One-Pot Synthesis of Rotationally Restricted, Conjugatable, BODIPY Derivatives from Phthalides

A BODIPY-picolinium-cation conjugate as a blue-light-responsive caged group

Caged compounds protected with photolabile protecting groups (PPGs) are useful for controlling various biological events with high spatiotemporal resolution. Most of the commonly used PPGs are controlled by ultraviolet light irradiation, but it is desirable to have PPGs controlled by visible light irradiation in order to minimize tissue damage. Here, we describe a boron-dipyrromethene (BODIPY)-picolinium conjugate (BPc group) that functions as a blue-light-controllable PPG. ESR experiments indicate that the photolysis mechanism is based on intramolecular photoinduced electron transfer. We illustrate the applicability of the BPc group to biologically active compounds by employing it firstly to photocontrol release of histamine, and secondly to photocontrol release of a soluble guanylyl cyclase (sGC) activator, GSK2181236A, which induces photovasodilation. The BPc group is expected to be a useful PPG for controlling various biological events with blue light irradiation.

Synthesis of carbohydrate–BODIPY hybrids

Owing to the relevance of fluorescently labeled carbohydrates in the study of biological processes, we have investigated several routes for the preparation of saccharides covalently linked to borondipyrromethene (BODIPY) fluorophores. We have shown that BODIPY dyes can be used as aglycons through synthetic saccharide protocols. In particular, a per-alkylated 8-(2-hydroxy-methylphenyl)-4,4′-dicyano-BODIPY derivative, which withstands glycosylation and protection/deprotection reaction conditions without decomposition, has been used in the stepwise synthesis of two fluorescently labeled trisaccharides. These saccharides displayed high water solubility and a low tendency to (H-)aggregation, a phenomenon that causes loss of photophysical efficiency in BODIPYs. Two additional synthetic strategies toward glyco-BODIPYs have also been described. The first method relies on a Ferrier-type C-glycosylation of the BODIPY core, leading to linker-free carbohydrate–BODIPY hybrids. Secondly, the application of the Nicholas propargylation reaction to 1,3,5,7-tetramethyl BODIPYs provides access to 2,6-dipropargylated BODIPYs that readily undergo CuAAC reactions with azido-containing sugars. From a photophysical standpoint, the BODIPY-labeled saccharides could be used as stable and fluorescent water-soluble chromophores, thereby addressing one of the current challenges in molecular imaging.

Phytochemical Study and Antiglioblastoma Activity Assessment of Plectranthus hadiensis (Forssk.) Schweinf. ex Sprenger var. hadiensis Stems

Glioblastoma (GB) is the most malignant form of primary astrocytoma, accounting for more than 60% of all brain tumors in adults. Nowadays, due to the development of multidrug resistance causing relapses to the current treatments and the development of severe side effects resulting in reduced survival rates, new therapeutic approaches are needed. The genus Plectranthus belongs to the Lamiaceae family and is known to be rich in abietane-type diterpenes, which possess antitumor activity. Specifically, P. hadiensis (Forssk.) Schweinf. ex Sprenger has been documented for the use against brain tumors. Therefore, the aim of this work was to perform the bioguided isolation of compounds from the acetonic extract of P. hadiensis stems and to investigate the in vitro antiglioblastoma activity of the extract and its isolated constituents. After extraction, six fractions were obtained from the acetonic extract of P. hadiensis stems. In a preliminary biological screening, the fractions V and III showed the highest antioxidant and antimicrobial activities. None of the fractions were toxic in the Artemia salina assay. We obtained different abietane-type diterpenes such as 7α-acetoxy-6β-hydroxyroyleanone (Roy) and 6β,7β-dihydroxyroyleanone (DiRoy), which was also in agreement with the HPLC-DAD profile of the extract. Furthermore, the antiproliferative activity was assessed in a glioma tumor cell line panel by the Alamar blue assay. After 48 h treatment, Roy exerted strong antiproliferative/cytotoxic effects against tumor cells with low IC₅₀ values among the different cell lines. Finally, we synthesized a new fluorescence derivative in this study to evaluate the biodistribution of Roy. The uptake of BODIPY-7α-acetoxy-6β-hydroxyroyleanone by GB cells was associated with increased intracellular fluorescence, supporting the antiproliferative effects of Roy. In conclusion, Roy is a promising natural compound that may serve as a lead compound for further derivatization to develop future therapeutic strategies against GB.

Bodipy-carbohydrate systems: synthesis and bio-applications

Luminescent BODIPY-sugar probes have stimulated the attention of researchers for the potential applications of such molecular systems in bio-imaging. The presence of carbohydrate units confers unique structural and biological features, beside enhancement of water solubility and polarity. On the other hand, BODIPY (BOronDiPYrromethene) derivatives represent eclectic and functional luminescent molecules because of their outstanding photophysical properties. This article provides a review on the synthesis and applications of BODIPY-linked glycosyl probes in which the labelling of complex carbohydrates with BODIPY allowed the disclosing of their in vivo behaviour or where the sugar constitutes a recognition element for specific targeting probes, or, finally, in which the stereochemical characteristics of the carbohydrate hydroxyl groups play as structural elements for assembling more than one photoactive subunit, resulting in functional supramolecular molecules with modulable properties. We describe the methods we have used to construct various multiBODIPY molecular systems capable of functioning as artificial antennas exhibiting extremely efficient and fast photo-induced energy transfer. Some of these systems have been designed to allow the modulation of energy transfer efficiency and emission color, and intensity dependent on their position within a biological matrix. Finally, future perspectives for such BODIPY-based functional supramolecular sugar systems are also highlighted.

Access to 2,6-Dipropargylated BODIPYs as "Clickable" Congeners of Pyrromethene-567 Dye: Photostability and Synthetic Versatility

Hitherto unreported 2,6-dipropargyl-1,3,5,7-tetramethyl BODIPYs can be efficiently prepared by a Nicholas reaction/decomplexation protocol from 1,3,5,7-tetramethyl BODIPYs. The title compounds, which improve the BODIPY photostability by retaining their inherent photophysical and photochemical properties, can be engaged in efficient copper(I)-catalyzed azide–alkyne cycloaddition (CuAAC) “click-type” reactions with azido derivatives to provide all-BODIPY-triads or conjugated BODIPYs.

Bringing Color to Sugars: The Chemical Assembly of Carbohydrates to BODIPY Dyes

The combination of carbohydrates with BODIPY fluorophores gives rise to a family of BODIPY-carbohydrate hybrids or glyco-BODIPYs, which mutually benefit from the encounter. Thus, from the carbohydrates standpoint, glyco-BODIPYs can be regarded as fluorescent glycoconjugate derivatives with application in imaging techniques, whereas from the fluorophore view the BODIPY-carbohydrate hybrids benefit from the biocompatibility, water-solubility, and reduced toxicity, among others, brought about by the sugar moiety. In this Account we have intended to present the collection of available methods for the synthesis of BODIPY-carbohydrate hybrids, with a focus on the chemical transformations on the BODIPY core.

A Concise Route to Water-Soluble 2,6-Disubstituted BODIPY-Carbohydrate Fluorophores by Direct Ferrier-Type C-Glycosylation

Novel, linker-free, BODIPY-carbohydrate derivatives containing sugar residues at positions C2 and C6 are efficiently obtained by, hitherto unreported, Ferrier-type C-glycosylation of 8-aryl-1,3,5,7-tetramethyl BODIPYs with commercially available tri-O-acetyl-d-glucal followed by saponification. This transformation, which involves the electrophilic aromatic substitution (S_EAr) of the dipyrrin framework with an allylic oxocarbenium ion, provides easy access to BODIPY-carbohydrate hybrids with excellent photophysical properties and a weaker tendency to aggregate in concentrated water solutions.

A Concise Synthesis of a BODIPY-Labeled Tetrasaccharide Related to the Antitumor PI-88

A convergent synthetic route to a tetrasaccharide related to PI-88, which allows the incorporation of a fluorescent BODIPY-label at the reducing-end, has been developed. The strategy, which features the use of 1,2-methyl orthoesters (MeOEs) as glycosyl donors, illustrates the usefulness of suitably-designed BODIPY dyes as glycosyl labels in synthetic strategies towards fluorescently-tagged oligosaccharides.

BODIPYs as Chemically Stable Fluorescent Tags for Synthetic Glycosylation Strategies towards Fluorescently Labeled Saccharides

A series of fluorescent boron-dipyrromethene (BODIPY, 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) dyes have been designed to participate, as aglycons, in synthetic oligosaccharide protocols. As such, they served a dual purpose: first, by being incorporated at the beginning of the process (at the reducing-end of the growing saccharide moiety), they can function as fluorescent glycosyl tags, facilitating the detection and purification of the desired glycosidic intermediates, and secondly, the presence of these chromophores on the ensuing compounds grants access to fluorescently labeled saccharides. In this context, a sought-after feature of the fluorescent dyes has been their chemical robustness. Accordingly, some BODIPY derivatives described in this work can withstand the reaction conditions commonly employed in the chemical synthesis of saccharides; namely, glycosylation and protecting-group manipulations. Regarding their photophysical properties, the BODIPY-labeled saccharides obtained in this work display remarkable fluorescence efficiency in water, reaching quantum yield values up to 82 %, as well as notable lasing efficiencies and photostabilities.

Tuning the Photonic Behavior of Symmetrical bis-BODIPY Architectures: The Key Role of the Spacer Moiety

Herein we describe the synthesis, computationally assisted spectroscopy, and lasing properties of a new library of symmetric bridged bis-BODIPYs that differ in the nature of the spacer. Access to a series of BODIPY dimers is straightforward through synthetic modifications of the pending ortho-hydroxymethyl group of readily available C-8 (meso) ortho-hydroxymethyl phenyl BODIPYs. In this way, we have carried out the first systematic study of the photonic behavior of symmetric bridged bis-BODIPYs, which is effectively modulated by the length and/or stereoelectronic properties of the spacer unit. The designed bis-BODIPYs display bright fluorescence and laser emission in non-polar media. The fluorescence response is governed by the induction of a non-emissive intramolecular charge transfer (ICT) process, which is significantly enhanced in polar media. The effectiveness of the fluorescence quenching and also the prevailing charge transfer mechanism (from the spacer itself or between the BODIPY units) rely directly on the electron-releasing ability of the spacer. Moreover, the linker moiety can also promote intramolecular excitonic interactions, leading to excimer-like emission characterized by new spectral bands and the lengthening of lifetimes. The substantial influence of the bridging moiety on the emission behavior of these BODIPY dyads and their solvent-sensitivity highlight the intricate molecular dynamics upon excitation in multichromophoric systems. In this regard, the present work represents a breakthrough in the complex relationship between the molecular structure of the chromophores and their photophysical signatures, thus providing key guidelines for rationalizing the design of tailored bis-BODIPYs with potential advanced applications.

Current Advances in the Synthesis of Valuable Dipyrromethane Scaffolds: Classic and New Methods

This review presents the most recent developments on the synthesis of dipyrromethanes, covering classical synthetic strategies, using acid catalyzed condensation of pyrroles and aldehydes or ketones, and recent breakthroughs which allow the synthesis of these type of heterocycles with new substitution patterns.

Towards Efficient and Photostable Red-Emitting Photonic Materials Based on Symmetric All-BODIPY-Triads, -Pentads, and -Hexads

The development of efficient and stable red and near-IR emitting materials under hard radiation doses and/or prolonged times is a sought-after task due to their widespread applications in optoelectronics and biophotonics. To this aim, novel symmetric all-BODIPY-triads, -pentads, and -hexads have been designed and synthesized as light-harvesting arrays. These photonic materials are spectrally active in the 655-730 nm region and display high molar absorption across UV-visible region. Furthermore, they provide, to the best of our knowledge, the highest lasing efficiency (up to 68 %) and the highest photostability (tolerance >1300 GJ mol^-1 ) in the near-IR spectral region ever recorded under drastic pumping conditions. Additionally, the modular synthetic strategy to access the cassettes allows the systematic study of their photonic behavior related to structural factors. Collectively, the outstanding behavior of these multichromophoric photonic materials provides the keystone for engineering multifunctional systems to expedite the next generation of effective red optical materials.

Mitochondria-Targeting Selenophene-Modified BODIPY-Based Photosensitizers for the Treatment of Hypoxic Cancer Cells

Two red-absorbing, water-soluble and mitochondria (MT)-targeting selenophene-substituted BODIPY-based photosensitizers (PSs) were realized (BOD-Se, BOD-Se-I), and their potential as photodynamic therapy (PDT) agents were evaluated. BOD-Se-I showed higher ¹ O₂ generation yield thanks to the enhanced heavy-atom effect, and this derivative was further tested in detail in cell culture studies under both normoxic and hypoxic conditions. BOD-Se-I not only effectively functioned under hypoxic conditions, but also showed highly selective photocytotoxicity towards cancer cells. The selectivity is believed to arise from differences in mitochondrial membrane potentials of healthy and cancerous cells. To the best of our knowledge, this marks the first example of a MT-targeted BODIPY PS that functions under hypoxic conditions. Remarkably, thanks to the design strategy, all these properties where realized by a compound that was synthesized in only five steps with 32 % overall yield. Hence, this material holds great promise for the realization of next-generation PDT drugs for the treatment of hypoxic solid tumors.

A Malonyl-Based Scaffold for Conjugatable Multivalent Carbohydrate-BODIPY Presentations

A concise synthetic route from methylmalonate to a tetravalent aliphatic scaffold has been developed. The ensuing tetra-tethered derivative is equipped with two hydroxyl groups, as well as orthogonal alkene and alkyne functionalities. The usefulness of the scaffold has been demonstrated with the preparation of two representative multivalent derivatives: (i) a tetravalent compound containing two D-mannose units, one fluorescent boron-dipyrromethene (BODIPY) dye and a suitably functionalized amino acid and (ii) by way of dimerization and saponification, a water-soluble tetramannan derivative containing two fluorescent BODIPY units. Additionally, photophysical measurements conducted on these derivatives support the viability of the herein designed single and double BODIPY-labeled carbohydrate-based clusters as fluorescent markers.

Carbohydrates and BODIPYs: access to bioconjugatable and water-soluble BODIPYs

Fluorescent difluoroboron dipyrromethenes (BODIPYs), have been accessed in a one-pot synthetic operation from phthalides and pyrroles, a process that involves O-ethylation of phthalides with Meerwein’s reagent (Et3OBF4) and reaction of the ensuing tetrafluoroborate salts with pyrrole, followed by treatment with BF3 · OEt2. These derivatives are endowed with a ortho-hydroxymethyl 8-C-aryl group for further derivatization and/or conjugation to, among others, carbohydrates. The new conjugate derivatives benefit from the optimal characteristics of BODIPYs as fluorescent dyes, including in some instances water-solubility (in the case of conjugation to unprotected carbohydrates). The different kinds of BODIPY-carbohydrate derivatives are compounds of potential interest for biological studies.

Solvent-Sensitive Emitting Urea-Bridged bis-BODIPYs: Ready Access by a One-Pot Tandem Staudinger/Aza-Wittig Ureation

Herein we describe the synthesis, and computationally aided photophysical characterization of a new set of urea-bridged bis-BODIPY derivatives. These new dyads are efficiently obtained by a one-pot tandem Staudinger/aza-Wittig ureation protocol, from easily accessible meso-phenyl ortho-azidomethyl BODIPYs. These symmetric bis-BODIPYs outstand by a high absorption probability and excellent fluorescence and laser emission in less polar media. Nevertheless, this emission ability decreases in more polar media, which is ascribed to a light-induced charge-transfer from the urea spacer to the dipyrrin core, a process that can be modulated by appropriate changes in the substitution pattern of the BODIPY core. Furthermore, this ureation protocol can also be employed for the direct conjugation of our BODIPY-azides to amine-containing compounds, thus providing access to fluorescent non-symmetric ureas.

Synthesis, Structure, and Properties of β-Vinyl Ketone/Ester Functionalized AzaBODIPYs from FormylazaBODIPYs

Postfunctionalization of azaBODIPY (the BF₂ complex of azadipyrromethene) is highly desirable due to the strong tunable absorption bands at wavelengths above 650 nm and the wide-ranging applications of this class of dyes in biomedicine and materials science. Currently available postfunctionalization methods for this class of dyes have been limited to the Pd-catalyzed coupling reactions on β-halogenated (brominated or iodinated) azaBODIPY platforms. In this work, we report a new strategy for the facile postfunctionalization of the azaBODIPY chromophore with various vinyl ketone and vinyl esters based on a Wittig reaction on our previously developed β-formylazaBODIPYs and our recently developed β-bromo-β'-formylazaBODIPYs. Our strategy uses easily accessible starting materials and mild reaction conditions. It is highly compatible with various common phosphonium ylides (aliphatic, aromatic, and ester substituted ones). These resultant bromo-containing β-vinyl ketone/ester functionalized azaBODIPYs are potential photosensitizers and can be further functionalized via coupling reactions. The ester groups on some of these resultant azaBODIPYs can be further hydrolyzed to achieve the desired water solubility and conjugate with the biomolecule and solid surface.

Asymmetric Dipyrrin and F-BODIPYs Conjugated to Terminal Alkynes and Alkenes

An asymmetric meso-H dipyrrin featuring a conjugated terminal alkyne substituent was converted to its corresponding difluoro boron complex, and the extent of π-conjugation was extended using Sonogashira cross-coupling. Treatment of the alkyne-substituted dipyrrin with BF₃·OEt₂ and NEt₃ revealed the reactivity of the conjugated terminal alkyne toward Lewis-activated electrophilic substitution and led to the isolation of F-BODIPYs bearing terminal bromovinyl and enol substituents.