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Catalytic aldol reaction

Selective retro-aldol has also been reported by using aqueous HC1 in THF.243 Recently, catalytic aldol reactions in aqueous media have generated great interest due to the atom-economy related to the reaction. Reaction of 2-alkyl-1,3-diketones with the aqueous formaldehyde using aqueous 6-10 M potassium carbonate as base afforded aldol reaction products, which are cleaved by the base to give vinyl ketones (Eq. 8.96).244... [Pg.267]

Organic-Base Catalyzed. Asymmetric direct aldol reactions have received considerable attention recently (Eq. 8.98).251 Direct asymmetric catalytic aldol reactions have been successfully performed using aldehydes and unmodified ketones together with chiral cyclic secondary amines as catalysts.252 L-proline and 5,5-dimethylthiazolidinium-4-carboxylate (DMTC) were found to be the most powerful amino acid catalysts for the reaction of both acyclic and cyclic ketones as aldol donors with aromatic and aliphatic aldehydes to afford the corresponding... [Pg.268]

Shibasaki et al. also developed catalytic reactions of copper, some of which can be applied to catalytic asymmetric reactions. Catalytic aldol reactions of silicon enolates to ketones proceed using catalytic amounts of CuF (2.5 mol%) and a stoichiometric amount of (EtO)3SiF (120 mol%) (Scheme 104).500 Enantioselective alkenylation catalyzed by a complex derived from CuF and a chiral diphosphine ligand 237 is shown in Scheme 105.501 Catalytic cyanomethyla-tion by using TMSCH2CN was also reported, as shown in Scheme 106.502... [Pg.475]

Saito, S. Kobayashi, S. Highly Anti-selective Catalytic Aldol Reactions of Amides with Aldehydes, f Am. Chem. Soc. 2006, 128, 8704-8705. [Pg.679]

The asymmetric catalytic aldol reaction of a silyl enol ether can be performed in a double and two-directional fashion to give the 1 2 adduct in the silyl enol ether form with >99% ee and 99% de in 77% isolated yield (Scheme 8C.25) [59]. The present catalytic asymmetric aldol reaction is characterized by a kinetic amplification phenomenon of the product chirality, going from the one-directional aldol intermediate to the two-directional product (Figure 8C.8). Further transformation of the pseudo C2 symmetric product, while still being protected as the silyl enol ether, leads to a potent analog of an HIV protease inhibitor. [Pg.562]

Aldol reactions using a carbocation as an organocatalyst An organocatalytic aldol reaction based on a different concept was developed by the Chen group. The chiral triarylcarbenium ion 34 was used as a novel non-metallic Lewis acid catalyst in a Mukaiyama-type aldol reaction which led to enantiomerically enriched aldol products (Scheme 6.17) [67]. Although non-chiral trityl salt-mediated catalytic aldol reactions had previously been reported by Mukaiyama and co-workers [68], the construction of a suitable chiral carbenium ion remained a challenge. Optically active salts of type 34 were synthesized as Lewis acids based on a reactive carbe-... [Pg.146]

It is worth noting that, in a similar manner to enzymatic conversions with type I or II aldolases, a direct asymmetric aldol reaction was achieved when L-proline was used as catalyst. Accordingly, the use of enol derivatives of the ketone component is not necessary, i.e. ketones (acting as donors) can be used directly without previous modification [72]. So far, most asymmetric catalytic aldol reactions with... [Pg.147]

In addition to the many intermolecular asymmetric (organo)catalytic aldol reactions, analogous intramolecular syntheses are also possible. In this connection it is worthy of note that the first example of an asymmetric catalytic aldol reaction was an intramolecular reaction using an organic molecule, L-proline, as chiral catalyst. This reaction - which will be discussed in more detail below - is the so-called Hajos-Parrish-Eder-Sauer-Wiechert reaction [97-101], which was discovered as early as the beginning of the 1970s. [Pg.166]

The intramolecular aldol reaction of triketones with asymmetric desymmetrization has been known for a long time. When Eder, Sauer, and Wiechert [97, 98], and in parallel Hajos and Parrish [99-101] reported this reaction in the early 1970s it was the first example of an asymmetric catalytic aldol reaction, and one of the first examples of an organocatalytic asymmetric synthesis [104]. [Pg.168]

The asymmetric catalytic aldol reaction of silyl allenolates ICH=C=CR2OSiMe3 with aldehydes R CHO has been achieved by Li et al. by using N-C3F7CO oxazaborolidine as the catalyst [43], The fluoroacyl group of the catalyst was found to be crucial for control of enantioselectivity. The reaction provides the first enantioselective approach to / -halo Baylis-Hillman-type adducts. [Pg.174]

The capability of L-proline - as a simple amino acid from the chiral pool - to act like an enzyme has been shown by List, Lemer und Barbas III [4] for one of the most important organic asymmetric transformations, namely the catalytic aldol reaction [5]. In addition, all the above-mentioned requirements have been fulfilled. In the described experiments the conversion of acetone with an aldehyde resulted in the formation of the desired aldol products in satisfying to very good yields and with enantioselectivities of up to 96% ee (Scheme 1) [4], It is noteworthy that, in a similar manner to enzymatic conversions with aldolases of type I or II, a direct asymmetric aldol reaction was achieved when using L-proline as a catalyst. Accordingly the use of enol derivatives of the ketone component is not necessary, that is, ketones (acting as donors) can be used directly without previous modification [6]. So far, most of the asymmetric catalytic aldol reactions with synthetic catalysts require the utilization of enol derivatives [5]. The first direct catalytic asymmetric aldol reaction in the presence of a chiral heterobimetallic catalyst has recently been reported by the Shibasaki group [7]. [Pg.179]

For a review about the asymmetric catalytic aldol reaction, see a) H. Groger, E. M. Vogl, M. Shibasaki, Chem. Eur. J. 1998,... [Pg.186]

Direct Catalytic Aldol Reactions 339 Claudio Nicolau and Mike Oiarbide... [Pg.672]

This catalytic aldol reaction has been extended to a,f -enals.2 Thus these reactions catalyzed by the chiral acyloxyboranes 3a and 3b in addition to Sn(OTf)2 and Bu3SnF lead to optically active products in as high as 97% ee even when 3 is... [Pg.315]

Sato/Kaneko [104] and Carreira [105] have independently employed acetoacetate-derived O-silyl dienolates as Si-substituted nucleophiles in asymmetric catalytic aldol reactions. The aldol products, d-hydroxy-/3-ketoesters, and the derived syn- and anti-yS,d-diol esters are ubiquitous structural subunits in biologically active natural products such as the polyene macrolide antibiotics. These structural subunits are also found in chemotherapeutics, most notably compactin analogs [106] that have been studied as... [Pg.823]

Catalytic aldol reactions are among the most useful synthetic methods for highly stereo-controlled asymmetric synthesis. In this account we discuss the recent development of a novel synthetic technique which uses tandem enzyme catalysis for the bi-directional chain elongation of simple dialdehydes and related multi-step procedures. The scope and the limitations of multiple one-pot enzymatic C-C bond formations is evaluated for the synthesis of unique and structurally complex carbohydrate-related compounds that may be regarded as metabolically stable mimetics of oligosaccharides and that are thus of interest because of their potential bioactivity. [Pg.87]

In the proline-catalyzed aldol reactions, enolizable achiral aldehydes and ketones are transformed into the corresponding enamines, which can then react with less enolizable carbonyl compounds, even in one-pot protocols. These reactions, unlike most catalytic aldol reactions, do not require preformed enolates, and constitute direct aldol reactions. [Pg.873]

Moreover, although it is known that aldol reactions that utilize acetone as a starting material are generally difficult to control, this reaction proceeds well in the presence of LLB (20 mol%) and 10 equiv of acetone to give the aldol adduct with up to 74% ee and in 82% yield. The postulated mechanism of this direct catalytic aldol reaction is presented in Scheme 12. [Pg.157]

Kobayashi s group37 developed a new enantioselective synthesis of Cis phytosphingosine using catalytic asymmetric aldol reactions as a key step (Scheme 23). The key catalytic aldol reaction of acrolein with the ketene silyl acetal 148 derived from phenyl a-benzyloxyacetate was carried out by using tin(II) triflate, chiral diamine 149, and tin(II) oxide. The desired aldol product... [Pg.485]

Reduction. Carboxylic acids, esin idly by Sm and hydrochloric acid. Ytterb reduction of a-halocarbonyl compound aprotic system (MejSiCl-Nal/MeCN ai Catalytic aldol reactions. ... [Pg.310]

Enantiomerically pure boron-based Lewis acids have also been used successfully in catalytic aldol reactions. Corey s catalyst (7.10a) provides good enantioselectivity with ketone-derived silyl enol ethers, including compound (7.11). Other oxazaborolidine complexes (7.13) derived from a,a-disubstituted a-amino acids give particularly high enantioselectivity, especially with the disubstituted ketene... [Pg.180]

Yamamoto s CAB catalysts (7.16) (see Section 8.1) have also been used in catalytic aldol reactions. This reaction is stereoconvergent as either geometry of silyl enol ether (7.18) affords syn selectivity in the product (7.19) indicating that the reaction proceeds via an open anticHnal transition state with the minimum of steric interactions between the aldehyde substituent and a-substituent of the enol ether, as depicted in Figure 7.2. ... [Pg.181]

Cyanopropionates have also been employed in catalytic aldol reactions. The enolisation of the nucleophile (7.98) by the rhodium complex of TRAP ligand (7.99) is the basis for the catalysis. The use of bulky esters affords high selectivity in the aldol reaction with formaldehyde (7.100), although only moderate antitsyn selectivity was observed when alternative aldehydes were employed. [Pg.193]

Trost has described very efficient and versatile bimetallic zinc catalyst 10 generated in situ from diethyl zinc and a chiral Ugand derived from proline and p-cresol (Scheme 10) [46], For example, this complex can promote catalytic aldol reactions with high enantiomeric excess. The role of two proximal zinc species is for one of them to form the enolate and for the second one to function as a Lewis acid to activate the aldehyde. [Pg.145]

Scheme 16.23 First example of an asymmetric phase-transfer catalytic aldol reaction. Scheme 16.23 First example of an asymmetric phase-transfer catalytic aldol reaction.
Andrus and coworkers utilised their glycolate skeleton in asymmetric phase-transfer catalytic aldol reactions to yield l,2-dihydro3yester deriva-... [Pg.111]

Scheme 16.26 Asymmetric phase-transfer catalytic aldol reactions of glycolate derivatives. Scheme 16.26 Asymmetric phase-transfer catalytic aldol reactions of glycolate derivatives.
It is difficult to summarize all the progress of the homogeneous catalytic aldol reaction due to the space limitations. With representative or selected examples, this article focuses on those efforts in homogeneous catalysis and will be discussed with selected examples according to the different types of catalysts il) transition metals based Lewis acids, such as palladium, copper, iron, etc (2)... [Pg.2208]


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See also in sourсe #XX -- [ Pg.261 , Pg.262 ]




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Asymmetric catalytic aldol reactions

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Catalytic Asymmetric Mukaiyama-Aldol Reactions

Catalytic Enantioselective Aldol Reaction

Catalytic enantioselective intermolecular aldol reaction

Shibasaki direct catalytic asymmetric aldol reaction

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