Highly efficient and solvent-free direct aldol reaction catalyzed by glucosamine-derived prolinamide

The catalytic activity of novel sugar-based prolinamides in the aldol reaction between ketones and aryl aldehydes has been examined. The prolinamide 1c was found to be an efficient organocatalyst for the asymmetric aldol reaction under solvent-free conditions. A variety of ketones and aldehydes were used as substrates and the corresponding aldol products were obtained in excellent chemical yields with high levels of anti diastereoselectivity (up to 99:1) and enantioselectivity (up to >99%).     

A new type of amino amide organocatalyzed enantioselective crossed aldol reaction of ketones with aromatic aldehydes

A new type of amino amide organocatalysts was designed and synthesized from commercially available amino acids in easy steps. Their catalytic activities were examined in enantioselective crossed aldol reaction of various acyclic and cyclic ketones with aromatic aldehydes to afford the corresponding chiral anti-aldol adducts with good to excellent chemical yields, diastereoselectivities and enantioselectivities (up to 99%, up to syn:anti?=?1:99, up to 97% ee).     

Highly efficient primary amine organocatalysts for the direct asymmetric aldol reaction in brine

Organocatalytic systems made up of six primary amine organocatalysts, derived from natural primary amino acids, in combination with 2,4-dinitrophenol (DNP) have proven to be efficient in the presence of brine without further addition of organic solvents. The system formed by 1f and DNP was the most efficient one; it can catalyze the direct aldol reaction with a broad range of ketones and aromatic aldehydes, giving the corresponding aldol products in high yields with up to nearly perfect diastereo- and enantioselectivities (up to 99/1 syn/anti, >99% ee).     

Novel prolinamide–ureas as organocatalysts for the asymmetric aldol reaction

Among the various versions of the aldol reaction, the enantioselective reaction between cyclic ketones and aldehydes constitutes a typical reaction model for the evaluation of novel organocatalysts. A multifunctional organocatalyst consisting of a prolinamide moiety, a gem diamine unit and a urea group was successfully employed in this asymmetric transformation. The products of the reaction between various ketones and aldehydes were obtained in high yields (up to 98%) with excellent diastereo- (up to >98:2 dr) and enantioselectivities (up to 99% ee).     

A chiral primary-tertiary-1,2-diamine as an efficient catalyst in asymmetric aldehyde–ketone or ketone–ketone aldol reactions

A novel chiral 1,2-diaminocyclohexane derivative, (1R,2R)-N¹-n-pentyl, N¹-benzyl-1,2-cyclohexanediamine, was designed, synthesized and applied as a catalyst in a number of aldol reactions between ketones and aryl aldehydes. Reactions between acetone and aryl aldehydes gave aldol products with moderate to good yields and with excellent enantioselectivity (up to yield 85%, ee 98%), while reactions between cyclohexanone and aryl aldehydes provided anti-β-hydroxyketone products with excellent yields, diastereoselectivity and with enantioselectivity (up to 82% yield, anti/syn ratio 99:1, ee 99%). The aldol reactions between acetone and isatins were investigated, which afforded excellent yields and enantioselectivity (up to 95% yield, 98% ee). The (R)- and (S)-isomers of convolutamydine A were obtained with 95% yield and 96% ee, and 95% yield and 94% ee, respectively.     

Acyclic amino acid-catalyzed direct asymmetric aldol reactions: alanine, the simplest stereoselective organocatalyst

The linear amino acid-catalyzed direct asymmetric intermolecular aldol reaction is presented; simple amino acids such as alanine, valine, isoleucine, aspartate, alanine tetrazole and serine catalyzed the direct catalytic asymmetric intermolecular aldol reactions between unmodified ketones and aldehydes with excellent stereocontrol and furnished the corresponding aldol products in up to 98% yield and with up to > 99% ee.     

Combining prolinamides with 2-pyrrolidinone: Novel organocatalysts for the asymmetric aldol reaction

Peptides and especially prolinamides have been identified as excellent organocatalysts for the aldol reaction. The combination of prolinamides with derivatives bearing the 2-pyrrolidinone scaffold, deriving from pyroglutamic acid, led to the identification of novel organocatalysts for the intermolecular asymmetric aldol reaction. The new hybrids were tested both in organic and aqueous media. Among the compounds tested, 22 afforded the best results in petroleum ether, while 25 afforded the products in brine in high yields and selectivities. Then, various ketones and aldehydes were utilized and the products of the aldol reaction were obtained in high yields (up to 100%) with excellent diastereo- (up to 97:3 dr) and enantioselectivities (up to 99% ee).     

Biocatalytic direct asymmetric aldol reaction using proteinase from Aspergillus melleus

The direct asymmetric aldol reaction of aromatic aldehydes with cyclic or acyclic ketones was catalyzed by proteinase from Aspergillus melleus (AMP) in acetonitrile in the presence of water. A wide range of substrates could be transformed into the corresponding aldol products in yields up to 89%, enantioselectivities up to 91% ee and diastereoselectivities up to >99:1 (anti/syn). This work provided an example of enzyme catalytic promiscuity that widens the applicability of this biocatalyst in organic synthesis without the need for additional cofactors or special equipment.     

Highly stereoselective direct aldol reactions catalyzed by (S)-NOBIN-l-prolinamide

(S)-NOBIN-l-prolinamide was employed as organocatalyst in the direct aldol reactions of different ketones and aromatic aldehydes using dioxane as solvent and in the presence of water as additive. Acetone led to the aldol products in up to 93% ee, while cyclic ketones furnished the anti-aldols in moderate to high yield, excellent diastereoselectivity (up to >99/<1 anti/syn ratio) and high ee (up to 95%).     

Organocatalysis of asymmetric aldol reaction in water: comparison of catalytic properties of (S)-valine and (S)-proline amides

(S)-Valine amides containing (S)- or (R)-α-phenylethyl substituents at N¹ atom efficiently catalyze asymmetric aldol reactions between cyclic (heterocyclic) ketones and aromatic aldehydes in water, predominantly giving rise to the aldol anti-diastereomers in high yields (up to 98%) and enantiomeric excess (up to 94%).     

Highly diastereo- and enantioselective direct aldol reaction under solvent-free conditions

An efficient, solvent-free protocol for the asymmetric aldol reaction between aldehydes and ketones using prolinamides 1–4 as organocatalysts is reported. Catalysts 2–4, in the presence of TFA (the ratio of catalyst and TFA = 1/1.5), proved to be excellent catalysts, giving the aldol products between aromatic aldehydes and ketones with nearly perfect diastereo- and enantioselectivities (up to >99:1 dr and >99% ee). This catalytic system can also be applied in the cross-aldol reaction of isatin with ketones and the Michael reaction between cyclohexane and nitroalkenes. A mechanism is proposed to account for the formation of the major enantiomer in this reaction.     

Highly enantioselective aldol reactions catalyzed by reusable upper rim-functionalized calix[4]arene-based l-proline organocatalyst in aqueous conditions

A series of upper rim-functionalized calix[4]arene-based l-Proline derivatives have been synthesized and employed for the enantioselective aldol reactions between cyclic ketones and aromatic aldehydes in the presence of water. Good to excellent yields (up to 96%), enantioselectivities (up to 99% ee), as well as diastereoselectivities (up to 99:1 dr) were obtained under the optimal reaction conditions. Detailed experiments clearly showed that the hydrophobic calixarene platform not only contributed to the good reactivities and enantioselectivities, but also exhibited size-selective catalysis function. Moreover, the organocatalyst can be readily recovered and reused for several runs without significant loss in its enantioselectivity.     

Highly diastereo- and enantioselective direct aldol reactions by 4-tert-butyldimethylsiloxy-substituted organocatalysts derived from N-prolylsulfonamides in water

A new set of 4-tert-butyldimethylsiloxy-substituted organocatalysts derived from N-prolylsulfonamide has been designed and proved to be effective in catalyzing the direct aldol reactions of cyclic ketones with a series of aromatic aldehydes. Furthermore, to the best of our knowledge, there are no reports on the aldol reaction generating the products in very good yields (>99%) and with excellent diastereoselectivities up to >99:1 and enantioselectivities up to >99% by using lower catalyst loadings (only 3 mol %), without using any additives in a large amount of water under mild conditions.     

Hydroxy-α-amino acids modified by ionic liquid moieties: recoverable organocatalysts for asymmetric aldol reactions in the presence of water

New chiral ionic liquids bearing proline, serine or threonine moieties were synthesized. Compounds that contain 1-dodecylimidazolium or 4-(5-n-nonyl)-pyridinium cations and NTf₂ or PF₆ anions efficiently catalyze the asymmetric aldol reaction between aldehydes and ketones in the presence of water to generate aldols with high distereo- (up to 98:2) and enantioselectivity (up to >99% ee). 4-Hydroxyproline modified by the 4-(5-n-nonyl)-pyridinium hexafluorophosphate moiety retains its activity and selectivity over at least eight reaction cycles.     

Highly Active Copper‐Network Catalyst for the Direct Aldol Reaction

The development of a highly active solid‐phase catechol–copper network catalyst for direct aldol reaction is described. The catalyst was prepared from an alkyl‐chain‐linked bis(catechol) and a copper(II) complex. The direct aldol reaction between carbonyl compounds (aldehydes and ketones) and methyl isocyanoacetate was carried out using 0.1–1 mol % [Cu] catalyst to give the corresponding oxazolines at yields of up to 99 % and a trans/cis ratio of >99:1. The catalyst was reused with no loss of catalytic activity. A plausible reaction pathway is also described.     

The application of a structurally simple,recyclable, and large-scale l-prolinamide catalyst for asymmetric aldol reactions

A highly efficient, bifunctional prolinamide catalyst, which consists of chiral proline and trans-cyclohexanediamine moieties, was prepared and evaluated in the direct asymmetric aldol reactions of various ketones and aldehydes. The catalyst displayed impressive catalytic activity toward heterocyclic ketones containing oxygen, sulfur, or nitrogen, which have not been sufficiently explored. The substrate scope also covered cyclic and acyclic ketones. With heterocyclic ketones or cyclohexanone, the aldol reactions gave products in high yields and with respectable enantioselectivities (87–99% ee) and diastereoselectivities (up to >99:1 anti/syn). The catalyst could be recycled and reused up to seven times resulting in good yields and with good selectivities. This catalyst is also efficient in large-scale reactions with the enantioselectivities remaining at the same level as in the experimental scale reactions.     

Organo-catalyzed highly diastereo- and enantio-selective direct aldol reactions in water

The asymmetric direct aldol reactions of a wide scope of aromatic aldehydes, with unmodified ketones in the presence of 1 mol % of organocatalyst prepared from (2R,3R)-diethyl 2-amino-3-hydroxysuccinate and trans-4-hydroxy-l-proline, were performed in water, affording aldol products in high yields with excellent diastereoselectivities of up to >99:1 and enantioselectivities of up to 98%.     

Carbohydrate-derived alcohols as organocatalysts in enantioselective aldol reactions of isatins with ketones

The catalytic activity of carbohydrate-derived amino alcohols in the enantioselective aldol reaction of ketones with isatin and its derivatives has been examined for the first time. The carbohydrate-derived amino alcohols 5 were found to be efficient organocatalysts for asymmetric aldol reactions. A variety of isatins were used as substrates and the corresponding aldol products were obtained in high yields (up to 99%) and with moderate enantioselectivities (up to 75%).     

Highly stereoselective anti-aldol reactions catalyzed by simple chiral diamines and their unique application in configuration switch of aldol products

Chiral derivatives of trans-1,2-diaminocyclohexane with different N,N-dialkyl groups in well-defined orientations have been synthesized, and applied as catalysts for the asymmetric aldol reaction between a variety of aldehydes and ketones. Enantiomeric catalyst 1j catalyzed the reaction in ethanol and provided excellent diastereoselectivity and enantioselectivity. Significantly, simple replacement of organic solvents with water switched the products of the aldol reactions from anti to syn configuration. Such catalytic reactions led to the products with anti to syn diastereoselectivity up to 99:1 in ethanol, while in water gave the products with syn to anti diastereoselectivity up to 99:1.     

Highly enantioselective hetero-Diels-Alder reaction between trans-1-methoxy-2-methyl-3-trimethylsiloxybuta-1,3-diene and aldehydes catalyzed by (R)-BINOL-Ti(IV) complex

An efficient enantioselective approach to 2,5-disubstituted dihydropyrones was developed. Some easily accessible inexpensive diol ligand metal complexes were employed, and [(R)-BINOL]₂-Ti(OiPr)₄ complex was found to be the most effective catalyst (up to 99% yield and 99% ee in the presence of 5 mol% catalyst) for the hetero-Diels-Alder reaction between trans-1-methoxy-2-methyl-3-trimethylsiloxybuta-1,3-diene (1) and aldehydes. The potential and generality of this catalyst were evaluated by a variety of aldehydes including aromatic, heteroaromatic, α,β-unsaturated and aliphatic aldehydes. Based on the isolated intermediate from the reaction of benzaldehyde being confirmed by ¹H, ¹³C NMR and HRMS data, the mechanism was proposed as a Mukaiyama aldol pathway.