Proline dipeptides containing fluorine moieties as oganocatalysts for the asymmetric aldol reaction

A series of dipeptide analogues consisting of proline, phenylalanine and aniline- or phenol-fluorine derivatives were synthesized. Their catalytic ability was evaluated in the intermolecular asymmetric aldol reaction, both in organic and aqueous media. Aniline-fluorine derivatives proved to be superior and the best results were obtained, when 2-CF₃ aniline was employed. A diverse substrate scope consisting of both aromatic and aliphatic aldehydes, as well as different ketones was demonstrated, where aromatic aldehydes afforded products in high yields (up to 100%) with excellent diastereo- (up to 95:5) and enantioselectivities (up to 97%), whereas the aliphatic aldehydes afforded also excellent selectivities, but relatively low yield. A simple addition of fluorine to a dipeptide analogue affords organocatalysts with new interesting properties that can catalyze the aldol reaction more efficiently.     

Novel sulfonylpolystyrene-supported prolinamides as catalysts for enantioselective aldol reaction in water

Five novel prolinamides derived from 1,n-diamines supported on commercially available sulfonylpolystyrene have been synthesized and successfully applied in diastereo- and enantioselective aldol reaction in water or, in one case, neat conditions. Excellent dr and er were obtained by reaction of different cyclohexanone derivatives with aromatic and heteroaromatic aldehydes in the presence of the catalysts and AcOH as co-catalyst. The supported organocatalysts work in low loading (5 mol %) exhibiting similar performances than homogeneous prolinamides, and can be reused, after activation, without loss of activity.     

tert-Butyl esters of tripeptides based on Pro-Phe as organocatalysts for the asymmetric aldol reaction in aqueous or organic medium

The enantioselective aldol reaction between ketones and aldehydes constitutes one the most common reaction models for the evaluation of novel organocatalysts. The last few years, it has been shown that the organocatalytic aldol reaction can be performed in water. A family of tripeptides consisting of proline, phenylalanine, and tert-butyl esters of amino acids was successfully employed in this asymmetric transformation. The products of the reaction between various ketones and aldehydes were obtained in high yields (up to 99%) with excellent diastereo- (up to 97:3 dr) and enantioselectivities (up to 99% ee). The C-terminal amino acid determines the ability of the tripeptide (Pro-Phe-AA-O^tBu) to act efficiently in aqueous or organic medium.     

Carbohydrate-based organocatalysts in direct asymmetric aqueous aldol reaction

Aldol reaction involving chiral amines as organocatalysts through enamine formation, like class-I aldolases, is one of the thriving areas of general interest and widely applicable asymmetric reactions. There are many natural and synthetic chiral templates known to work as efficient organocatalysts, but using carbohydrate templates for chiral induction in asymmetric aldol reactions is a relatively new area developed in the recent years. This review focuses on carbohydrates alone or their conjugates with previously known chiral moieties as organocatalysts for asymmetric aldol reactions.     

Enantiopure azetidine-2-carboxamides as organocatalysts for direct asymmetric aldol reactions in aqueous and organic media

A family of enantiopure azetidine-2-caboxamides was asymmetrically synthesized, and was examined as organocatalyst in direct aldol reactions. A well chosen chiral azetidine-2-caboxamide was found to smoothly catalyze the direct aldol reaction of various benzaldehydes with acetone in brine, and β-hydroxy ketones were produced with enantiomeric excess up to 96%. The reaction of benzaldehydes with cyclic ketones also led to the formation of anti-products in diastereomeric ratio up to 99:1 and enantiomeric excess up to 99%.     

Camphor containing organocatalysts in asymmetric aldol reaction on water

A new class of bifunctional organocatalysts were synthesized and proved to be effective in catalyzing aldol reaction on water with high to excellent diastereo- and enantioselectivities.     

Reusable shuttles for exchangeable functional cargos: Reversibly assembled,magnetically powered organocatalysts for asymmetric aldol reactions

A supramolecular approach has been followed to support adamantyl substituted proline organocatalysts onto the surface of magnetite nanoparticles decorated with a β-cyclodextrin motif. The resulting magnetic nanoparticles (ca. ∼10 nm diameter) were used as modular, magnetically recyclable catalysts in the asymmetric aldol reaction of aromatic aldehydes with cyclic ketones in water. The catalytic assemblies can be easily dismantled in organic media, and the recovered nanoparticles (magnetically powered chemical shuttles) re-complexed with another suitably substituted catalytic unit (replaceable functional cargo).     

Synthesis of a biphenyl-based axially chiral amino acid as a highly efficient catalyst for the direct asymmetric aldol reaction

A biphenyl-based axially chiral amino acid (S)-2 has been designed and synthesized. The new amino acid (S)-2 has been found to be a more efficient catalyst than (S)-1 in the direct asymmetric aldol reaction of acetone with aldehydes. For instance, the use of only 0.1 mol % of (S)-2 was sufficient to complete the reaction between acetone and 4-nitrobenzaldehyde, giving the corresponding aldol adduct in good yield with an excellent enantioselectivity.     

Highly efficient prolinamide-based organocatalysts for the direct asymmetric aldol reaction in brine

Four prolinamide-based organocatalysts were readily synthesized and applied to the direct asymmetric aldol reactions of ketones and aromatic aldehydes in brine. When 2,4-dinitrophenol (DNP) was used as an acidic additive, 1 mol % low loading of 2b afforded aldol products with excellent diastereoselectivity of up to 98/2 dr and high enantioselectivity of up to 97% ee.     

Sulfonamides of homoproline and dipeptides as organocatalysts for Michael and aldol reactions

Sulfonamides of the non-natural amino acid homoproline and the dipeptide Pro-Phe were synthesised and evaluated for their catalytic activity in Michael and aldol reactions. Sulfonamides of homoproline outperform proline and Pro-Phe in the Michael reaction, whereas sulfonamides of Pro-Phe lead to better results in the aldol reaction. The results of the present study show that the conversion of the carboxylic group of either homoproline or dipeptide Pro-Phe to the bioisosteric acyl sulfonamide group lead to improved organocatalysts.     

Evaluation of mono- and dipeptides as organocatalysts for enantioselective aldol reaction

Catalytic activities of (3S)-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid (THIQA)-based mono- and dipeptides and their l-proline analogs in asymmetric aldol reaction were investigated. THIQA-based dipeptides showed better enantioselectivity than proline analogs, whereas proline-based dipeptides gave higher yield in the aldol reaction of cyclohexanone with several aldehydes in dichloromethane at −10 °C in the presence of benzoic acid.     

N-Prolinylanthranilic acid derivatives as bifunctional organocatalysts for asymmetric aldol reactions

Several N-prolinylanthranilic acid derivatives were prepared and tested as bifunctional organocatalysts in the direct asymmetric aldol reaction of cyclohexanone with p-nitrobenzaldehyde. It was found that methyl substitution ortho to the carboxylic acid improves enantioselectivity, but substitution ortho to the anilide group does not. The catalyst derived from 2-amino-6-methylbenzoic acid was tested over a range of direct asymmetric aldol reactions and its overall performance is equal to or better than many of the known prolinamide catalysts in terms of yield, diastereoselectivity, and enantioselectivity.     

Development of an N-sulfinyl prolinamide for the asymmetric aldol reaction

A new class of organocatalysts is reported that incorporates an N-sulfinyl amide in place of the carboxylic acid of proline to serve as a hydrogen bond donor, chiral directing group, and solubilizing element. The successful application of this type of catalyst to the asymmetric aldol reaction of acetone and aryl aldehydes, for which proline performs poorly, is also described.     

Prolinamides derived from aminophenols as organocatalysts for asymmetric direct aldol reactions

N-(2-Hydroxyphenyl)-prolinamides were synthesized with the aim to introduce an additional hydrogen bonding site to the prolinamide structure. These compounds were evaluated as organocatalysts for asymmetric aldol reactions between aromatic aldehydes and cyclohexanone. Very good yields, diastereoselectivities, and enantioselectivities were achieved in both organic solvents and water. The importance of the additional hydrogen bonding site was confirmed by comparative experiments with prolinamide derivatives without the hydroxyl group.     

l-Prolinamide derivatives as efficient organocatalysts for aldol reactions on water

A series of l-prolinamide derivatives have been evaluated as potential organocatalysts for aldol reactions. To synthesize these catalysts, a novel and simple route has been developed using proline N-carboxyanhydride (proline-NCA). A new catalyst with a trifluoromethyl-sulfonamide group has been found exhibiting excellent diastereoselectivity (anti/syn: 97/3) and enantioselectivity (99% ee) with 10 mol % loading and being easily recyclable in water.     

Rationally designed organocatalyst for direct asymmetric aldol reaction in the presence of water

A rationally designed organocatalyst for direct asymmetric aldol reaction in the presence of water has been developed. High yield (up to 99%), diastereoselectivity (up to 99:1) and enantioselectivity (up to 97%) were obtained under optimal conditions.     

Pyrrolidine as an efficient organocatalyst for direct aldol reaction of trifluoroacetaldehyde ethyl hemiacetal with ketones

Pyrrolidine-catalyzed aldol reaction of trifluoroacetaldehyde ethyl hemiacetal (1) with ketones or aldehydes was described. In the presence of 20 mol % of pyrrolidine, the reaction proceeded smoothly at room temperature to afford the aldol products in good to excellent yields (up to 95%). Pyrrolidine showed a much higher catalytic activity than piperidine in the reaction with less reactive ketones. GC analysis clearly indicated that the catalyst and the enamine intermediates were kept at extremely low concentration during the reaction. Based on these observations, we suggested that formation of the enamine would be a rate-determining step for the catalytic aldol reaction. In addition, the asymmetric aldol reaction of 1 with cyclohexanone catalyzed by l-proline derivatives was also discussed.     

Synthesis and application of proline based organocatalyst for highly enantioselective aldol reaction by hydrogen bonding

Using hydrogen bond based concept we have synthesized new organocatalysts and have applied them to catalyze direct asymmetric aldol reaction. Reaction proceeded efficiently and gave high yield with excellent diastereoselectivity and enantioselectivity.     

Proline-based dipeptides with two amide units as organocatalyst for the asymmetric aldol reaction of cyclohexanone with aldehydes

A series of proline-based dipeptide organocatalysts with two amide units (1-16) have been developed and evaluated in the direct catalytic asymmetric aldol reactions of aldehydes with cyclohexanone. These catalysts showed good solubility in organic solvents compared with their corresponding carboxyl terminal dipeptides. The robust amide bond formation allowed structural modifications and fine tuning of catalyst properties by varying the stereo and electronic effects of the terminal amide to affect the ability of hydrogen bonding formation between the catalysts and the substrates. The reactions proceeded smoothly in high yields (up to 99%), enantioselectivities (up to 98% ee) and anti-diastereoselectivities (up to 99:1) in the presence of bifunctional organocatalyst 4 under the optimal reaction conditions.     

New N-terminal prolyl-dipeptide derivatives as organocatalysts for direct asymmetric aldol reaction

A series of new N-terminal prolyl-dipeptide derivatives have been synthesized and evaluated as organocatalysts for the direct asymmetric aldol reaction of acetone with electron-deficient aromatic aldehydes. At room temperature, the presence of 10 mol % of catalysts 2 and 5 efficiently catalyzes the direct asymmetric aldol reaction to give the aldol adducts with modest to excellent enantiomeric excesses (ee) values, which are up to 96%.