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Chiral silyl ethers

Instead of the absorption of chiral modifiers on metal surfaces, a new method using a slightly different approach attaches chiral moieties directly to metal surfaces through chemical bonds. Chiral silyl ethers have been attached to Pd surface atoms these new catalysts have the form (Pd)s=Si-0-R(,< orS) 42 Their synthesis arose from studies of the effects of siliconation on the catalytic activities and selectivities of dispersed, supported Pd and Pt.43-47 The results from... [Pg.110]

A somewhat similar approach has been developed by Smith and coworkers [224], who published a method to deposit a chiral silyl ether moiety onto a Pd surface through Si-metal bonds. [Pg.518]

The catalytic asymmetric hydrosilation of a prochiral ketone to the corresponding chiral silyl ether followed by a mild hydrolysis is in principle an attractive route for the preparation of chiral alcohols that has die advantage that it would not require high hydrogen pressure to effect the reduction (1-6). Despite the synthetic potential only limited application of this technique to the synthesis of complex organic molecules has been made (7-9). This is in part due to the relatively low optical... [Pg.63]

Chiral silyl ether 210 was prepared and subjected to azidonation with trimethylsilyl azide (TMSN3) and iodoben-zene diacetate [PhI(OAc)2], which served as a substitute of IN3, to give a-azido-dioxasilepine 211 as a colorless oil, in diastereoselective ratio 1 3 (Equation 40) <20050BC816>. [Pg.1007]

Grafted auxiliaries. Smith et al. [48] grafted chiral silyl ethers to a Pd surface through a Pd-Si bond. A borneoxysilyl-Pd catalyst was able to hydrogenate a-methylcinnamic acid with ee s up to 22%. Santini et al. [49] reported the preparation of a menthyl-Sn-Rh catalyst that hydrogenated ketopantolactone with 11% ee. [Pg.1283]

Hydrosilylation and silylformylation of carbonyls. Condensation of RR SiHj with arbonyl compounds provides chiral silyl ethers in the presence of [RhCl(cod)]2 and a hiral ligand. In the presence of CO, a-siloxy carbonyl compounds are obtained. ... [Pg.31]

Indeed, as it was mentioned above. Smith accomplished the enantioselective hydrogenation of unsaturated carboxylic acids using Pt or Pd catalysts modified with chiral silyl ethers. [Pg.180]

Ironically, auxiliary-induced control via the alkene failed to generate synthetically useful selectivities, but direct substrate-induced control did. In particular, chiral silyl enol ethers with stereocenters in the y-position allowed the synthesis of enantiomerically... [Pg.47]

Variations and Improvements on Alkylations of Chiral OxazoUnes Metalated chiral oxazolines can be trapped with a variety of different electrophiles including alkyl halides, aldehydes,and epoxides to afford useful products. For example, treatment of oxazoline 20 with -BuLi followed by addition of ethylene oxide and chlorotrimethylsilane yields silyl ether 21. A second metalation/alkylation followed by acidic hydrolysis provides chiral lactone 22 in 54% yield and 86% ee. A similar... [Pg.240]

Aldehydes, 43 a-Chiral, 112 a/MJnsaturated, 85,110 /3-Aldchydosi lanes, 22 Aldol reaction, directed, 139 Alkoxytrimethylsilanes, 122 Alkyl lithium. 67 Alkyl silyl ethers, 91-97,127 Alkylation, 33 of ethyl glycinate, 88-89 t-Alkylation, 111-135... [Pg.167]

The enolates of other carbonyl compounds can be used in mixed aldol reactions. Extensive use has been made of the enolates of esters, thiol esters, amides, and imides, including several that serve as chiral auxiliaries. The methods for formation of these enolates are similar to those for ketones. Lithium, boron, titanium, and tin derivatives have all been widely used. The silyl ethers of ester enolates, which are called silyl ketene acetals, show reactivity that is analogous to silyl enol ethers and are covalent equivalents of ester enolates. The silyl thioketene acetal derivatives of thiol esters are also useful. The reactions of these enolate equivalents are discussed in Section 2.1.4. [Pg.78]

Copper-catalyzed systems have been developed that reduce ketones directly to silyl ethers. The reactions involve chiral biphenyl diphosphine type ligands and silane or siloxane hydride donors.187... [Pg.429]

Kibayashi and coworkers have used enantiometrically pure allylic silyl ethers obtained from amino acids in cycloaddition with nitrones (Eq. 8.49).71 Cyclic nitrone reacts with a chiral allyl ether to give selectively the exo and erythro isomer (de 90%). Optically active alkaloids containing a piperidine ring such as (+)-monomorine,71c (+)-coniine,71a and (-)-oncinotine71b have been prepared from the addition product. [Pg.252]

Another useful method for the asymmetric oxidation of enol derivatives is osmium-mediated dihydroxylation using cinchona alkaloid as the chiral auxiliary. The oxidation of enol ethers and enol silyl ethers proceeds with enantioselectivity as high as that of the corresponding dihydroxylation of olefins (vide infra) (Scheme 30).139 It is noteworthy that the oxidation of E- and Z-enol ethers gives the same product, and the E/Z ratio of the substrates does not strongly affect the... [Pg.226]

In order to establish the correct absolute stereochemistry in cyclopentanoid 123 (Scheme 10.11), a chirality transfer strategy was employed with aldehyde 117, obtained from (S)-(-)-limonene (Scheme 10.11). A modified procedure for the conversion of (S)-(-)-limonene to cyclopentene 117 (58 % from limonene) was used [58], and aldehyde 117 was reduced with diisobutylaluminium hydride (DIBAL) (quant.) and alkylated to provide tributylstannane ether 118. This compound underwent a Still-Wittig rearrangement upon treatment with n-butyl lithium (n-BuLi) to yield 119 (75 %, two steps) [59]. The extent to which the chirality transfer was successful was deemed quantitative on the basis of conversion of alcohol 119 to its (+)-(9-methyI mande I ic acid ester and subsequent analysis of optical purity. The ozonolysis (70 %) of 119, protection of the free alcohol as the silyl ether (85 %), and reduction of the ketone with DIBAL (quant.) gave alcohol 120. Elimination of the alcohol in 120 with phosphorus oxychloride-pyridine... [Pg.249]

Besides their application in asymmetric alkylation, sultams can also be used as good chiral auxiliaries for asymmetric aldol reactions, and a / -product can be obtained with good selectivity. As can be seen in Scheme 3-14, reaction of the propionates derived from chiral auxiliary R -OH with LICA in THF affords the lithium enolates. Subsequent reaction with TBSC1 furnishes the 0-silyl ketene acetals 31, 33, and 35 with good yields.31 Upon reaction with TiCU complexes of an aldehyde, product /i-hydroxy carboxylates 32, 34, and 36 are obtained with high diastereoselectivity and good yield. Products from direct aldol reaction of the lithium enolate without conversion to the corresponding silyl ethers show no stereoselectivity.32... [Pg.148]

In the presence of a chiral promoter, the asymmetric aldol reaction of pro-chiral silyl enol ethers 71 with prochiral aldehydes will also be possible (Table 3-6). In this section, a chiral promoter, a combination of chiral diamine-coordinated tin(II) triflate and tributyl fluoride, is introduced. In fact, this is the first successful example of the asymmetric reactions between prochiral silyl enol ethers and prochiral aldehyde using a chiral ligand as promoter. [Pg.156]

Hagiwara et al.107 reported the chiral Pd(II) complex-catalyzed asymmetric addition of enol silyl ethers to imines, based on the belief that Pd(II) enolate was involved in the reaction. They found that with compound 171a as the catalyst, very low enantioselectivity was obtained in the asymmetric reactions between silyl enol ether and imine compounds (Scheme 3-58). However, in the... [Pg.184]

Ferraris et al.108 demonstrated an asymmetric Mannich-type reaction using chiral late-transition metal phosphine complexes as the catalyst. As shown in Scheme 3-59, the enantioselective addition of enol silyl ether to a-imino esters proceeds at —80°C, providing the product with moderate yield but very high enantioselectivity (over 99%). [Pg.185]

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]

Table II. MABR-Catalyzed Rearrangement of Chiral Epoxy Silyl Ethers... [Pg.205]

Hydrosilylation can be applied to alkenes, alkynes, and aldehydes or ketones. A wide range of metal compounds can be used as a catalyst. The most common and active ones for alkenes and alkynes are undoubtedly based on platinum. Hydrosilylation of C-0 double bonds gives silyl ethers, which are subsequently hydrolysed to their alcohols. The reaction is of interest in its enantioselective version in organic synthesis for making chiral alcohols, as the achiral hydrogenation of aldehydes or ketones does not justify the use of expensive silanes as a reagent. [Pg.373]

TABLE 6. Results of the regio- and diastereoselective ene reaction of singlet oxygen with chiral aUyUc alcohols, acetates, amines (and acyl derivatives), silyl ethers and ethers... [Pg.345]

The oxygen atom has also been used to generate other functionalities, such as the aldehyde moiety in Kibayashi s syntheses of (—)-coniine (197) and its enantiomer (Scheme 1.43) (253). Here, reaction of tetrahydropyridine N-oxide 93 with a silylated chiral allyl ether dipolarophile 198 delivered the adduct 199 with the desired bridgehead stereochemistry via the inside alkoxy effect . Desilylation and hydrogenolytic N—O bond rupture with palladium(II) chloride provided the diol 200... [Pg.35]


See other pages where Chiral silyl ethers is mentioned: [Pg.162]    [Pg.186]    [Pg.162]    [Pg.186]    [Pg.327]    [Pg.159]    [Pg.778]    [Pg.302]    [Pg.355]    [Pg.137]    [Pg.111]    [Pg.302]    [Pg.329]    [Pg.98]    [Pg.91]    [Pg.181]    [Pg.742]    [Pg.444]    [Pg.218]    [Pg.203]    [Pg.209]    [Pg.702]    [Pg.215]    [Pg.183]    [Pg.5]    [Pg.79]    [Pg.353]    [Pg.431]   
See also in sourсe #XX -- [ Pg.49 ]




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