Design, Synthesis, and Photophysical Properties of BODIPY-Labeled Lupane Triterpenoids

: Novel boron-dipyrromethene difluoride (4,4-difluoro-4-bora-3 α ,4 α -diaza-s-indacene) (BODIPY)-lupane triterpenoid conjugates bearing a fluorescent marker at the C-2 position of ring A of the triterpene core were obtained via the Sonogashira reaction as a key step. The starting compounds in the cross-coupling reaction were C-2 propynyl derivatives of betulinic or betulonic acids and fluorescent dyes with an iodo-group at C-2 or meso position of BODIPY-platform. The newly elaborated coupling procedure might have applicability in the synthesis of fluorescently-labeled triterpenoid conjugates suitable for biological assays.


Introduction
Pentacyclic triterpenic acids, including the lupane family of triterpenoids (betulin, betulinic, and betulonic acids), are an important class of natural plant products.The widespread availability in nature, beneficial biological and pharmacological properties (antitumor, antiviral, and antiparasitic effects), and easy transformation of native 3-OH and 17-COOH groups make these secondary plant metabolites promising scaffolds for the discovery of new drug candidates [1][2][3].To date, the derivatization of the C-3 hydroxyl and C-17 carboxyl functions in natural triterpenic acids has been applied to obtain numerous semi-synthetic derivatives, which in some cases have surpassed the parent compounds in their biological action and pharmacological parameters [4,5].Thus, some derivatives of betulinic acid with C-3 and/or C-17 side chains, including 3-O-(3′,3′-dimethylsuccinyl)betulinic acid, known as beverimate, exhibited a high inhibitory effect against HIV-1 human immunodeficiency virus [6][7][8][9].The addition of lipophilic mitochondria-targeted cationic groups to the triterpene skeleton significantly increased the cytotoxicity of triterpenoids.The resulting cationic derivatives of triterpene acids exhibited antitumor activity at low micromolar or even nanomolar concentrations [10][11][12][13][14][15][16][17].There are works detailing some aspects of the mechanism of antiviral or antitumor action of the identified lead compounds of the triterpene structure [10,14,15,17].Studies detailing molecular and cellular events involving these compounds have not yet been reported.
Over the last few years, fluorescently-labeled probes of small bioactive molecules have provided powerful means for studying biological phenomena and the mechanism of action of these molecules.Fluorescent labeling technologies offer a good opportunity to analyze the interaction of drugs with molecular targets at the cellular, subcellular levels, as well as in vivo at the level of the whole organism.Meanwhile, among the series of low molecular weight fluorescent compounds used for labeling biologically active molecules and analysis of biological phenomena, the BODIPY family fluorophores are in wide demand [18,19].The fluorophore, boron-dipyrromethene difluoride (4,4-difluoro-4bora-3α,4α-diaza-s-indacene), known under the BODIPY trademark, stand out with its many attractive spectral properties such as high absorption coefficient, high fluorescence (FL) quantum yield, photochemical stability, stability in a physiological environment, good solubility in organic solvents, and great potential for structural derivatization [19,20].The BODIPY family fluorophores have been covalently linked to numerous classes of biomolecules, including proteins [20,21], carbohydrates [22], fatty acids, and steroids [23][24][25][26][27][28].Still, only two research papers on the synthesis and fluorescent biological analysis of BODIPY-labeling of triterpenoid compounds have been reported so far [29,30].In these works, fluorescent pentacyclic triterpene conjugates have been prepared by covalent binding to the known (BODIPY-FL) BODIPY-propanoic acid fluorophore through the 3-OH and 17-COOH functional groups.Unfortunately, this resulted in a decrease or even a complete loss of the cytotoxic effect of the new compounds compared to the parent compounds.The research results are consistent with the already well-known facts about the key role of the C-3 and C-17 functional groups of triterpenoids as pharmacophores [4,5].
Here we aimed to work out a new approach for the synthesis of BODIPY-triterpenoid acids conjugates avoiding covalent binding of the triterpene core to the BODIPY-platform at the C-3 and C-17 positions.We have recently developed an efficient method for introducing a propynyl substituent at the C-2 position of the ring A of triterpenic acids and demonstrated that the terminal acetylene moiety in these compounds could be effectively involved in the CuAAC reaction and in the Sonogashira coupling reaction [31][32][33].In this research project, we applied C-2 propynyl derivatives of betulinic and betulonic acids 3-5 as initial substances for conjugation with some BODIPY dyes through the Sonogashira coupling reaction.
BODIPY iodine derivatives containing an iodine atom in the phenyl ring 17, 18 were linked directly to triterpenoids 3-5, while meso-aryl-substituted BODIPY 15, 16 containing an electron-donor (Me) or electron-withdrawing (Br) substituent in the aryl group were subjected to iodination to obtain the target monoiodo-BODIPY derivatives at the two-position 19, 20 (Scheme 2).Syntheses of meso-aryl-substituted BODIPY were carried out by a classical three-step method, starting with condensation of pyrrole rings with aryl aldehydes.In these reactions, trifluoroacetic acid or BF3•OEt2 is traditionally involved as acid catalysts protonating or chelating the carbonyl oxygen atom, and the reactions are carried out in CH2Cl2 [19].We synthesized dipyrromethanes 11-14 by the method [35] with InCl3 as an acid catalyst.
The reactions were carried out at a large (20 molar) excess of the pyrrole or dimethylpyrrole, serving as a reagent and solvent at once.These conditions offered a significant reduction in the formation of pyrrole oligomerization by-products and preparation of the target dipyrromethanes 11-14 in high yields (67-87%).Oxidation of dipyrromethanes by 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) gave dipyrromethenes which were reacted with a 16-fold excess of BF3•OEt2 without isolation and purification under typical conditions involving Et3N [36].Two-step one-pot synthesis helped to achieve target BOD-IPY derivatives 15-18 in 28-67% yields (Scheme 2).The oxidation and complexation reactions of dipyrromethane 14 were considerably complicated by the formation of oligomerization products; thorough chromatographic purification of the reaction mixture on silica gel gave fluorophore 18 in a relatively low yield (28%).
The record shows that the introduction of halogens, usually Br or I atoms, at positions two and six or three and five of the BODIPY platform bring about a bathochromic shift in the absorption and emission spectra and FL quenching as compared to parent dyes [37].The products of halogenation of pyrrole fragments at these positions are usually further applied to implement Pd-catalyzed coupling reactions, including Sonogashira coupling [37][38][39][40][41][42].The analysis of the mesomeric structures of BODIPY revealed that the two-and six-positions have a small positive charge compared to other carbon atoms of the pyrrole fragments.Therefore, these positions can be most susceptible to electrophilic attacks.The study [37] illustrated that iodination of a BODIPY dye with unsubstituted pyrrole rings using ICl in an equimolar ratio gives the C-2 monoiodo derivative of BODIPY in a relatively high yield and selectivity.In our study, iodination of meso-substituted derivatives of BODIPY 15, 16 according to the method [37] at an equimolar ratio of fluorophores and ICl in a mixture of solvents CH2Cl2/MeOH produced the target iodides 19, 20 after their purification by column chromatography on silica gel in 76 and 77% yields, respectively.
The synthesis of BODIPY-triterpenoid 21-26 conjugates linked through propynyl or phenylpropynyl bridges at the meso position of the dye or at the C-2 position of the BOD-IPY platform was carried out by cross-coupling according to the Sonogashira reaction.The reaction proceeded for 1-3 h at room temperature in Et3N/DMF (1.5:1) medium under the action of PdCl2(PPh3)2 and CuI catalysts.The yields of target products 21-26 were 53-88% after a silica gel column chromatography (Scheme 3).It should be pointed out that the propynyl derivative of betulonic acid 3 was not involved in the cross-coupling reaction with the meso-(4-bromophenyl) substituted derivative BODIPY 15.Under the above conditions, dimerization of the methyl ester of betulonic acid was registered; the target product could not be observed even in trace amounts (Scheme 3).The compounds 21-26 were studied by various spectroscopic techniques.The molecular ion peak in mass spectra and matching elemental analysis with the expected composition of compounds confirmed the identity of the compounds 21-26.The NMR spectra data of compounds 21-26 slightly differed.As such, an extensive analysis of the NMR spectrum for compound 21 is presented here.Thus, the signals of the carbon atoms of the acetylene bond C-2′ and C-3′ were observed to shift downfield (to 92.1 and 80.9 ppm, respectively) in the 13C NMR spectrum of compound 21, compared to the original propynyl derivative 3 (82.8and 69.1 ppm, respectively).Moreover, a signal of a quaternary carbon atom in the region of 126.9 ppm was registered in the 13C NMR spectrum, which we identified as the carbon atom bonded to the acetylene fragment.The 1H NMR spectrum of compound 21 revealed the presence of a new multiplet signal in the region of 7.54 ppm, belonging to the signals of the phenyl substituent, as well as the presence of signals of pyrrole protons in the region of 6.94 (H-1′'), 6.56 (H-2′'), 7.96 (H-3′') ppm.The collected spectral data conclude that there is a covalent bond in the structure of compound 21 between the carbon of the phenyl substituent BODIPY and the carbon of the acetylene fragment of the triterpene framework and, consequently, the involvement of functional groups in the Sonogashira cross-coupling reaction.
In fact, the aryl substituent in the meso position of the BODIPY fluorophores is located almost perpendicular to the BODIPY nucleus.Therefore, it participates little in electronic conjugation and does not have a significant impact on the change in the absorption and emission wavelengths of the dye.At the same time, the introduction of π-electron donors such as phenylethynyl or ethynyl groups in the 2,6-or 3,5-position of the BODIPY skeleton can noticeably increase the absorption and emission wavelengths compared to the unsubstituted BODIPY molecule [41,42].In this regard, we decided to investigate the spectroscopic properties of the fluorescent conjugates of tritepenoid-BODIPY 21-24 in comparison with conjugates 25, 26.
The spectroscopic properties of BODIPY-fluorophores 17-20 and conjugates 21-26 were studied in MeOH.The findings of this study are summarized in Table 1 and Figure 1 (absorption and photoluminescence (PL) spectra of compounds 17-26 in MeOH).The form of the absorption and emission spectra of BODIPY-derivatives 17-20 corresponds to the previously reported similar compounds [37,38,41].Characteristic maxima with a rather high molar extinction coefficients and small Stokes shifts are recorded in the absorption and PL spectra of these compounds.The iodine substituent at the C-2 atom of the BODIPY core caused a significant red-shift of the absorption and emission maxima, while quenching of the quantum yield was observed.The presence of methyl substituents in the pyrrolic fragments of BODIPY 18 and its conjugate with triterpenoid 22 considerably increased the quantum yield and caused a slight hypsochromic shift of the PL maximum in comparison to unsubstituted analogs 17 and 21.Attachment of the triterpenoid 2 to BODIPY moiety at C-2 position via propynyl spacer produced a noticeable bathochromic shift in the absorption maximum in conjugates 25 and 26 (Table 1 and Figure 1c,d).The luminescence of conjugates 25 and 26 (570-580 nm) also shifted to the long-wavelength region of the spectrum relative to unsubstituted BODIPY (518 nm).Furthermore, compared to the initial iodine derivatives BODIPY 19 and 20, a noticeable increase in the quantum yield and Stokes shifts were observed in conjugates 25 and 26.

Conclusions
In this article, an efficient synthesis has been developed, and six new fluorescent conjugates of lupane triterpenoids were synthesized, with the triterpene core linked to the BODIPY fluorophore at the C-8 or C-2 positions of the dye through propynyl or phenylpropynyl spacers.The study of the fluorescent properties of the resulting conjugates revealed that the conjugates (compounds 21-24) retained the fluorescent properties of the initial chromophores upon covalent binding of terpenoids to the BODIPY nucleus at the meso position.Meanwhile, the acetylene fragment in the propynyl bridge at the C-2 atom of the pyrrole ring increased the π-electronic delocalization of BODIPY-backbone in compounds 25 and 26.Consequently, conjugates 25 and 26 demonstrated a significant bathochromic shift of the absorption maximum (25, λabs 551 nm) and the luminescence maximum (λem 578 nm) relative to BODIPY (λem 518 nm).Moreover, compared to the initial substances, iodine derivatives of BODIPY 19 and 20, conjugates 25 and 26 exhibited an increase in quantum yields and Stokes shifts.We believe that the novel approach developed by our research group can find application in the synthesis of BODIPY-triterpenoid conjugates as potential fluorescent probes for biological studies of triterpene compounds.
Conjugation of the triterpenoid core to BODIPY at the C-8 atom of the dye through the phenylethynyl spacer in compounds 21-24 did not change the position of the absorption and PL maxima in comparison with the initial fluorophores (for example, compounds 17 and 21).

Author Contributions:
Validation and writing-review and editing, A.S.; performing the chemistry experiments, R.G. and E.D.; performing the photoluminescent (PL) experiments, A.T. The manuscript was prepared through the contributions of A.S., R.G., and D.N.All authors have read and agreed to the published version of the manuscript.Funding: This research was funded by RUSSIAN SCIENCE FOUNDATION, grant number 19-73-00155.

Table 1 .
Spectral and luminescent properties of compounds 17-26 at T = 297 K in MeOH.