Aldol reactions catalyst

Mukaiyama aldol reactions. Catalyst 69 has been used in the synthesis of bryosta-tin and epothilone segments and, for example, using silyl enol ether 70 high levels of stereocontrol are possible (Scheme 9-23) [48]. This methodology has recently been used in an approach to acutiphycin [49]. 

The dipeptide-derived 64 is a suitable aldol reaction catalyst for handling haloacetones and a-hydroxyacetone, and the the water-compatibility of the analogous 65 underscores its utility in the reaction involving a,a -dihydiDxyacetone. ... 

Metal chelates that find use as aldol reaction catalysts are further exemplified hy the A1 complex of salen 60, which brings together a-isocyano amides and aldehyds to provide chiral 2-(a-hydroxyalkyl)-5-aminooxazoles. ... 

In 2001, Yamamoto prepared various chiral diamines (pyrrolidine based secondary and tertiary amines) " and screened a range of protic acids to catalyse the aldol reaction. Catalyst la TfOH, had no catalytic activity alone. However, the combination of diamine la and la TfOH catalysed the aldol reaction of ketones with p-nitrobenzaldehyde. The aldol products were obtained with high chemical yields and reasonable to high enantio- and diastereoselectivities (Scheme 9.1). ... 

Intrigued by results obtained with the catalyst 79, Corey et al. developed amino-acid-based oxazaborolidines (e.g., 85). Evans et al. proposed tin (II)- and copper (Il)-bisoxazoline-catalyzed aldol reactions (catalysts 86,... 

It turned out that the dodecylsulfate surfactants Co(DS)i Ni(DS)2, Cu(DS)2 and Zn(DS)2 containing catalytically active counterions are extremely potent catalysts for the Diels-Alder reaction between 5.1 and 5.2 (see Scheme 5.1). The physical properties of these micelles have been described in the literature and a small number of catalytic studies have been reported. The influence of Cu(DS)2 micelles on the kinetics of quenching of a photoexcited species has been investigated. Interestingly, Kobayashi recently employed surfactants in scandium triflate catalysed aldol reactions". Robinson et al. have demonshuted that the interaction between metal ions and ligand at the surface of dodecylsulfate micelles can be extremely efficient. ... 

A useful catalyst for asymmetric aldol additions is prepared in situ from mono-0> 2,6-diisopropoxybenzoyl)tartaric acid and BH3 -THF complex in propionitrile solution at 0 C. Aldol reactions of ketone enol silyl ethers with aldehydes were promoted by 20 mol % of this catalyst solution. The relative stereochemistry of the major adducts was assigned as Fischer- /ir o, and predominant /i -face attack of enol ethers at the aldehyde carbonyl carbon atom was found with the (/ ,/ ) nantiomer of the tartaric acid catalyst (K. Furuta, 1991). 

The 0X0 and aldol reactions may be combined if the cobalt catalyst is modified by the addition of organic—soluble compounds of 2inc or other metals. Thus, propylene, hydrogen, and carbon monoxide give a mixture of aldehydes and 2-ethylhexenaldehyde [123-05-7] which, on hydrogenation, yield the corresponding alcohols. 

Butyraldehyde undergoes stereoselective crossed aldol addition with diethyl ketone [96-22-0] ia the presence of a staimous triflate catalyst (14) to give a predominantiy erythro product (3). Other stereoselective crossed aldol reactions of //-butyraldehyde have been reported (15). 

Neo acids are prepared from selected olefins using carbon monoxide and acid catalyst (4) (see Carboxylic Acids, trialkylacetic acids). 2-EthyIhexanoic acid is manufactured by an aldol condensation of butyraldehyde followed by an oxidation of the resulting aldehyde (5). Isopalmitic acid [4669-02-7] is probably made by an aldol reaction of octanal. 

Stannous triflate is an efficient catalyst for aldol-type condensations [ 23, 124, 125 Under conditions of kinetic control, it provides excellent diastereo-selectivity in various cross-aldol reactions (equation 61)... 

The last isomer, the so-called aldotripiperideine (185), is obtained by the action of acid catalysts on a-tripiperideine at its boiling point (298,299), or in aqueous solution at pH 9.2 and 100°C. Further aldol reaction between tetrahydroanabasine and A -piperideine obviously occurs. Hydrogenolysis of this compound gives dihydroaldotripiperideine (186) which is convertible into matridine (187), a reduction product of the alkaloid matrine. 

Chiral salen chromium and cobalt complexes have been shown by Jacobsen et al. to catalyze an enantioselective cycloaddition reaction of carbonyl compounds with dienes [22]. The cycloaddition reaction of different aldehydes 1 containing aromatic, aliphatic, and conjugated substituents with Danishefsky s diene 2a catalyzed by the chiral salen-chromium(III) complexes 14a,b proceeds in up to 98% yield and with moderate to high ee (Scheme 4.14). It was found that the presence of oven-dried powdered 4 A molecular sieves led to increased yield and enantioselectivity. The lowest ee (62% ee, catalyst 14b) was obtained for hexanal and the highest (93% ee, catalyst 14a) was obtained for cyclohexyl aldehyde. The mechanism of the cycloaddition reaction was investigated in terms of a traditional cycloaddition, or formation of the cycloaddition product via a Mukaiyama aldol-reaction path. In the presence of the chiral salen-chromium(III) catalyst system NMR spectroscopy of the crude reaction mixture of the reaction of benzaldehyde with Danishefsky s diene revealed the exclusive presence of the cycloaddition-pathway product. The Mukaiyama aldol condensation product was prepared independently and subjected to the conditions of the chiral salen-chromium(III)-catalyzed reactions. No detectable cycloaddition product could be observed. These results point towards a [2-i-4]-cydoaddition mechanism. 

The reaction was studied in the absence, and presence, of (MeO)2AlMe as a model catalyst for the BINOL-AlMe system. The change in relative energy for the concerted hetero-Diels-Alder reaction, and formation of the hetero-Diels-Alder adduct 11 via a Mukaiyama aldol reaction, is shown in Fig. 8.13. The conclusion of the study was that in the absence of a catalyst the concerted reaction is the most... 

An enantioselective aldol reaction may also be achieved with non-chiral starting materials by employing an asymmetric Lewis acid as catalyst ... 

Stork and Takahashi took -glyceraldehyde synthon from the chiral pool and condensed it with methyl oleate, using lithium diisopropyl amide as catalyst for the mixed aldol reaction, leading to The olefinic linkage is a latent form... 

A syn-selective asymmetiic nih o-aldol reaction has been reported for structurally simple aldehydes using a new catalyst generated from 6,6-bis[(tiiethylsilyl)ethynyl]BINOL (g in Scheme 3.18). The syn selectivity in the nitro-aldol reaction can be explained by steric hindrance in the bicyclic transition state as can be seen in Newman projection. In the favored h ansition state, the catalyst acts as a Lewis acid and as a Lewis base at different sites. In conbast, the nonchelation-controlled transition state affords anti product with lower ee. This stereoselective nitro-aldol reaction has been applied to simple synthesis of t/ireo-dihydrosphingosine by the reduction of the nitro-aldol product with H2 and Pd-C (Eq. 3.79). 

The LLB catalysts requires at least 3.3 mol% of asymmehic catalyst for efficient nitro-aldol reactions, and the reactions are rather slow (first generation). Second-generation LLB catalysts are prepared by addition of 1 equiv of H2O and 0.9 equiv of n-BuLi. The second-generation-catalysts are more reactive than the first generation LLB as shown in Eq. 3.80. The proposed mechanism of asymmetiic niti o-aldol reaction using these catalysts is presented in Scheme 3.20. ... 

On the other hand, carbonyl condensation reactions require only a catalytic amount of a relatively weak base rather than a full equivalent so that a small amount of enolate ion is generated in the presence of unreacted carbonyl compound. Once a condensation has occurred, the basic catalyst is regenerated. To carry out an aldol reaction on propanal, for instance, we might dissolve the aldehyde in methanol, add 0.05 equivalent of sodium methoxide, and then warm the mixture to give the aldol product. 

The gold complex, generated in situ from bis(4-isocyanocyclohexyl)gold(I) tetrafluoroborate and (A)-A-methyl-,V-[2-(dialkylamino)ethyl]-l-[(5)-r,2-bis(diphenylphosphino)ferrocenyl]eth-ylamine, is an effective catalyst for the aldol reaction of various aldehydes with methyl iso-cyanoacetate to give the trans- and cw-4,5-dihydro-l,3-oxazoles. Depending on the aldehyde, the transjeis product ratio ranges from 84 16 to 100 0, and the ee of the main diastereomer is between 72 and 97%26. 

Cationic bis(oxazoline) and pyridil-bis(oxazoline) Cu(ll) and Zn(ll) Lewis-acid catalysts. A comparative study in catalysis of Diels-Alder and aldol reactions [101]... 

Figure 3.8 Aldol reaction and peptide dendrimer catalyst.

The aldol reaction can also be performed with acid catalysts, as mentioned above, in which case dehydration usually follows. Here there is initial protonation of the carbonyl group, which attacks the a carbon of the enol form of the other... 

Mukaiyama aldol reactions have been reported, usually using chiral additives although chiral auxiliaries have also been used. This reaction can also be run with the aldehyde or ketone in the form of its acetal R R C(OR )2> in which case the product is the ether R COCHR2CR R OR instead of 27. Enol acetates and enol ethers also give this product when treated with acetals and TiCLi or a similar catalyst. When the catalyst is dibutyltin bis(triflate), Bu2Sn(OTf)2, aldehydes react, but not their acetals, while acetals of ketones react, but not the ketones themselves. 

Gruttadauria, M., Riela, S., Lo Meo, P., D Anna, F., Noto, R. (2004) Supported Ionic Liquid Asymmetric Catalysis A New Method for Chiral Catalysts Recycling, the Case of ProUne-Catalysed Aldol Reaction. Tetrahedon Letters, 45(32), 6113-6116. 

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