2022 Volume 5 Issue 1

Open Access

INEOS OPEN, 2022, 5 (1), 15–20  Journal of Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences Download PDF Electronic supplementary information DOI: 10.32931/io2201a

Carborane–Bodipy Conjugates: Synthesis and Characterization

Abstract

A series of new carborane–BODIPY conjugates bearing succinimide–triazole or maleimide functional groups are prepared from synthetically available amino-substituted BODIPYs. The resulting compounds are characterized by UV–vis, IR, and NMR spectroscopy as well as mass spectrometry.

Key words: BODIPY, carborane, maleimide, succinimide, triazole.

 

Introduction

4,4-Difluoro-4-bora-3a,4a-diaza-s-indacenes (BODIPYs) are important compounds that find application in various fields, including biology, medicine, and materials science [1–5]. They display valuable photophysical and optoelectronic properties, such as high quantum yields of fluorescence, small absorption and emission bandwidths, high stability at physiological pH values, and chemical inertness along with ample opportunities to modify a structure of the chromophoric BODIPY ligand and thus impart new properties to the system [6]. The BODIPY fluorescent dyes, being structural analogs of porphyrins, are widely used in biochemistry, biophysics, and biotechnology as fluorescent markers, and in medicine for imaging living cells and animals in preclinical research [7, 8]. A number of reports are devoted to the structural modification of the BODIPY compounds aiming at improving and extending their application scope [9]. Recently, the BODIPY dyes have been proposed as photosensitizers (PSs) for photodynamic therapy (PDT). Today, PDT is one of the efficient binary methods for the treatment of cancer diseases. PDT is based on the selective destruction of pathological cells by the action of cytotoxic reactive oxygen species (ROS) (¹О₂, HO₂^–, HO^·), which are locally generated upon excitation with monochromatic light selectively accumulated in tumor PS, thus limiting undesired effects on healthy tissues [10]. At the same time, the BODIPY derivatives are not only efficient PSs for PDT but have remarkable fluorescence properties, which make them useful compounds for diagnosis by fluorescence bioimaging [11]. Furthermore, owing to their unique spectroscopic features, the BODIPY dyes are of interest for the modification with carborane clusters. Carboranes have unique structural and electronic properties, such as high chemical and thermal stability [12], three-dimensional aromaticity [13, 14], high hydrophobicity and electron-withdrawing character [15], as well as high biocompatibility [16]. The high synthetic potential of carboranes and their derivatives provides the basis for the preparation of promising compounds for practical application in different areas, such as catalysis [17], creation of luminescent materials and materials for nonlinear optics [18], thermally stable polymers [19], dendrimers and precursors for nanostructured systems [20]. Moreover, carborane derivatives are attractive building blocks for the design of new pharmaceuticals [21]. Owing to their synthetic flexibility, high thermal and hydrolytic stability, general robustness, remarkable biostability, as well as high boron content, carboranes are useful as potential components of drugs for boron neutron capture therapy (BNCT). BNCT is a binary method for the treatment of cancer that is based on the selective absorption of the non-radioactive ¹⁰B isotope in the tumor tissue followed by irradiation with low energy thermal neutrons. The ¹⁰B(n,α)⁷Li reaction produces the energetic α particles and residual ⁷Li nuclei (with the energies of 200 and 350 keV μm^–1, respectively) that have a cell killing effect within a 10 µm range (about one cell diameter) causing the lethal damage of the ¹⁰B enriched tumor cells. Therefore, BNCT selectively destroys cancer cells without damaging the surrounding healthy tissue. In this context, the modification of the BODIPY dyes with carboranes opens up a possibility to use them both as photosensitizers for PDT and as potential drugs for BNCT [22–28]. In this article, different ways of the targeted modification of a BODIPY core with carborane polyhedra were developed in order to create multifunctional drug candidates for PDT, BNCT, and other applications. Herein, we report on the synthesis and characterization of new maleimide-containing BODIPY dyes as efficient compounds for selective introduction of carborane clusters via the Michael addition followed by the formation of the corresponding succinimide-substituted BODIPYs. These conjugates were prepared by the copper(I)-catalyzed 1,3-dipolar cycloaddition of the azido-substituted BODIPYs with N-propargyl maleimide. The carborane BODIPY-substituted maleimides were also synthesized by the reaction of carborane bromomaleimide with amino-substituted BODIPYs. The novel conjugates are promising building blocks for the synthesis of biologically active molecules suitable for PDT and BNCT of cancer and perhaps for other diseases in which this modality is currently used [29–31]. The incorporation of the maleimide fragment into organic scaffolds can significantly affect the biological activity. The maleimide group reacts specifically with sulfhydryl groups in biomolecules, allowing for conjugation with peptides via the formation of biologically active succinimide derivatives [32]. Therefore, along with the known modifications for BNCT and PDT (e. g., boronation), this study offers an opportunity of a specific peptide targeting compounds via the interaction of the sulfhydryl residues with the exogenous maleimide moiety. The new conjugates can emerge as versatile biochemical tools.

Results and discussion

For the synthesis of the BODIPY derivatives containing carborane clusters, readily available 8-(4-aminophenyl)-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (1) and 8-(4-amino-2,3,5,6-pentafluorophenyl)-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (2) were chosen as the key precursors. They were smoothly obtained according to the published procedures in 90–95% yields [33, 34]. The acylation of the amino groups in compounds 1 and 2 with chloroacetyl chloride in CH₂Cl₂ in the presence of Et₃N for 1–6 h led to the formation of chloroacetylamido derivatives 3 and 4 in 91% and 42% yields, respectively (Scheme 1).

Scheme 1. Synthesis of the BODIPY–carborane conjugates.

Compounds 3 and 4 easily formed the corresponding azides in 86% yields under the action of NaN₃ in DMF at the ambient temperature. The azide groups in 5, 6 were then reduced with SnCl₂ in MeOH at the ambient temperature, affording amines 7, 8 as red crystalline solids in 94–96% yields. BODIPY derivatives 5, 6 were conjugated to N-propargyl maleimide 9 [35] via the click reaction [36] in the presence of Cu(OAc)₂ and sodium ascorbate in dry DMSO for 15–60 min to give corresponding BODIPYs 10, 11 containing triazole linkers modified with maleimides in 59–86% yields. Owing to the presence of the activated double bond, the maleimide substituents in 10, 11 can act as the Michael acceptors in the reactions with S-, N-, and O-nucleophiles [37]. The selective reactivity of the maleimide substituents in compounds 10, 11 towards S-nucleophiles was demonstrated by the example of 9-mercapto-m-carborane 12 [38]. It was shown that the reactions of mercaptocarborane 12 readily proceed at the maleimide double bonds of compounds 10, 11 in THF in the presence of NaOAc to generate thiosuccinimide products 13, 14 in 70–85% yields as red crystalline solids (Scheme 1). Furthermore, the alkylation of the amino groups in compounds 7, 8 with 3-bromo-1-(N-(o-carborane-3'-yl)maleimide (15) [39] in THF in the presence of NaOAc resulted in the formation of carborane maleimide BODIPYs 16, 17 in 28–47% yields.

The structures of all the new compounds were confirmed by UV–vis, IR, ¹H, ¹¹B, and ¹⁹F NMR spectroscopy as well as mass spectrometry. In the IR spectra, the characteristic absorption bands of the amino groups were observed at 3272–3465 cm^–1. The absorption bands at ca. 1700 cm^–1 correspond to the stretching vibrations of the C=O group for all the compounds obtained. Compounds 13, 14, 16, and 17 showed an intense absorption band in the infrared spectrum at 2597–2606 cm^–1 that was assigned to the stretching vibrations of the BH groups in the closo-carborane polyhedron. The azide absorption bands for compounds 5, 6 were detected at 2106 cm^–1 and 2114 cm^–1, respectively. All the compounds showed the absorption bands at 1260 cm^–1 attributed to the BODIPYs BF₂ groups.

For all the new compounds, the six protons of the pyrrole rings appeared as singlet signals at 8.14–7.91 ppm, as doublets in the regions of 6.87–8.03 ppm and 6.53–7.26 ppm. The four protons of phenyl ring appeared as doublet signals at 7.94–7.72 ppm and 8.56–7.51 ppm. The spacer methylene protons were observed as singlets at δ_H = 4.09–5.32 ppm. The methylene protons in the succinimide ring (compounds 13, 14) showed the doublet of doublet signals at δ_H = 3.25–3.90 ppm, the maleimide CH=CH protons in compounds 10, 11 were found as singlets at δ_H = 6.72–6.94 ppm. The maleimide CH protons in compounds 16, 17 appeared as singlets at 5.14 and 5.07 ppm, respectively. The protons of the carborane CH groups in compounds 13, 14, 16, and 17 were found as broaden singlets in the range of 3.02–4.41 ppm. The ¹¹B{¹H} NMR spectra of compounds 13, 14, 16, and 17 exhibited the signals at 0.2 ppm (13, 14, 17) or 0.3 ppm (16), which can be attributed to the BF₂ units, along with the signals of the carborane BH units in the region from −1.1 to −17.8 ppm, confirming the closo-structure of the carborane clusters. The detailed spectroscopic data of all the compounds obtained can be found in the Electronic Supplementary Information.

Experimental

General remarks

All reactions were performed in an atmosphere of dry argon. All solvents were dried according to the standard protocols. Unless otherwise specified, the reagents were purchased from Sigma-Aldrich. The ¹H, ¹¹B, and ¹⁹F NMR spectra were recorded on a Bruker Avance-400 spectrometer operating at 400 MHz for ¹H NMR, 128 MHz for ¹¹B NMR, and 376 MHz for ¹⁹F NMR or a Bruker Avance-300 spectrometer operating at 282 MHz for ¹⁹F NMR. The chemical shifts (δ) were referenced internally to the residual solvent peak (CDCl₃, ¹H: 7.26 ppm; (CD₃)₂CO, ¹H: 2.05 ppm) for ¹H or externally relative to BF₃·OEt₂ for ¹¹B and CFCl₃ for ¹⁹F. The IR spectra were recorded on a Bruker Tensor 37 FTIR spectrometer in KBr tablets. The UV–vis spectra were measured on a Carl Zeiss Specord M 40 spectrophotometer in CH₂Cl₂ or acetone. The MALDI mass spectra were recorded on a Bruker autoflex speed TOF mass spectrometer (Germany) equipped with a solid-state UV laser of 355 nm and operating in the positive reflection mode. The MALDI mass spectra were registered using steel targets (MTP 384 ground steel; Bruker Daltonics Inc., Germany) containing 384 cells for deposition of the analyte and matrix. Dithranol and DHB were tested as the matrices and used in THF solution. The course of the reactions and purity of the compounds were monitored by TLC using Kieselgel 60 F254 plates (Merck) (elution with CH₂Cl₂, CH₂Cl₂–acetone (4:1), or CH₂Cl₂–methanol (5:1)). Merck silica gel L 0.040–0.060 mesh was used for column chromatography (elution with CH₂Cl₂ for compounds 3–6, CH₂Cl₂–acetone (4:1) for compounds 10, 11, 13, 14, 16, and 17, or CH₂Cl₂–methanol (5:1) for compounds 7 and 8).

Syntheses

8-[4-(2-Сhloro-N-acetylamido)phenyl]-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (3). Chloroacetyl chloride (300 mg (211 µL), 2.66 mmol) and Et₃N (287 mg (395 µL), 2.66 mmol) were sequentially added dropwise to a solution of BODIPY 1 (500 mg, 1.77 mmol) in dry CH₂Cl₂ (100 mL). The reaction mixture was stirred at room temperature under an argon atmosphere in the dark for 1 h. Then, water (400 mL) was added to the mixture. The organic phase was separated, washed with water (3×200 mL), dried over Na₂SO₄, and evaporated to dryness. The resulting residue was purified by column chromatography on silica gel (eluent: CH₂Cl₂). Yield: 574 mg (90%). UV–vis (CH₂Cl₂, λ_max/nm (e·10^–3)): 241 (14.4), 375 (11.5), 502 (43.2). IR (KBr, ν/cm^–1): 3272 (NH), 1683 (C=O), 1549, 1476 (C=C, C=N), 1257 (BF₂). ¹Н NMR (400 MHz, CDCl₃): d 8.49 (br. s, 1H, NH), 7.96 (br. s, 2H, pyrrole), 7.78 (d, J = 8.6 Hz, 2H, Ph), 7.61 (d, J = 8.3 Hz, 2H, Ph), 6.96 (d, J = 3.7 Hz, 2H, pyrrole), 6.58 (d, J = 2.4 Hz, 2H, pyrrole), 4.28 (s, 2H, CH₂) ppm. MS (MALDI): m/z [М–F]⁺ calcd. for C₁₇H₁₃BClFN₃O 340.082; found 340.106.

8-[4-(2-Chloro-N-acetylamido)-2,3,5,6-tetrafluorophenyl]-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (4). Chloroacetyl chloride (3.18 g (2.24 mL), 28.2 mmol) and Et₃N (2.85 g (3.92 mL), 28.2 mmol) were sequentially added dropwise to a solution of BODIPY 2 (1.0 g, 2.82 mmol) in dry CH₂Cl₂ (200 mL). The reaction mixture was stirred at room temperature under an argon atmosphere in the dark for 6 h. Then, water (500 mL) was added to the mixture. The organic phase was separated, washed with water (3×200 mL), dried over Na₂SO₄, and evaporated to dryness. The resulting residue was purified by column chromatography on silica gel (eluent: CH₂Cl₂). Yield: 512 mg (42%). UV–vis (CH₂Cl₂, λ_max/nm (e·10^–3)): 340 (7.6), 519 (50.0). IR (KBr, ν/cm^–1): 3441 (NH), 1710 (C=O), 1568, 1478 (C=C, C=N), 1258 (BF₂). ¹Н NMR (400 MHz, CDCl₃): d 8.15 (s, 1H, NH), 8.00 (s, 2H, pyrrole), 6.87 (d, J = 3.8 Hz, 2H, pyrrole), 6.60 (d, J = 4.1 Hz, 2H, pyrrole), 4.36 (s, 2H, CH₂) ppm. ¹⁹F NMR (376 MHz, CDCl₃): d –137.0 (d, J = 13.8 Hz, 2F, ortho-F), –142.5 (d, J = 13.8 Hz, 2F, meta-F), –144.8 (dd, J = 57.7, 27.5 Hz, 2F, BF₂) ppm. MS (MALDI): m/z [М–F]⁺ calcd. for C₁₇H₉BClF₅N₃O 412.045; found 412.074.

8-[4-(2-Azido-N-acetylamido)phenyl]-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (5). Sodium azide (108 mg, 1.69 mmol) was added to a solution of BODIPY 3 (500 mg, 1.39 mmol) in dry DMF (20 mL). The reaction mixture was stirred at room temperature under an argon atmosphere in the dark for 1.5 h. After addition of water (500 mL), the target product was extracted with EtOAc (200 mL). The organic phase was separated, washed with water (2×200 mL), dried over Na₂SO₄, and evaporated to dryness. The resulting residue was purified by column chromatography on silica gel (eluent: CH₂Cl₂). Yield: 436 mg (86%). UV–vis (CH₂Cl₂, λ_max/nm (e·10^–3)): 244 (16.1), 379 (12.4), 502 (43.8). IR (KBr, ν/cm^–1): 3435 (NH), 2106 (N₃), 1683 (C=O), 1553, 1477 (C=C, C=N), 1258 (BF₂). ¹Н NMR (400 MHz, CDCl₃): d 8.26 (br. s, 1H, NH), 7.96 (br. s, 2H, pyrrole), 7.78 (d, J = 8.3 Hz, 2H, Ph), 7.61 (d, J = 8.6 Hz, 2H, Ph), 6.96 (d, J = 3.8 Hz, 2H, pyrrole), 6.58 (d, J = 2.7 Hz, 2H, pyrrole), 4.25 (s, 2H, CH₂) ppm. MS (MALDI): m/z [М–F]⁺ calcd. for C₁₇H₁₃BFN₆O 347.123; found 347.162.

8-[4-(2-Azido-N-acetylamido)-2,3,5,6-tetrafluorophenyl]-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (6). Sodium azide (181 mg, 2.79 mmol) was added to a solution of BODIPY 4 (400 mg, 0.93 mmol) in dry DMF (20 mL). The reaction mixture was stirred at room temperature under an argon atmosphere in the dark for 3 h. After addition of water (500 mL), the target product was extracted with EtOAc (200 mL). The organic phase was separated, washed with water (2×200 mL), dried over Na₂SO₄, and evaporated to dryness. The resulting residue was purified by column chromatography on silica gel (CH₂Cl₂). Yield: 350 mg (86%). UV–vis (CH₂Cl₂, λ_max/nm (e·10^–3)): 341 (4.4), 519 (29.8). IR (KBr, ν/cm^–1): 3441 (NH), 2114 (N₃), 1717 (C=O), 1568, 1479 (C=C, C=N), 1258 (BF₂). ¹Н NMR (400 MHz, CDCl₃): d 8.04 (s, 1H, NH), 8.00 (s, 2H, pyrrole), 6.87 (d, J = 3.5 Hz, 2H, pyrrole), 6.60 (d, J = 3.8 Hz, 2H, pyrrole), 4.35 (s, 2H, CH₂) ppm. ¹⁹F NMR (282 MHz, CDCl₃): d –137.1 (d, J = 12.6 Hz, 2F, ortho-F), –142.5 (d, J = 12.6 Hz, 2F, meta-F), –144.8 (dd, J = 55.0, 27.5 Hz, 2F, BF₂) ppm. MS (MALDI): m/z [М–F]⁺ calcd. for C₁₇H₉BF₅N₆O 419.085; found 419.036.

8-[4-(2-Amino-N-acetylamido)phenyl]-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (7). Anhydrous SnCl₂ (313 mg, 1.65 mmol) was added to a solution of BODIPY 5 (200 mg, 0.55 mmol) in dry methanol (100 mL). The reaction mixture was stirred at room temperature under an argon atmosphere in the dark for 72 h. After the removal of the solvent in vacuo, the resulting residue was purified by column chromatography on silica gel (eluent: CH₂Cl₂–methanol (5:1)). Yield: 176 mg (94%). UV–vis (acetone, λ_max/nm (e·10^–3)): 372 (4.0), 497 (8.1). IR (KBr, ν/cm^–1): 3396 (NH), 1687 (C=O), 1554, 1474 (C=C, C=N), 1259 (BF₂). ¹Н NMR (400 MHz, (CD₃)₂CO): d 8.21 (br. s, 1H, NH), 8.01 (s, 2H, pyrrole), 7.94 (d, J = 8.6 Hz, 2H, Ph), 7.71 (d, J = 8.6 Hz, 2H, Ph), 7.10 (d, J = 3.8 Hz, 2H, pyrrole), 6.68 (d, J = 3.8 Hz, 2H, pyrrole), 4.38 (s, 2H, CH₂), 2.25 (s, 2H, NH₂) ppm. MS (MALDI): m/z [М–F]⁺ calcd. for C₁₇H₁₅BFN₄O 321.132; found 321.156.

8-[4-(2-Amino-N-acetylamido)-2,3,5,6-tetrafluorophenyl]-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (8). Anhydrous SnCl₂ (97 mg, 0.51 mmol) was added to a solution of BODIPY 6 (150 mg, 0.34 mmol) in dry methanol (100 mL). The reaction mixture was stirred at room temperature under an argon atmosphere in the dark for 12 h. After the removal of the solvent in vacuo, the resulting residue was purified by column chromatography on silica gel (eluent: CH₂Cl₂–methanol (5:1)). Yield: 135 mg (96%). UV–vis (acetone, λ_max/nm (e·10^–3)): 513 (33.4). IR (KBr, ν/cm^–1): 3444 (NH), 1713 (C=O), 1569, 1478 (C=C, C=N), 1258 (BF₂). ¹Н NMR (400 MHz, (CD₃)₂CO): d 8.36 (s, 1H, NH), 8.14 (s, 2H, pyrrole), 7.27 (d, J = 3.8 Hz, 2H, pyrrole), 6.72 (d, J = 4.1 Hz, 2H, pyrrole), 4.57 (s, 2H, CH₂), 2.10 (s, 2H, NH₂) ppm. ¹⁹F NMR (282 MHz, (CD₃)₂CO): d –137.2 (br. s, 2F, ortho-F), –140.2 (m, 4F, meta-F, BF₂) ppm. MS (MALDI): m/z [М–F]⁺ calcd. for C₁₇H₁₁BF₅N₄O 393.095; found 393.102.

8-{4-[2-(1-Maleimido-1,2,3-triazol-4-yl)-N-acetylamido]phenyl}-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (10). Cu(OAc)₂ (5 mg, 0.028 mmol) and sodium ascorbate (15 mg, 0.076 mmol) were added to a solution of BODIPY 5 (150 mg, 0.55 mmol) and propargyl maleimide 9 (371 mg, 2.75 mmol) in dry DMSO (6 mL). The reaction mixture was stirred at room temperature under an argon atmosphere in the dark for 1 h. After addition of water (200 mL), the target product was extracted with EtOAc (100 mL). The organic phase was separated, washed with water (2×100 mL), dried over Na₂SO₄, and evaporated to dryness. The resulting residue was purified by column chromatography on silica gel (eluent: CH₂Cl₂–acetone (4:1)). Yield: 162 mg (59%). UV–vis (CH₂Cl₂, λ_max/nm (e·10^–3)): 376 (13.0), 502 (46.0). IR (KBr, ν/cm^–1): 3465 (NH), 1709 (C=O), 1555, 1476 (C=C, C=N), 1260 (BF₂). ¹Н NMR (400 MHz, CDCl₃): d 9.38 (s, 1H, triazole CH), 7.94 (br. s, 1H, NH), 7.92 (br. s, 2H, pyrrole), 7.72 (d, J = 8.3 Hz, 2H, Ph), 7.51 (d, J = 8.3 Hz, 2H, Ph), 6.90 (d, J = 3.5 Hz, 2H, pyrrole), 6.72 (s, 2H, maleimide CH=CH), 6.53 (d, J = 2.2 Hz, 2H, pyrrole), 5.32 (s, 2H, CH₂), 4.87 (s, 2H, CH₂) ppm. MS (MALDI): m/z [М–F]⁺ calcd. for C₂₄H₁₈BFN₇O₃ 482.155; found 482.104.

8-{4-[2-(1-Maleimido-1,2,3-triazol-4-yl)-N-acetylamido]-2,3,5,6-tetrafluorophenyl}-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (11). Cu(OAc)₂ (5 mg, 0.028 mmol) and sodium ascorbate (15 mg, 0.076 mmol) were added to a solution of BODIPY 6 (150 mg, 0.34 mmol) and propargyl maleimide 3 (137 mg, 1.02 mmol) in dry DMSO (6 mL). The reaction mixture was stirred at room temperature under an argon atmosphere in the dark for 15 min. After addition of water (200 mL), the target product was extracted with EtOAc (100 mL). The organic phase was separated, washed with water (2×100 mL), dried over Na₂SO₄, and evaporated to dryness. The resulting residue was purified by column chromatography on silica gel (eluent: CH₂Cl₂–acetone (4:1)). Yield: 167 mg (86%). UV–vis (CH₂Cl₂, λ_max/nm (e·10^–3)): 342 (6.3), 519 (47.6). IR (KBr, ν/cm^–1): 3439 (NH), 1711 (C=O), 1569, 1479 (C=C, C=N), 1260 (BF₂). ¹Н NMR (400 MHz, (CD₃)₂CO): d 10.25 (br. s, 1H, triazole CH), 8.14 (br. s, 2H, pyrrole), 8.03 (s, 1H, NH), 7.26 (d, J = 3.5 Hz, 2H, pyrrole), 6.94 (s, 2H, maleimide CH=CH), 6.71 (d, J = 3.8 Hz, 2H, pyrrole), 5.60 (s, 2H, CH₂), 4.80 (s, 2H, CH₂) ppm. ¹⁹F NMR (282 MHz, (CD₃)₂CO): d –141.7 (m, 2F, ortho-F), –144.6 (m, 4F, meta-F, BF₂) ppm. MS (MALDI): m/z [М–F]⁺ calcd. for C₂₄H₁₄BF₅N₇O₃ 554.117; found 554.175.

8-{4-[2-(3-((m-Carboran-9-yl)thio)pyrrolidine]-2,5-dione-1-yl)-N-acetylamido]phenyl}-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (13). Carborane 12 (176 mg, 1.0 mmol) and NaOAc (17 mg, 0.2 mmol) were added to a solution of BODIPY 10 (100 mg, 0.2 mmol) in dry THF (10 mL). The reaction mixture was stirred at room temperature under an argon atmosphere in the dark for 1 h. After addition of water (100 mL), the target product was extracted with EtOAc (50 mL). The organic phase was separated, washed with water (2×100 mL), dried over Na₂SO₄, and evaporated to dryness. The resulting residue was purified by column chromatography on silica gel (eluent: CH₂Cl₂–acetone (4:1)). Yield: 95 mg (70%). UV–vis (CH₂Cl₂, λ_max/nm (e·10^–3)): 241 (18.2), 374 (11.5), 502 (38.5). IR (KBr, ν/cm^–1): 3441 (NH), 3070 (carborane CH), 2605 (carborane  BH), 1709 (C=O), 1555, 1476 (C=C, C=N), 1260 (BF₂). ¹Н NMR (400 MHz, CDCl₃): d 9.52 (br. s, 1H, triazole CH), 7.95 (br. s, 1H, NH), 7.91 (br. s, 2H, pyrrole), 7.78 (d, J = 7.6 Hz, 2H, Ph), 7.53 (d, J = 7.3 Hz, 2H, Ph), 6.92 (br. s, 2H, pyrrole), 6.53 (br. s, 2H, pyrrole), 5.31 (br. s, 2H, CH₂), 4.86 (br. s, 2H, CH₂), 3.90 (br. s, 1H, succinimide CH), 3.28 (dd, J = 18.3, 8.7 Hz, 1H, succinimide CH₂), 3.02 (br. s, 2H, carborane CH), 2.77 (dd, J = 18.3, 1.6 Hz, 1H, succinimide CH₂) ppm. ¹¹B NMR (128 MHz, CDCl₃): d 0.2 (t, J = 31 Hz, 1В, BF₂), –1.1 (br. s, 1B), –6.4 (m, 2В), –9.8 (d, J = 147 Hz, 1В), –13.6 (d, J = 139 Hz, 5В), –17.3 (d, J = 184 Hz, 1В) ppm. MS (MALDI): m/z [М–F]⁺ calcd. for C₂₆H₃₀B₁₁FN₇O₃S 658.321; found 658.365.

8-{4-[2-(3-((m-Carboran-9-yl)thio)pyrrolidine-2,5-dione-1-yl)-N-acetylamido]-2,3,5,6-tetrafluorophenyl}-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (14). Carborane 12 (150 mg, 0.85 mmol) and NaOAc (14 mg, 0.17 mmol) were added to a solution of BODIPY 11 (100 mg, 0.17 mmol) in dry THF (10 mL). The reaction mixture was stirred at room temperature under an argon atmosphere in the dark for 3 h. After addition of water (100 mL), the target product was extracted with EtOAc (50 mL). The organic phase was separated, washed with water (2×100 mL), dried over Na₂SO₄, and evaporated to dryness. The resulting residue was purified by column chromatography on silica gel (eluent: CH₂Cl₂–acetone (4:1)). Yield: 108 mg (85%). UV–vis (CH₂Cl₂, λ_max/nm (e·10^–3)): 340 (4.5), 519 (42.7). IR (KBr, ν/cm^–1): 3441 (NH), 3078 (carborane CH), 2600 (carborane BH), 1709 (C=O), 1570, 1477 (C=C, C=N), 1260 (BF₂). ¹Н NMR (400 MHz, CDCl₃): d 10.09 (br. s, 1H, triazole CH), 8.03 (br. s, 1H, NH), 7.95 (br. s, 2H, pyrrole), 6.91 (br. s, 2H, pyrrole), 6.54 (br. s, 2H, pyrrole), 5.57 (br. s, 2H, CH₂), 4.82 (br. s, 2H, CH₂), 3.86 (br. s, 1H, succinimide CH), 3.25 (dd, J = 18.3, 8.7 Hz, 1H, succinimide CH₂), 3.02 (br. s, 2H, carborane CH), 2.75 (dd, J = 18.3, 2.4 Hz, 1H, succinimide CH₂) ppm. ¹¹B NMR (128 MHz, CDCl₃): d 0.2 (t, J = 28 Hz, 1В, BF₂), –1.1 (br. s, 1B), –6.4 (m, 2В), –9.8 (d, J = 149 Hz, 1В), –13.6 (m, 5В), –17.2 (d, J = 168 Hz, 1В) ppm. ¹⁹F NMR (282 MHz, CDCl₃): d –138.0 (d, J = 13.8 Hz, 2F, ortho-F), –142.5 (d, J = 13.8 Hz, 2F, meta-F), –144.7 (dd, J = 59.7, 27.5 Hz, 2F, BF₂) ppm. MS (MALDI): m/z [М–F]⁺ calcd. for C₂₆H₂₆B₁₁F₅N₇O₃S 730.283; found 730.233.

8-{4-[2-(3-Amino-1-(N-(o-carborane-3-yl))maleimido)-N-acetylamido]phenyl}-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (16). Carborane 15 (184 mg, 0.58 mmol) and NaOAc (48 mg, 0.58 mmol) were added to a solution of BODIPY 7 (100 mg, 0.29 mmol) in dry THF (10 mL). The reaction mixture was stirred at room temperature under an argon atmosphere in the dark for 1 h. After addition of water (100 mL), the target product was extracted with EtOAc (50 mL). The organic phase was separated, washed with water (2×100 mL), dried over Na₂SO₄, and evaporated to dryness. The resulting residue was purified by column chromatography on silica gel (eluent: CH₂Cl₂–acetone (4:1)). Yield: 79 mg (47%). UV–vis (CH₂Cl₂, λ_max/nm (e·10^–3): 241 (15.0), 367 (8.6), 503 (25.4). IR (KBr, ν/cm^–1): 3429 (NH), 3076 (carborane CH), 2597 (carborane BH), 1707 (C=O), 1555, 1478 (C=C, C=N), 1260 (BF₂). ¹Н NMR (400 MHz, CDCl₃): d 8.22 (br. s, 1H, NH), 7.93 (s, 2H, pyrrole), 7.69 (d, J = 8.6 Hz, 2H, Ph), 7.54 (d, J = 8.6 Hz, 2H, Ph), 6.91 (d, J = 4.0 Hz, 2H, pyrrole), 6.55 (dd, J = 4.1, 1.9 Hz, 2H, pyrrole), 6.38 (br. s, 1H, NH), 5.07 (s, 1H, maleimide CH), 4.41 (br. s, 2H, carborane CH), 4.09 (d, J = 5.3 Hz, CH₂) ppm. ¹¹B NMR (128 MHz, CDCl₃): d 0.3 (t, J = 28 Hz, 1В, BF₂), –3.5 (m, 1B), –13.1 (m, 9В) ppm. MS (MALDI): m/z [М–F]⁺ calcd. for C₂₃H₂₆B₁₁FN₅O₃ 558.312; found 558.346.

8-{4-[2-(3-Amino-1-(N-(o-carborane-3-yl))maleimido)-N-acetylamido]-2,3,5,6-tetrafluorophenyl}-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (17). Carborane 15 (153 mg, 0.48 mmol) and NaOAc (39 mg, 0.48 mmol) were added to a solution of BODIPY 8 (100 mg, 0.24 mmol) in dry THF (10 mL). The reaction mixture was stirred at 45–50 °C under an argon atmosphere in the dark for 6 h. After addition of water (100 mL), the target product was extracted with EtOAc (50 mL). The organic phase was separated, washed with water (2×100 mL), dried over Na₂SO₄, and evaporated to dryness. The resulting residue was purified by column chromatography on silica gel (eluent: CH₂Cl₂–acetone (4:1)). Yield: 44 mg (28%). UV–vis (CH₂Cl₂, λ_max/nm (e·10^–3)): 519 (29.2). IR (KBr, ν/cm^–1): 3439 (NH), 3063 (carborane CH), 2606 (carborane BH), 1710 (C=O), 1569, 1479 (C=C, C=N), 1260 (BF₂). ¹Н NMR (400 MHz, CDCl₃): d 7.99 (br. s, 2H, pyrrole), 7.95 (br. s, 1H, NH), 6.84 (br. s, 2H, pyrrole), 6.58 (br. s, 2H, pyrrole), 6.30 (br. s, 1H, NH), 5.14 (s, 1H, maleimide CH), 4.41 (br. s, 2H, carborane CH), 4.23 (d, J = 5.1 Hz, CH₂) ppm. ¹¹B NMR (128 MHz, CDCl₃): d 0.2 (t, J = 28 Hz, 1В, BF₂), –3.4 (m, 1B), –12.1 (m, 9В) ppm. ¹⁹F NMR (282 MHz, CDCl₃): d –136.8 (d, J = 17.2 Hz, 2F, ortho-F), –142.5 (d, J = 12.6 Hz, 2F, meta-F), –144.6 (dd, J = 57.4, 28.7 Hz, 2F, BF₂) ppm. MS (MALDI): m/z [М–F]⁺ calcd. for C₂₃H₂₂B₁₁F₅N₅O₃ 630.274; found 630.248.

Conclusions

The practical synthesis of the carborane BODIPY dyes bearing succinimide and maleimide groups was developed based on the transformations of the chloroacetylamido-substituted BODIPY derivatives through the formation of the corresponding azido and amino functionalized compounds. The succinimide-substituted BODIPYs were obtained via the synthesis of the corresponding maleimides using the click reaction conjugations of the BODIPY azide groups to N-propargyl maleimide followed by the Michael addition reaction with 9-mercapto-m-carborane. The carborane maleimide-substituted BODIPYs were prepared by the reactions of the corresponding amino-substituted BODIPYs with 3-bromo-1-(N-(o-carborane-3'-yl)maleimide. All the reactions proceeded smoothly at room temperature, affording the desired compounds in good yields. These compounds are promising candidates for further biomedical investigations.

Acknowledgements

This work was supported by the Ministry of Science and Higher Education of the Russian Federation (agreement no. 075-00697-22-00) and was performed using the equipment of the Center for Molecular Composition Studies of INEOS RAS.

Electronic supplementary information

Electronic supplementary information (ESI) available online: ¹H, ¹¹B, and ¹⁹F NMR spectra as well as mass spectra of the newly synthesized compounds. For ESI, see DOI: 10.32931/io2201a.

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