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Asymmetric epoxidation natural products synthesis

Application of asymmetric epoxidation to multistep synthesis of natural products... [Pg.283]

Since the first report by Katsuki and Sharpless in 1980 [21], the asymmetric epoxidation of allylic alcohols has been widely applied to the synthesis of various compounds [22]. Among several asymmetric epoxidations developed, the Sharpless asymmetric epoxidation (AE) is undoubtedly the most popular and versatile method, which has greatly contributed to the progress of natural product synthesis. [Pg.188]

Several a,j5-unsaturated (5-lactones are produced by higher plants, and it is speculated that they originate biogenetically from the respective 1,3-polyhydroxylated acids. The synthesis of (— )-tarchonanthuslactone (615) exemplifies the utility of asymmetric 1,3-polyols in natural product synthesis (Scheme 90) [145]. All- y -epoxide 606 is transformed into the natural product 615 in good overall yield using only standard synthetic operations. [Pg.243]

The asymmetric epoxidation of functionalized alkenes still attracts considerable attention. Synthetic chemists continue to be in search of new and improved routes to epoxides, since they provide versatile intermediates for natural product synthesis. The topic of preparative techniques for chiral epoxides is seldom broached without the mention of the Sharpless epoxidation. Indeed, the impact of this protocol cannot be overestimated, as new applications continue to be reported. For example, linear poly(tartrate ester) ligands have been used this past year to generate a solid-supported Sharpless-type catalyst <97CC123>. [Pg.49]

Another class of reaction for which chiral tertiary amines are privileged catalysts is the Morita-Baylis-Hillman type (477, 478). One of the first applications of Cinchona alkaloids to mediate an asymmetric Morita-Baylis-Hillman reaction in a natural product synthesis was reported by Hatakeyama et al. in 2001 (479). Using a stoichiometric amount of (3-isocupreidine (568), a stereoselective addition of hexafluoroisopropyl acrylate (569) to aldehyde 570 could be carried out in good yield and with excellent selectivity (99% ee) (Scheme 119). The chiral p-hydroxy ester 571 was converted further into the epoxide 572, a known intermediate in the synthesis of epopromycin B (573). Epopromycin B (573) is a plant cell wall... [Pg.119]

Asymmetric Buchner reactions using chiral auxiliary have also been undertaken. The diazoketo substrate 126 for the chiral tethered Buchner reaction is prepared from optically pure (2/ ,4/f)-2,4-pentanediol in three steps the Mitsunobu reaction with 3,5-dimethylphenol, esterification with diketene, and diazo formation/deacetylation. Treatment of 126 with rhodium(II) acetate results in a quantitative yield of 127 with more than 99% ee. This compound is reduced with lithium aluminium hydride, and the resulting diol 128 undergoes epoxidation and concurrent acetal formation to give 129 as a single diastereomer. Hydrogenation of 129 with Raney nickel proceeds stereoselectively to yield saturated diol 130, which is converted to aldehyde 132 via acid hydrolysis followed by oxidation. Compound 132 is a versatile intermediate for natural product synthesis. [Pg.442]

The chiral (salen)Co catalysts have also been applied to cyclization reaction and preparation of intermediates for natural product synthesis [85]. In addition, chiral (salen)Ru catalysts proved to be effective for kinetic resolution of racemic epoxides [86]. Tridentate Schiff base Cr(III) complex (201) derived l-amino-2-indanol acts as a potent catalyst for asymmetric ring-opening reaction of meso-aziridines with trimethylsilyl azide (Scheme 16.60) [87]. The aziridine (200) was readily converted at —30 °C to the corresponding amino-azide in 95% yield with 94% ee. [Pg.366]

A reiterative application of a two-carbon elongation reaction of a chiral carbonyl compound (Homer-Emmonds reaction), reduction (DIBAL) of the obtained trans unsaturated ester, asymmetric epoxidation (SAE or MCPBA) of the resulting allylic alcohol, and then C-2 regioselective addition of a cuprate (Me2CuLi) to the corresponding chiral epoxy alcohol has been utilized for the construction of the polypropionate-derived chain ]R-CH(Me)CH(OH)CH(Me)-R ], present as a partial structure in important natural products such as polyether, ansamycin, or macro-lide antibiotics [52]. A seminal application of this procedure is offered by Kishi s synthesis of the C19-C26 polyketide-type aliphatic segment of rifamycin S, starting from aldehyde 105 (Scheme 8.29) [53]. [Pg.290]

Allylic alcohols can be converted to epoxy-alcohols with tert-butylhydroperoxide on molecular sieves, or with peroxy acids. Epoxidation of allylic alcohols can also be done with high enantioselectivity. In the Sharpless asymmetric epoxidation,allylic alcohols are converted to optically active epoxides in better than 90% ee, by treatment with r-BuOOH, titanium tetraisopropoxide and optically active diethyl tartrate. The Ti(OCHMe2)4 and diethyl tartrate can be present in catalytic amounts (15-lOmol %) if molecular sieves are present. Polymer-supported catalysts have also been reported. Since both (-t-) and ( —) diethyl tartrate are readily available, and the reaction is stereospecific, either enantiomer of the product can be prepared. The method has been successful for a wide range of primary allylic alcohols, where the double bond is mono-, di-, tri-, and tetrasubstituted. This procedure, in which an optically active catalyst is used to induce asymmetry, has proved to be one of the most important methods of asymmetric synthesis, and has been used to prepare a large number of optically active natural products and other compounds. The mechanism of the Sharpless epoxidation is believed to involve attack on the substrate by a compound formed from the titanium alkoxide and the diethyl tartrate to produce a complex that also contains the substrate and the r-BuOOH. ... [Pg.1053]

Another microwave-mediated intramolecular SN2 reaction forms one of the key steps in a recent catalytic asymmetric synthesis of the cinchona alkaloid quinine by Jacobsen and coworkers [209]. The strategy to construct the crucial quinudidine core of the natural product relies on an intramolecular SN2 reaction/epoxide ringopening (Scheme 6.103). After removal of the benzyl carbamate (Cbz) protecting group with diethylaluminum chloride/thioanisole, microwave heating of the acetonitrile solution at 200 °C for 2 min provided a 68% isolated yield of the natural product as the final transformation in a 16-step total synthesis. [Pg.178]

The synthesis of the spiroisoxazoline natural product (+ )-calafianin 447 has been reported, using asymmetric nucleophilic epoxidation and nitrile oxide cycloaddition as key steps. Syntheses and spectral analyses of all calafianin stereoisomers lead to unambiguous assignments of relative and absolute stereochemistry (494). [Pg.100]

Sharpless "asymmetric epoxidation" has been used in the enantioselective synthesis of several natural products, including the kinetic resolution of allylic alcohols [11] and the creation of ... [Pg.283]

Optically active epoxides are important building blocks in asymmetric synthesis of natural products and biologically active compounds. Therefore, enantio-selective epoxidation of olefins has been a subject of intensive research in the last years. The Sharpless [56] and Jacobsen [129] epoxidations are, to date, the most efficient metal-catalyzed asymmetric oxidation of olefins with broad synthetic scope. Oxidative enzymes have also been successfully utilized for the synthesis of optically active epoxides. Among the peroxidases, only CPO accepts a broad spectrum of olefinic substrates for enantioselective epoxidation (Eq. 6), as shown in Table 8. [Pg.91]

Peroxidases have been used very frequently during the last ten years as biocatalysts in asymmetric synthesis. The transformation of a broad spectrum of substrates by these enzymes leads to valuable compounds for the asymmetric synthesis of natural products and biologically active molecules. Peroxidases catalyze regioselective hydroxylation of phenols and halogenation of olefins. Furthermore, they catalyze the epoxidation of olefins and the sulfoxidation of alkyl aryl sulfides in high enantioselectivities, as well as the asymmetric reduction of racemic hydroperoxides. The less selective oxidative coupHng of various phenols and aromatic amines by peroxidases provides a convenient access to dimeric, oligomeric and polymeric products for industrial applications. [Pg.103]

In conjunction with the chiral anion TRIP (156) (10 mol%), diamine 157 (10 mol%) can be used in the catalytic asymmetric epoxidation of a,p-unsaturated ketones (>90% ee) [196], while the secondary amine 158 (10 mol%) can be used for the epoxidation of both di- and trisubstituted a,P-unsaturated aldehydes (92-98% ee) (Fig. 15) [211], The facile nature of these reactions, using commercially available peroxides as the stoichiometric oxidant, together with the synthetic utility of the epoxide products suggests application in target oriented synthesis. [Pg.331]

Terashima et al. 231) reported an asymmetric halolactonization reaction. This highly stereoselective reaction permits the synthesis of intermediates for the preparation of chiral a,a-disubstituted a-hydroxycarboxylic acids (227)231c), a-hydroxyketones (228) 231c), functionalized epoxides (229) 231d,e) and natural products 231h,j). Only amino acids have so far been used as a source of the chiral information in the asymmetric halolactonization reaction. Again, the best results have been obtained by using cyclic imino acid enantiomers, namely proline. [Pg.227]

Compatibility of asymmetric epoxidation with acetals, ketals, ethers, and esters has led to extensive use of allylic alcohols containing these groups in the synthesis of polyoxygenated natural products. One such synthetic approach is illustrated by the asymmetric epoxidation of 15, an allylic alcohol derived from (S)-glyceraldehyde acetonide [59,62]. In the epoxy alcohol (16) obtained from 15, each carbon of the five-carbon chain is oxygenated, and all stereochemistry has been controlled. The structural relationship of 16 to the pentoses is evident, and methods leading to these carbohydrates have been described [59,62a]. [Pg.245]

Nucleophilic ring opening of epoxy sugars is a valuable method for the synthesis of many modified sugar derivatives. The reaction is accompanied by Walden inversion, and a wide range of nucleophiles can be used. The cyclic nature of epoxides renders the competing elimination process stereoelectronically unfavourable. For asymmetric epoxides, in principle, two regio-isomeric products can be formed however, in... [Pg.84]

Sharpless asymmetric epoxidation ° is an enantioselective epoxidation of an allylic alcohol with ferf-butyl hydroperoxide (f-BuOOH), titanium tetraisopropoxide [Ti(0-fPr)4] and (-b)- or (—)-diethyl tartrate [(-b)- or (—)-DET] to produce optically active epoxide from achiral allylic alcohol. The reaction is diastereoselective for a-substituted allylic alcohols. Formation of chiral epoxides is an important step in the synthesis of natural products because epoxides can be easily converted into diols and ethers. [Pg.22]

The Shaipless asymmetric epoxidation reaction is often used as a key step in synthetic protocols involving the synthesis of natural products such as terpenes, carbohydrates, insect pheromones, and pharmaceutical products. The SAE reaction is characterized by its simplicity and reliability. The epoxides are obtained with predictable absolute configuration and in high enantiomeric excess (ee). Moreover, 2,3-epoxy alcohols serve as versatile intermediates for a host of stereospecific transformations. [Pg.176]

The enantioselective total synthesis of the annonacenous acetogenin (+)-parviflorin was accomplished by T.R. Hoye and co-workers." The b/s-tetrahydrofuran backbone of the natural product was constructed using a sequential double Sharpless asymmetric epoxidation and Sharpless asymmetric dihydroxylation. The bis allylic alcohol was epoxidized using L-(+)-DET to give the essentially enantiopure bis epoxide in 87% yield. [Pg.409]

Allyl epoxides are produced by the acclaimed Sharpless asymmetric epoxidation reaction [75], and are important intermediates and products. For example, an allyl epoxide is a vital part of the structure of amphidinolides, a series of unique macrolides isolated from dinoflagellates (Amphidinium sp.). Amphidinolide H (AmpH) is a potent cytotoxic 26-membered macrolide with potent cytotoxicity for several carcinoma cell lines [76]. An allyl epoxide is involved in the total synthesis of prostaglandin A2 with a cuprate reagent [77]. Allyl epoxides derived from Sharpless chemistry are a practical method for construction of polypropionate structures by Lewis acid-induced rearrangement [78,79]. Other allyl epoxides such as l,2-epoxy-3-methyl-3-butanol are useful organic intermediates for the production of a-hydroxyketones, which are used for the synthesis of various natural... [Pg.9]


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See also in sourсe #XX -- [ Pg.1045 , Pg.1046 , Pg.1047 , Pg.1048 ]




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