Stereoselective organic reactions in water

Negative and Positive Confinement Effects in Chiral Separation Chromatography Monitored with Molecular-Scale Precision by In-Situ Electron Paramagnetic Resonance Techniques

Separation of compounds using liquid chromatography is a process of enormous technological importance. This is true in particular for chiral substances, when one enantiomer has the desired set of properties and the other one may be harmful. The degree of development in liquid chromatography is extremely high, but still there is a lack in understanding based on experimental data how selectivity works on a molecular level directly at the surfaces of a porous host material. We have prepared amino-acid containing organosilica as such host materials. Watching the rotational dynamics of chiral spin probes using electron paramagnetic resonance spectroscopy allows us to differentiate between surface adsorbed and free guest species. Diastereotopic selectivity factors were determined, and the influence of chiral surface group density, chemical character of the surface groups, pore-size, and temperature was investigated. We found higher selectivity values in macroporous solids with a rather rigid organosilica network and at lower temperature, indicating the significant effect of confinement effects. In mesoporous materials features are opposed with regards to the T-dependent behavior. From EPR imaging techniques and the resulting (macroscopic) diffusion coefficients, we could confirm that the correlations found on the microscopic level transform also to the macroscopic behavior. Thus, our study is of value for the development of future chromatography materials by design.

Catalyst-Free One-Pot Three-Component Synthesis of Diversely Substituted 5-Aryl-2-oxo-/thioxo-2,3-dihydro-1H-benzo[6,7]chromeno[2,3-d]pyrimidine-4,6,11(5H)-triones Under Ambient Conditions

A simple, catalyst-free, straightforward, and highly efficient one-pot synthesis of pharmaceutically interesting diverse kind of a new series of functionalized 5-aryl-2-oxo-/thioxo-2,3-dihydro-1H-benzo[6,7]chromeno[2,3-d]pyrimidine-4,6,11(5H)-triones 4 (4-1-4-37) and substituted 5,5'-(1,4-phenylene)bis(2-oxo-/thioxo-2,3-dihydro-1H-benzo[6,7]chromeno[2,3-d]pyrimidine-4,6,11(5H)-trione) derivatives 4' (4'-1-4'-3) has been developed based on a three-component reaction between barbituric acid/N,N-dimethylbarbituric acid/2-thiobarbituric acid (1), aromatic aldehydes (2), and 2-hydroxy-1,4-naphthoquinone (3) in aqueous ethanol at room temperature (25-30 °C). The salient features of this protocol are mild reaction conditions, use of no catalyst, no need of column chromatographic purification, excellent yields, high atom economy, eco-friendliness, easy isolation of products, and reusability of reaction media.

"On-Water" Facile Synthesis of Novel Pyrazolo[3,4-b]pyridinones Possessing Anti-influenza Virus Activity

A facile and versatile "on-water" protocol for the synthesis of pyrazolo[3,4-b]pyridinones was developed by the unprecedented construction of two rings and five new bonds in one-pot. It was proved that water was an important promoter of the reaction and PEG2000 was found to improve the reaction in terms of yield. 32 Derivatives were newly synthesized and most of them were prepared in an hour. The scope and limitation indicated that electron withdrawing groups substituted on synthons, substituted benzoyl acetonitriles or aryl aldehydes, were helpful to construct the pyrazolo[3,4-b]pyridinones. The reaction media PEG2000/H₂O was successfully recycled and reused at least 5 times without any obvious decrease in yield. The anti-influenza activities of the derivatives were evaluated and the screening results highlighted two derivatives, which exhibited strong inhibitory activity against H5N1 pseudovirus. These positive bioassay results implied that the library of potential anti-influenza virus agent candidates could be rapidly prepared in an eco-friendly manner, and provided a new insight into drug discovery for medicinal chemists.

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.

Direct Substitution of Alcohols in Pure Water by Brønsted Acid Catalysis

With the increasing concern for sustainability, the use of environmentally friendly media to perform chemical processes has attracted the attention of many research groups. Among them, the use of water, as the unique solvent for reactions, is currently an active area of research. One process of particular interest is the direct nucleophilic substitution of an alcohol avoiding its preliminary transformation into a good leaving group, since one of the by-products in this approach would be water. The direct substitution of allylic, benzylic, and tertiary alcohols has been achieved through S_N1-type reactions with catalytic amounts of Brønsted or Lewis acids; however, organic solvents are often required. In this review, the pioneering S_N1 approaches performed in pure water and in the absence of a metal based Lewis acid are compiled and discussed.

A Multicomponent Reaction as a Versatile Tool for the Synthesis of Spirooxindoles using N-Alkylisatins; Efficient Catalysis by ZnO Nanoparticles

An efficient and green protocol for the synthesis of functionalised spirooxindole derivatives is described. This involves the reaction of in situ-generated keto-enaminones with N-alkylisatins in the presence of isothiocyanates.

Revisiting the Juliá–Colonna enantioselective epoxidation: supramolecular catalysis in water

With only water as solvent, poly-l-leucine serves as supramolecular catalyst for the epoxidation of α,β-unsaturated ketones.

Palladium-catalyzed tandem addition/cyclization in aqueous medium: synthesis of 2-arylindoles

Pd-catalyzed tandem reaction of potassium aryltrifluoroborates with nitriles for the synthesis of 2-arylindoles in aqueous medium.

Room-temperature Cu-catalyzed N-arylation of aliphatic amines in neat water

A room-temperature copper-catalyzed N-arylation of aliphatic amines with great selectivity and substrate scope tolerance in neat water has been developed.

Cp*Rh(iii)-catalyzed C(sp3)–H alkylation of 8-methylquinolines in aqueous media

A mild and efficient approach for the cross-coupling reaction of 8-methylquinolines with a range of allylic alcohols in water as a solvent under rhodium(iii) catalysis is described.

An efficient Friedel–Crafts alkylation for the synthesis of 3-indolyl-3-hydroxy oxindoles and unsymmetrical 3,3-diaryl oxindoles catalyzed by Dabco-based ionic liquids in water

An efficient, green and reusable catalytic system for controlled 3-indolylation of isatins.

Application of a 2-aryl indenylphosphine ligand in the Buchwald–Hartwig cross-coupling reactions of aryl and heteroaryl chlorides under the solvent-free and aqueous conditions

An efficient solvent-free and aqueous protocol for the Buchwald–Hartwig cross-coupling reaction has been developed. Notably, the catalytic system also efficiently catalyzed the reaction under aqueous conditions.