Water opportunities: catalyst and solvent in Mukaiyama aldol addition of Rawal’s diene to carbonyl derivatives

γ-Cyclodextrins as Supramolecular Reactors for the Three-component Aza-Darzens Reaction in Water

In recent decades, many efforts have been devoted to studying reactions catalyzed in nanoconfined spaces. The most impressive aspect of catalysis in nanoconfined spaces is that the reactivity of the molecules can be smartly driven to disobey classical behavior. A green and efficient three-component aza-Darzens (TCAD) reaction using a catalytic amount of γ-cyclodextrins (CDs) in water has been developed to synthesize N-phenylaziridines. CDs effectively performed this reaction in an environmentally friendly setting, achieving good yields. The same reaction was then performed using polymeric γ-CD such as a γ-cyclodextrin polymer crosslinked (GCDPC) with epichlorohydrin, a sponge-like macroporous γ-cyclodextrin-based cryogel (GCDC), and a γ-cyclodextrin-based hydrogel (GCDH). The homogeneous and heterogeneous catalyst recovery was then studied, and it was proved to be easily recycled several times without relevant activity loss. Water, as a unique and eco-friendly reaction medium, has been utilized for the first time, to the best of our knowledge, in this reaction. The inclusion of the reagents in CDs has been studied and rationalized by NMR spectroscopy experiments and molecular modeling calculations. The credit of the presented protocol includes good yields and catalyst reusability and precludes the use of organic solvents.

Construction of sulfur-containing compounds with anti-cancer stem cell activity using thioacrolein derived from garlic based on nature-inspired scaffolds

Sulfur-containing compounds, such as cyclic compounds with a vinyl sulfane structure, exhibit a wide range of biological activities including anticancer activity. Therefore, the development of efficient strategies to synthesize such compounds is a remarkable achievement. We have developed a unique approach for the rapid and modular preparation of nature-inspired cyclic and acyclic sulfur-containing compounds using thioacrolein, a naturally occurring chemically unstable intermediate. We constructed thiopyranone derivatives through the regioselective sequential double Diels-Alder reaction of thioacrolein produced by allicin, a major component in garlic, and two molecules of silyl enol ether as the diene partner. The cytotoxicity toward cancer stem cells of the thiopyranones was equal to or higher than that of (Z)-ajoene (positive control) derived from garlic, and the thiopyranones had higher chemical stability than (Z)-ajoene.

Catalyst-free and additive-free reactions enabling C-C bond formation: a journey towards a sustainable future

This review focuses on the catalyst- and additive-free C-C bond forming reactions reported mostly from the year 2005 to date. C-C bond forming reactions are highly important as large and complex organic molecules can be derived from simpler ones via these reactions. On the other hand, catalyst- and additive-free reactions are economical, environmentally friendly and less sensitive to air/moisture, allow easy separation of products and are operationally simple. Hence, a large number of research articles have been published in this area. Though a few reviews are available on the catalyst-free organic reactions, most of them were published a few years ago. The current review excludes catalysts as well as additives and is specific to only C-C bond formation. Besides many organic name reactions, catalyst/additive-free C-H functionalizations, coupling reactions and UV-visible-light-promoted reactions are also discussed. Undoubtedly, the contents of this review will motivate readers to do more novel work in this area which will accelerate the journey towards a sustainable future.

Cucurbit[7]uril as a catalytic nanoreactor for one-pot synthesis of isoxazolidines in water

The main objective of supramolecular chemistry is to mimic the macrosystems present in nature, a goal that fits perfectly with the green chemistry guidelines. The aim of our work is to use the hydrophobic cavity of cucurbit[7]uril (CB[7]) to mimic nature for performing different dehydration and cycloaddition reactions in water. The hydrophobic cavity of CB[7] made it possible to synthesize nitrones and isoxazolidines in a one-pot fashion using water as a reaction solvent. Substituted isoxazolidines were obtained from the cycloaddition of nitrones with various styrenes and cinnamates, under microwave irradiation, with a catalytic amount of CB[7], and a moderate increase in the formation of the trans adduct was observed, compared to the reaction being carried out in toluene. The mechanism of the reaction and the inclusion of reagents and products in the CB[7] cavity have been studied and rationalized by NMR spectroscopy, ESI-MS experiments, and molecular modeling calculations.

New compounds for a good old class: Synthesis of two Β-lactam bearing cephalosporins and their evaluation with a multidisciplinary approach

Antimicrobial resistance is spreading massively in the world and is becoming one of the main health threats of the 21st century. One of the possible strategies to overcome this problem is to modify the known classes of antibiotics in a rational way, with the aim of tuning their efficacy. In this paper, we present the synthesis and the evaluation of the biological activity of a series of two β-lactam bearing cephalosporin derivatives, in which an additional isolated azetidinone ring, bearing different substituents, is joined to the classical cephalosporanic nucleus by a chain of variable length. A computational approach has been also applied in order to predict the molecular interactions between some representative derivatives and selected penicillin-binding proteins, the natural targets of β-lactam antibiotics. All these derivatives are active against Gram-positive bacteria, with MIC₁₀₀ comparable or even better than that of the reference antibiotic ceftriaxone, and show no or very low cytotoxic activity on different cell lines. Overall, these molecules appear to be able to exert their activity in particular against microorganisms belonging to some of the species more involved in the development of multidrug resistance.

Green, Mild, and Efficient Friedel-Crafts Benzylation of Scarcely Reactive Arenes and Heteroarenes under On-Water Conditions

Metal-free Friedel-Crafts benzylation (FCB) of scarcely reactive arenes and heteroarenes was performed under on-water conditions by an environmentally sustainable procedure. The catalytic strategy exploits the hydrophobicity of the resorcinarene macrocycle 1 a. The proposed mechanism is based on the activation of benzyl chloride by H-bonding interactions with catalyst 1 a. In fact, under on-water conditions the hydrophobic amplification of the strength of the H-bonding interactions between the OH groups of the resorcinarene catalyst and the chlorine atom of benzyl chloride leads to polarization of the C-Cl bond, which consequently promotes electrophilic attack of the π nucleophile. Thus, many arenes and heteroarenes were efficiently benzylated under mild on-water conditions by using resorcinarene 1 a as catalyst. The FCB of benzene is industrially relevant for the synthesis of diphenylmethane, and hence the on-water procedure was extended to the gram-scale synthesis of diphenylmethane, starting from benzene and benzyl chloride in the presence of resorcinarene catalyst 1 a.

A hexameric resorcinarene capsule as a hydrogen bonding catalyst in the conjugate addition of pyrroles and indoles to nitroalkenes

A hexameric resorcinarene capsule (C) acts as a hydrogen bonding catalyst for the addition of nitroalkenes to pyrroles and indoles.

The hexameric resorcinarene capsule as an artificial enzyme: ruling the regio and stereochemistry of a 1,3-dipolar cycloaddition between nitrones and unsaturated aldehydes

The hexameric resorcinarene capsule as an artificial enzyme to rule the regio and stereochemistry of a 1,3-dipolar cycloaddition.

A Simple Tetraminocalix[4]arene as a Highly Efficient Catalyst under "On-Water" Conditions through Hydrophobic Amplification of Weak Hydrogen Bonds

The simple tetraminocalix[4]arene 1, which contains weak H-bond-donor NH₂ groups, is reported to be a highly efficient organocatalyst for the Vinylogous Mukaiyama Aldol Reaction (VMAR) of 2-(trimethylsilyloxy)furan 5 with α-ketoesters 6 a-l under "on-water" conditions owing to the hydrophobic amplification of weak H-bond interactions. The catalytic efficiency of calixarene catalyst 1 was shown to be closely related to its recognition abilities towards the reactants 5 and 6 through a multipoint recognition model. The proposed model provided good explanations for the differences on the reaction rate acceleration and on the stereoselectivity observed with different substrates.

γ-Cyclodextrin as a Catalyst for the Synthesis of 2-Methyl-3,5-diarylisoxazolidines in Water

A green and efficient 1,3-dipolar cycloaddition of nitrones with different styrenes and cinnamates using a catalytic amount of γ-cyclodextrin (γ-CD) in water has been developed to give substituted isoxazolidines. γ-CD was found to be highly efficacious in carrying out this reaction under an eco-friendly environment, affording moderate to excellent yields and, in some cases, excellent diastereomeric excess (up to >95%) at 100 °C in 8-12 h. The catalyst can be easily recuperated and recycled for several times without loss of activity. Water, an eco-friendly reaction medium, has been utilized for the first time, to the best of our knowledge, in this reaction. The credit of the presented protocol includes high yields and catalyst reusability, and precludes the use of organic solvents. The use of in silico calculations allowed us to rationalize the obtained results and to improve the stereoselectivity.

Supramolecular synthons in the gamma-hydroxybutenolides

The supramolecular organization in the solid state of five novel gamma-hydroxybutenolides is described.

Exploiting the hydrophobicity of calixarene macrocycles for catalysis under “on-water” conditions

Thioureido-calixarene derivatives provide a rate acceleration for the vinylogous Mukaiyama aldol reaction (VMAR) under on-water conditions.

Iridium-Catalyzed Enantioselective Allylic Substitution of Enol Silanes from Vinylogous Esters and Amides

The enol silanes of vinylogous esters and amides are classic dienes for Diels-Alder reactions. Here, we report their reactivity as nucleophiles in Ir-catalyzed, enantioselective allylic substitution reactions. A variety of allylic carbonates react with these nucleophiles to give allylated products in good yields with high enantioselectivities and excellent branched-to-linear ratios. These reactions occur with KF or alkoxide as the additive, but mechanistic studies suggest that these additives do not activate the enol silanes. Instead, they serve as bases to promote the cyclometalation to generate the active Ir catalyst. The carbonate anion, which was generated from the oxidative addition of the allylic carbonate, likely activates the enol silanes to trigger their activity as nucleophiles for reactions with the allyliridium electrophile. The synthetic utility of this method was illustrated by the synthesis of the anti-muscarinic drug, fesoterodine.

The mechanism of tautomerisation and geometric isomerisation in thioformic acid and its water complexes: exploring chemical pathways for water migration

A systematic and automated search for chemical pathways of isomerisation between geometric and tautomeric forms of gas-phase thioformic acid (TFA) and its water complexes is performed using a global reaction route mapping (GRRM) method, and an uncovered pathway for cis-trans isomerisation in the thiol form of TFA has been explored through computations performed at CCSD(T)/6-311++G(2d,2p)//B3LYP/6-311++G(2d,2p) level of the coupled cluster and density functional theories. To explore the routes for water migration, a detailed analysis of complexes of TFA with a single-water molecule is presented. Notably, during the isomerisation process in TFA, a positive catalytic effect of water was observed that can arise either by the stabilization of the reactant and/or of the transition state through extensive hydrogen bonding. Interesting behavior of isomeric forms of TFA along the pathways analysed is revealed through the Gibbs free-energy change and its temperature-dependence. The cis form of TFA(thiol) in the complexes of TFA with a single-water molecule is found to be thermodynamically equally feasible as the trans form which though is known to be the most dominating among the isomeric forms of TFA. Besides these, various complexes of TFA with two-water molecules have also been explored to study the hydrogen-bonding interaction through natural bond-orbital (NBO) analysis. The complexes of TFA with two-water molecules have also been characterized using spectral features including vibrational frequency analysis, and the effect of complexation has been observed by noting frequency shift.

Mukaiyama Aldol Reactions in Aqueous Media

Mukaiyama aldol reactions in aqueous media have been surveyed. While the original Mukaiyama aldol reactions entailed stoichiometric use of Lewis acids in organic solvents under strictly anhydrous conditions, Mukaiyama aldol reactions in aqueous media are not only suitable for green sustainable chemistry but are found to produce singular phenomena. These findings led to the discovery of a series of water-compatible Lewis acids such as lanthanide triflates in 1991. Our understanding on these beneficial effects in the presence of water will be deepened through the brilliant examples collected in this review. 1 Introduction 2 Rate Enhancement by Water in the Mukaiyama Aldol Reaction 3 Lewis Acid Catalysis in Aqueous or Organic Solvents 3.1 Water-Compatible Lewis Acids 4 Lewis-Base Catalysis in Aqueous or Organic Solvents 5 The Mukaiyama Aldol Reactions in 100% Water 6 Asymmetric Catalysts in Aqueous Media and Water 7 Conclusions and Perspective.