Bifunctional Squaramide Organocatalysts for the Asymmetric Addition of Formaldehyde tert-Butylhydrazone to Simple Aldehydes

Oxocarbon Acids and their Derivatives in Biological and Medicinal Chemistry

The biological and medicinal chemistry of the oxocarbon acids 2,3- dihydroxycycloprop-2-en-1-one (deltic acid), 3,4-dihydroxycyclobut-3-ene-1,2-dione (squaric acid), 4,5-dihydroxy-4-cyclopentene-1,2,3-trione (croconic acid), 5,6-dihydroxycyclohex- 5-ene-1,2,3,4-tetrone (rhodizonic acid) and their derivatives is reviewed and their key chemical properties and reactions are discussed. Applications of these compounds as potential bioisosteres in biological and medicinal chemistry are examined. Reviewed areas include cell imaging, bioconjugation reactions, antiviral, antibacterial, anticancer, enzyme inhibition, and receptor pharmacology.

Brønsted acid catalyzed enantioselective addition of hydrazones to 3-indolylmethanols

A metal-free synthesis of enantiopure β-substituted tryptophan derivatives was developed. A chiral Brønsted acid enabled the addition of donor-substituted hydrazones to 3-indolylmethanols in excellent yields and enantioselectivities.

Ruthenium- and Copper-Catalyzed Propargylic Substitution Reactions of Propargylic Alcohol Derivatives with Hydrazones

Ruthenium- and copper-catalyzed propargylic substitution reactions of propargylic alcohol derivatives with N-monosubstituted hydrazones as ambident nucleophiles are achieved in which N-monosubstituted hydrazones exhibit impressive different reactivities depending on different catalytic systems, behaving as carbon-centered nucleophiles to give the corresponding propargylic alkylated products in ruthenium catalysis, or as nitrogen-centered nucleophiles to afford the corresponding propargylic aminated products in copper catalysis. DFT calculations were carried out to investigate the detailed reaction pathways of these two systems. Further transformation of propargylic substituted products affords the corresponding multisubstituted pyrazoles as cyclization products in good to high yields.

A new asymmetric activation strategy for hydrazones as acyl anion equivalents in the bimetallic catalyzed carbonyl-ene reaction

A new asymmetric activation strategy for hydrazones as acyl anion equivalents is developed in the bimetallic catalyzed carbonyl-ene reaction of isatins and hydrazones. Under mild conditions, optically active functionalized 3-hydroxy-2-oxindoles were furnished in up to 98% yield with up to 97% enantioselectivity. In this process, formaldehyde tert-butylhydrazone which is seldom employed in asymmetric carbonyl-ene reactions accelerated by a metallic catalyst can be activated well by a Brønsted base. A possible catalytic cycle is proposed.

Enantio- and Diastereoselective Nucleophilic Addition of N-tert-Butylhydrazones to Isoquinolinium Ions through Anion-Binding Catalysis

A highly enantio- and diastereoselective thiourea-catalyzed dearomatization of isoquinolines employing N-tert-butylhydrazones as neutral α-azo carbanions and masked acyl anion equivalents has been developed. Experimental and computational data supports the generation of highly ordered complexes wherein the chloride behaves as a template for the catalyst, the hydrazone reagent, and the isoquinolinium cation, providing excellent stereocontrol in the formation of two contiguous stereogenic centers. The ensuing selective and high-yielding transformations provide appealing dihydroisoquinoline derivatives.

Probing α-Amino Aldehydes as Weakly Acidic Pronucleophiles: Direct Access to Quaternary α-Amino Aldehydes by an Enantioselective Michael Addition Catalyzed by Brønsted Bases

The high tendency of α-amino aldehydes to undergo 1,2-additions and their relatively low stability under basic conditions have largely prevented their use as pronucleophiles in the realm of asymmetric catalysis, particularly for the production of quaternary α-amino aldehydes. Herein, it is demonstrated that the chemistry of α-amino aldehydes may be expanded beyond these limits by documenting the first direct α-alkylation of α-branched α-amino aldehydes with nitroolefins. The reaction produces densely functionalized products bearing up to two, quaternary and tertiary, vicinal stereocenters with high diastereo- and enantioselectivity. DFT modeling leads to the proposal that intramolecular hydrogen bonding between the NH group and the carbonyl oxygen atom in the starting α-amino aldehyde is key for reaction stereocontrol.

Room Temperature Ionic Liquids in Asymmetric Hetero-Ene Type Reactions: Improving Organocatalyst Performance at Lower Temperatures

Room temperature ionic liquids (RTILs) have been widely used as (co)solvents in several catalytic processes modifying, in most of the cases, the catalyst activity and/or the selectivity for the studied reactions. However, there are just a few examples of their use in hydrogen bonding organocatalysis. In this paper, we show the positive effect of a set of imidazole-based ionic liquids ([bmim]BF₄ and [hmim]PF₆) in the enantioselective addition of formaldehyde tert-butylhydrazone to prochiral α-keto esters catalyzed by a sugar-based chiral thiourea. Reactions performed in the presence of low percentages of RTILs led to an increase of the catalyst activity, thereby making possible to work at lower temperatures. Thus, the chiral tert-butyl azomethyl tertiary alcohols could be obtained with moderate to good conversions and higher enantioselectivities for most of the studied substrates when using up to 30 vol% of [hmim]PF₆ as a cosolvent in processes performed in toluene.

α-Keto hydrazones in asymmetric aminocatalysis: reactivity through β-amino aza-dienamine intermediates

α-keto hydrazones activated by a bifunctional amino-thiourea catalyst generate β-amino aza-dienamine intermediates which react with nitroalkenes in highly enantioselective fashion.

Cooperative Transport and Selective Extraction of Sulfates by a Squaramide-Based Ion Pair Receptor: A Case of Adaptable Selectivity

The use of a squaramide-based ion pair receptor offers a solution to the very challenging problem of extraction and transport of extremely hydrated sulfate salt. Herein we demonstrate for the first time that a neutral receptor is able not only to selectively extract but also to transport sulfates in the form of an alkali metal salt across membranes and to do so in a cooperative manner while overcoming the Hofmeister bias. This was made possible by an enhancement in anion binding promoted by cation assistance and by diversifying the stoichiometry of receptor complexes with sulfates and other ions. The existence of a peculiar 4:1 complex of receptor 2 with sulfates in solution was confirmed by UV-vis and 1H NMR titration experiments, DOSY and DLS measurements, and supported by solid-state X-ray measurements. By varying the separation technique and experimental conditions, it was possible to switch the depletion of the aqueous layer into extremely hydrophilic or less lipophilic salts, thus obtaining the desired selectivity.

Formaldehyde tert-butyl hydrazone as a formyl anion equivalent: asymmetric addition to carbonyl compounds

The asymmetric 1,2-addition of formyl anion equivalents to carbonyl compounds is a powerful synthetic tool that ideally provide access to highly functionalizable α-hydroxy aldehydes in an enantioselective fashion. In this context, the nucleophilic character of formaldehyde hydrazones, together with their remarkable stability as monomeric species, has been exploited for the functionalization of diverse carbonyl compounds, using initially auxiliary-based methodologies and, more recently, catalytic enantioselective versions. This feature article highlights our research progress employing formaldehyde tert-butyl hydrazone as a versatile formyl anion equivalent, in combination with bifunctional H-bonding organocatalysis. The design and optimization of different catalytic systems, focusing on a dual activation of both reagents, is reviewed, as well as the racemization free unmasking of the formyl group and representative product transformations for the construction of valuable, densely functionalyzed chiral building blocks.

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).

Development of C2-Symmetric Chiral Bifunctional Triamines: Synthesis and Application in Asymmetric Organocatalysis

The synthesis and application of a newly designed C₂-symmetric chiral bifunctional triamine family ( C₂-CBT) is reported. These enantiopure chiral triamine scaffolds can be accessed in multigram amounts from simple amino acids while avoiding chromatographic purification. As a proof of principle, C₂-CBT has been studied in the aldol reaction of cyclic ketones with isatins, with the target tertiary alcohols being formed in a highly efficient manner. Catalyst recovery by simple extraction techniques and subsequent reuse has been performed.

Catalytic enantioselective ene-type reactions of vinylogous hydrazone: construction of α-methylene-γ-butyrolactone derivatives

Catalytic asymmetric ene-type reactions of vinylogous hydrazone with isatins, α-ketoester, imines and aldehydes were accomplished which gave an environmentally friendly, straightforward approach to afford bioactive chiral α-methylene-γ-butyrolactone derivatives.