New Simple Hydrophobic Proline Derivatives as Highly Active and Stereoselective Catalysts for the Direct Asymmetric Aldol Reaction in Aqueous Medium

Enantioselective Catalytic Aldol Reactions in the Presence of Knoevenagel Nucleophiles: A Chemoselective Switch Optimized in Deep Eutectic Solvents Using Mechanochemistry

In the presence of different nucleophilic Knoevenagel competitors, cyclic and acyclic ketones have been shown to undergo highly chemoselective aldol reactions with aldehydes. In doing so, the substrate breadth for this emerging methodology has been significantly broadened. The method is also no longer beholden to proline-based catalyst templates, e.g., commercially available O-t-Bu-L-threonine is advantageous for acyclic ketones. The key insight was to exploit water-based mediums under conventional (in-water) and non-conventional (deep eutectic solvents) conditions. With few exceptions, high aldol-to-Knoevenagel chemoselectivity (>10:1) and good product profiles (yield, dr, and ee) were observed, but only in DESs (deep eutectic solvents) in conjunction with ball milling did short reaction times occur.

New boro amino amide organocatalysts for asymmetric cross aldol reaction of ketones with carbonyl compounds

Distinct types of new boron fused primary amino amide organocatalysts were designed and synthesized from commercially available amino acids. Their catalytic activities were investigated in asymmetric crossed aldol reaction of ketones with aromatic aldehydes to afford the corresponding chiral anti-aldol adducts with good chemical yields, moderate diastereoselectivity and good to excellent enantioselectivities (up to 94% yields, up to 90 : 10 dr, up to 94% ee).

Synthesis of antiviral drugs by using carbon–carbon and carbon–heteroatom bond formation under greener conditions

Antiviral medications are a branch of medicines notably used to treat that cause many significant diseases in humans and animals. This monograph mainly focuses on recent developments and synthesis of antiviral drugs using carbon-carbon and carbon–hetero bond cross-coupling chemistry. Viral infections exact several severe human diseases, accounting for remarkably high mortality rates. In this sense, academia and the pharmaceutical industry continuously search for novel compounds with better antiviral activity. The researchers face the challenge of developing greener and economical ways to synthesize these compounds and make significant progress.

Organocatalysts Derived from Unnatural α-Amino Acids: Scope and Applications

The organocatalytic properties of unnatural α-amino acids are reviewed. Post-translational derivatives of natural α-amino acids include 4-hydroxy-l-proline and 4-amino-l-proline scaffolds, and also proline homologues. The activity of synthetic unnatural α-amino acid-based organocatalysts, such as β-alkyl alanines, alanine-based phosphines, and tert-leucine derivatives, are reviewed herein. The organocatalytic properties of unnatural monocyclic, bicyclic, and tricyclic proline derivatives are also reviewed. Several families of these organocatalysts permit the efficient and stereoselective synthesis of complex natural products. Most of the reviewed organocatalysts accelerate the reported reactions through covalent interactions that raise the HOMO (enamine intermediates) or lower the LUMO (iminium intermediates).

Binding abilities of a chiral calix[4]resorcinarene: a polarimetric investigation on a complex case of study

Polarimetry was used to investigate the binding abilities of a chiral calix[4]resorcinarene derivative, bearing L-proline subunits, towards a set of suitably selected organic guests. The simultaneous formation of 1:1 and 2:1 host–guest inclusion complexes was observed in several cases, depending on both the charge status of the host and the structure of the guest. Thus, the use of the polarimetric method was thoroughly revisited, in order to keep into account the occurrence of multiple equilibria. Our data indicate that the stability of the host–guest complexes is affected by an interplay between Coulomb interactions, π–π interactions, desolvation effects and entropy-unfavorable conformational dynamic restraints. Polarimetry is confirmed as a very useful and versatile tool for the investigation of supramolecular interactions with chiral hosts, even in complex systems involving multiple equilibria.

Catalytic enantioselective aldol reactions

Recent developments in catalytic asymmetric aldol reactions have been summarized.

The smallest organocatalyst in highly enantioselective direct aldol reaction in wet solvent-free conditions

The catalytic efficacy of the smallest organocatalyst, l-proline hydrazide, prepared from a cheaply available natural amino acid, such as l-proline, was studied for the direct asymmetric aldol reaction of various ketones with aromatic aldehydes at room temperature in the presence of several acid additives.

Chiral picolylamines for Michael and aldol reactions: probing substrate boundaries

Here we report on inroads concerning increased substrate breadth via the picolylamine organocatalyst template, a vicinal chiral diamine based on a pyridine-primary amine motif. The addition of cyclohexanone to β-nitrostyrene has many catalyst solutions, but cyclopentanone and isobutyraldehyde additions continue to be challenging. PicAm-3 (10 mol%) readily allows the Michael addition of cyclopentanone or isobutyraldehyde (5.0 equiv.) to β-nitrostyrene derivatives. By contrast, PicAm-1 (7.0 mol%) is optimal for catalyzing the aldol reaction of cyclohexanone or cycloheptanone (3.3 equiv.) with aromatic aldehydes. Eighteen products are reported and for each reaction type new products are reported (4b-d, 9c). Very good yields and stereoselectivities are generally noted. The reactions, which require an acid additive, proceed via a transient chiral enamine and a mechanistic case is put forth for a bifunctional catalysis model.

Hydrophobic substituent effects on proline catalysis of aldol reactions in water

Derivatives of 4-hydroxyproline with a series of hydrophobic groups in well-defined orientations have been tested as catalysts for the aldol reactions. All of the modified proline catalysts carry out the intermolecular aldol reaction in water and provide high diastereoselectivity and enantioselectivity. Modified prolines with aromatic groups syn to the carboxylic acid are better catalysts than those with small hydrophobic groups (1a is 43.5 times faster than 1f). Quantum mechanical calculations provide transition structures, TS-1a(water) and TS-1f(water), that support the hypothesis that a stabilizing hydrophobic interaction occurs with 1a.

D-Glucosamine in a chimeric prolinamide organocatalyst for direct asymmetric aldol addition

O-TBDPS D-glucosamine coupled with l-proline is reported to act as an efficient organocatalyst in the accomplishment of direct aldol reactions. Excellent results, in terms of chemical yields, as well as diastereomeric and enantiomeric ratios, are reported for the catalyzed additions of cyclohexanone and acetone to variously substituted benzaldehydes.

Organocatalyzed enantioselective aldol reaction of 1H-pyrrole-2,3-diones

The enantioselective cross aldol reaction of 1-benzyl 4,5-dioxo-2-aryl-4,5-dihydro-1H-pyrrole-3-carboxylates and ketones was studied with proline derivatives or cinchona alkaloid-derived primary amines as the catalysts for the first time. trans-4-Benzoyloxy-L-proline (15) was found to be the best catalyst for acyclic ketones. For cyclohexanone, the best results were achieved with 9-deoxy-9-epi-aminoquinine (18) as the catalyst in the presence of racemic 1,1'-binaphthyl-2,2'-diyl hydrogen phosphate (19) as the cocatalyst. Using these protocols, 3-alkyl-3-hydroxy-1H-pyrrol-2(3H)-one derivatives were obtained in excellent yields and good to high ee values (up to 94% ee).