Design and synthesis of multivalent neoglycoconjugates by click conjugations

Article abstract of DOI:10.3762/bjoc.10.134

A highly stereoselective BF₃·OEt₂-promoted tandem hydroamination/glycosylation on glycal scaffolds has been developed to form propargyl 3-tosylamino-2,3-dideoxysugars in a one-pot manner. Subsequent construction of multivalent 3-tosy

Full text of DOI:10.3762/bjoc.10.134

Design and synthesis of multivalent
neoglycoconjugates by click conjugations
Feiqing Ding, Li Ji, Ronny William, Hua Chai and Xue-Wei Liu*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2014, 10, 1325–1332.
Division of Chemistry and Biological Chemistry, School of Physical
and Mathematical Sciences, Nanyang Technological University, 21
Nanyang Link, Singapore 637371
doi:10.3762/bjoc.10.134
Received: 24 February 2014
Accepted: 21 May 2014
Published: 10 June 2014
Email:
Xue-Wei Liu* - xuewei@ntu.edu.sg
This article is part of the Thematic Series "Multivalent glycosystems for
nanoscience".
* Corresponding author
Keywords:
Guest Editor: B. Turnbull
click conjugations; copper-catalyzed; microwave irradiation;
multivalent glycosystems; neoglycoconjugates; one-pot
© 2014 Ding et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
A highly stereoselective BF3∙OEt2-promoted tandem hydroamination/glycosylation on glycal scaffolds has been developed to form
propargyl 3-tosylamino-2,3-dideoxysugars in a one-pot manner. Subsequent construction of multivalent 3-tosylamino-2,3-dideoxy-
neoglycoconjugates with potential biochemical applications was presented herein involving click conjugations as the key reaction
step. The copper-catalyzed regioselective click reaction was tremendously accelerated with assistance of microwave irradiation.
Introduction
Oligosaccharides and glycopeptides are the key constituents of moiety found in nature. However, unlike amides, the triazole
the cellular membrane and extracellular matrix, and play a moiety proved to be robust and resistant to chemical and enzy-
pivotal role in various key cellular events such as cell–cell matic cleavage [17-20]. Moreover, the inertness of both azide
recognition, host–pathogen or host–symbiont interactions, and alkyne groups towards a majority of functional groups
molecular recognition of antibodies and metastasis [1-5]. The connected to the core of a variety of biomolecules also renders
construction of a 1,4-disubstituted-1,2,3-triazole unit via a the click reaction particularly suitable for covalently linking
copper(I)-catalyzed modern version of the Huisgen-type bioactive molecular entities [21,22]. For example, the click
azide–alkyne cycloaddition [6-10] has been considered to be a strategy is especially versatile for the effective construction of
powerful ligation method for glycoconjugation [11-16]. In addi- complex glycosylated structures such as clusters, dendrimers,
tion to the simplicity of this reaction and the ease of purifica- polymers, peptides and macrocycles. In all the cases the tria-
tion, 1,4-disubstituted-1,2,3-triazoles, the regiospecific product zole ring plays a crucial role in combining divergent units
of this reaction, exhibit similarities to the ubiquitous amide together to establish a complex molecular architecture [23-31].
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The α-GalNAc-linked glycopeptides, α-N-glycosidically linked
to the polypeptide chain through the amido nitrogen of an
asparagine residue at the N-terminal [32], were found to be the
most important semi-synthetic glycoconjugates, usually modi-
fied from their naturally occurring parent precursors [33-39].
Over the years, many structural analogues of this class of anti-
biotics have been synthesized. In addition, triazoles are consid-
ered as peptidic linkage surrogates. Surprisingly, despite the
enormous research interests associated with their synthesis, only
a few examples of oligosaccharides and glycopeptides mimics
Figure 1: Our reported strategy for quick access to 3-amino-2,3-
dideoxysugars via regio- and stereoselective tandem hydroamination/
glycosylation of glycals.
have so far been prepared by a click chemistry strategy [40-48]. continuation of our previous work, herein we report a direct and
Most recently, we developed a strategy for the synthesis of reliable synthetic approach to multivalent 3-tosylamino-2,3-
3-amino-2,3-dideoxysugars using a regio- and stereoselective dideoxyneoglyco conjugates 4 with potential biochemical appli-
tandem hydroamination/glycosylation of the glycal shown in cations involving click conjugations as the key reaction step
Figure 1 [49-53]. Extending the synthetic utility of this (Figure 3).
protocol, herein, we wish to report the synthetic modification of
α-GalNAc-linked glycopeptides to 3-tosylamino-2,3-dideoxy- Results and Discussion
neoglycoconjugates via click conjugations (Figure 2).
Primarily, we successfully synthesized propargyl 3-p-toluene-
sulfonamido-4,6-di-O-acetyl-2,3-dideoxy-α-D-allohexopyra-
Given the success in using “click chemistry” in the glycosyl- noside (2a) in gram scale via BF3∙OEt2-promoted one-pot three-
ation reactions, we aspired to apply the highly efficient triazole component α-selective tandem hydroamination/glycosylation
formation employing an azide 3 and a suitable alkyne appended reaction (Scheme 1). In fact, when 3,4,6-tri-O-acetyl-D-glucal
to the 3-amino-2,3-dideoxysugars moiety 2 (Figure 3). In (1a), propargyl alcohol and p-toluenesulfonamide were
Figure 2: Synthetic modification of α-GalNAc linked glycopeptides to 3-tosylamino-2,3-dideoxyneoglycoconjugates via click conjugation.
Figure 3: Our proposal for access to 3-tosylamino-2,3-dideoxyneoglycoconjugates via tandem hydroamination/glycosylation of glycals followed by
click conjugations.
Scheme 1: Synthesis of propargyl 3-tosylamino-2,3-dideoxy-α-D-allohexopyranoside (2a).
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Beilstein J. Org. Chem. 2014, 10, 1325–1332.
subjected to a one-pot reaction in the presence of 2.2 equiv of Next, the required α-propargyl 3-tosylamino-2,3-dideoxyglyco-
BF3∙OEt2 in DCE at room temperature for 20 min, the desired sides 2 were synthesized by BF3∙OEt2-promoted one-pot three-
aminoglycoside 2a was obtained in good yield with exclusive component tandem hydroamination/glycosylation reaction on a
α-stereoselectivity [50]. Later, a systematic screening was glycal scaffold including tri-O-acetyl-D-glucal (1a), tri-O-
executed using 3-tosylamino-2,3-dideoxysugar 2a and benzyl acetyl-D-allal (1b), tri-O-acetyl-D-galactal (1c), di-O-acetyl-D-
azide (3a) as our model system under varied conditions of cata- rhamnal (1d), hexa-O-acetyl-D-maltal (1e). Accordingly, a
lysts, additives, solvents and reaction temperatures (Table 1). series of α-propargyl 3-tosylamino-2,3-dideoxyglycosides
The initial evaluation involved no catalyst and additives at 2a–2d were obtained exclusively with α-stereoselectivity in
100 °C and DMF, MeCN/H2O 3:1 or MeOH as the solvent good yields (Table 2, entries 1–5).
system, which resulted in unsuccessful reactions (Table 1,
entries 1–3). However, a trace amount of the desired product With pure α-propargyl 3-tosylamino-2,3-dideoxyglycosides and
was detected in the presence of 10 mol % of copper(I) iodide the optimized conditions in hand, we focused on performing a
(Table 1, entry 4). The combination of CuSO4·5H2O Huisgen cycloaddition reaction. The scope and generality of
(10 mol %) and sodium ascorbate (10 mol %) was found to be a this method to prepare 3-tosylamino-2,3-dideoxyneoglycocon-
suitable catalyst leading regiospecifically to the 1,4-disubsti- jugates 4 with the assistance of copper sulfate and sodium
tuted-1,2,3-triazole 4a with moderate yield of 46% in t-BuOH/ ascorbate was examined extensively. A range of α-alkyne-3-
H2O 1:1 after 20 hours at 70 °C (Table 1, entry 5). The yield tosylamino-2,3-dideoxysugars and azides with various
was further improved to 97% by employing DMF as solvent in substituent groups (R2) were screened and the summarized
a shorter period of 12 hours (Table 1, entry 6). Encouraged by results are shown in Table 3. Overall, the yields obtained were
these results, we attempted to improve the assemblies and to from good to excellent while preserving the anomeric selec-
shorten the reaction times further; reactions were subjected to tivity and regioselectivity. In general, the analogous reaction of
microwave irradiation, which is best known to accelerate tran- a set of azides with different substituent groups (3a–3e) with
sition metal-catalyzed homogeneous reactions [54]. Microwave- α-propargyl 3-tosylamino-2,3-dideoxy glycosides 2 afforded the
assisted organic reactions are rapidly becoming recognized as a corresponding 3-tosylamino-2,3-dideoxyneoglycoconjugates
valuable tool for facilitating a wide variety of organic transfor- (4a–4h) in good to excellent yields with exclusive anomeric
mations [55,56]. Finally, we found that the rate of conversion selectivity (Table 3, entries 1–8). This encouraging result
accelerated dramatically when microwave irradiation was used prompted us to apply these conditions to alkyne 2a and a series
under 70 °C. To our delight under microwave conditions and in of azido-linked monosaccharides 3f, 3g and 3h as well as the
DMF with addition of 1 mol % of CuSO4·5H2O and 10 mol % propargyl disaccharide 2d with α-GlaNAc azido 3g which were
of sodium ascorbate, a quantative yield of desired 3-tosylamino- also obtained in good yields and selectivities (Table 3, entries
2,3-dideoxyneoglycoconjugate 4a was obtained in 15 min 9–13). Subsequently, to shorten the reaction times, we subjected
(Table 1, entry 7).
all the click conjugations to microwave irradiation. All the reac-
Table 1: Optimization for synthesis of 3-tosylamino-2,3-dideoxyneoglycoconjugate 4a.
Entry
Catalyst (mol %)
Solvent
Temperature (°C)
Time (h)
Yield (mol %)a
1
2
3
4
5
6
7
none
none
DMF
MeCN/H2O
MeOH
100
100
100
60
20
20
NRb
NRb
NRb
trace
46
none
20
CuI (10)
THF
12
CuSO4·5H2O (1)
CuSO4·5H2O (1)
CuSO4·5H2O (1)
t-BuOH/H2O
DMF
70
20
70
12
97
DMF
70c
0.25
98
aIsolated yield after purification. bNR = no reaction. cAssisted by microwave irradiation, 200 W.
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Beilstein J. Org. Chem. 2014, 10, 1325–1332.
Table 2: One-pot synthesis of α-propargyl 3-tosylamino-2,3-dideoxyglycosides 2.
Entry
1
1
2
Yield (%)a
86
1a
1b
2a
2a
2
84
3
4
81
74
1c
1d
2b
2c
5
67
1e
2d
aIsolated yields after purification.
tions were completed in considerably shorter reaction times of parts, there has been a constant development of new glycocon-
less than 30 min for the Huisgen cycloaddition of alkenes and jugates such as glycodendrimers [57]. Hence, as continuation of
azides catalyzed by copper sulfate and sodium ascorbate, previous encouraging results, we have further designed the use
affording the corresponding products in good to excellent yields of noncarbohydrate diazide 5a in the cycloaddition reaction
in each case (Table 3, method B). This result showed that the with the α-propargyl 3-tosylamino-2,3-dideoxyalloside 2a and
synthesis of 3-tosylamino-2,3-dideoxyneoglycoconjugates via α-propargyl 3-tosylamino-2,3,6-trideoxy-α-L-ribohexopyra-
copper-catalyzed Huisgen cycloaddition is highly efficient noside 2c (Scheme 2) to obtain divalent 3-tosylamino-2,3-
under microwave irradiation.
dideoxyneoglycoconjugates 6a and 6b in 83% and 61% yield
respectively. The synthesis of trivalent 3-tosylamino-2,3-
In carbohydrate recognition events, higher multivalent interac- dideoxyneoglycoconjugates 6c was also feasible by using
tions are absolutely essential as the monovalent affinities of triazide 5b in 66% yield (Scheme 3). Interestingly, for all the
carbohydrate monosaccharides are comparatively low and reactions under microwave irradiation, reaction times were
weak. To enhance this multivalent effect, thereby increasing the reduced to 15 minutes. As such, this novel synthetic protocol
binding efficiencies of carbohydrates with the coupling counter- provides a straightforward access to a wide range of 3-tosyl-
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Table 3: Scope for synthesis of 3-tosylamino-2,3-dideoxyneoglycoconjugates.
Yield (%)a
Entry
2
3
4
Ab
Bc
1
2a
3a
97
98
4a
2
2b
3a
89
93
4b
4c
3
4
2c
2d
3a
3a
74
71
81
78
4d
4e
4f
5
6
7
8
2a
2a
2a
2a
82
91
86
87
85
92
89
92
3b
3c
3d
3e
4g
4h
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Beilstein J. Org. Chem. 2014, 10, 1325–1332.
Table 3: Scope for synthesis of 3-tosylamino-2,3-dideoxyneoglycoconjugates. (continued)
9
2a
2a
2d
76
93
80
80
95
82
3f
4i
4j
10
11
3g
3g
4k
12
2a
72
78
3h
4l
aIsolated yields after purification. b70 °C under conventional heating, 12 hours. c70 °C under microwave irradiation, 200 W, 15 minutes.
Scheme 2: Synthesis of divalent 3-tosylamino-2,3-dideoxyneoglycoconjugates 6a and 6b.
amino-2,3-dideoxyneoglycoconjugate derivatives which may dideoxyneoglycoconjugate architectures was successfully
find numerous biochemical applications [40-48].
achieved by using cycloaddition reactions of alkynes and
azides. It is expected that this strategy will find extensive appli-
cations in glycoscience, because triazole-linked glycoconju-
Conclusion
In conclusion, it has been established that the construction of gates can exhibit very interesting biological properties, offering
well-defined multivalent, anomerically pure 3-amino-2,3- a convenient access toward oligosaccharides, glycopeptide
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Beilstein J. Org. Chem. 2014, 10, 1325–1332.
Scheme 3: Synthesis of trivalent 3-tosylamino-2,3-dideoxyneoglycoconjugate 6c.
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underway and the results will be reported in due course.
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