Tetrahedron Letters 52 (2011) 894–898

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Tetrahedron Letters

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‘Click’ to bidentate bis-triazolyl sugar derivatives with promising biologicaland optical features

Zhuo Song a, Xiao-Peng He a,c, Xiao-Ping Jin a, Li-Xin Gao b, Li Sheng b, Yu-Bo Zhou b, Jia Li b,⇑,Guo-Rong Chen a,⇑a Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, Shanghai 200237, PR Chinab National Center for Drug Screening, State Key Laboratory of Drug Research, Shanghai Institute of Materia Medica, Shanghai Institutes of Biological Sciences,Chinese Academy of Sciences, Shanghai 201203, PR Chinac School of Pharmacy, East China University of Science and Technology, Shanghai 200237, PR China

a r t i c l e i n f o a b s t r a c t

Article history:Received 7 November 2010Revised 3 December 2010Accepted 14 December 2010Available online 21 December 2010

Keywords:Click reactionPTP1B inhibitorNi2+ sensorGlycosyl bis-triazoleBidentate O-glucoside

0040-4039/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.tetlet.2010.12.055

⇑ Corresponding authors. Tel.: +86 021 64253016;E-mail addresses: jli@mail.shcnc.ac.cn (J. Li), mr

(G.-R. Chen).

Bidentate 1-O-methyl-a-D-pyranoglucosides bearing two triazolyl a-ketoester groups on the 2,6- or 3,4-positions of sugar scaffold were efficiently synthesized via Cu(I)-catalyzed azide-alkyne 1,3-dipolar cyclo-addition (click reaction) in good yields. These newly featured sugar derivatives displayed favorable inhib-itory activity on protein tyrosine phosphatase 1B (PTP1B) and unexpected selective fluorescencequenching in the presence of Ni2+.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Sugars are one of the most abundant natural ingredients, whichgovern myriad biological and pathological processes.1 Consideringtheir functional diversity, natural abundance and high biocompat-ibility, the versatile sugar moieties have been employed as theessential structure for the design of various functionalized glyco-conjugates, facilitating a multitude of practical physical, chemicaland biological studies.

For example, modified sugars could be immobilized on properlyderivatized surfaces to fabricate glycan microarrays for the elabo-ration of carbohydrate-receptor interactions.2 Intact sugars mayparticipate in the glycoprotein synthesis,3a serving as a leadingprecursor for lectin-directed enzyme activated prodrug therapy(LEAPT).3b Recent studies also indicated the sugar-containingamphiphilic glycolipids favorable as non-ionic and green surfac-tants with low toxicity and high biodegradation.4 In addition tothese compelling functionalities, the structurally and configura-tionally diverse sugars have fueled considerable interest as a cen-tral scaffold for drug design.5

Since the definition of click chemistry by Sharpless and co-workers,6 the sugar family has turned up its robust chemical ally

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fax: +86 021 64252758.s_guorongchen@ecust.edu.cn

toward the proliferation of novel functional heterocyclic glyco-ser-ies.7,8 One of the latest fascinating successes of click-assisted sugarchemical biology shall be the revelation of Cu-free click reaction9

in conjunction with fluorescent chemistry toward the in-vivolabeling of living organisms, enabling spatiotemporal tracking oftheir development processes by Bertozzi and co-workers.7c,10 Inaddition, the modular, quick, and regioselective click chemistryhas also proven efficacious toward the combinatorial synthesis ofsugar-based bioactive compounds.7a,8a

We have recently synthesized triazole-linked dimeric aryl C-glycosides11a via Cu(I)-catalyzed azide-alkyne 1,3-dipolar cycload-dition (i.e., click reaction) as inhibitors of protein tyrosine phos-phatase 1B,11b a negative signaling enzyme associated with type2 diabetes, obesity, and breast cancer.12 With a continuous attemptto prepare ‘bidentate’ sugar-based PTP1B inhibitors via clickreaction as a strategy for enhancing the binding affinity with theenzyme target,13 we sought to synthesize chelator-like14 disubsti-tuted glycosides based on a monosaccharide scaffold. The previ-ously validated a-ketoester derivative 6 (Scheme 1)15 was usedas the primary pharmacophore, which was envisioned to be simul-taneously coupled on two adjacent positions (C2,6 or C3,4) of anaryl 1-O-methyl-a-D-pyranoglucosyl ring.

Furthermore, enlightened by a large number of recent investi-gations on click-based fluorescent chemistry,16 we have tentativelyassessed the ion-sensing property of the synthesized compoundsvia fluorescence spectroscopy. To our surprise, these bis-triazolyl

Scheme 1. Reagents and conditions: (a) propargyl bromide, NaH, DMF, 0 �C to rt;(b) (i) AcCl, MeOH, rt; (ii) BnBr, NaH, DMF, 0 �C to rt; (c) VcNa, CuSO4�5H2O, CH2Cl2/water, rt.

Table 1Inhibitory activity of compounds 7 and 8 tested on PTP1B

Entry Compound Inhibition ratea (%) IC50a (lM)

1 7 96.3 1.52 8 94.0 4.8

a Values are means of three experiments.

Figure 1. UV–vis and fluorescence spectra of (a)

Z. Song et al. / Tetrahedron Letters 52 (2011) 894–898 895

sugar derivatives exhibited marked and selective fluorescencequenching in the presence of Ni2+ over a panel of tested metal cat-ions. We thus report here the ‘click’ synthesis, preliminary biolog-ical activity, and unexpected optical properties of two newlyfeatured difunctional triazolyl O-glycosides.

2. Results and discussion

2.1. Synthesis

To achieve the triazole-linked bidentate glycosides, 2,6- or 3,4-di-2-O-propynyl 1-O-methyl-a-D-pyranoglucosides were first pre-pared while the a-ketoester azide 6 (Scheme 1) was synthesizedaccording to a known literature report.15 Since the aryl substitu-ents have proven crucial for PTP1B inhibition via our formerstudy,11b benzylated sugar alkynes were prepared. As shown inScheme 1, The 2,6-di-2-O-propynyl glucoside 2 were synthesizedby introducing propargyl bromide onto the 2,6-positions of 3,4-di-O-benzyl glucoside 117a in the presence of NaH. For the prepara-tion of benzylated 3,4-di-2-O-propynyl glucoside 5, selectivelysilylated 317b was first propargylated and then followed by AcClpromoted desilylation and O-benzylation.

The click reaction of the azide with sugar alkynes was sequen-tially performed. However, our initial attempt toward such reac-tion under the promotion of catalytic amount of sodiumascorbate (VcNa, 0.4 equiv) and CuSO4�5H2O (0.2 equiv) failed tocompletely give bis-triazoles after over night stirring. This mightbe ascribed to the spatial hindrance of the previously formedmonomeric triazolyl intermediates, which largely impede the fur-ther formation of the difunctional final cycloadducts.14a We thentended to optimize the reaction condition by increasing the cata-lyst loading. To our delight, in the presence of 6 equiv VcNa and

compound 7 and (b) compound 8 in MeCN.

Figure 2. Fluorescence spectra of (a) compound 7 (10 lM) in CH3CN upon addition of various NO3� salt (10 equiv) and (b) compound 8 (10 lM) in CH3CN upon addition of

various NO3� salt (40 equiv).

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3 equiv CuSO4�5H2O with 6 h stirring, both desired bis-triazoles 7and 8 were achieved efficiently in good yields of 80% and 87%,respectively (Scheme 1).19

2.2. Biological activity

The inhibitory activity of the prepared glucosides 7 and 8 wasassayed on PTP1B via our previously developed methods.18 Asshown in both entries 1 and 2 in Table 1, the two compounds firstexhibited excellent inhibitory effect against PTP1B as reflected bytheir high inhibitory rates (96% for 7 and 94% for 8). Their IC50 val-ues were then calculated from the nonlinear curve fitting of per-cent inhibition (inhibition (%)) vs inhibitor concentration [I] byusing the following equation: inhibition (%) = 100/{1 + (IC50/[I])k},where k is the Hill coefficient and were assigned to 1.5 lM for com-pound 7 and 4.8 lM for compound 8.18 In addition, the 2,6-disub-stitued glucoside 7 (entry 1) possesses more than 3.5-foldenhanced inhibitory activity comparing to 3,4-disubstituted gluco-side 8 reflected by IC50 values. Such biological results suggestdisubstitution on monosaccharide scaffold viable as a strategy to-ward the development of novel enzymatic inhibitors and for theprobing of structural preference of the targeted enzyme by varyingsubstitution position(s).

2.3. Photochemistry

Click chemistry has recently been fruitfully introduced into thedesign of fluorescent ion-chemosensors due to its high regioselec-tivity and synthetic expediency. Meanwhile, the formed triazolegroup(s) may simultaneously enhance the fluorescence inten-sity16a and/or act as an ion binding site owning to its N-rich nature.A large number of sugar-based ion-sensors have been disclosed inthe past few years via click chemistry.16 Enlightened by theseinteresting results, we envisioned our newly constructed chela-tor-like glycosyl bis-triazoles applicable as potential metal sensorsthat are optically detectable.

The optical properties of compounds 7 and 8 were first studiedby monitoring their UV–vis absorption and fluorescence spectra. Asgiven in Figure 1, compound 7 showed an absorption band cen-tered at 268 nm and a major fluorescence emission at 408 nm(Fig. 1a) while compound 8 displayed an absorption band centered

at 265 nm and a major fluorescence emission at 409 nm (Fig. 1b) inCH3CN.

We then tended to monitor the fluorescence changes uponaddition of the nitrate of a wide range of metal cations in MeCN,including Na+, K+, Mn2+, Ca2+, Co2+, Ag+, Mg2+, Cd2+, Cu2+, andNi2+. To our surprise, upon addition of Ni2+, the fluorescence inten-sity of both compounds 7 and 8 decreased remarkably as reflectedin their fluorescence emission spectra (Fig. 2). In contrast, othermetal cations added gave unapparent changes in fluorescenceintensity, suggesting they could act as selective fluorescence sen-sor for Ni2+. The emission of compound 7 was almost totallyquenched by 10 equiv of Ni2+ (Fig. 2a) whereas 40 equiv of Ni2+

was required for quenching the emission of compound 8(Fig. 2b), suggesting its relatively worse sensitivity.

The fluorescence spectra of compounds 7 and 8 observed duringthe titration of various concentrations of Ni2+ were then shown inFigure 3. When solutions containing 10 lM of compound 7 weregradually added with Ni2+ (0-6 equiv) in CH3CN, the fluorescenceemission (excited at 268 nm) peaked at 408 nm precipitouslydropped (Fig. 3a). Meanwhile, when solutions containing 10 lMof compound 8 were gradually added with Ni2+ (0-40 equiv) inCH3CN, the fluorescence intensity (excited at 265 nm) decreasedremarkably at 409 nm (Fig. 3b). In addition, the complexationtimes were determined and were shown in Supplementary Fig. 1(Supplementary data, 2.0 min for 7 (Supplementary Fig. 1a) and1.4 min for 8 (Supplementary Fig. 1b)).

Next, for further ascertaining their selectivity toward Ni2+, thecompetition experiment by adding 10 equiv Ni2+ to the competingmetal ion (10 equiv)-ligand 7 and 40 equiv Ni2+ to metal ion(40 equiv)-ligand 8 mixtures was conducted, respectively, andwas illustrated in Figure 4. The I0/In ratio directly reflects their fluo-rescence quenching efficiency. Clearly, in the presence of a repre-sentative selection of alkali metal ions (Na+, K+), alkaline earthmetal ions (Mg2+, Ca2+), and transition-metal ions (Ag+, Co2+,Cd2+, Mn2+, Cu2+), the fluorescence intensity of both compounds 7(Fig. 4a) and 8 (Fig. 4b) was not significantly affected (gray col-umns). In contrast, upon successive addition of Ni2+, the emissionof compound 7 (10 equiv Ni2+) and compound 8 (40 equiv Ni2+)was similarly quenched (black columns) as in the case of Ni2+ alonewith compound 8 (Fig. 4b) being comparatively less interfered byother competing metal cations.

Figure 4. Fluorescence intensity change profiles of (a) compound 7 (10 lM) at 408 nm in CH3CN with selected cations (10 equiv) in the absence or presence of Ni2+ (100 lM),kex = 268 nm and (b) compound 8 (10 lM) at 409 nm in CH3CN with selected cations (40 equiv) in the absence or presence of Ni2+ (40 equiv), kex = 265 nm.

Figure 3. Fluorescence spectra obtained during the titration of (a) compound 7 (10 lM) with Ni2+ ion (from top to bottom: 0, 0.2, 0.4, 0.6, 0.8, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0 equiv)excited at 268 nm and (b) compound 8 (10 lM) in CH3CN with Ni2+ ion (from top to bottom: 0, 4.0, 8.0, 12, 16, 20, 24, 28, 32, 36, 40 equiv) excited at 265 nm.

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The preliminary optical spectra depicted above have thus re-vealed an unexpected selective Ni2+-quenching property of thebidentate bis-triazolyl glucosides prepared in this study, thusprompting more advanced relevant studies in the future.

3. Conclusion

In summary, we have achieved in this study the efficient syn-thesis of two multifunctional bidentate sugar derivatives via clickreaction. They were determined to serve as favorable PTP1B inhib-itors, which simultaneously displayed unexpected selective fluo-rescence quenching in the presence of Ni2+. Such newly featuredchelator-like glyco-structures may furnish new insight towardthe development of both sugar-based PTP1B inhibitors and fluores-cent Ni2+ chemosensors. Furthermore, the ‘click-to-bidentate-su-gar-triazole’ strategy employed here is deemed desirable forconstructing more multi-functionalized glyco-derivatives based

on the versatile sugar scaffold and such study is currently under-way in our laboratories.

Acknowledgments

Project supported by National Natural Science Foundation ofChina (Grant No. 20876045, No. 30801405), National Basic Re-search Program of China (No. 2007CB914201), National Science &Technology Major Project of China ‘Key New Drug Creation andManufacturing Program’ (No. 2009ZX09302-001), ShanghaiScience and Technology Community (No. 10410702700,09DZ2291200) and Chinese Academy of Sciences (No. KSCX2-YW-R-168). X.-P.H. also gratefully acknowledges the French Embassyin Beijing, PR China for a co-tutored doctoral fellowship and hisFrench co-advisor Professor Juan Xie (ENS Cachan, Paris, France).Professor Jian-Li Hua (East China University of Science and Technol-ogy) is warmly thanked for her critical reading on the paper.

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Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.tetlet.2010.12.055.

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19. General procedure for click reaction: To a biphasic solution of di-2-O-propynylsugar (1.0 equiv) and the azide 6 (2.2 equiv) in CH2Cl2 (5 mL) and H2O (5 mL),Na ascorbate (6 equiv) and CuSO4�5H2O (3 equiv) were added, stirring for 8 h atrt. The resulting mixture was then directly diluted with CH2Cl2 and washedwith water. The combined organic layers were dried over MgSO4, filtered andevaporated in vacuum to give a crude residue which was then purified bycolumn chromatography. Characterization of compound 7. Dimethyl 2,2’-((4,4’-((((2R,3R,4S,5R,6S)-5-(benzyloxy)-2-((benzyloxy)methyl)-6-methoxytetrahydro-2H-pyran-3,4-diyl)bis(oxy))bis(methylene))bis(1H-1,2,3-triazole-4,1-diyl))bis(4,1-phenylene))bis(2-oxoacetate) (7): From compound 5 (270.0 mg, 0.60 mmol) and6 (246.1 mg, 1.2 mmol) afforded 7 as a white powder (411.2 mg, 79.7%).Rf = 0.25 (EtOAc/petroleum ether, 1:1); [a]D = +58.0 (c 0.4, CH2Cl2); 1H NMR(400 MHz, CDCl3): d = 8.19–8.15 (m, 4H), 7.93 (s, 1H), 7.74 (d, 2H, J = 8.8 Hz),7.69 (d, 2H, J = 8.8 Hz), 7.39 (d, 2H, J = 7.2 Hz), 7.35–7.22 (m, 8H), 5.02 (d, 1H,J = 11.6 Hz), 4.99 (d, 1H, J = 8.4 Hz), 4.97 (d, 1H, J = 8.4 Hz), 4.97 (d, 1H,J = 7.6 Hz), 4.93–4.88 (m, 2H), 4.80 (d, 1H, J = 12.0 Hz), 4.66 (d, 1H, J = 12.0 Hz),4.53 (d, 1H, J = 12.0 Hz), 4.02 (s, 6H), 4.00–3.95 (m, 1H), 3.82 (d, 1H,J = 10.4 Hz), 3.75 (d, 1H, J = 5.2 Hz), 3.71–3.68 (m, 2H), 3.44 (s, 3H); 13C NMR(100 MHz, CDCl3): d = 184.1, 163.3, 146.4, 141.1, 138.8, 137.9, 132.0, 128.6,128.4, 127.9, 127.7, 127.6, 127.5, 120.6, 120.3, 120.0, 97.5, 81.6, 79.7, 77.8, 75.5,73.6, 70.1, 68.4, 65.9, 64.2, 55.3, 53.1; HRMS: calcd for C45H44N6O12+H:861.3095, found: 861.3096. Characterization of compound 8. Methyl 2-(4-(4-((((2S,3R,4S,5R,6R)-4,5-bis(benzyloxy)-2-methoxy-6-(((1-(4-(2-methoxy-2-oxoacetyl)phenyl)-1H-1,2,3-triazol-4-yl)methoxy)methyl)tetrahydro-2H-pyran-3-yl)oxy)methyl)-1H-1,2,3-triazol-1-yl)phenyl)-2-oxoacetate (8): From compound2 (207.1 mg, 0.46 mmol) and 6 (188.6 mg, 0.92 mmol) afforded 8 as a whitepowder (343.1 mg, 86.7%). Rf = 0.23 (EtOAc/petroleum ether, 1:1); [a]D = +20.4(c 0.1, CH2Cl2); 1H NMR (400 MHz, CDCl3): d = 8.21 (d, 2H, J = 8.4 Hz), 8.13 (d,2H, J = 8.4 Hz), 8.01 (s, 1H), 7.86 (s, 1H), 7.83 (d, 2H, J = 8.8 Hz), 7.63 (d, 2H,J = 8.8 Hz), 7.40–7.17 (m, 10H), 5.17 (d, 1H, J = 12.8 Hz), 5.09 (d, 1H,J = 12.8 Hz), 4.87 (d, 1H, J = 11.2 Hz), 4.83 (d, 1H, J = 12.8 Hz), 4.80 (d, 1H,J = 12.0 Hz), 4.74–4.68 (m, 3H), 4.60 (d, 1H, J = 11.2 Hz), 4.02 (s, 6H), 4.00 (t, 1H,J = 9.6 Hz), 3.88 (dd, 1H, J = 3.2, 10.0 Hz), 3.80–3.77 (m, 2H), 3.62 (t, 1H,J = 9.2 Hz), 3.58 (dd, 1H, J = 3.6, 9.6 Hz), 3.39 (s, 3H); 13C NMR (100 MHz,CDCl3): d = 184.3, 163.1, 147.0, 146.0, 141.1, 141.0, 138.2, 138.0, 132.1, 132.0,131.9, 131.8, 128.5, 128.4, 128.0, 127.9, 127.7, 127.4, 120.3, 120.0, 119.8, 97.7,81.5, 80.0, 74.8, 73.1, 68.9, 66.3, 64.6, 55.3, 53.0; HRMS: calcd forC45H44N6O12+H: 861.3095, found: 861.3096.