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.
The last few decades have witnessed a growing awareness on the importance of carbohydrates in biological processes . For instance, it has now became clear that intercellular carbohydrate-receptor interactions are crucial for physiological events, e.g. cell-cell adhesion and cell differentiation, as well as in pathological processes, e.g. cancer metastasis and bacterial infection , . On the other hand, the use of small fluorescent molecules has enabled significant advances in chemical biology and biomedicine when combined with fluorescence spectroscopy or microscopy, detection techniques , . Opposite to proteins and nucleic acids, fluorescent labeling of carbohydrates has received far less attention, and early work have focused on the study of glycosidase activities with the help of fluorescent glycosides . More recent examples of fluorescently-labeled carbohydrates have already shown the usefulness of small-molecule carbohydrate fluorophores in the investigation of biological processes , , , , .
Our research group has focused, over the last two decades, in the study of carbohydrates with particular interest in their synthetic transformations, including glycosylation studies , , . Very recently, we have turned our attention to the preparation of fluorophores aiming at the development of novel fluorescent labels for carbohydrates. In this manuscript, we include details of our efforts in that direction.
The most representative small-molecule fluorescent probes used in the labeling of biomolecules are coumarin, fluorescein, rhodamine, and cyanine and BODIPY dyes (Fig. 1) , , , . We were initially inclined to use BODIPY (boron dipyrromethene or 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) dyes , . These dyes present a series of features that make them very appealing as potential fluorescent labels for biomolecules, these include: (i) relatively high photostability, (ii) neutral total charge, (iii) sharp absorption and emission spectra, (iv) strong-UV-vis absorption spectra, (v) high fluorescence quantum yield (φ), (vi) reasonable stability under physiological conditions, (vii) negligible triplet-state formation, and (viii) photophysical properties, relatively insensitive to solvent polarity. Conversely, two general drawbacks have also been associated to BODIPY dyes: their small Stokes shifts, which may cause re-absorption effects, and the poor water-solubility associated to most of these derivatives .
Synthesis of conjugatable BODIPY fluorophores. Access to water-soluble carbohydrate-BODIPY hybrids
At the beginning of our studies, only a few examples on the preparation of carbohydrate-BODIPY hybrids existed in the literature, , and a key factor to the success on the investigations compiled in this manuscript was the launching of a collaboration with Prof. Eduardo Peña-Cabrera’s group at the University of Guanajuato (Mexico). Accordingly, we developed two complementary routes to monosaccharide-BODIPY hybrids based on copper(I)-catalyzed azide alkyne cycloaddition (CuAAC) , , , , of glycosyl derived alkynes, i.e. 6, or azides, i.e. 7–9, with BODIPY molecules containing azido, i.e. 4, or alkyne, i.e. 5, moieties, respectively (Scheme 1).
The Peña-Cabrera’s approach to BODIPYs
Azido and alkyne BODIPYs 4 and 5, respectively, were obtained according to methodologies previously described in Prof. Peña-Cabrera laboratories. Thus, Liebesking-Srögl cross-coupling reaction (LSCCR)  between Biellmann’s BODIPY (1)  and o-azidobenzyl boronic acid 2, according to Peña-Cabrera’s procedure , produced rotationally restricted, vide infra, BODIPY 4, as a reddish crystalline solid, whereas blue-emitting 8-propargylamino BODIPY 5, was easily prepared upon treatment of BODIPY 1 with propargyl amine in methylene chloride . Subsequent application of the CuAAC process to BODIPYs 4 and 5 in combination with their sugar-counterparts, propargyl-mannoside 6 and 2-azidoethyl glycosides 7–9 (manno- gluco- and galacto-configured), permitted the preparation of carbohydrate-BODIPY hybrids 10, and 11–13, respectively (Scheme 1). However, a further refinement came about when it was noted that both processes (LSCCR and CuAAC) in each approach, could be carried out as one-pot operations.
Subsequent saponification (NEt3/MeOH, 4:1; reflux) of carbohydrate-BODIPY hybrids 11a–13a, led to the desired sugar tetraols 11b–13b (Scheme 1). These compounds, as well as tetraacetyl BODIPY-mannose derivative 10, proved to be water-soluble, the latter to a lesser extent and most likely due to the presence of the triazole moiety. The spectral bands of these derivatives spanned from the blue to yellow owing to the changes on the spacer, amino or aryl, respectively (see Table 1). Compound 10, where the triazole was linked to the C-8 aryl group through its ortho position, displayed a very high fluorescent quantum yield (φ=0.91, Table 1, entry (i) compared to that of its C-8 amino analogs 11b–13b (φ≈0.16, see Table 1, entries (ii–iv). This phenomenon is due to the steric hindrance of the ortho-substituent at the C-8 phenyl ring with the hydrogen atoms at C-1 and C-7, which hampers free-rotation and therefore minimizes non-radiative relaxation processes .
|Entry||Compound||λab [nm]||λfl [nm]||% φ|
aλab, Absorption wavelength; λfl, fluorescence wavelength; φ, fluorescence quantum yield.
An expeditious approach to rotationally restricted, conjugatable, BODIPY derivatives from phthalide
Based on the aforementioned findings, we decided to focus our investigations on ortho-substituted C8-aryl derivatives, e.g. 10, rather than C-8 amino substituted BODIPYs, e.g. 11, due to the improved fluorescent quantum yields displayed by the former compounds. In this context, and despite the efficiency and versatility of Peña-Cabrera’s LSCC approach to 8-aryl substituted BODIPYs , we were concerned with the multistep route to 8-thiomethyl BODIPY 1 (Scheme 2). Thus, access to Biellmann’s BODIPY (1) from pyrrole took place in three steps, one of them involving the use of thiophosgene.
Synthesis of BODIPYs from phthalides
Accordingly, we devised a straightforward method based on the use of phthalide(s), e.g. 16, as the 8-C-aryl ortho-substituted-component precursors of the targeted BODIPY derivatives, i.e. 19 (Scheme 3) . Our approach made use of the unprecedented and mild reaction of salt 17 with pyrrole that, via the intermediacy of bis-pyrrolyl phthalan 18, led ultimately to ortho-hydroxymethyl substituted C-8 aryl BODIPYs, i.e. 19. The reaction pathway proposed for this transformation (Scheme 3) was indeed supported by the isolation of intermediate 18 (R1=H), which upon treatment with BF3·OEt2 and NEt3, led to BODIPY 19a (R1=H) . Salt 17, in its turn, was uneventfully generated upon treatment of phthatlide (16) with triethyloxonium tetrafluoroborate (Meerwein’s reagent) . Interestingly, the three-step process displayed in Scheme 3, gave better yields when it was carried out as a one-pot operation, probably due to the lability of the intermediate bis-pyrrolyl phthalans (18).
Accordingly, from a practical standpoint, the synthetic protocol (Scheme 4) involves: (i) treatment of the corresponding phthalide (16) in anhydrous dichloromethane, in the presence of 4 Å molecular sieves, with triethyloxonium tetrafluoroborate for 24 h at room temperature (r.t.); (ii) subsequent addition of the desired pyrrole at 0°C (3.0 equiv. of 2-alkyl pyrroles, and 10.0 equiv. of pyrrole) and keeping of the reaction mixture for 3 h, at r.t., and (iii) cooling to 0°C, and final addition of triethylamine (6.0 equiv.) and borontrifluoride diethyl etherate complex (9.0 equiv.) to the reaction mixture (2 h).
This synthetic methodology proved to be fairly general, and it was applied to the preparation of a series of substituted BODIPYs by combining differently substituted pyrroles and phthalides (Fig. 2). In general, the reaction proceeded well with substituted pyrroles and either electron-rich or electron-deficient phthalides. In each case the expected BODIPY could be isolated in moderate to good yields, even in gram-scale reactions (Fig. 2). This phthalide coupling reaction, unlike the Peña-Cabrera’s approach more sensitive to steric factors , gave better BODIPY yields when alkyl-substituted pyrroles, rather than pyrrole itself, were employed (compare the yields obtained in the preparation of compounds 19a–c, Fig. 2).
On the role of the ortho-hydroxymethyl substituent
Our interest in ortho-substituted 8-C-hydroxymethyl BODIPYs was associated with several aspects of the ortho-hydroxymethyl group : (i) the ortho-nature of the substituent prevents the rotation of the aryl moiety around the 8C-aryl bond, which ultimately results in much higher fluorescence quantum yield; (ii) the hydroxyl moiety, as such or after functional group modification, could provide a handle for conjugation to different molecules; and (iii) the incorporation of a bulky apical group in the fluorophore avoids aggregation, a phenomenon that is known to diminish its fluorescence quantum yield .
In relation with the first aspect, even though arylation at the C-8 (meso) position has hardly any effect on the absorption and emission wavelengths of the BODIPYs (Table 2), the quantum yield is appreciably reduced when free rotation of the meso-aryl group is possible, as in compound 20 (compare % φ, for compounds 19a and 20, Table 2). Consistent with this, introduction of the ortho-substituent and methyl groups at C-1 and C-7, also increased the quantum yields in compounds 19b–c (see % φ, Table 2).
|Entry||Compound||λab [nm]||λfl [nm]||% φ||Δυst (cm−1)||Eff (%)|
aDye concentration: 2 μM. λab, absorption wavelength; λfl, fluorescence wavelength; φ, fluorescence quantum yield; Δυst, stokes shift; bdye concentration: 2 mM. Eff (%): lasing efficiency, as the ratio between the energy of the laser output and the pump energy incident on the cell surface.
An interesting additional aspect of BODIPYs relates to their use as laser dyes with outstanding lasing efficiency upon laser irradiation , . Thus the lasing behavior of compounds 19a–c was studied, and it showed good correlation with their photophysical properties, in particular with the Stokes shift. In general, an increase in the Stokes shift enhances the laser action [Eff (%), Table 2] and it might outweigh the decrease in the fluorescence quantum yield [e.g. compare Eff (%) in compounds 8H-BODIPY and 19a, Table 2].
From the standpoint of photostability, a key feature in fluorescent dyes, photodecomposition studies were performed on derivatives 19b and 19c, which were shown to display high photostability when submitted to drastic pumping conditions.
In connection with the second issue, the ortho-hydroxymethyl group in compound 19b could be uneventfully transformed into formyl, isothiocyanylmethyl and azidomethyl derivatives 21a–c, respectively. The easiness in the conjugation of these derivatives with carbohydrates was illustrated by the preparation of β-glucosides 22 and 23 from 19b and 21a, respectively (Fig. 3). Accordingly, glycosylation of 19b with D-glucose pentaacetate provided BODIPY-glucoside 22a, whereas reaction of isothiocyanate 21a with 2-aminoethyl-2,3,4-tetra-O-acetyl-β-D-galactopyranose, provided thiourea derivative 23. On the other hand, conjugation of BODIPY-aldehyde 21b to amino-containing molecules could be carried out under reductive amination conditions, as it was demonstrated in its reaction with benzylamine to give benzylamino derivative 24. Azidomethyl derivative 21c, could be used in CuAAC reactions with alkyne-containing molecules, as was previously shown with related BODIPY 4 (Scheme 1).
Structural modifications on the BODIPY core
An additional facet of general interest in the design of fluorescent probes, has been the search for organic chromophores with strong absorption in the near infrared (NIR) region (650–900 nm). These NIR-fluorophores, compared to those with absorption and fluorescence bands in the ultraviolet/visible (UV/Vis) region, will display minimum interference from background autofluorescence by biomolecules, causing less photo-damage, and improved tissue penetration , . In this context, borodipyrromethene dyes have been considered privileged platforms since, well-documented, synthetic postmodifications on the dipyrromethene core might led to NIR-fluorescent BODIPY dyes .
Along this line, we have performed synthetic modifications compatible with the ortho-hydroxymethyl group, aimed at extending the conjugation of the BODIPY core and resulting in red-shifted BODIPY fluorophores. Accordingly, di-iodo derivative 25, readily accessible from 19b after treatment with N-Iodosuccinide (NIS), was transformed into 2,6-dialkynyl BODIPYs 26a–c by Sonogashira cross-coupling reaction with the corresponding alkynes (Fig. 4) . Interestingly, BODIPY-carbohydrate derivative 26b could be accessed by a Sonogashira cross-coupling reaction of 25 with propargyl α-D-mannopyranoside tetraacetate, in moderate yield. The former could be uneventfully deprotected to yield bis-mannopyranoside derivative 26c. On the other hand, the π-extension of the conjugation at positions C-3 and C-5, can uneventfully be attained by a Knoevenagel-type condensation  of either 19b or 25, leading to styryl derivatives 27 and 28, respectively (Fig. 4). Alternatively, Vilsmeier-Haak formylation of 25, allows the synthesis of mono-formyl derivative 29 (Fig. 4) .
A one-pot entry to urea-bridged bis-BODIPYs dimers from a serendipitous discovery
As a continuation of our studies on conjugatable BODIPY derivatives, we had obtained azidomethyl compounds 4, 21c, and 29–31, from the corresponding hydroxymethyl derivatives (Fig. 5). Thus, even though these derivatives could be engaged in CuAAC reactions for conjugation, we were interested in their transformation to ortho-aminomethyl BODIPY derivatives, to explore their conjugation with formyl-containing compounds, via reductive amination protocols.
In this context, we first examined the well-known Staudinger azido→amino transformation reaction (PPh3, aq. NaOH) ,  on azide 31. Contrary to our expectations, no traces of the expected aminomethyl derivative 32 could be observed in the Staudinger reaction of 31, and instead a dimeric urea derivative 33 was isolated in 75 % yield (Scheme 5) .
From the results shown in Scheme 5, it was clear that formation of 33 from azidomethyl derivative 31, required the presence of an external CO2 source in the reaction media, to account for the observed “carbonyl” bridge. Thus, on the assumption that “CO2” was present in the reaction media, we postulated a pertinent reaction pathway for this transformation (Scheme 6). Accordingly, reaction of 31 with PPh3 would provide iminophosphorane 34 , whose reaction with “CO2” could lead to isocyanate 36 by way of an intermolecular aza-Wittig reaction . The latter species already accounted for the additional carbonyl bridge in urea 33 . Finally, reaction of 36 with a postulated aminomethyl derivative intermediate 32, will give rise to dimer 33. Formation of amine 32, in its turn, could be visualized by hydrolysis of either iminophosphorane 34 or isocyanate 35, which in either case will have to be understood as the rate limiting step of the overall transformation, since no amine 32 could be detected in the reaction media. We visualized the addition of amine 32 to isocyanate 35, leading to 33, as a very rapid and efficient process.
Since BODIPY dimers have become a topic of interest , , we decided to search for reaction conditions leading to the reliable preparation of urea-bridged dimers. We had hypothesized that the origin of the required carbon dioxide might have been CO2 absorbed in the aqueous NaOH solution employed . Subsequent experiments were directed to find a consistent CO2 source and to that end we drew our attention to triethylammonium hydrogen carbonate buffer (TEAB), a reagent recommended by Azhayev and co-workers for related transformations . Along this line, the reaction of azidomethyl BODIPYs in the presence of PPh3 and TEAB in 1,4-dioxane at r.t., paved the way to dimeric BODIPYs 33 and 36a–d (Fig. 6).
Access to urea-BODIPY conjugates from azidomethyl BODIPYs
On the other hand, mindful of the interest in BODIPY species amenable to conjugation, we have also found that BODIPY azides can react with amine-containing derivatives leading to the non-symmetric BODIPY ureas, e.g. 37 (Scheme 7). This novel approach for conjugation was inspired on our proposed reaction pathway, depicted in Scheme 6, provided that amine 32, could be replaced with an “external” amine-containing compound. From a practical standpoint, reaction of azidomethyl BODIPYs with either ammonia, primary, or secondary amines, in the presence of PPh3 and TEAB, in dioxane at r.t. resulted in the formation of non-symmetric ureas 37 (Scheme 7), in good to excellent yields .
The study of the photophysical properties of these new derivatives was also carried out, displaying excellent fluorescence and laser emission properties in less polar media. This emission, however, decreased in more polar media, owing to a light induced charge transfer from the urea linker to the dipyrromethene core.
In summary, we have shown that borondipyrromethene fluorophores (BODIPYs) are useful dyes for conjugation with carbohydrates, which might help in the solubilization of the former, aimed at their use in biological studies. In this context, we have developed a one-pot, efficient, protocol for the synthesis of ortho-hydroxymethyl 8-C-aryl BODIPYs based in the coupling of phthalides with pyrrole derivatives. The ensuing BODIPYs possess an anchoring point that can be used as such, or after functional group modification, in conjugation reactions with a variety of partners, including: alkynes, amino derivatives, glycosyl derivatives and isocyanates. The photophysical properties of the conjugates retain the good characteristics of the BODIPY derivatives and, therefore, are promising derivatives with potential use in biological studies containing carbohydrates.
This paper was presented (AMG) at the 29thInternational Carbohydrate Symposium, Lisbon, Portugal, July 2018.
We would like to thank our students, as well as the members of the various research groups involved in the investigations reported in this Conference Paper for their contribution. Special thanks are given to Profs. Eduardo Peña-Cabrera (Universidad de Guanajuato, Mexico), Inmaculada Garcia-Moreno (IQFR-CSIC, Spain), and Jorge Bañuelos (Universidad del Pais Vasco, Spain). This research was funded by the Ministerio de Ciencia e Innovación (MICINN) and Ministerio de Economia y Competitividad (MINECO, Spain), grants CTQ2012-32114 and CTQ2015-66702-R, respectively.
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