Investigation of the Pyridinium Ylide--Alkyne Cycloaddition as a Fluorogenic Coupling Reaction

Color-Tunable Indolizine-Based Fluorophores and Fluorescent pH Sensor

A new fluorescent indolizine-based scaffold was developed using a straightforward synthetic scheme starting from a pyrrole ring. In this fluorescent system, an N,N-dimethylamino group in the aryl ring at the C-3 position of indolizine acted as an electron donor and played a crucial role in inducing a red shift in the emission wavelength based on the ICT process. Moreover, various electron-withdrawing groups, such as acetyl and aldehyde, were introduced at the C-7 position of indolizine, to tune and promote the red shift of the emission wavelength, resulting in a color range from blue to orange (462-580 nm). Furthermore, the ICT effect in indolizine fluorophores allowed the design and development of new fluorescent pH sensors of great potential in the field of fluorescence bioimaging and sensors.

Single-Step Approach toward Nitrones via Pyridinium Ylides: The DMAP-Catalyzed Reaction of Benzyl Halides with Nitrosoarenes

A single-step approach is reported for the preparation of nitrones from benzyl halides and nitrosoarenes via pyridinium ylides, utilizing 4-dimethylaminopyridine (DMAP) catalyst and mild reaction conditions (Li₂CO₃, dimethylacetamide, and room temperature). The reaction provides both keto- and aldonitrones in high yields with a wide scope for benzyl halides and nitrosoarenes. In the same reaction system, 2-methyl-2-nitrosopropane, which does not have an aryl group, also affords the corresponding N-tert-butyl nitrones from primary benzyl bromides that have an electron-withdrawing group. As an application of the reaction, methyl 2-bromo-2-phenylacetate was used to prepare the corresponding isoxazolidine by a sequential one-pot synthesis.

Heterocyclization Reagents for Rapid Assembly of N-Fused Heteroarenes from Alkenes

N-Fused heterocycles are of particular use and upmost importance in multiple fields. Herein, we disclose a conceptually new approach for the rapid assembly of N-fused heteroarenes from alkenes. A portfolio of strategically designed heterocyclization reagents are readily prepared for the cascade reaction. A plethora of N-fused heteroarenes including seven types of heterocyclic core are furnished. The protocol features a broad functional-group compatibility and high product diversity, and provides a practical tool for late-stage heteroarene elaboration.

4-Dimethylaminopyridine-Catalyzed Metal-Free Aerobic Oxidation of Aryl α-Halo Esters to Aryl α-Keto Esters

A novel, metal-free aerobic oxidation method is described. 4-Dimethylaminopyridine (DMAP) successfully catalyzed the oxidation of aryl α-halo esters to corresponding aryl α-keto esters (up to 95% yield) under mild reaction conditions (Li₂CO₃, dimethylacetamide, air, and room temperature). A mechanism has been proposed where the oxidation proceeds through a [3 + 2] cycloaddition between O₂ in an air atmosphere and pyridinium ylides. The ylides are supposedly generated from aryl α-halo esters and DMAP in the presence of carbonates. Based on the plausible mechanism, the potential of DMAP as a catalyst in oxidation reactions was extended.

Microwave-Assisted Synthesis of Fluorescent Pyrido[2,3-b]indolizines from Alkylpyridinium Salts and Enaminones

Pyridinium ylides are well recognized as dipoles for cycloaddition reactions. In its turn, the microwave-assisted interaction of N-(cyanomethyl)-2-alkylpyridinium salts with enaminones unexpectedly proceeds as a domino sequence of cycloisomerization and cyclocondensation reactions, instead of a 1,3-dipolar cycloaddition. The reaction takes place in the presence of sodium acetate as base and employs benign solvents. The optical properties of the resulting pyrido[2,3-b]indolizines were studied, showing green light emission with high fluorescence quantum yields.

Computational and Spectral Means for Characterizing the Intermolecular Interactions in Solutions and for Estimating Excited State Dipole Moment of Solute

The results obtained both in quantum chemical computation and in solvatochromic study of pyridinium di-carbethoxy methylid (PCCM) are correlated in order to estimate the electric dipole moment in the excited state of this molecule. This estimation is made by a variational method in the hypothesis that the molecular polarizability does not change in time of the absorption process. Ternary solutions of PCCM in protic binary solvents are used here, both establishing the contribution of each type of interaction to the spectral shift and to characterize the composition of the first solvation shell of PCCM. Results are compared with those obtained before for other binary solvents. The difference between the interaction energies in molecular pairs of PCCM-active solvent and PCCM-less active solvent was also estimated based on the cell statistical model of the ternary solutions.

Quinoline-galactose hybrids bind selectively with high affinity to a galectin-8 N-terminal domain

Quinolines, indolizines, and coumarins are well known structural elements in many biologically active molecules. In this report, we have developed straightforward methods to incorporate quinoline, indolizine, and coumarin structures into galactoside derivatives under robust reaction conditions for the discovery of glycomimetic inhibitors of the galectin family of proteins that are involved in immunological and tumor-promoting biological processes. Evaluation of the quinoline, indolizine and coumarin-derivatised galactosides as inhibitors of the human galectin-1, 2, 3, 4N (N-terminal domain), 4C (C-terminal domain), 7, 8N, 8C, 9N, and 9C revealed quinoline derivatives that selectively bound galectin-8N, a galectin with key roles in lymphangiogenesis, tumor progression, and autophagy, with up to nearly 60-fold affinity improvements relative to methyl β-d-galactopyranoside. Molecular dynamics simulations proposed an interaction mode in which Arg59 had moved 2.5 Å and in which an inhibitor carboxylate and quinoline nitrogen formed structure-stabilizing water-mediated hydrogen bonds. The compounds were demonstrated to be non-toxic in an MTT assay with several breast cancer cell lines and one normal cell line. The improved affinity, selectivity, and low cytotoxicity suggest that the quinoline-galactoside derivatives provide an attractive starting point for the development of galectin-8N inhibitors potentially interfering with pathological lymphangiogenesis, autophagy, and tumor progression.

Indium- and zinc-catalyzed enantioselective amide propargylation of aldehydes with stannylated allenyl amides

The catalytic enantioselective propargylation of aldehydes with newly prepared stannyl allenyl amides is described. The reaction has been accomplished by using catalytic amounts of indium chloride, zinc chloride, and a chiral BINOL derivative, affording amide-functionalized homopropargyl alcohols in excellent yields and enantioselectivities.

Synthesis of a crystallochromic indolizine dye by a base- and catalyst-free photochemical route

Irradiation of 2-benzoyl-N-benzylpyridinium derivatives and dimethyl acetylene dicarboxylate resulted in the formation of inolizine derivatives, one which was found to be the first example of a crystallochromic indolizine.

Indolizine-Based Scaffolds as Efficient and Versatile Tools: Application to the Synthesis of Biotin-Tagged Antiangiogenic Drugs

We describe the design and optimization of polyfunctional scaffolds based on a fluorescent indolizine core derivatized with various orthogonal groups (amines, esters, oximes, alkynes, etc.). To show one application as tools in biology, the scaffold was used to prepare drug-biotin conjugates that were then immobilized onto avidin-agarose for affinity chromatography. More specifically, the antiangiogenic drug COB223, whose mechanism of action remained unclear, was chosen as a proof-of-concept drug. The drug-selective discrimination of proteins observed after elution of the cell lysates through the affinity columns, functionalized either with the biologically active COB223 or a structurally related inactive analogue (COB236), is a clear indication that the presence of the indolizine core does not limit drug-protein interaction and confirms the usefulness of the indolizine scaffold. Furthermore, the separation of COB223-interacting proteins from human placental extracts unveiled unanticipated protein targets belonging to the family of regulatory RNA-binding proteins, which opens the way to new hypotheses on the mode of action of this antiangiogenic drug.