Revisiting Biginelli-like reactions: solvent effects, mechanisms, biological applications and correction of several literature reports

This study critically reevaluates reported Biginelli-like reactions using a Kamlet-Abboud-Taft-based solvent effect model. Surprisingly, structural misassignments were discovered in certain multicomponent reactions, leading to the identification of pseudo three-component derivatives instead of the expected MCR adducts. Attempts to replicate literature conditions failed, prompting reconsideration of the described MCRs and proposed mechanisms. Electrospray ionization (tandem) mass spectrometry, NMR, melting points, elemental analyses and single-crystal X-ray analysis exposed inaccuracies in reported MCRs and allowed for the proposition of a complete catalytic cycle. Biological investigations using both pure and "contaminated" derivatives revealed distinctive features in assessed bioassays. A new cellular action mechanism was unveiled for a one obtained pseudo three-component adduct, suggesting similarity with the known dihydropyrimidinone Monastrol as Eg5 inhibitors, disrupting mitosis by forming monoastral mitotic spindles. Docking studies and RMSD analyses supported this hypothesis. The findings described herein underscore the necessity for a critical reexamination and potential corrections of structural assignments in several reports. This work emphasizes the significance of rigorous characterization and critical evaluation in synthetic chemistry, urging a careful reassessment of reported synthesis and biological activities associated with these compounds.

The two-pronged approach of heteroatoms and substituents to achieve a synergistic regulation of the ESIPT process in amino 2-(2'-hydroxyphenyl)benzoxazole derivatives

Amino 2-(2'-hydroxyphenyl)benzazole derivatives are a class of molecules with excellent photophysical properties. Most of them can be applied as a fluorescent probe via the excited-state intramolecular proton transfer (ESIPT) process. In this work, we focus on the effects of heteroatoms (O, S) and substituents (acetylacetone, hydrogen) in the derivatives. Using DFT/TDDFT methods with the B3LYP-D3BJ functionals, the absorption and emission peaks are in good agreement with the experimental data. Results of optimized structures, infrared vibrational spectra, and reduced density gradient present the existence of the ESIPT process in the S₁ state in these molecules, it also indirectly shows that the heteroatom S is more than O, and the substituent acetylacetone is more than hydrogen has stronger hydrogen bonds. The proton transfer (PT) potential energy curves (PECs) qualitatively show that it is easier for the heteroatom S to induce ESIPT than that of O. The same for the substituent acetylacetone than that of hydrogen. Under the joint influence of the simultaneous stacking of heteroatom S and acetylacetone substituent, the energy barrier of the PT process can be effectively lowered, realizing a synergistic strategy, which can provide some guidance for the design of fluorescent materials.

Multicomponent synthesis of chromophores – The one-pot approach to functional π-systems

Multicomponent reactions, conducted in a domino, sequential or consecutive fashion, have not only considerably enhanced synthetic efficiency as one-pot methodology, but they have also become an enabling tool for interdisciplinary research. The highly diversity-oriented nature of the synthetic concept allows accessing huge structural and functional space. Already some decades ago this has been recognized for life sciences, in particular, lead finding and exploration in pharma and agricultural chemistry. The quest for novel functional materials has also opened the field for diversity-oriented syntheses of functional π-systems, i.e. dyes for photonic and electronic applications based on their electronic properties. This review summarizes recent developments in MCR syntheses of functional chromophores highlighting syntheses following either the framework forming scaffold approach by establishing connectivity between chromophores or the chromogenic chromophore approach by de novo formation of chromophore of interest. Both approaches warrant rapid access to molecular functional π-systems, i.e. chromophores, fluorophores, and electrophores for various applications.

Interaction Study between ESIPT Fluorescent Lipophile-Based Benzazoles and BSA

In this study, the interactions of ESIPT fluorescent lipophile-based benzazoles with bovine serum albumin (BSA) were studied and their binding affinity was evaluated. In phosphate-buffered saline (PBS) solution these compounds produce absorption maxima in the UV region and a main fluorescence emission with a large Stokes shift in the blue–green regions due to a proton transfer process in the excited state. The interactions of the benzazoles with BSA were studied using UV-Vis absorption and steady-state fluorescence spectroscopy. The observed spectral quenching of BSA indicates that these compounds could bind to BSA through a strong binding affinity afforded by a static quenching mechanism (K_q~10¹² L·mol⁻¹·s⁻¹). The docking simulations indicate that compounds 13 and 16 bind closely to Trp134 in domain I, adopting similar binding poses and interactions. On the other hand, compounds 12, 14, 15, and 17 were bound between domains I and III and did not directly interact with Trp134.

Hybrids of 4-hydroxy derivatives of goniothalamin and piplartine bearing a diester or a 1,2,3-triazole linker as antiproliferative agents

A library of nine hybrids of 4-hydroxygoniothalamin (2), 4-hydroxypiplartine (4), monastrol (5) and oxo-monastrol (6) was prepared via a modular synthetic route with a diester or a 1,2,3-triazole as linkers. The compounds were assayed against a panel of human cancer cell lines, including MCF-7 (breast adenocarcinoma), HeLa (cervical adenocarcinoma), Caco-2 (colorectal adenocarcinoma) and PC3 (prostate adenocarcinoma), as well as against normal breast (MCF10A) and prostate (PNT2) cells. In general, hybrids with an ester linker containing 4-hydroxypiplartine (4) were more potent than the corresponding hybrids with 4-hydroxygoniothalamin (2). On the other hand, compounds presenting the 1,2,3-triazole linker displayed enhanced cytotoxicity and selectivity when compared to their corresponding hybrids with the diester linker. The 4-hydroxypiplartine-based hybrids 12 and 22 displayed high cytotoxicity (IC50 values below 10 μM) against all cancer cells studied, especially in MCF-7 cells with IC50 values of 1.7 ± 0.1 and 1.6 ± 0.9 μM, respectively. Furthermore, the 4-hydroxygoniothalamin-monastrol hybrid (compound 21) and the 4-hydroxypiplartine-oxo-monastrol hybrid (compound 25), both bearing a 1,2,3-triazole linker, displayed high selectivity and potency towards breast cancer cell line (MCF-7 vs. MCF10 cells, selectivity index = 15.8 and 7.1, respectively), while the 4-hydroxypiplartine -4-hydroxymethylgoniothalamin hybrid with a diester linker (compound 33) showed high selectivity towards melanoma cancer cells (selectivity index = 9.6). Antiproliferative and pro-apoptotic potential of compounds 12 and 22 against MCF-7 cancer cells were further investigated. Cell cycle studies revealed increased G2/M population in MCF-7 cultures as well as reduced G0/G1 population compared to the control groups indicating cell cycle arrest in G2/M phase. In addition, the frequency of positive cells for annexin V was higher in treated samples suggesting that compounds 12 and 22 induce apoptosis in estrogen-positive MCF-7 cells.

Fluorescent Benzothiadiazole Derivatives as Fluorescence Imaging Dyes: A Decade of New Generation Probes

The current review describes advances in the use of fluorescent 2,1,3-benzothiadiazole (BTD) derivatives after nearly one decade since the first description of bioimaging experiments using this class of fluorogenic dyes. The review describes the use of BTD-containing fluorophores applied as, inter alia, bioprobes for imaging cell nuclei, mitochondria, lipid droplets, sensors, markers for proteins and related events, biological processes and activities, lysosomes, plasma membranes, multicellular models, and animals. A number of physicochemical and photophysical properties commonly observed for BTD fluorogenic structures are also described.

1,3-Bis(carboxymethyl)imidazolium Chloride as a Sustainable, Recyclable, and Metal-Free Ionic Catalyst for the Biginelli Multicomponent Reaction in Neat Condition

A simple and novel methodology has been developed for the synthesis of 1,3-bis(carboxymethyl)imidazolium chloride [BCMIM][Cl] salt. The ionic salt [BCMIM][Cl] catalyzed the reaction among arylaldehydes; the substituted 1,3-dicarbonyl compounds and urea/thiourea at 80 °C with 5 mol % under neat condition provided the substituted dihydropyrimidin-2(1H)-one/thiones in the synthesis step with yields of up to 96%. In addition, we synthesized the commercially available drug Monastrol by employing this methodology. The new synthesis method employs the benefits of a broad substrate scope, short reaction time, and high atom economy along with low catalyst loading in neat conditions, and is devoid of chromatographic purification. The ionic salt [BCMIM][Cl] was recycled and reused up to six cycles without substantial damage of its catalytic efficiency.

Sulfamic acid pyromellitic diamide-functionalized MCM-41 as a multifunctional hybrid catalyst for melting-assisted solvent-free synthesis of bioactive 3,4-dihydropyrimidin-2-(1H)-ones

This study introduces a practical approach to fabricate a novel hybrid acidic catalyst, namely sulfamic acid pyromellitic diamide-functionalized MCM-41 (MCM-41-APS-PMDA-NHSO₃H). Various techniques such as FTIR, TGA, XRD, BET, FESEM, and EDX were used to confirm its structural characteristics. The efficiency of the new MCM-41-APS-PMDA-NHSO₃H organosilica nanomaterials, as a heterogenous nanocatalyst, was examined in the synthesis of biologically active 3,4-dihydropyrimidin-2-(1H)-one derivatives under solvent-free conditions. It was found that the nanoporous MCM-41-APS-PMDA-NHSO₃H, demonstrating acidic nature and high surface area, can activate all the Biginelli reaction components to afford desired 3,4-dihydropyrimidin-2-(1H)-ones under solvent-free conditions in short reaction time. Furthermore, easy and quick isolation of the new introduced hybrid organosilica from the reaction mixture as well as its reusability with negligible loss of activity in at least five consecutive runs are another advantages of this green protocol.

Substituent effect on ESIPT mechanisms and photophysical properties of HBT derivatives

Substituent effects on excited-state intramolecular proton transfer (ESIPT) and photophysical properties of 2-(2-Hydroxyphenyl) benzothiazole (HBT) derivatives have been theoretically unveiled via the density functional theory (DFT) and time-dependent DFT (TDDFT). The optimized geometrical configurations and normal mode analyses confirm that the proton transfer processes are more reactive in excited state. Through calculating the activation energies and rate constants of ESIPT processes, finding that the processes are increasingly inactive when substituent group changes from -CN, -CO₂Me, -Cl, -Me, -NMe₂ to -NO₂. In addition, the photophysical properties analyses indicate the vertical transition energies are in good agreement with those observed in experiment. Note that all the absorption and emission maxima of enol and keto forms have the significant red-shift. In order to clarify the substituent effect on ESIPT and photophysical properties, we draw the frontier molecular orbitals (FMOs) isosurfaces and calculate the distances of electrons and holes and atomic charges. It follows that the intramolecular charge transfer (ICT) degrees are increasingly enlarged as substituting from -CN, -CO₂Me, -Cl, -Me, -NMe₂ to -NO₂ groups, which not only causes the red-shift of absorption and emission of enol and keto forms, but also affects the charge distribution of proton donor and acceptor, inhibiting the occurrence of ESIPT processes.

Molecular design of amino-type hydrogen-bonding molecules for excited-state intramolecular proton transfer (ESIPT)-based fluorescent probe using the TD-DFT approach

A molecular screening of a new series of NH-type molecules for ESIPT-based fluorescent probes has been carried out using time-dependent density. functional theory.