Click Chemistry Inspired Synthesis of Hydroxyanthracene Triazolyl Glycoconjugates

Novel hydroxyanthracene-based terminal alkynes 3 and 5a/b were synthesized by the acetylide addition reaction at the 9,10-position of anthraquinone 1 under mild conditions. The developed alkynes 3, 5a, and 5b on Huisgen azide-alkyne cycloaddition reaction with azido-sugars 6 in the presence of Cu(I) catalyst provided a series of triazole fasten hydroxyanthracene glycoconjugates 7, 8, and 9, respectively, in good yields. The representative compounds 9 and 7h were successfully deprotected under room-temperature conditions to liberate the corresponding free glycoconjugates 10 and 11, respectively. Further, structures of a few compounds were unmaliciously evidenced by their single-crystal X-ray.


INTRODUCTION
Anthranoids are a class of abundant anthracene derivatives, e.g., anthranol, anthrone, anthraquinone, etc., widely distributed in the plant kingdom. 1,2 Many anthranoids, e.g., emodin, rhein, chrysophanol, physcion, etc., isolated from roots and barks of several plants, molds, and lichens have displayed some noticeable biological activities. 3 They can be easily embedded in the DNA double helix and often exhibit potential antimalarial, antiplatelet, and neuroprotective properties. 4,5 A diverse range of anthracyclines, e.g., idarubicin, daunorubicin, epirubicin, etc., have been already approved by the US FDA for the treatment of infectious (bacterial, fungal, and viral) and inflammatory diseases and oncology. 6−10 Besides, many anthraquinones have profound applications as dyes in the textile and agrochemicals industries.
There has been increasing attention in the design and synthesis of novel anthraquinone glycosides with a belief that they may possess stronger laxative effects than the free aglycone. 11 Numerous studies have established that the anthraquinone glycosides are very less likely to enter into the systemic circulation, hence, possess laxative effects even at lower doses in comparison to the corresponding aglycones. Therefore, there is interest in the development of new strategies that may offer a potentially more reliable route to the homogeneous glycoconjugates, leading to either the exact copies of naturally occurring glycosides or, alternatively, mimicking the glycosidic linkages found in nature. 12 An appreciable benefit of the latter approach is that the artificial glycoconjugates may retain the geometric and special characteristic of native glycoform and exhibit hydrolytic stability toward glycosidases and glycosyltransferases.
Cu(I)-catalyzed azide−alkyne cycloaddition (CuAAC) has emerged as a straightforward molecular linking strategy to obtain products relevant to medicine, biology, and materials science. 13,14 Earlier, several functionalized anthraquinone derivatives have been synthesized for a diverse range of applications, e.g., photoinitiators in polymerization, electochemical enhancers, chemo-sensor in metal ion sensing, colorimetric reagent for anions, enzyme inhibitors, etc. 15,16 However, in all such compounds, the functionalization has been carried out exclusively at the benzene residues of the anthraquinone skeleton. There are no reports on "click" chemistry inspired functionalization at quinone-carbons (9,10postion) of anthraquinone. Therefore, we herein report the synthesis of some terminal alkynes, such as, ethynyl-10hydroxyanthracen-9-one 3, and diastereoisomeric compounds, cis/trans-diethynyl-9,10-dihydroanthracene-diols 5a/b, in good yields under the mild reaction conditions from commercially available anthraquinone 1 as a substrate. The CuAAC reaction of terminal alkynes 3, 5a, and 5b with diverse range of azido-sugars 6 furnished a series of regioselective glycoconjugates 7, 8, and 9 in 80−90% yields. We also investigated the deprotection of compounds 9 and 7h to afford the corresponding free glycoconjugates 10 and 11, respectively, in good yields. Click chemistry inspired functionalization at the 9,10-position of anthraquinone 1 has never been attempted; hence, the developed terminal alkynes 5a/b in addition to the triazolyl glycoconjugates 7−11 are new.

RESULTS AND DISCUSSION
The strategy begins with the reaction of anthraquinone 1 with lithium (trimethysilyl)acetylide (2.0 equiv) in the presence of n-butyl lithium under an inert condition giving an acetylide addition product 2 in good yields (Scheme 1). The compound 2 after purification with silica gel (Flash) column chromatography (CC) was characterized using spectroscopic methods (UV, FT-IR, NMR, and HRMS). The unambiguous structure of 2 was evidenced by single-crystal X-ray diffraction analysis (see Supporting Information Table S2).
Generally, the acetylide ion attacks a carbonyl group of compound 1 to produce an alkynol 2 (alcohol on carbon adjacent to triple bond) as a sole product. However, in the presence of an excess of lithium (trimethysilyl)acetylide, both the carbonyl groups in compound 1 undergo acetylide addition giving a mixture of cis/trans-4a/b with a high diastereoselectivity. The diastereoselectivity of the reaction was found to be solvent dependent, such as, the silylation of compound 1 in the presence of tetrahydrofuran (THF) resulted in the formation of a mixture of cis/trans-4a/b (1:9). However, a reverse diastereoselectivity in product formation was observed while carrying out the reaction in the presence of anhydrous toluene as a reaction solvent under an inert reaction condition (Scheme 2). 17 Both the diastereomers were isolated successfully using silica gel (Flash) CC followed by characterization through UV, FT-IR, NMR, and HRMS analyses. Unambiguous structure of diastereomer 4a was evidenced by single-crystal Xray diffraction analysis (see Supporting Information Figure S51 and Table S4).
The desilylation of TMS derivatives 2 and 4a/b was affected by treatment with methanolic KOH solution in the presence of THF to afford the corresponding terminal alkynes 3 and 5a/b, respectively, in good yields (Scheme 2). All the developed terminal alkynes 3, 5a, and 5b were isolated from reaction mixture via silica gel (Flash) CC. Their structures were deduced by FT-IR, NMR, and HRMS analyses. The unambiguous structure of compound 5a was evidenced by single-crystal X-ray analysis (see Supporting Information Figure S52 and Table S5).
The CuAAC reaction of alkynes 3 and 5a/b was carried out with numerous deoxy-azido-sugars 6 readily prepared from commercially available monosaccharides via application of protection and modification methods reported in literature (see Supporting Information Table S1). 18,19 Generally, the CuAAC reaction is known to proceed in protic and aprotic solvents. They are well tolerated by most of the functional groups. Therefore, reaction of terminal alkyne 3 with azidosugar 6a in the presence of DIPEA and CuI (catalytic amount) in dry dichloromethane (DCM) as reaction solvent under the inert atmosphere afforded 2-(4-(9-hydroxy-10-oxo-9,10-dihydroanthracen-9-yl)-1H-1,2,3-triazol-1-yl)-2,3,4-tri-O-acetyl-β-D-xylopyranose 7a in 90% yield. The reaction was completed smoothly in 12 h at room temperature (RT). function resonated as a singlet at δ 7.39 ppm. The anomeric protons of sugar residue appeared as a multiplet between δ 5.72−5.68 ppm. The oxy-methylene group resonated as a multiplet observed between δ 5.37−4.11, while a broad one proton singlet at δ 3.52 was attributed to the presence of a hydroxyl group in the compound 7a. Signals for methyl hydrogens were observed at δ 2.05, δ 2.01, and δ 1.85 ppm. The unambiguous structure of compound 7a ( Figure 1) was established by single-crystal X-ray analysis (see Supporting Information Table S6).
The methodology was further used successfully for the cycloaddition of other alkynes with azido-sugars to afford the libraries of 10-hydroxyanthracen-9(10H)-one linked triazolylglycoconjugates 7 (Table 1) and bis-triazolyl glycoconjugates 8 and 9 (Table 2). Although, the regioselective CuAAC reaction of terminal alkyne 5a 20−22 with azido-sugars 6 took a slightly longer time for completion. However, the yields of resulting bis-triazolyl derivatives 8 were found to be consistent, such as, the unreacted terminal alkynes 5a could not be detected in the product mixture. The structure of compounds 8 was deduced by NMR spectroscopy, wherein the 1 H NMR spectrum of compounds 8 was observed clean and simple due to a relatively symmetric proton environment compared to the compound 9 that was obtained by CuAAC reaction of trans-alkyne 5b with the azido-sugar 6b.
In view of potential applications of the developed triazolyl glycoconjugates 7, 8, and 9 in future biological assays, we also investigated their deprotection reactions to obtain the corresponding free glycoconjugates. A representative acetylprotected compound 9 could be deprotected successfully by reaction with sodium alkoxide to afford the corresponding free glycoconjugate 10 in good yield (Scheme 3). 23 Similarly, the compounds bearing isopropylidene function could also be deprotected easily, such as a representative compound 7h on reaction with trifluoroacetic acid (TFA) in water as a medium resulted in the formation of corresponding free triazolyl glycoconjugate 11 (Scheme 4). The compound 11 could be isolated in good yields via silica gel (Flash) CC. The HR-MS spectrum of compound 11 exhibited a molecular ion peak [M  8 Hz) was assigned to the two aromatic protons, while the triazolylhydrogen appeared as singlet at δ 7.96 ppm. The compound 11 exhibited a total of three multiplets, two protons each observed between δ 7.75−7.43 for the corresponding six hydroxyanthracen-9(10H)-one nucleus. A singlet corresponding to the hydroxyl-group was observed at δ 6.94. The four hydroxyl protons of the sugar residue resonated as four multiplets appeared between δ 4.84−4.52 ppm. Two methylene protons adjacent to triazole ring resonated as multiplet between δ 4.33−4.27. The signal of tertiary hydrogens in sugar residue appeared as a multiplet between δ 4.14−3.45 ppm.

CONCLUSION
A diverse range of hydroxyl-anthracene-based terminal alkynes were prepared in good yields by the acetylide addition at the 9,10-position of anthraquinone via reaction with lithium-(trimethysilyl)acetylide in the presence of n-butyl lithium under inert conditions. The CuAAC reaction of the developed terminal alkynes with deoxy-sugar azides resulted in the synthesis of a series of mono-and bis-triazolyl glycoconjugates in excellent yields. The representative compounds 9 and 7h were successfully deprotected to liberate the corresponding free triazolyl glycoconjugates 10 and 11, respectively, for application in future biological assays.

General Information.
Pure analytical grade reagents and solvents were used for syntheses. Experiments were performed in glassware dried in an oven at 100°C. Reactions were monitored on silica gel 60 F 254 aluminum TLC plates (Merck) and visualized under UV lamp or using spraying reagent (Draggendorff reagent, iodine vapors, or methanolic-H 2 SO 4 solution) followed by charring at 100°C. 1 H (500 MHz) and 13 C NMR (125 MHz) were recorded in CDCl 3 or  DMSO-d 6 as solvents with chemical shift values given in δ (ppm) relative to trimethylsilyl (TMS). Data shown as chemical shift, integration, multiplicity (s = singlet, d = doublet, t = triplet, m = multiplet) and coupling constants (Hz). High-resolution mass spectra (HR-MS) were obtained by ESI-TOF mode. IR spectra were obtained on a FT-IR Agilent Cary 660 series spectrometer. Single-crystal X-ray data of compounds were obtained from Super-Nova X-ray diffractometer with scan speed set at 5 s/scan, and data reduction was done by CrysAlisPro software. The melting point of compounds was determined on M565 Buchi melting apparatus.

Procedure for Synthesis of Trimethylsilylanthracen-9-one (2).
A three neck RB flask (250 mL) was charged with anhydrous toluene (100 mL) and trimethylsilyl acetylene (2.84 mL, 20 mmol). To the solution after purging with a positive pressure of argon for 30 min, n-butyllithium (2.5 M in hexane) (8.0 mL, 20 mmol) was added in a dropwise manner   under argon pressure at 0°C. The reaction mixture was allowed to attain RT under stirring for 45 min. Solid anthraquinone (2.08 g, 10 mmol) was added to the reaction mixture in one lot. The resulting mixture was stirred for 14 h at RT. After time elapsed, the reaction was further heated at 50− 55°C for a period of 36 h. After quenching with ammonium chloride solution under cooling and extraction with ethyl acetate, the organic phase was collected over dried Na 2 SO 4 . Concentration of organic layer under the reduced pressure afforded a brown solid which was chromatographed over silica gel (Flash) to afford compound 2 as a white crystalline solid in 80% yield.

Procedure for Synthesis of bis-Trimethylsilylanthracen-9-one (4a/b).
A three neck RB flask (250 mL) was charged with anhydrous toluene (100 mL) and trimethylsilyl acetylene (7.4 mL, 52 mmol, 5.2 equiv). To the solution after purging with a positive pressure of argon for 30 min, nbutyllithium (2.5 M in hexane) (20.0 mL, 50 mmol) was added in a dropwise manner under argon pressure at 0°C. The reaction mixture was allowed to attain RT under stirring condition of 45 min. Solid anthraquinone (2.08 g, 10 mmol, 1 equiv) was added to the reaction mixture in one lot. The resulting mixture was stirred for 14 h at RT. After time elapsed, the reaction was further heated at 50−55°C for 36 h. After quenching with ammonium chloride solution under cooling and extraction with ethyl acetate, the organic phase was collected over dried Na 2 SO 4 . Concentration of organic layer under reduced pressure afforded a brown solid, where diastereoisomers 4a and 4b were isolated through silica gel (Flash) CC in ∼80% and 12% yield, respectively.     [3,4-d] [1,3]