Sequential Au/Cu Catalysis: A Two Catalyst One-Pot Protocol for the Enantioselective Synthesis of Oxazole α-Hydroxy Esters via Intramolecular Cyclization/Intermolecular Alder-Ene Reaction

Catalytic (4+2) Annulation via Regio- and Enantioselective Interception of in-situ Generated Alkylgold Intermediate

A regio- and stereoselective stepwise (4+2) annulation of N-propargylamides and α,β-unsaturated imines/ketones has been accomplished with synergetic catalysis by a combination of a gold-complex and a chiral quinine-derived squaramide (QN-SQA), leading to highly functionalized chiral tetrahydropyridines/dihydropyrans in good to high yields with generally excellent enantioselectivity. Mechanistic studies and DFT calculations indicate that the in situ formed alkylgold species is the key intermediate in this transformation, and the amide group served as a traceless directing group in this highly selective transformation. This method complements the enantioselective (4+2) annulation of allene reagents, providing the formal internal C-C π-bond cycloaddition products, which is challenging and remains elusive.

Distinction of Plasmonic Intrananoparticle and Internanoparticle Molecular Reaction Rates at the Three-Phase Contact Line of an Evaporating Sessile Droplet

Molecular reactions on the surface of a plasmonic nanoparticle (intrananoparticle) and between nanoparticles (internanoparticle) may differ in kinetics, dynamics, and product selectivity. We report that the difficulty in distinguishing the kinetics in a dispersion medium could be overcome by probing the reactions at the three-phase contact line (TPCL) of an evaporating sessile droplet using surface-enhanced Raman spectroscopy (SERS). Thus, when an evaporating aqueous droplet on glass containing 4-aminothiophenol-stabilized Ag nanoparticles was monitored by SERS at the TPCL, dimerization into 4,4'-dimercaptoazobenzene followed two steps, each preceded by the loss of H-bonded water accordingly. On the basis of the results, we assigned the first step with a higher relative kinetic rate (∼3 times) to be an intrananoparticle reaction and the second one as an internanoparticle reaction. In D2O medium, the ratio of the rates was ∼1.8. The observed vibrational signatures of the losses of water molecules before reactions and product formations were accounted for by using density functional theoretical calculations.

Synthesis and Antiproliferative Evaluation of d-Glucuronamide-Based Nucleosides and (Triazolyl)methyl Amide-Linked Pseudodisaccharide Nucleosides

The synthesis and antiproliferative evaluation of novel d-glucopyranuronamide-containing nucleosides is described. Based on our previously reported anticancer d-glucuronamide-based nucleosides, new analogues comprising N/O-dodecyl or N-propargyl substituents at the glucuronamide unit and anomerically-N-linked 2-acetamido-6-chloropurine, 6-chloropurine or 4-(6-chloropurinyl)methyl triazole motifs were synthesized in 4-6 steps starting from acetonide-protected glucofuranurono-6,3-lactone. The methodologies were based on the access to N-substituted glycopyranuronamide precursors, namely 1,2-O-acetyl derivatives or glucuronoamidyl azides for further nucleobase N-glycosylation or 1,3-dipolar cycloaddition with N9 - and N7 -propargyl-6-chloropurines, respectively. N-Propargyl glucuronamide-based N9 -purine nucleosides were converted into (triazolyl)methyl amide-6,6-linked pseudodisaccharide nucleosides via cycloaddition with methyl 6-azido-glucopyranoside. A CuI/Amberlyst A-21 catalytic system employed in the cycloaddition reactions also effected conversion into 6-dimethylaminopurine nucleosides. Antiproliferative evaluation in chronic myeloid leukemia (K562) and breast cancer (MCF-7) cells revealed significant effects exhibited by the synthesized monododecylated purine-containing nucleosides. A N-propargyl 3-O-dodecyl glucuronamide derivative comprising a N9 -β-linked 6-chloropurine moiety was the most active compound against MCF-7 cells (GI50 =11.9 μM) while a related α-(purinyl)methyltriazole nucleoside comprising a N7 -linked 6-chloropurine moiety exhibited the highest activity against K562 cells (GI50 =8.0 μM). Flow cytometry and immunoblotting analysis of apoptosis-related proteins in K562 cells treated with the N-propargyl 3-O-dodecyl glucuronamide-based N9 -linked 6-chloropurine nucleoside indicated that it acts via apoptosis induction.

Bimetallic tandem catalysis-enabled enantioselective cycloisomerization/carbonyl-ene reaction for construction of 5-oxazoylmethyl α-silyl alcohol

A bimetallic tandem catalysis-enabled enantioselective cycloisomerization/carbonyl-ene reaction was developed. The reaction proceeded well with a broad range of N-propargylamides and acylsilanes, affording the target chiral 5-oxazoylmethyl α-silyl alcohols in up to 95% yield and 99% ee under mild conditions. Importantly, this facile protocol was available for the late-stage modification of several bioactive molecules. Based on the mechanistic study and control experiments, a possible catalytic cycle and transition state are proposed to elucidate the reaction process and enantioinduction.

Gold-based enantioselective bimetallic catalysis

Multimetallic catalysis is a powerful strategy to access complex molecular scaffolds efficiently from easily available starting materials. Numerous reports in the literature have demonstrated the effectiveness of this approach, particularly for capitalizing on enantioselective transformations. Interestingly, gold joined the race of transition metals very late making its use in multimetallic catalysis unthinkable. Recent literature revealed that there is an urgent need to develop gold-based multicatalytic systems based on the combination of gold with other metals for enabling enantioselective transformations that are not possible to capitalize with the use of a single catalyst alone. This review article highlights the progress made in the field of enantioselective gold-based bimetallic catalysis highlighting the power of multicatalysis for accessing new reactivities and selectivities which are beyond the reach of individual catalysts.

Formal nucleophilic pyrrolylmethylation via palladium-based auto-tandem catalysis: switchable regiodivergent synthesis and remote chirality transfer

Although nucleophilic benzylation-type reaction to introduce various aromatic systems into molecules has been widely explored, the related pyrrolylmethylation version remains to be disclosed. Reported herein is a palladium-catalysed multiple auto-tandem reaction between N-Ts propargylamines, allyl carbonates and aldimines in the presence of an acid, proceeding through sequential allylic amination, cycloisomerisation, vinylogous addition and aromatisation steps. A diversity of formal pyrrolylmethylated amine products were finally furnished efficiently. In addition, switchable regiodivergent 3-pyrrolylmethylation and 4-pyrrolylmethylation were realised by tuning catalytic conditions. Moreover, remote chirality transfer with readily available enantioenriched starting materials was well achieved with an achiral ligand, relying on diastereoselective generation of η2-Pd(0) complexes between Pd(0) and chiral 1,3-diene intermediates in the key vinylogous addition step. A few control experiments were conducted to elucidate the palladium-involved tandem reaction and regiodivergent synthesis.

Zn(OTf)2-Promoted Isocyanide-Based Three-Component Reaction: Direct Access to 2-Oxazolines and β-Amino Amides

An efficient one-pot reaction of propargylamides, isocyanides, and water catalyzed by zinc was developed for the rapid construction of 2-oxazolines with a wide functional group tolerance. The methylene-3-oxazoline was proven to play a vitally important role to start the tandem cascade transformation through unfunctionalized alkynes with sequential nucleophilic addition approaches of isocyanide and water. Notably, with a slight alteration of the reaction temperature and the addition of one molecule of water, various β-amino amide derivatives were synthesized in good to excellent yields.

Asymmetric Formal Nucleophilic o-Cresolylation with Morita-Baylis-Hillman Carbonates of 2-Cyclohexenones via Palladium Catalysis

Here we report an asymmetric formal nucleophilic o-cresolylation reaction with the Morita-Baylis-Hillman (MBH) carbonates from 2-cyclohexanones and diverse aldehydes under palladium catalysis, by in situ generation of electron-neutral and HOMO-raised η²-Pd(0)-dienone complexes via an oxidative insertion/π-σ-isomerization/β-H elimination activation sequence. The subsequent umpolung vinylogous addition to a variety of imines is realized upon Pd(0)-mediated π-Lewis base catalysis, finally furnishing o-cresolylated products followed by another cascade of a π-σ-isomerization/β-H elimination/aromatization process. Moderate to excellent diastereo- and enantioselectivity are achieved for substantial substrate assemblies by employing a newly designed bulky chiral phosphonamidite ligand, and the resultant multifunctional products can be facilely elaborated to access diverse enantioenriched architectures. In addition, the catalytic reaction pathway is finely illuminated by control experiments.

Enantioselective Oxidative Multi-Functionalization of Terminal Alkynes with Nitrones and Alcohols for Expeditious Assembly of Chiral α-Alkoxy-β-amino-ketones

Catalytic oxidative functionalization of alkynes has emerged as an effective method in synthetic chemistry in recent decades. However, enantioselective transformations via metal carbene intermediates are quite rare due to the lack of robust chiral catalysts, especially in the intermolecular versions. Herein, we report the first asymmetric three-component reaction of commercially available alkynes with nitrones and alcohols, which affords α-alkoxy-β-amino-ketones in good yields with high to excellent enantioselectivity using combined catalysis by an achiral gold complex and a chiral spiro phosphoric acid (CPA). Mechanistically, this atom-economic reaction involves a catalytic alkyne oxidation/ylide formation/Mannich-type addition sequence that uses nitrone as the oxidant and the leaving fragment imine as the electrophile, providing a novel method for multi-functionalization of commercially available terminal alkynes.

Iron-Catalyzed Cycloaddition of Amides and 2,3-Diaryl-2H-azirines To Access Oxazoles via C-N Bond Cleavage

A novel and efficient iron-catalyzed cycloaddition reaction using readily available 2,3-diaryl-2H-azirines and primary amides is reported. A wide range of trisubstituted oxazoles could be achieved in good yields with good functional group compatibility. In this transformation, two C-N bonds were cleaed and new C-N and C-O bonds were formed.

Enantiomeric Separation of New Chiral Azole Compounds

Twelve new azole compounds were synthesized through an ene reaction involving methylidene heterocycles and phenylmaleimide, producing four oxazoles, five thiazoles, and one pyridine derivative, and ethyl glyoxylate for an oxazole and a thiazole compound. The twelve azoles have a stereogenic center in their structure. Hence, a method to separate the enantiomeric pairs, must be provided if any further study of chemical and pharmacological importance of these compounds is to be accomplished. Six chiral stationary phases were assayed: four were based on macrocyclic glycopeptide selectors and two on linear carbohydrates, i.e., derivatized maltodextrin and amylose. The enantiomers of the entire set of new chiral azole compounds were separated using three different mobile phase elution modes: normal phase, polar organic, and reversed phase. The most effective chiral stationary phase was the MaltoShell column, which was able to separate ten of the twelve compounds in one elution mode or another. Structural similarities in the newly synthesized oxazoles provided some insights into possible chiral recognition mechanisms.

InCl3-catalyzed 5-exo-dig cyclization/1,6-conjugate addition of N-propargylamides with p-QMs to construct oxazole derivatives

An InCl₃-catalyzed tandem intramolecular 5-exo-dig cyclization/1,6-conjugate addition/aromatization of N-propargylamides with p-QMs to produce oxazoles tethering diarylmethane has been successfully developed.