Riboflavin Crystals with Extremely High Water Solubility.

New insights into the unique biochemical properties of riboflavin (Rf), also known as vitamin B2, are leading to the development of its use not only as a vitamin supplement but also as a potential anti-inflammatory, immunomodulatory, antioxidant, anticancer, and antiviral agent, where it may play a role as an inhibitor of viral proteinases. At the same time, the comparison of the pharmacoactivity of Rf with its known metabolites, namely, flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), is very complicated due to its poor water solubility: 0.1-0.3 g/L versus 67 g/L for FMN and 50 g/L for FAD, which is the limiting factor for its administration in clinical practice. In this study, we report the recrystallization procedure of the type A Rf crystals into the slightly hydrophobic type B/C and a new hydrophilic crystal form that has been termed the P type. Our method of Rf crystal modification based on recrystallization from dilute alkaline solution provides an unprecedented extremely high water solubility of Rf, reaching 23.5 g/L. A comprehensive study of the physicochemical properties of type P riboflavin showed increased photodynamic therapeutic activity compared to the known types A and B/C against clinical isolates of Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli, and Salmonella typhimurium. Importantly, our work not only demonstrates a simple and inexpensive method for the synthesis of riboflavin with high solubility, which should lead to increased bioactivity, but also opens up opportunities for improving both known and new therapeutic applications of vitamin B2.

Multifunctional fluorescent diarylethene: time-resolved study of photochemistry.

A study of luminescence and photochromic properties of (E)-2,3-bis(2,5-dimethylthiophen-3-yl)-5-(4-(pyrrolidin-1-yl)benzylidene)cyclopent-2-en-1-one, which is a diarylethene with a push-pull system between carbonyl and dimethylamino groups, was performed using time-resolved methods. The intramolecular charge transfer (ICT) process as well as 6π-electrocyclization and E-/Z-isomerization contribute to the complex light-induced properties of this molecule. Formation of unexpected short-lived intermediates was detected in the time range from 100 fs to 100 µs. A model based on two processes (additional photocyclization and interconversion between conformers) was proposed to rationalize this result. The key intermediates existing in the picosecond time domain are so-called precursors, which are proposed for both parallel (p) and anti-parallel (ap) isomers of the open form. In general, fast light-induced processes for the fluorescent diarylcyclopentenones are much more complicated than for the parent cyclopentenone-based DAE.

Novel pentacyclic derivatives and benzylidenes of the progesterone series cause anti-estrogenic and antiproliferative effects and induce apoptosis in breast cancer cells.

The promising antitumor effects of progesterone derivatives have been identified in many studies. However, the specific mechanism of action of this class of compounds has not been fully described. Therefore, in this study, we investigated the antiproliferative and (anti)estrogenic activities of novel pentacyclic derivatives and benzylidenes of the progesterone series. The antiproliferative effects of the compounds were evaluated on hormone-dependent MCF7 breast cancer cells using the MTT test. Estrogen receptor α (ERα) activity was assessed by a luciferase-based reporter assay. Immunoblotting was used to evaluate the expression of signaling proteins. All benzylidenes demonstrated inhibitory effects with IC50 values below 10 µM, whereas pentacyclic derivatives were less active. These patterns may be associated with the lability of the geometry of benzylidene molecules, which contributes to an increase in the affinity of interaction with the receptor. The selected compounds showed significant anti-estrogenic potency. Benzylidene 1d ((8 S,9 S,10R,13 S,14 S,17 S)-17-[(2E)-3-(4-fluorophenyl)prop-2-enoyl]-10,13-dimethyl-1,2,6,7,8,9,11,12,14,15-decahydrocyclopenta[a]phenanthren-3-one) was the most active in antiproliferative and anti-estrogenic assays. Apoptosis induced by compound 1d was accompanied by decreases in CDK4, ERα, and Cyclin D1 expression. Compounds 1d and 3d were characterized by high inhibitory potency against resistant breast cancer cells. Apoptosis induced by the leader compounds was confirmed by PARP cleavage and flow cytometry analysis. Compound 3d caused cell arrest in the G2/M phase. Further analysis of novel derivatives of the progesterone series is of great importance for medicinal chemistry, drug design, and oncology.

Divergent Reactivity of Triaryldivinyl Ketones: Competing 4π and Putative 6π Electrocyclization Pathways.

This work describes an acid-promoted cyclization of triaryldivinyl ketones containing a thiophene moiety in the α-position. Two cyclization pathways are accessible: one a 4π-Nazarov cyclization and the other we propose to proceed through a 6π electrocyclic mechanism. The relative proportion of products from these divergent pathways is affected by reaction conditions and steric bulk in the substrate. We present experimental and computational evidence that when using HCl in dioxane, the 4π-conrotatory electrocyclization is more favorable, whereas GaCl3 in methylene chloride shifts the chemoselectivity toward a putative 6π-disrotatory electrocyclization. DFT calculations suggest that a complex interplay between kinetic and thermodynamic factors is implicated in the chemodivergent behavior.

Photostability and Antiproliferative Activity of Furan Analogues of Combretastatin A-4.

Cancer is one of the most serious health problems that usually require heavy medical treatment. It is important to ensure that no additional burden is placed on patients due to the modes of administration and/or poor quality of pharmaceuticals. In this regard, understanding, quantifying, and improving the photostability (resistance to UV light or sunlight) of drugs is among the important elements that can improve the patient's quality of life. In this work, the photochemical properties of a wide range of furanone analogues of combretastatin A-4 and their antiproliferative activity against A-431 epidermoid carcinoma cells were studied in a search for compounds with improved photostability and antiproliferative activity. It was found that the incorporation of an arylidene moiety led to a significant improvement in photostability, while the antiproliferative activity strongly depends on the nature of the aryl residue in the arylidene moiety. The high photostability of arylidenes was achieved due to the delocalization of the central double bond of the 1,3,5-hexatriene system, which limited the 6π-electrocyclization. The best results in terms of antiproliferative activity were obtained for thiophene arylidene (IC50 = 0.6 µM) and 3,4-diarylfuran (IC50 = 0.047 µM). The obtained results address the lack of data available now in scientific literature on the photodegradation of combretastatin A-4 analogues and should be taken into account in studies of the side effects of pharmaceuticals based on them.

Photoinduced Skeletal Rearrangement of Diarylethenes: Photorelease of Lewis Acid and Synthetic Applications.

The skeletal photorearrangement including 6π-electrocyclization induced by UV light of ortho-halogen-substituted diarylethenes has been studied. It has been found that the reaction pathways leading to bi- or tricyclic frameworks depend on the kind of halogen substituent and solvent. Photocyclization with halogen abstraction leads to bicyclic fused aromatics, while the tricyclic frameworks are formed due to the tandem 6π-electrocyclization/sigmatropic shift reaction. THF is preferred as the solvent in the former process and chloroform in the latter reaction. It was found for the first time that, owing to the ability of this series of diarylethenes to undergo skeletal photorearrangement with the release of the bromide cation, they can be used both as brominating agents and as Lewis acids for catalyzing electrophilic reactions.

Photocyclization of Diarylethenes: The Effect of Electron and Proton Acceptors as Additives.

The effect of electron and proton acceptors on the photocyclization of diarylethenes has been studied. Without any additives, the deprotonation reaction is predominant, although other processes, including the sigmatropic shift, are not excluded. A deuterium exchange experiment has shown that a strong base (DABCO) facilitates the deprotonation reaction, thereby limiting the sigmatropic shift. In the presence of an oxidizing agent or additional sources of radicals (O2, I2, TEMPO), the processes of deprotonation and rearrangement (H-shift) are practically not observed, and the reaction proceeds along a radical pathway with the formation of phenanthrene or its heterocyclic analogue.

Photocontrollable Modulation of Frontier Molecular Orbital Energy Levels of Cyclopentenone-Based Diarylethenes.

Photoswitchable diarylethenes provide a unique opportunity to optically modulate frontier molecular orbital energy levels, thereby opening an avenue for the design of electronic devices such as photocontrollable organic field-effect transistors (OFETs). In the present work, the absolute position of the frontier orbital levels of nonsymmetrical diarylethenes based on a cyclopentenone bridge has been studied using cyclic voltammetry and density functional theory (DFT) calculations. It has been shown that varying heteroaromatic substituents make it possible to change the absolute positions of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of both diarylethene photoisomers. The data obtained are used to refine the operation mechanism of the previously developed OFET devices, employing the cyclopentenone-derived diarylethenes at the dielectric/semiconductor interface.

Novel d-Annulated Pentacyclic Steroids: Regioselective Synthesis and Biological Evaluation in Breast Cancer Cells.

The acid-catalyzed cyclization of benzylidenes based on 16-dehydropregnenolone acetate (16-DPA) was studied. It was found that these compounds readily undergo regioselective interrupted Nazarov cyclization with trapping chloride ion and an efficient method of the synthesis of d-annulated pentacyclic steroids based on this reaction was proposed. The structures of the synthesized pentacyclic steroids were determined by NMR and X-ray diffraction. It was found that the reaction affords a single diastereomer, but the latter can crystallize as two conformers depending on the structure. Antiproliferative activity of synthesized compounds was evaluated against two breast cancer cell lines: MCF-7 and MDA-MB-231. All tested compounds showed relatively high antiproliferative activity. The synthetic potential of the protocol developed was illustrated by the gram-scale experiment.

Fe(iii)-Catalyzed synthesis of steroidal imidazoheterocycles as potent antiproliferative agents.

An efficient and practical method has been developed for the synthesis of steroidal imidazoheterocycles via cost-effective and environmentally benign FeCl3-catalyzed oxidative amination. A library of steroidal imidazo[1,2-a]pyridines was directly synthesized from readily available 2-aminopyridines and steroidal ketones in aerobic conditions. The synthesized compounds were screened for activity on human microsomal cytochrome P450s CYP7, CYP17 and CYP21. Antiproliferative activity of two lead compounds 3ia and 3la was additionally evaluated against the human MCF-7 (breast cancer), SKOV3 (ovarian cancer), and 22Rv1 (prostate cancer) cell lines. Steroidal imidazo[1,2-a]pyridine 3la which is a substrate molecule for CYP17A1 with IC50 = 1.7 µM (MCF-7), 3.0 (SKOV3), and 6.0 µM (22Rv1) has proved to be more active than reference drug cisplatin.

Light-Sensitive Material Structure-Electrical Performance Relationship for Optical Memory Transistors Incorporating Photochromic Dihetarylethenes.

Photoswitchable organic field-effect transistors (OFETs) with embedded photochromic materials are considered as a promising platform for development of organic optical memory devices. Unfortunately, the operational mechanism of these devices and guidelines for selection of light-sensitive materials are still poorly explored. In the present work, a series of photochromic dihetarylethenes with a cyclopentenone bridge moiety were investigated as a dielectric/semiconductor interlayer in the structure of photoswitchable OFETs. It was shown that the electrical performance and stability of the devices can be tuned by variation of the substituents in the structure of the photochromic material. In particular, it was found that dihetarylethenes with donor substituents demonstrated the best light-induced switching effects (wider memory windows and higher switching coefficients) in the devices. The operation mechanism of the light-triggered memory devices was proposed based on the differential in situ Fourier transform infrared (FTIR) spectroscopy data and regression analysis of the threshold voltage-programming time experimental dependencies. The established relationships will facilitate further rational design of new photochromic materials, thus paving a way to fast and durable organic optical memories and memory transistors (memristors).

Mechanism of photochromic transformations and photodegradation of an asymmetrical 2,3-diarylcyclopentenone.

A mechanistic study of the photochromic properties and photodegradation processes of an asymmetrical diarylcyclopentenone bearing thiophene and benzothiophene units using stationary photolysis, nanosecond laser flash photolysis and time-resolved luminescence was performed. It was found that the light-induced reversible isomerization of (3-(2,5-dimethyltiophen-3-il)-2-(2-methyl-1-benzylthiophen-3-il)cyclopent-2-en-1-one, compound 1) from open to closed form is a common photochromic transformation inherent to diarylethenes, while the photodegradation process proceeds in two ways. The first is a formal 1,2-dyotropic rearrangement, proceeding without the participation of oxygen. The second is the oxygen-dependent mechanism involving the excitation of the open form 1A into the triplet state, quenching of the latter by dissolved oxygen, and oxidation of the initial compound by singlet oxygen.

Reversible Shifting of a Chemical Equilibrium by Light: The Case of Keto-Enol Tautomerism of a ß-Ketoester.

Manipulating the equilibrium between a ketone and an enol by light opens up ample opportunities in material chemistry and photopharmacology. By incorporating ß-ketoester into the ethene bridge of a photoactive diarylethene, we achieved reversible light-induced tautomerization to give thermally stable enol. In a pristine state, the tautomeric equilibrium is almost completely shifted toward the ketone. Photocyclization of diarylethene results in a new equilibrium containing a significant fraction of the enol tautomer.

Post-Modification of the Ethene Bridge in the Rational Design of Photochromic Diarylethenes.

Fine-tuning of the molecular structure of organic bistable compounds to improve their photochromic performance or to introduce additional functions remains an important issue in the development of photoresponsive materials. Diarylethenes bearing heterocyclic moieties belong to the most intensively studied class of organic photochromes due to their excellent photochemical properties. A huge number of diarylethenes have been synthesized so far. Analysis of the literature data shows that there are very worthy examples of diarylethenes developed by the Irie and Feringa groups, which can be the common starting material for a number of diarylethenes functionalized in hetaryl moieties. We refer to these structures as photochromic diarylethene precursors. These diarylethenes have proved to be very useful in the construction of functional molecules with desired properties. On the other hand, in our groups, we have elaborated on diarylethene precursors with modifiable ethene bridges. In this review, we have collected examples of such structures and their chemical modifications, leading to the improvement or fine-tuning of photochromic switching.

Photoswitching off the Antiproliferative Activity of Combretastatin A-4 Analogues.

The photostability and antiproliferative activity of combretastatin A-4 (CA-4) analogues against human epidermoid carcinoma cells A-431 were studied. For the first time, it was shown that UV or sunlight irradiation of furanone analogues of CA-4 results in a photorearrangement giving products with relatively low antiproliferative activity. The observed ability of this series CA-4 to the photodegradation can be used for the design of a new class of drug candidates with high selectivity to cancer cells.

Photochromic diarylethene ligands featuring 2-(imidazol-2-yl)pyridine coordination site and their iron(II) complexes.

A new family of photochromic diarylethene-based ligands bearing a 2-(imidazol-2-yl)pyridine coordination unit has been developed. Four members of the new family have been synthesized. The photoactive ligands feature non-aromatic ethene bridges (cyclopentene, cyclopentenone, and cyclohexenone), as well as closely spaced photoactive and metal coordination sites aiming a strong impact of photocyclization on the electronic structure of the coordinated metal ion. The ligands with cyclopentenone and cyclohexenone bridges show good cycloreversion quantum yields of 0.20-0.32. The thermal stability of closed-ring isomers reveals half-lives of up to 20 days in solution at room temperature. The ligands were used to explore coordination chemistry with iron(II) targeting photoswitchable spin-crossover complexes. Unexpectedly, dinuclear and tetranuclear iron(II) complexes were obtained, which were thoroughly characterized by X-ray crystallography, magnetic measurements, and Mössbauer spectroscopy. The formation of multinuclear complexes is facilitated by two coordination sites of the diarylethene, acting as a bridging ligand. The bridging nature of the diarylethene in the complexes prevents photocyclization.

Novel steroidal 1,3,4-thiadiazines: Synthesis and biological evaluation in androgen receptor-positive prostate cancer 22Rv1 cells.

A flexible approach to previously unknown spirofused and linked 1,3,4-thiadiazine derivatives of steroids with selective control of heterocyclization patterns is disclosed. (N-Arylcarbamoyl)spiroandrostene-17,6' [1,3,4]thiadiazines and (N-arylcarbamoyl)17-[1',3',4']thiadiazine-substituted androstenes, novel types of heterosteroids, were prepared from 16ß,17ß-epoxypregnenolone and 21-bromopregna-5,16-dien-20-one in good to high yields by the treatment with oxamic acid thiohydrazides. The synthesized compounds were screened for antiproliferative activity against the human androgen receptor-positive prostate cancer cell line 22Rv1. Most of (N-arylcarbamoyl)17-[1',3',4']thiadiazine-substituted androstenes exhibit better antiproliferative potency (IC50 = 2.1-6.6 µM) than the antiandrogen bicalutamide. Compounds 7d with IC50 = 3.0 µM and 7j with IC50 = 2.1 µM proved to be the most active in the series under study. Lead synthesized compound 7j downregulates AR expression and activity in 22Rv1 cells. NF-κB activity is also blocked in 7j-treated 22Rv1 cells. Apoptosis is considered as a possible mechanism of 7j-induced cell death.

Photorearrangement of dihetarylethenes as a tool for the benzannulation of heterocycles.

A general strategy for the preparative benzannulation of aromatic heterocycles via photocyclization of 1,2-dihetarylethenes was proposed for the first time. The strategy includes two steps, namely, modular assembly of dihetarylethenes from widely available 3-hetarylacetic acids and 2-bromo-1-hetarylethanones, and subsequent preparative photorearrangement (using a UV lamp at 365 nm as the light source). This approach is efficient for the annulation of a wide range of heterocycles and provides C-, N-, O- or S-substituents in the benzoheterocycles obtained. The photochemical step is a metal-, acid-, and oxidant-free reaction, which requires non-inert conditions, and can be easily monitored by NMR spectroscopy. Applicability of the proposed strategy was tested in the synthesis of a wide range of substituted carbazoles and benzo[b]thiophenes as well as on a gram-scale benzannulation of 3-indoleacetic acid. Our study disclosed how to overcome two notable obstacles to the successful photorearrangement of dihetarylethenes: undesired reactions associated with photogenerated singlet oxygen, and the instability of desired products. The first problem was successfully solved by the addition of DABCO, while development of an in situ alkylation protocol to trap unstable photoproducts allowed us to overcome the second issue.

Mechanistic Aspects of Photoinduced Rearrangement of 2,3-Diarylcyclopentenone Bearing Benzene and Oxazole Moieties.

Photoinduced rearrangement of diarylethenes to naphthalenes or isoelectronic benzoannulated heterocycles is a novel reaction in preparative organic photochemistry. Recently it was shown that unsymmetrical diarylethenes containing benzene and oxazole derivatives efficiently undergo this transformation leading to amide derivatives of naphthalene. Mechanistic study of skeletal rearrangement for a typical representative of these compounds, namely 3-(5-methyl-2-phenyl-1,3-oxazol-4-yl)-2-phenylcyclopent-2-en-1-one, was performed by stationary and laser flash photolysis as well as density functional theory (DFT) calculations. The mechanism of the rearrangement was found to comprise several thermal stages. Both singlet and triplet states of the initial compound can be transformed to the reaction product, which results in the dependence of the quantum yield vs concentration of dissolved oxygen. Three reactive intermediates were registered in the laser flash photolysis experiment; the predicted structures were in accordance with DFT calculations of the electronic absorption spectra. In addition to the previously proposed mechanism of skeletal rearrangement based on a sigmatropic shift of hydrogen, two new parallel pathways based on formation of a carbanion/carbocation were determined.

Synthesis and evaluation of the antiproliferative activity of benzylidenes of 16-dehydroprogesterone series.

Novel benzylidenes (chalcones) of the 16-dehydroprogesterone series have been characterized and their antitumor activity against two breast cancer cell lines was evaluated. Benzylidenes exhibit significant antiproliferative effect on cells and inhibit cell growth in hormone-dependent MCF-7 and hormone-independent MDA-MB-231 breast cancer cell lines. Compound 3d exhibits the highest activity against two breast cancer cell lines, with the IC50 value of about 2 µM. Compounds 3e,m,n display considerable selectivity for hormone-dependent breast cancer cells, with the IC50 value lower than 6 µM. Moreover, these steroidal benzylidenes regulate ERα signaling and reveal p53-independent mechanism of pro-apoptotic action in MCF-7 cells. The new class of antitumor compounds holds promise as the basis for the design of agents for cancer therapy.