will be controlled both by the substrate (substrate-controlled) and by the chiral catalyst (catalyst-controlled). On occasion, this will amplify the stereoselectivity, or suppress the selectivity, and is termed double stereo-differentiation or double asymmetric induction [68]. If the directions of substrate-control and catalyst-control are the same this is a matched pair, but if the directions of the two types of control are opposite then it is a mismatched pair. [Pg.670]

A method for highly efficient asymmetric cyclopropanation with control of both relative and absolute stereochemistry uses vinyldiazomethanes and inexpensive a-hydroxy esters as chiral auxiliaries263. This method was also applied for stereoselective preparation of dihydroazulenes. A further improvement of this approach involves an enantioselective construction of seven-membered carbocycles (540) by incorporating an initial asymmetric cyclopropanation step into the tandem cyclopropanation-Cope rearrangement process using rhodium(II)-(5 )-N-[p-(tert-butyl)phenylsulfonyl]prolinate [RhjtS — TBSP)4] 539 as a chiral catalyst (equation 212)264. [Pg.843]

A Lewis-acid-mediated intramolecular cyclization of allenyl stannane 344 furnishes 2,6- //-tetrahydropyran as the major product, the stereochemistry of which can be switched to syn with moderate effect if a propargylstannane 345 is used as a substrate (Equation 147, Table 16) <1996TL3059>. The stereoselectivity observed in an analogous system, the intramolecular cyclization of y-alkoxyallyl stannanes 346 with a tethered aldehyde, can be controlled by changing the geometry of the alkene (Scheme 83) <1997JOC7439>. y-Alkoxyallyl stannanes are also known to cyclize both diastereoselectively and enantioselectivity, by incorporation of both a chiral auxiliary and a chiral catalyst respectively into the reaction <1999JOC4901>. [Pg.498]

Keywords Stereogenic reactions. Mechanism control. Substrate and reagent control. Stereoselectivity, Simple diastereoselectivity, exo-endo-Diastereoselectivity, Double stereodifferentiation, Chiral catalysts, Chiraphor and catalaphor. Hybrid catalysts. Chiral auxiliary... [Pg.44]

Other chiral catalysts have also been developed that do not fall into the families shown in Fig. 3 [25-30], but to date they tend to give low enantioselectivities and will not be discussed in this chapter. In addition, chiral auxiliaries are sometimes utilized instead of or in addition to chiral catalysts to control stereoselectivity [31-34], but this is becoming increasingly less popular as the catalysts become more selective. [Pg.310]

A variety of methods are now available to control the relative and absolute stereochemistry of the cyclocondensation reaction of activated dienes with aldehydes. Chiral aldehydes that posses a high degree of diastereofacial selectivity with a variety of dienes can be used and the stereoselectivity controlled by the choice of diene, aldehyde and catalyst. Chiral dienes in combination with chiral catalysts can also be used in an unusual process of double diastereofacial selectivity. The resulting adducts can be purified and eliminated to give optically pure dihydropyrones. The chiral auxiliary can be isolated and reused if... [Pg.688]

Amino acids are practical chiral educts (chirons), and they also steer stereoselective reactions as catalysts. Phenylalanine-controlled aldol condensations with 2-dialkyl-cyclopenta-l,3-dione give just one enantiomer (Danishefsky and Cain, 1976) Intramolecular cyclization reactions of serine yield chiral P-lactams via an active hydroxyl amide and after protection of the amino group of serine. The addition of aldehydes to cysteine produces chiral thiazolidinecarboxylic acids in quantitative yield. Aspartic and glutamic acids have been converted to... [Pg.499]

If a chiral catalyst is used to promote the aldol reaction, the determination of stereoselectivity is shifted from substrate control to catalyst control (see Scheme 98). Consequently, when either ( S)-2- er -butyldimethylsilyloxypropanal (689) or its enantiomer (R)-2-tert-butyldimethylsilyloxypropanal (741) is reacted with 738 in the presence of tin(II) triflate and... [Pg.100]

While the detailed structures of most catalyst sites are still unknown, it was established that stereoselectivity does not come from the chirality of the growing chain end. Rather, it is built into the catalyst site itself. Normal preparations of the catalysts give equal numbers of (/ ) and (5) chiral catalyst sites. These coordinate selectively with (R) and (5) monomers, respectively, in the process of catalytic-site control. ... [Pg.175]

From the results also shown in Table 16, the stereochemical control by the (—)-menthyl group itself was found to produce predominantly ( )-menthyl (5)-mandelate (21% e.e.), an antipode of that obtained from the lithium aluminum hydride reduction. In process (b), it was shown that a counteracting asymmetric induction by the chiral catalyst of (—)-DIOP exceeded the effect of ( )-menthyl group to give the (jR)-mandelate with rather low stereoselectivity (37% e.e.). Production of the (R)-mandelate was scarcely favored in case of the asymmetric reduction of ethyl benzoyl-... [Pg.213]

There are different approaches for stereocontrol for the Robinson annulation the control can either arise fi-om the inherent nature of the starting ketone and/or the vinyl ketones substituents in combination with the reaction conditions, or by the use of a chiral catalyst. In the first case, an example is the stereoselective aldol cyclization to give the ketol intermediate 50. In this case the cyclization is kinetically controlled under protic basic conditions of sodium ethoxide and ethanol as it gives the cw-fused adduct rather than the more stable trans-fased ketol, which is not detected at any time during the reaction. [Pg.396]

There is significant interest in controlling the absolute stereochemistry of ring opening in epoxide/C02 copolymerization. Cyciohexene oxide, a meso molecule, is an ideal substrate for desymmetrization using chiral catalysts. In 1999, Nozaki et al7 reported that a 1 1 mixture of ZnEt2 and (S)-diphenyl (pyrrolidine-2-yl)methanol (11) (Scheme 13) was active for stereoselective cyciohexene oxide/C02 copolymerization at 40 °C and 30 atm. CO2 (Scheme 14). The resultant polycarbonate contained 100% carbonate linkages, had an Ain of... [Pg.172]


See other pages where Stereoselective control chiral catalysts is mentioned: [Pg.219]    [Pg.298]    [Pg.671]    [Pg.312]    [Pg.162]    [Pg.250]    [Pg.185]    [Pg.33]    [Pg.133]    [Pg.496]    [Pg.507]    [Pg.798]    [Pg.1009]    [Pg.336]    [Pg.402]    [Pg.121]    [Pg.93]    [Pg.150]    [Pg.427]    [Pg.55]    [Pg.806]    [Pg.203]    [Pg.637]    [Pg.63]    [Pg.325]    [Pg.90]    [Pg.3]    [Pg.140]    [Pg.73]    [Pg.245]    [Pg.100]   
See also in sourсe #XX -- [ Pg.517 , Pg.518 , Pg.519 , Pg.520 , Pg.521 , Pg.522 , Pg.523 , Pg.524 ]




SEARCH



Catalyst control

Catalyst stereoselective

Chiral catalysts

Chiral compounds catalyst controlled stereoselectivity

Chiral control

Chiral stereoselectivity

Chirality chiral controllers

Chirality control

Controlling, stereoselectivity

Stereoselective control

Stereoselectivity chiral catalysts

Stereoselectivity control

© 2024 chempedia.info