Uridine triphosphate hybrid catalyst for carbon‑carbon bond formation reactions with enhanced enantioselectivity by mercury(II) ions

DNA hybrid catalysts are constructed by embedding active metal species into the chiral scaffolds of DNA, which have been successfully applied to some important aqueous-phase enantioselective transformations. Owing to simple components and inherent chirality, nucleotide hybrid catalysts are emerging in response to soving the unclear locations of catalytic centers and the plausible catalytic mechanisms in DNA-based asymmetric catalysis. However, the tertiary structure of nucleotides lacks tunability, severely impeding further design of nucleotide hybrid catalysts for potential applications. To this end, a design strategy for tunable nucleotide hybrid catalysts is put forward by introducing metal-mediated base pairs. Herein, we found that the formation of uracil‑mercury(II)-uracil (U-Hg2+-U) base pairs could enhance the enantioselectivity in uracil-containing nucleotide-based asymmetric reactions. Compared with uracil triphosphate (UTP) complexing with Cu2+ ions (UTP∙Cu2+), the presence of Hg2+ ions gave rise to an increased enantiomeric excess (ee) of 38 % in Diels-Alder reactions and 22 % ee in Michael reactions. The Hg2+-tuning behaviors of UTP hybrid catalyst have been demonstrated to largely depend on nucleotides, Hg2+ concentrations, metal cofactors, additives and reaction types. Based on ultraviolet-visible, circular dichroism and nuclear magnetic resonance spectroscopic techniques, the chiral enhancement of Hg2+-containing UTP hybrid catalyst is proved to largely depend on the formation of U-Hg2+-U base pairs and the plausible cross-linked structure of UTP-Hg2+-UTP/Cu2+ assembly. This work provides a tunable strategy based on the concept of metal-mediated base pairs, allowing further design of potent oligonucleotide-based catalysts for other enantioselective reactions.

Efficient Silver(I)-Containing I-Motif DNA Hybrid Catalyst for Enantioselective Diels-Alder Reactions

The inherent chiral structures of DNA serve as attractive scaffolds to construct DNA hybrid catalysts for valuable enantioselective transformations. Duplex and G-quadruplex DNA-based enantioselective catalysis has made great progress, yet novel design strategies of DNA hybrid catalysts are highly demanding and atomistic analysis of active centers is still challenging. DNA i-motif structures could be finely tuned by different cytosine-cytosine base pairs, providing a new platform to design DNA catalysts. Herein, we found that a human telomeric i-motif DNA containing cytosine-silver(I)-cytosine (C-Ag+-C) base pairs interacting with Cu(II) ions (i-motif DNA(Ag+)/Cu2+) could catalyze Diels-Alder reactions with full conversions and up to 95 % enantiomeric excess. As characterized by various physicochemical techniques, the presence of Ag+ is proved to replace the protons in hemiprotonated cytosine-cytosine (C : C+) base pairs and stabilize the DNA i-motif to allow the acceptance of Cu(II) ions. The i-motif DNA(Ag+)/Cu2+ catalyst shows about 8-fold rate acceleration compared with DNA and Cu2+. Based on DNA mutation experiments, thermodynamic studies and density function theory calculations, the catalytic center of Cu(II) ion is proposed to be located in a specific loop region via binding to one nitrogen-7 atom of an unpaired adenine and two phosphate-oxygen atoms from nearby deoxythymidine monophosphate and deoxyadenosine monophosphate, respectively.

PyBox-La(OTf)3-Catalyzed Enantioselective Diels-Alder Cycloadditions of 2-Alkenoylpyridines with Cyclopentadiene

The PyBox-La(OTf)3-catalyzed enantioselective Diels-Alder cycloaddition of 2-alk-2-enoylpyridines with cyclopentadiene is realized, producing enantiopure disubstituted norbornenes, which possess four contiguous stereocenters and are biologically relevant structures in up to 92:8 dr and 99:1 er.

Fe-BPsalan Complex-Catalyzed Asymmetric [4 + 2] Cycloaddition of Cyclopentadiene with α,β-Unsaturated Heterocycles

An efficient iron-catalyzed asymmetric [4 + 2] cycloaddition of cyclopentadiene with α,β-unsaturated acyl imidazoles or 2-cinnamoylisoindoline-1,3-dione derivatives was developed to afford the addition products in high yield and selectivity. Interestingly, the absolute structures of the addition products were controlled by the auxiliaries via different coordination modes with the same type of catalyst.

Enantioselective construction of cis-hydroindole scaffolds via an asymmetric inverse-electron-demand Diels-Alder reaction: application to the formal total synthesis of (+)-minovincine

cis-Hydroindole scaffolds widely exist in a large number of natural products, pharmaceuticals, and organocatalysts. Therefore, the development of efficient and enantioselective methods for the construction of cis-hydroindoles is of great interest and importance. Herein, a novel approach for the enantioselective synthesis of cis-hydroindole scaffolds has been realized through a chiral N,N'-dioxide/Mg(OTf)2 complex catalyzed asymmetric inverse-electron-demand Diels-Alder (IEDDA) reaction of 2-pyrones and cyclic enamines. A series of substituted cis-hydroindole derivatives bearing multiple contiguous stereocenters and functional groups were obtained in good to excellent yields and enantioselectivities (up to 99% yield, and 95% ee) under mild reaction conditions. Moreover, the enantioselective formal total synthesis of (+)-minovincine was concisely furnished with high efficiency and stereoselectivity to demonstrate the synthetic potential of this method.

Applications of Nickel(II) Compounds in Organic Synthesis

This review article summarizes the applications of nickel(II) compounds in organic synthesis since 2016. In recent years, the field of nickel(II) catalysis is gaining considerable interest due to readily available, low-cost nickel(II)-compounds and several key properties of nickel. This review article is organized by the reaction type, although some reactions can be placed in multiple sections.

Fe-BPsalan complex catalyzed highly enantioselective Diels–Alder reaction of alkylidene β-ketoesters

A practical Fe-BPsalan-catalyzed asymmetric Diels–Alder reaction of various alkylidene β-ketoesters and dienes was developed to afford estrone analogues in excellent yields, good to high diastereoselectivities and excellent enantioselectivities.

Asymmetric Catalytic Diverse Ring Opening/Cycloadditions of Cyclobutenones with (E)-Alkenyloxindoles and (E)-Dioxopyrrolidines

Highly enantioselective ring-opening/cycloaddition reactions of cyclobutenones were achieved by employing chiral N,N'-dioxide/metal complexes as the catalysts. The Diels-Alder type cycloaddition with (E)-alkenyloxindoles yielded spirocyclohexaneoxindoles with excellent results. Meanwhile, a hetero-Diels-Alder process occurred with (E)-dioxopyrrolidines to afford spiropyrrolidinone-dihydropyranone derivatives.

Copper Catalyzed Diastereo- and Enantioselective 1,4-Addition Michael Reaction of 2,3-Dioxopyrrolidines with Nitroalkanes in Aqueous Media

A good diastereo- and enantioselective 1,4-addition Michael reaction catalyzed by a chiral copper complex was developed in aqueous media. A series of nitro-containing pyrrolidones could be gained in high yields with excellent diastereoselectivities and good ee values by virtue of this developed method. It affords a facile access to construct carbon-carbon bonds with water and air tolerance. Furthermore, the gram scale synthesis was conducted successfully to give rise to the corresponding products.

Cocatalyst-controlled divergent cascade cycloaddition reaction of arylalkynols and dioxopyrrolidienes: access to spiroketals and oxa-bridged eight-membered cyclic ethers

A cocatalyst-controlled divergent cascade cycloaddition reaction was developed for the synthesis of two different complex oxygen-containing heterocyclic compounds from arylalkynols and dioxopyrrolidienes in the presence of Au(i) catalyst.

Squaramide-catalysed asymmetric cascade reactions of 2,3-dioxopyrrolidines with 3-chlorooxindoles

An efficient squaramide-catalysed asymmetric Michael addition/cyclization cascade reaction of 3-chlorooxindoles with 2,3-dioxopyrrolidines produced the corresponding chiral spiro-compounds in high yields with excellent stereoselectivities.

Diastereodivergent construction of bispiro[oxindole-bi-pyrrolidine]s with four consecutive stereocentersviaasymmetric [3 + 2] cycloaddition of 2,3-dioxopyrrolidines using identical catalysts

The diastereodivergent asymmetric [3 + 2] cycloaddition ofN-2,2,2-trifluoroethylisatin ketimines with the identical catalysts was firstly performed, which leads to two diastereomeric bispiro[oxindole-bi-pyrrolidine]s.

Lewis acid catalyzed annulation of spirocyclic donor–acceptor cyclopropanes with exo-heterocyclic olefins: access to highly functionalized bis-spirocyclopentane oxindole frameworks

Oxindole-activated spiro-DAC is used as synthon to prepare complex bis-spirocyclopentane oxindole frameworks via ring-enlargement.

Enantioselective Hetero-Diels-Alder Reaction and the Synthesis of Spiropyrrolidone Derivatives

An efficient enantioselective hetero-Diels-Alder reaction was developed under catalysis of a chiral copper complex. A variety of spiropyrrolidones, which bear a tetra-substituted carbon stereocenter, can be obtained in good yields with excellent enantioselectivities by virtue of this method. Furthermore, a substrate-dependent reaction pathway was proposed on the basis of the isolated intermediates.

Catalytic Asymmetric Inverse-Electron-Demand Hetero-Diels-Alder Reaction of Dioxopyrrolidines with Hetero-Substituted Alkenes

A highly efficient enantioselective inverse-electron-demand hetero-Diels-Alder reaction of dioxopyrrolidines with heterosubstituted alkenes has been realized by using a N, N'-dioxide/Ni(II) complex as the catalyst. A series of chiral bicyclic dihydropyranes were generated in excellent yields, good dr, and excellent ee values. Besides, a catalytic cycle with a possible transition state was proposed.

Halo-Substituted Chalcones and Azachalcones-Inhibited, Lipopolysaccharited-Stimulated, Pro-Inflammatory Responses through the TLR4-Mediated Pathway

A series of B-ring, halo-substituted chalcones and azachalcones were synthesized to evaluate and compare their anti-inflammatory activity. Mouse BALB/c macrophage RAW 264.7 were pre-treated with 10 μg/mL of each compound for one hour before induction of inflammation by lipopolysaccharide (1 μg/mL) for 6 h. Some halo-chalcones and -azachalcones suppressed expression of pro-inflammatory factors toll-like receptor 4 (TLR4), IκB-α, transcription factor p65, interleukine 1β (IL-1β), IL-6, tumor necrosis factor α (TNF-α), and cyclooxygenase 2 (COX-2). The present results showed that the synthetic halo-azachalcones exhibited more significant inhibition than halo-chalcones. Therefore, the nitrogen atom in this series of azachalcones must play a more crucial role than the corresponding C-2 hydroxyl group of chalcones in biological activity. Our findings will lay the background for the future development of anti-inflammatory nutraceuticals.

Highly enantioselective synthesis of fused bicyclic dihydropyranones via low-loading N-heterocyclic carbene organocatalysis

Highly diastereo and enantioselective [4+2] cycloadditions have been achieved between pyrrolidone-derived cyclic enones and α-haloaldehydes under mild conditions. Relying on extremely reactive in-situ generated chiral N-heterocyclic carbenes, this stereoselective annulation proceeds efficiently even on the gram scale with the catalyst loading as low as 0.025 mol% (250 ppm). A variety of cis-substituted bicyclic dihydropyranones can be produced in up to 96% yield with up to >99% ee. In addition, simple, inexpensive linear aldehydes such as n-propanal can be used directly in asymmetric cycloadditions via oxidative N-heterocyclic carbene organocatalysis with low catalyst loading. This method may provide an economical and practical approach for the asymmetric synthesis of medicinally relevant molecules.

Electrostatic self-assembled nanoparticles based on spherical polyelectrolyte brushes for magnetic resonance imaging

Electrostatic self-assemblies based on SPBs and Gd-DTPA-NO-C₄ exhibit perfect relaxometric performance.

l-Phenylalanine potassium catalyzed asymmetric formal [3 + 3] annulation of 2-enoyl-pyridine N-oxides with acetone

A highly enantioselective formal [3 + 3] annulation of 2-enoyl-pyridine N-oxides with acetone was developed.

Regiodivergent condensation of 5-alkoxycarbonyl-1H-pyrrol-2,3-diones with cyclic ketazinones en route to spirocyclic scaffolds

The condensation of 5-alkoxycarbonyl-1H-pyrrolediones with cyclic ketazinones was systematically investigated. It was discovered that the regioselectivity of this reaction can be easily swapped between two alternative directions affording derivatives of partially hydrogenated indole or benzofurane. The control of this regioselectivity is efficiently governed by steric effects at the hydrazone moiety of the ketazinone reagent.

Asymmetric Cycloaddition and Cyclization Reactions Catalyzed by Chiral N,N'-Dioxide-Metal Complexes

Catalytic asymmetric cycloadditions and cascade cyclizations are a major focus for the enantioselective construction of chiral carbo- and heterocycles. A number of chiral Lewis acids and organocatalysts have been designed for such reactions. The development of broadly applicable catalysts bearing novel chiral backbones to meet the demands of various applications is an ongoing challenge. Approximately 10 years ago, we introduced a group of conformationally flexible C₂-symmetric N,N'-dioxide amide compounds, which represent a new class of privileged ligands. The coordination of the four oxygens of a chiral N,N'-dioxide around a central metal generates an octahedral tricyclometalated Lewis acid catalyst that can carry out various enantioselective reactions. In this Account, we summarize our recent studies on asymmetric cycloadditions between various dienophiles and dienes, dipoles and dipolarophiles, and cascade cyclizations catalyzed by chiral N,N'-dioxide-metal complexes. In principle, these unique chiral catalysts lower the LUMO energy of electron-deficient 2π components or heterodienes by coordination with the functional groups via various binding modes. With N-Boc-3-alkenyloxindole and alkylidene malonate as the electron-deficient 2π components, N,N'-dioxide-metal complexes provided excellent catalytic activities and asymmetric inductions for a variety of transformations, including [2 + 1], [3 + 2], [4 + 2], and [8 + 2] cycloadditions. Mechanistically, these substrates could be efficiently activated through bidentate coordination. The strategy was also useful for asymmetric cascade cyclizations to form polycyclic adducts. Monodentate or bidentate coordination of other α,β-unsubstituted carbonyl compounds to metal centers enabled both normal Diels-Alder reactions and inverse-electron-demand hetero-Diels-Alder reactions as well as [2 + 2] additions. Furthermore, hetero-Diels-Alder reactions of aldehydes, ketones, and imines are well-tolerated and afford various heterocycles. This includes allowing the concise synthesis of the antimalarial compound KAE609. Asymmetric Michael/cyclization reactions of bidentate α,β-unsaturated pyrazolamides gave efficient access to the chiral drugs (-)-paroxetine and (R)-thiazesim. The formal [3 + 2] cycloadditions of donor-acceptor epoxides and aziridines enantioselectively gave a series of five-membered oxo- and aza-heterocycles. The reaction of cyclopropane diketones showed unprecedented reactivities and provided a new route for the synthesis of dihydropyrrole and benzimidazole derivatives. General models for the catalytic reactions emerged from knowledge of the absolute configurations of the products of several reactions and X-ray crystal structures of the catalysts. In the field of chirality created by the coordination of an N,N'-dioxide to a metal center, the bonding of one or two reactants establishes a perfect reaction template for generation of the target adducts. Representative examples have been used to demonstrate how the substructures of the ligands and other reaction components affect the stereoselectivity and how metal salts impact the reactivity. These results reveal the importance of tunability and compatibility of the ligands and metal precursors for achieving high stereoinduction and activity.

β2, 2 -Amino Acid N-Carboxyanhydrides Relying on Sequential Enantioselective C(4)-Functionalization of Pyrrolidin-2,3-diones and Regioselective Baeyer-Villiger Oxidation

A catalytic enantioselective entry to β^2, 2 -amino acids enabling their direct coupling with nucleophiles is described. The approach is based upon an effective bifunctional Brønsted base catalyzed construction of a quaternary carbon stereocenter at C₄ position of pyrrolidin-2,3-diones. Subsequent regioselective Baeyer-Villiger oxidation of the resultant adducts gives β^2, 2 -amino acid N-carboxyanhydrides as the reactive species, which can further react with nucleophiles. Following this strategy both, β^2, 2 -amino acid derivatives with different functionalities at the newly created stereocenter, and spirocyclic structures can be efficiently prepared.

Chiral GAP catalysts of phosphonylated imidazolidinones and their applications in asymmetric Diels-Alder and Friedel-Crafts reactions

The design and synthesis of recyclable imidazolidinone catalysts using GAP chemistry/technique was described. Their applications in asymmetric Diels-Alder and Friedel-Crafts reactions with α,β-unsaturated aldehydes resulted in excellent yields and higher enantioselectivities than previous processes. As recyclable small molecular catalysts, phosphonylated imidazolidinones can be recovered and reused for up to three runs without costing significant decrease in catalytic activity.