Construction of triazolyl bidentate glycoligands (TBGs) by grafting of 3-azidocoumarin to epimeric pyranoglycosides via a fluorogenic dual click reaction

Article abstract of DOI:10.1016/j.carres.2012.10.001

Glycoligands, which feature a glycoside as the central template incorporating Lewis bases as metal chelation sites and various fluorophores as the chemical reporter, represent a range of interesting scaffolds for development of chemosensors. Here, new typ

Full text of DOI:10.1016/j.carres.2012.10.001

Carbohydrate Research 363 (2012) 38–42
Contents lists available at SciVerse ScienceDirect
Carbohydrate Research
journal homepage: www.elsevier.com/locate/carres
Note
Construction of triazolyl bidentate glycoligands (TBGs) by grafting
of 3-azidocoumarin to epimeric pyranoglycosides via a fluorogenic
dual click reaction
a
a
a
Jia-Lu Xue a, Xiao-Peng He a, , Jin-Wei Yang , De-Tai Shi , Chao-Ying Cheng ,
⇑
Juan Xie b, Guo-Rong Chen a, , Kaixian Chen a,c
⇑
a Key Laboratory for Advanced Materials & Institute of Fine Chemicals, East China University of Science and Technology, Shanghai 200237, PR China
b PPSM, Institut d’Alembert, ENS Cachan, av du Pt Wilson, F-94235 Cachan, France
c State Key Laboratory of Drug Research, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai 201203, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Glycoligands, which feature a glycoside as the central template incorporating Lewis bases as metal che-
lation sites and various fluorophores as the chemical reporter, represent a range of interesting scaffolds
for development of chemosensors. Here, new types of triazolyl bidentate glycoligands (TBGs) based on
the grafting of 3-azidocoumarin to the C2,3- or C4,6-positions of three epimeric pyranoglycosides includ-
ing a glucoside, a galactoside, and a mannoside were efficiently synthesized via a fluorogenic dual click
reaction assisted by microwave irradiation. The desired TBGs were afforded in high conversion rates
(>90%) and reasonable yields (ꢀ70%). Moreover, a preliminary optical study of two hydroxyl-free gluco-
side-based TBGs indicates that these compounds are strongly fluorescent in pure water, implying their
potential for ion detections in aqueous media.
Received 27 August 2012
Received in revised form 1 October 2012
Accepted 3 October 2012
Available online 12 October 2012
Keywords:
Triazolyl bidentate glycoligand (TBG)
Click chemistry
Coumarin
Fluorogenic chemistry
Chemosensor
Ó 2012 Elsevier Ltd. All rights reserved.
There grows increasing interest in the employment of sugars
that are among the most ample raw materials provided by nature
as the central scaffold for the development of functional com-
pounds due to their well-defined stereoinformation, benign water
solubility, and especially, high biocompatibility. Sugar-based
chemosensors are a class of recently identified small-molecule
probes that consist of sugar moieties as a core template in conjunc-
tion with Lewis bases as metal chelation sites and diverse fluoro-
phores as the chemical reporter.1–10 Of the various chemical
entities disclosed, triazolyl bidentate glycoligands (we christen
them as TBGs) have more recently emerged to be of particular
interest in the selective detection of ions.11–15
The construction of these compounds is succinct considering
that it simply requires the pre-introduction of an azide or an al-
kyne functionality to the selected sugar or fluorophore skeletons
via readily accessible methods to obtain the intermediates, and a
subsequent CuI-catalyzed azide-alkyne 1,3-dipolar cycloaddition
click reaction (CuAAC),16–19 which has fulfilled the preparation of
countless functional glyco-molecules,20–36 efficiently forms the de-
sired triazolyl fluorescent glycoligands. Notably, the triazole rings
generated in these glycoligands not only may serve as a linker,
but can also function as an ion coordination site.11,12 However,
former studies mainly stressed on the synthesis of TBGs by using
furanoglycosides as the sugar template (Fig. 1), whereas correlative
examples based on the six-membered ring pyranoses such as
glucose, galactose, and mannose, which are abundant natural
materials and are also structurally more diverse compared to the
five-membered ring furanoses, are rare.
Therefore, we sought to use the pyranoglycosides as the tem-
plate for the diversification of TBGs (Fig. 1). We have reported in
previous studies that C3,4-disubstituted triazolyl glucosides and
galactosides may perform as selective cation or anion probes in
aqueous media.13–15 In this study, three natural epimeric pyrano-
glycosides, that is, a glucoside, a galactoside, and a mannoside that
differ in their C4- or C2-configuration were used as the sugar tem-
plate, whereon the fluorescently deactivated 3-azidocoumarin was
introduced efficiently via a fluorogenic CuAAC to their C2,3- or
C4,6-positions.
Synthesis: Coumarins are prevalent fluorophores and have been
used extensively as the substrate of fluorogenic click chemistry in
many biochemical studies due to their small size and high biocom-
patibility.37 Furthermore, some triazole-connected coumarin–su-
gar conjugates have been synthesized recently.38–41
We therefore chose 3-azidocoumarin a (Scheme 1), a fluorogen
with quenched fluorescence because of the presence of an elec-
⇑
Corresponding authors. Tel./fax: +86 21 64253016.
E-mail addresses: xphe@ecust.edu.cn (X.-P. He), mrs_guorongchen@ecust.edu.cn
tron-rich a
-nitrogen atom on its 3-position,37 for the construction
(G.-R. Chen).
0008-6215/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.carres.2012.10.001

J.-L. Xue et al. / Carbohydrate Research 363 (2012) 38–42
39
O
O
N
N
∗
∗
∗
∗
N
N
N
N
N
N
N
N
N
N
∗
∗
Figure 1. Triazolyl bidentate glycoligands (TBGs).
R2O
deprotection of benzylidene of the known c43 and e,45 followed
by an O-alkylation in the presence of NaH and propargyl bromide.
The CuAAC ligation of the azide with the six alkynyl sugar tem-
plates was then performed (Scheme 2). It has been well-noted that
the microwave irradiation can powerfully enhance the efficiency of
CuAAC,46,47 especially of those involving polyvalent alkynyl scaf-
folds as the substrate with large steric hindrance that hampers
the reactivity.14,42
6
R2O
R1O
HO
O
O
O
4
1
3
2
OMe
N3
OR1
As a consequence, CuAACs of 3-azidocoumarin (2.2 equiv) a
with different di-propargyloxy glycosides 1–6 (1 equiv) were per-
formed under microwave irradiation (60 °C, Yalian-YL8023B1
R4O
Ph
O
48
microwave oven) by using [Cu(CH3CN4)]PF6 as the catalyst in a
solvent system of tBuOH/H2O/CH2Cl2 (1:1:1, V/V/V).49 The Huisgen
[3+2] cycloaddition of a with 1 was proceeded as a model reaction,
from which we first noticed that relatively low catalyst loadings
ranging from 0.1 to 1.0 equiv resulted in the generation of only
trace amounts of new products as monitored by TLC, even by stir-
ring for 12 h. While the catalyst loading was increased to more
than 2.0 equiv, the consumption of the starting materials tended
to become greater. However, limited reaction times from 30 min
to 2 h led generally to the partial formation of three main products
with gradually increased polarities, which are deduced to be the
C2- or C3-mono-triazole-linked coumarin–sugar conjugate and
the title bis-triazolyl compound.14,50
Eventually, the optimal condition was determined to be the
addition of 4.0 equiv of [Cu(CH3CN4)]PF6 to the reaction system to-
gether with a reaction time of 4 h, which led to the formation of the
desired C2,3-bis-triazolyl coumarin–glucoside conjugate 7 (con-
firmed by NMR and MS) in an excellent conversion rate of 94%
and a reasonable isolated yield of 71% (Table 1). To test the supe-
riority of the microwave technique employed, the same reaction
was performed conventionally by stirring at 60 °C in the presence
of 4.0 equiv of the catalyst.
After stirring over night, only 60% of the sugar alkyne 1 has been
consumed and 7 was isolated with a poor yield of 12.8% along with
the generation of the monotriazolyl byproducts. This demonstrates
that despite the relatively long reaction time (4 h), the microwave
irradiation is helpful for obtaining the title product with an accept-
able efficiency. We then performed the CuAAC of a with the rest of
the alkynes 2–6 under the optimized condition, and the results are
shown in Table 1. To our delight, all title bis-triazolyl products 8–
12 were similarly obtained in high conversion rates (ꢀ90%) and
reasonable yields (ꢀ70%).
R4O
R3O
O
O
O
*
R3O
OMe
OMe
OR3
OR3
R6O
Ph
O
R6O
R5O
O
O
O
*
R5O
OMe
OMe
OR5
OR5
Scheme 1. Reagents: (i) (a) NaH, propargyl bromide, DMF, then (b) p-TsOH, MeOH,
then (c) NaH, BnBr, DMF; (ii) (a) p-TsOH, MeOH, then (c) NaH, propargyl bromide,
DMF.
of the TBGs. The fluorogenic coumarin was envisaged to be intro-
duced simultaneously to the epimeric position and a neighboring
position of the side chain of glucose, galactose, and mannose, three
readily available natural sugar epimers. As shown in Scheme 1, the
commercially available methyl
a-O-glycosides of these monosac-
charides were used to undergo facile and regioselective protec-
tion–deprotection reactions for preparing the di-propargyloxy
glycosyl templates.
The C2,3- and C4,6-di-propargyloxy methyl
and 2 were synthesized via previous methods.42 The known C4,6-
benzylidene methyl
-O-galactoside and mannoside b43 and c44
were first propargylated via Williamson method in the presence
of NaH and propargyl bromide. Subsequent removal of the benzyl-
idene group by p-TsOH, followed by a benzylation in the presence
of NaH and BnBr led to C2,3-di-propargyloxy methyl O-
side 3 and mannoside 4 in 56.4% and 50.6% yields (three-step),
respectively. Likewise, the C4,6-di-propargyloxy methyl -O-galac-
Optical study: Glucoside-based benzyl TBGs 7 and 8 were se-
lected for a preliminary optical study, which were first subject to
a hydrogenolysis in the presence of H2/PdCl2,47,51 giving the hydro-
xyl-free TBGs 13 (94%) and 14 (96%) in excellent yields (Scheme 3).
The UV absorbance and fluorescence intensity of 13 and 14 were
then recorded in pure water.
As shown in Figure 2, both compounds display a single peak
centered at 345 nm in their UV spectra (left), while by excitation
at 345 nm, they exhibit strong fluorescence and interpret a broad
emission band ranging from 400 to 550 nm akin to that of
coumarin. This indicates that the fluorescence of coumarin has
been successfully restored upon the sugar template. Moreover,
a-O-glucosides 1
a
a-galacto-
a
toside 4 and mannoside 6 were obtained in 78.2% and 75.1% yields,
respectively, through a successive two-step sequence involving

40
J.-L. Xue et al. / Carbohydrate Research 363 (2012) 38–42
N
O
N
N
R1
O
R2
O
N
N
O
OMe
N
N
O
10
O
N
O
N
BnO
OMe
N
OBn
HO
O
O
N
N
=
OBn
O
N
N
N
BnO
O
O
N
N
OMe
N
N
O
O
N
N
O
BnO
OMe
N
R3 R4
N
N
Scheme 2. Reagents and conditions: (i) [Cu(CH3CN4)]PF6, tBuOH/H2O/CH2Cl2, microwave irradiation.
Table 1
OH
O
Microwave-assisted CuAAC of azide a with alkynes 1–6
HO
O
Azide
Alkyne
Product
Conversion rate (%)
Yield (%)
a
1
2
3
4
5
6
7
8
9
10
11
12
94
94
92
95
90
91
71.4
70.2
66.5
72.6
64.8
66.8
N
OMe
N
O
N
O
O
N
N
N
the C4,6-modified TBG 14 shows much stronger florescence inten-
sity than its C2,3-modified counterpart 13, suggesting that the
optical property of the TBGs can be tuned by grafting the fluoro-
genic precursor to different substitution sites of a sugar moiety.
In conclusion, we have realized in this study the efficient con-
struction of new types of pyranoglycosyl TBGs by grafting 3-azido-
coumarin to the C2,3- or C4,6-positions of three epimeric
monosaccharides via a microwave-assisted fluorogenic CuAAC
ligation in high conversion rates and reasonable yields. A prelimin-
ary optical study of two hydroxyl-free glucoside-based TBGs shows
that these compounds are strongly fluorescent in pure water,
implying their potential for ion detections in aqueous media. De-
tailed optical studies as well as tests of their biological activities
are currently underway and will be reported in due course.
O
HO
HO
O
OH
O
O
N
N
N
O
OH
O
O
N
N
O
N
O
HO
OMe
1. Experimental section
1.1. General information
OH
Scheme 3. Reagents: (i) PdCl2, H2, MeOH.
Solvents were purified by standard procedures. Petroleum ether
(PE) used refers to the fraction boiling in the range of 60–90 °C. The
organic extracts were dried over MgSO4. 1H and 13C NMR spectra
were recorded on a Bruker AM-400 spectrometer in CDCl3 or
DMSO-d6 solutions using TMS as the internal standard (chemical
shifts in parts per million). Standard abbreviations are used to de-
scribe the signal multiplicity. All reactions were monitored by TLC.
polarimeter at room temperature in a 10-cm, 1-mL cell. High and
low resolution mass spectra (HRMS and LRMS) were recorded on
a Waters LCT Premier XE spectrometer using standard conditions
(ESI, 70 eV). Analytical HPLC was measured using Agilent 1100 ser-
ies equipment. All UV–vis absorption spectra were measured on a
Varian Cary 500 UV–vis spectrophotometer and fluorescence
Optical rotations were measured using
a Perkin–Elmer 241

J.-L. Xue et al. / Carbohydrate Research 363 (2012) 38–42
41
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
250
200
150
100
50
(d, J = 6.0 Hz, 1H), 6.84 (s, 1H), 6.51 (br s, 6H), 4.92 (s, 1H), 4.81
(d, J = 9.5 Hz, 2H), 3.67–3.58 (m, 2H), 3.52–3.34 (m, 4H), 3.30–
3.28 (m, 3H); 13C NMR (100 MHz, DMSO-d6): d 162.5, 162.3,
156.2, 154.5, 136.1, 130.8, 127.3, 124.7, 124.4, 119.1, 114.2,
113.8, 110.1, 102.0, 96.9, 81.3, 78.8, 72.5, 69.8, 65.5, 62.9, 60.5,
54.2; HRMS (ESI): m/z calcd for C31H28N6O12-H: 675.1687; found:
675.1711. Analytical HPLC: tR = 4.6 min (solvent: MeOH, 0.5 mL/
min over 8.9 min, purity 98.8%).
a
1.3.2. 3-(4-((((2R,3S,4R,5R,6S)-4,5-Dihydroxy-2-(((1-(7-hydroxy-
2-oxo-2H-chromen-3-yl)-1H-1,2,3-triazol-4-
yl)methoxy)methyl)-6-methoxytetrahydro-2H-pyran-3-
yl)oxy)methyl)-1H-1,2,3-triazol-1-yl)-7-hydroxy-2H-chromen-
2-one (14)
0
275 300 325 350 375 400 425 450 475 500 525 550 575
Wavelength (nm)
From 8 (60 mg, 0.07 mmol), afforded 14 as a light yellow solid
800
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
(47.4 mg, 96.3%). Rf = 0.27 (CH2Cl2/MeOH, 10:1). ½a D25
ꢁ
+22.0 (c 0.1,
700
600
500
400
300
200
100
0
MeOH). 1H NMR (400 MHz, DMSO-d6): d 8.49 (d, J = 12.0 Hz, 2H),
8.46 (d, J = 12.0 Hz, 2H), 7.68 (d, J = 8.1 Hz, 2H), 6.91 (t, J = 6.2 Hz,
2H), 6.85 (s, 2H), 4.96 (d, J = 11.8 Hz, 1H), 4.69-4.60 (m, 4H), 3.61
(br s, 2H), 3.58 (d, J = 8.7 Hz, 1H), 3.51 (d, J = 9.5 Hz, 1H), 3.31–
3.26 (m, 2H), 3.26 (s, 3H); 13C NMR (100 MHz, DMSO-d6): d
162.5, 162.4, 156.2, 154.5, 144.5, 143.9, 136.1, 130.8, 127.3,
124.8, 124.6, 119.1, 114.3, 110.1, 102.1, 99.6, 77.8, 73.5, 71.9,
69.4, 68.7, 64.8, 63.3, 54.5; HRMS (ESI): m/z calcd for
b
C
31H28N6O12+H: 677.1843; found: 677.1845. Analytical HPLC:
tR = 4.7 min (solvent: MeOH, 0.5 mL/min over 9.1 min, purity
98.0%).
275 300 325 350 375 400 425 450 475 500 525 550 575
Wavelength (nm)
Acknowledgments
Figure 2. Stacked UV–vis (red) and fluorescence (blue) spectra (excited at 345 nm)
of 10ꢂ4 M of (a) TBG 7 and (b) TBG 8 in deionized water. (For interpretation of the
references to color in this figure legend, the reader is referred to the web version of
this article.)
We gratefully acknowledge the National Natural Science Foun-
dation of China (Nos. 21176076, 21202045) and the Fundamental
Research Funds for the Central Universities (No. WK1013002) for
generously providing the funding. Dr. X.-P. He is also supported
by China Postdoctoral Science Foundation Funded Projects (Nos.
2011M500069, 2012T50400).
spectra measured on
spectrophotometer.
a Varian Cary Eclipse Fluorescence
1.2. General procedure for the microwave-assisted CuAAC
Supplementary data
To a solution of the sugar alkyne (1.0 equiv) and 3-azidocoum-
arin (2.2 equiv) in tBuOH/H2O/CH2Cl2 (5 mL/5 mL/5 mL) was added
[Cu(CH3CN4)]PF6 (4.0 equiv). This mixture was then transferred to
a Yalian (YL8023B1) microwave oven and stirred at 60 °C with a
ramp time of 6 min and a hold time of 4 h. Solvent was then re-
moved in vacuum and the resulting residue was diluted with
CH2Cl2 and washed successively with water and brine. The com-
bined organic layer was dried over MgSO4, filtered, and then con-
centrated in vacuum to give a crude product which was purified
by column chromatography.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.carres.2012.10.
001.
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