Application of 2-azido-2-deoxythioglycosides for β-glycoside formation and oligosaccharide synthesis

Article abstract of DOI:10.1002/ejoc.201200173

Most natural 2-acetamido-2-deoxyglycosides exist in a 1,2-trans-β- glycosidic configuration. This study investigated the use of 2-azido-2-deoxythioglycosides for 1,2-trans-β-glycosidic bond formation under low-concentration glycosylation conditions. Furth

Full text of DOI:10.1002/ejoc.201200173

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FULL PAPER
DOI: 10.1002/ejoc.201200173
Application of 2-Azido-2-deoxythioglycosides for β-Glycoside Formation and
Oligosaccharide Synthesis
Kwok-Kong Tony Mong,*[a] Yu-Fang Yen,[a] Wei-Cheng Hung,[a] Yen-Hsun Lai,[a] and
Jiun-Han Chen[a]
Keywords: Azides / Carbohydrates / Glycosylation / Glycopeptides / Regioselectivity
Most natural 2-acetamido-2-deoxyglycosides exist in a 1,2-
trans-β-glycosidic configuration. This study investigated the
use of 2-azido-2-deoxythioglycosides for 1,2-trans-β-gly-
thioglycosyl substrates for one-pot oligosaccharide synthesis
was demonstrated in the synthesis of protected β-(1Ǟ6)-N-
acetylglucosamine trimer, core-6–serine conjugate, and F1–α
cosidic bond formation under low-concentration glycosyla- serine conjugate.
tion conditions. Further application of the 2-azido-2-deoxy-
trichloroacetimidate derivatives occasionally suffer from
Introduction
glycosyl trichloroacetamide formation. Thus, development
of a β-glycosylation method based on 2-azido-2-deoxythio-
glycoside donors is highly desirable.
2-Acetamidoglycosides including GlcNAc and GalNAc
constitute approximately 36.0% of the monosaccharide
units in mammalian oligosaccharides, which are often con-
jugated to proteins and lipids as glycoproteins and glycolip-
ids.[1,2] Such glycoconjugates are prevalent in nature and as-
sociated with different biological processes.[3–5] With a few
exceptions, most of the 2-acetamidoglycosides are con-
nected to other saccharide units by 1,2-trans-β-glycosidic
bonds. In general, such bonds are constructed from glycosyl
donors bearing a participating function at the C2 posi-
tion,[6–8] which is applicable for both non-amino- and
aminoglycosyl donors. Concerning 2-amino-2-deoxy-
glycosyl donors, a range of amine protecting groups with a
participating function have been developed.[9–11]
In 2009, we presented a low-concentration glycosylation
method for the construction of 1,2-trans-β-glycosidic bonds
that employs nonparticipating thioglycoside donors, par-
ticularly in the d-gluco and d-galacto series.[22] This method
employs CH2Cl2/alkanenitrile as solvent to generate 1,2-
trans-β-glycosides under low-substrate-concentration and
low-temperature conditions. Subsequent studies revealed a
participating role of the C-2 ether function in nitrile solvent
(Figure 1a).[23]
In contrast to the participating glycoside donors, 2-
azido-2-deoxyglycosides are often recruited for 1,2-cis-α-
glycosidic bond formation.[12–14] The azido protecting
group has a number of advantages over other participating
functions such as lower steric bulk, stability under a range
of reaction conditions, and a lack of rotamer formation.[11]
In addition, the azido function can be reduced to an amine
under mild reaction conditions.[15,16] For these reasons, the
use of 2-azido-2-deoxyglycosides for β-glycosidic bond for-
mation has always been attractive for synthetic chemists.
Previously, 2-azido-2-deoxyglycosyl trichloroacetimidates
and phosphates were exploited for β-glycoside forma-
tion.[17–21] However, these donors are less, stable and their
preparation requires additional synthetic steps. Moreover,
[a] Applied Chemistry Department, National Chiao Tung
University (Taiwan),
1001 Ta Hsueh Road, Hsinchu, Taiwan, R.O.C.
Fax: +886-3-5723764
E-mail: tmong@mail.ncut.edu.tw
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201200173.
Figure 1. (a) C2 ether participation in low-concentration glycosyl-
ations. (b) Possible participation of 2-azido-2-deoxythioglycosides.
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FULL PAPER
Along this line, we speculated that comparable participa-
tion may occur for 2-azido-2-deoxythioglycosyl donors, be-
cause the azido function bears lone-electron pairs, which
would probably stabilize the electrophilic α-glycosylnitril-
ium ion (Figure 1b). Putting the idea to practice, we under-
took an investigation of the glycosylation behavior of 2-
azido-2-deoxythioglycosides under low-concentration gly-
cosylation conditions. The investigation included: (i) devel-
opment of an efficient thioglycosidation method for per-
acetyl-2-azido-2-deoxyglycosyl acetates; (ii) exploration of
the scope and limitations of 2-azido-2-deoxythioglycoside
donors in β-glycosidic bond formation, and (iii) application
of 2-azido-2-deoxythioglycoside substrates for one-pot oli-
gosaccharide synthesis.
Results and Discussion
1. Preparation of 2-Azido-2-deoxyglycosyl Substrates
This study commenced with the multigram preparation
of 2-azido-2-deoxythioglucosides. Readily available glucosa-
mine–hydrogen chloride was selected as the starting mate-
rial. Treatment of the glucosamine salt with imidazole-1-
sulfonyl azide hydrochloride salt, followed by acetylation,
produced per-O-acetyl 2-azido-2-deoxyglucosyl acetate 1
(Scheme 1a).[24] However, thioglycosidation of 1 with thioc-
resol and boron trifluoride–diethyl ether was sluggish when
the reaction was performed by stirring at room temp., and
the desired thioglycoside 2 was produced in 60% yield ac-
companied by 5–10% unreacted acetate 1. Although such
an inefficient thioglycosidation was reported in previous
studies, no solution has yet been provided.[21,25] To ensure
a sufficient supply of thioglycosides, we needed to address
this problem. To this end, we explored the use of sonic-
ation.[26] When a sonication procedure was applied, the time
of thioglycosidation was reduced from 72 to 18 h, and the
reaction yield increased from 60 to 80% (Scheme 1a). Upon
securing the preparative route, thioglycoside 2 was deacety-
lated and benzylated to furnish 2-azidoper-O-benzyl-2-de-
oxythioglucoside 3. To prepare glycosyl donors for an or-
thogonal glycosylation study, a portion of thioglycoside 3
was converted into 2-azido-2-deoxyglucosyl phosphate 4 by
using the procedure developed by Sabesan and Neira.[27]
In addition to 2-azido-2-deoxythioglucoside donor 3, 2-
azido-2-deoxythioglucoside acceptors 6[28] and 7 were pre-
pared for the orthogonal glycosylation study (Scheme 1b).
Thus, deacetylation of 2-azido-2-deoxythioglucoside 2 fol-
lowed by benzylidenation and benzylation furnished 2-
azido-3-O-benzyl-4,6-O-benzylidene-2-deoxythioglucoside
(5). The α-anomer of 5 could be separated by precipitation
and was further elaborated to thioglycoside acceptors 6 and
7. It should be noted that use of the pure anomeric form
Scheme 1. (a) Preparation of 2-azido-2-deoxythioglucosyl donor 3
and glucosyl phosphate donor 4. (b) Preparation of 2-azido-2-de-
oxythioglucoside acceptors 6 and 7. (c) Preparation of 2-azido-2-
deoxythiogalactoside donors 11 and 12.
of acceptors 6 and 7 in glycosylations facilitated product with triethylsilane (Et3SiH) and trifluoroacetic acid
characterization. 2-Azido-2-deoxythioglucoside 6 with an (TFA).[29] Treatment of 5 with borane–tetrahydrofuran
unprotected 4-hydroxy function was obtained by treating 5 complex (BH3·THF) and trimethylsilyl triflate (TMSOTf)
2
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β-Glycoside Formation and Oligosaccharide Synthesis
at 0 °C produced the 2-azido-2-deoxythioglucoside 7 with
an unprotected 4-hydroxy function. It should be noted that
the previous benzylidene ring-opening reaction employed
CuOTf2 as acid catalyst, but, in our hands, the use of
CuOTf2 led to the formation of a complex reaction mix-
ture.[30]
In contrast to its d-gluco counterpart, preparation of 2-
azido-2-deoxythiogalactoside donors followed a different
synthetic route that began with inexpensive and readily
available galactose (Scheme 1c). First, unprotected ga-
lactose was converted into galactal, which was transformed
into per-O-acetyl-2-azido-2-deoxygalactosyl acetate 8 by
standard azidonitration and nitrate Ǟ acetate substitu-
tion.[31] Similar to acetate 1, the thioglycosidation of 8 was
accelerated by using sonication. Thus, a high yield (95%)
of 2-azido-2-deoxythiogalactoside 9 was obtained within
5 h. It should be noted that different reaction times (4 h or
3 d) have been reported for the thioglycosidation of 8.[14,21]
In our hands, the reaction required 24 h to reach comple-
tion in the absence of sonication. Subsequent deacetylation
of 9 produced unprotected thioglycoside 10, which was con-
verted into benzylidene-protected 2-azido-2-deoxy-β-thio-
galactoside 11 and 2-azidoper-O-benzyl-2-deoxy-α-thioga-
lactoside 12 by standard protecting-group manipulation.
2. Glycosylations with 2-Azido-2-deoxyglycosyl Donors
Once the 2-azido-2-deoxyglycosyl donors were obtained,
the stage was set for an investigation into their glycosylation
properties. To this end, glycosyl acceptors 7 and 13–16, and
aglycon acceptor 17 were employed (Figure 2).[32] Acti-
vation of the thioglycoside donors 3, 11, and 12 was
achieved by using N-iodosuccinimide (NIS) and trimethyl-
silyl trifluoromethanesulfonate (TMSOTf),[33] whereas acti-
vation of the glycosyl phosphate donor 4 required a stoi-
chiometric amount of TMSOTf.[19,20] The results of the gly-
cosylation reactions are summarized in Table 1.
Figure 2. Acceptors 13–17 and glycosylation products 18–26.
Glycosylations of galactosyl acceptor 13 and rhamnoside
acceptor 14 with 2-azido-2-deoxythioglucoside 3 produced
the respective disaccharides 18[19,22] and 19 in 83 and 60%
yields, respectively, with excellent β-selectivities (Table 1,
Entries 1 and 3). However, when the same reactions were
repeated in CH2Cl2, the selectivity was significantly eroded,
and the α/β-anomeric ratios of 18 and 19 decreased to 1:2.5
and 1:6, respectively (Table 1, Entries 2 and 4). Other than
the 2-azido-2-deoxythioglucoside donor 3, the low-concen-
tration glycosylation method performed well for 2-azido-2-
deoxyglucosyl phosphate donor 4 (Table 1, Entry 5). Be-
sides the glycosylations of primary acceptors, 2-azido-2-de-
oxythioglucoside 3 was found to be effective for glycosyl-
ations of hindered secondary glycosyl acceptors, as wit-
nessed by glycosylations of the glucoside acceptor 15 and
mannoside acceptor 16. Both glycosylations yielded the
corresponding disaccharides 20 and 21 in 76 and 55%
yields, respectively (Table 1, Entries 6 and 7). It should be
noted that GlcNAc-β(1Ǟ2)-Man glycoside 21 constitutes
Table 1. Glycosylation of 2-azido-2-deoxythioglycosides 3, 11, 12,
and 2-azido-2-deoxyglocosyl phosphate 4.
Entry Donor/acceptor Method[a]
Glycosylation product
Product Yield [%] Ratio α/β[b]
1
2
3
4
5
6
7
8
9
3/13
3/13
3/14
3/14
4/13
3/15
3/16
4/7
11/14
11/15
11/17
12/15
A
B
A
B
A
A
A
A
A
A
A
A
18
18
19
19
18
20
21
22
23
24
25
26
83
85
60
60
82
76
55
77
70
80
80
65
1:Ͼ49
1:5
1:32
1:6
1:Ͼ49
1:33
1:24
1:19
1:19
1:33
1:19
1:25
10
11
12
[a] Method A: low-substrate-concentration glycosylation; Meth-
od B: conventional glycosylation in CH2Cl2. [b] The α/β anomeric
ratios were determined by HPLC analysis (Mightysil column: Si-
the disaccharide unit found in complex N-linked glycans.[34] 60 250-4.6; hexane/CH2Cl2/EtOAc; flow-rate 1.0 mL min–1).
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Concerning the orthogonal glycosylation, 2-azido-2-deoxy-
glucosyl phosphate 4 was coupled with 2-azido-2-deoxy-
thioglucoside acceptor 7. The glycosylation furnished the
desired thiodisaccharide 22 in high yield (77%) with a 1:19
α/β-anomeric ratio (Table 1, Entry 8). Gratifyingly, no self-
condensation of acceptor 7 was detected. However, when
less reactive 4-hydroxy-unprotected acceptor 6 was em-
ployed for glycosylation with 4, significant levels of thio-
transfer reaction occurred, whereby the thioacetal function
in 6 was transferred to the anomeric center of 4 (unpub-
lished data).
After studying 2-azido-2-deoxythioglucoside 3, we
shifted the focus to 2-azido-2-deoxythiogalactoside donors
11 and 12. Although these donors had different protecting-
group patterns, this did not affect the glycosylation effi-
ciency as evidenced by the glycosylations of acceptors 14,
15, and 17 (Table 1, Entries 9–12). However, such glycosyl-
ation properties did not extend to 4,6-O-benzylidene-pro-
tected 2-azidothioglucoside; in this case, donor activation
was ineffective under low-concentration glycosylation con-
ditions (unpublished data). We attributed this behavior to
the inherently less reactive nature of d-gluco donors. The
β-configurations of the newly formed glycosidic bonds in
disaccharides 18–26 were confirmed by the chemical shifts
of the signals of the anomeric carbon atoms at δ = 100.0–
Scheme 2. (a) Glycosylation of 2-azido-2-deoxythioglucoside 6 with
2-O-benzoyl-protected thioglucoside donor 27; (b) glycosylation of
2-azido-2-deoxythiogalalctoside 29 with 2-O-benzoyl-protected
thiogalalctoside donor 28; (c) glycosylation of 2-azido-2-deoxy-
thioglucoside 6 with 2-O-benzoyl-protected thiogalactoside 28.
1
102.4 ppm and by the corresponding JC–H coupling con-
stant of 156–163 Hz.[35]
4. Application of 2-Azido-2-deoxythioglycoside Substrates
in Oligosaccharide Synthesis
3. Glycosylations of 2-Azido-2-deoxythioglycoside
Acceptors
Encouraged by the aforementioned investigations, we at-
tempted the synthesis of oligosaccharides using 2-azido-2-
After studying the donor properties of the 2-azido-2-de- deoxythioglycoside substrates. For model studies, protected
oxythioglycosides, we evaluated their acceptor properties β-(1Ǟ6)-N-acetylglucosamine (PNAG) trimer 35, core-6–
under conventional glycosylation conditions. This infor- serine conjugate 38, and F1–α-serine conjugate 39 were se-
mation is relevant for the design of saccharide building lected as the synthetic targets.
blocks for oligosaccharide synthesis. It is well recognized
The β(1Ǟ6)-N-acetylglucosamine trimer 35 constitutes
that the azido function at the C2 position reduces the reac- the backbone of the β-(1Ǟ6)-N-acetylglucosamine poly-
tivity of the anomeric function; however, whether such a mers (PNAG), which is found in various pathogens includ-
disarming effect influences the nucleophilicity of hydroxy ing Staphylococcus aureus,[40] Escherichia coli,[41] and Acti-
functions remains unclear. To investigate this effect, 2-O- nobacter spp.[42] Previous syntheses of the PNAG oligomers
benzoyl-protected thioglucoside 27[36] and thiogalactoside often used the phthalimide function (N-Phth) for amine
28[37] were selected as donors for glycosylations of 2-azido- protection.[43,44] However, removal of the N-Phth protection
2-deoxythioglucoside 6 and 2-azido-2-deoxythiogalactoside is nontrivial and often requires harsh reaction conditions.
29 (Scheme 2).[38]
The glycosylations of 6 with 27 and 29 with 28 provided thioglycoside building blocks would be desirable.
thiodisaccharides 30 and 31, respectively, in yields of 62 and Assembly of the protected N-acetylglucosamine trimer
Thus, an alternative strategy that employs 2-azido-2-deoxy-
70%. No self-glycosylation of thioglycoside acceptors was 35 required three 2-azido-2-deoxyglucosyl (GlcN3) building
detected. This result is attributed to the stronger disarming blocks 4, 7, and 34. The reducing-end GlcN3 building block
effect of the azido function compared to the 2-benozyl 34 was prepared from the 2-azido-2-deoxythioglucoside 7
function.[39] Based on the inherently higher reactivity of the by (i) protecting the 6-hydroxy function in 7 with p-meth-
d-galacto donor and the disarming nature of the 2-azido oxybenzyl ether (PMB) to obtain thioglycoside 33; (ii) cou-
function, we predicted a highly efficient reactivity-based pling 33 with 6-chlorohexanol 17 under low-concentration
glycosylation would occur between 2-benzoyl protected glycosylation conditions to produce fully protected 2-azido-
thiogalactoside 28 and 2-azidothioglucoside 6. As expected, 2-deoxy O-glucoside, and (iii) removal of the PMB function
the desired LacNAc thioglycoside 32 was produced in excel- in the preceding O-glycoside leading to the target building
lent yield (85%) despite the hindered 4-hydroxy function of block 34 (Scheme 3a). In a one-pot synthesis of 35, 2-azido-
6 (Scheme 2c).
2-deoxythioglucoside 7 was glycosylated with 2-azido-2-de-
4
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β-Glycoside Formation and Oligosaccharide Synthesis
oxyglucosyl phosphate 4 under low-concentration glyc-
osylation conditions to obtain thiodisaccharide 22
(Scheme 3b). Without intermediate isolation, a supplemen-
tary dose of NIS and the reducing-end GlcN3 building
block 34 were added to react with the thiodisaccharide 22.
The glycosylation furnished the desired N-acetylglucos-
amine trimer 35 in 72% yield in one pot.
Scheme 3. (a) Preparation of the reducing-end saccharide acceptor
34; (b) one-pot synthesis of protected β(1Ǟ6)-N-acetylglucosamine
trimer 35.
Core-6–serine conjugate is composed of a GlcNAc-
β(1Ǟ6)-GalNAc disaccharide α-linked to the serine of a
protein, which belongs to one type of core-unit O-linked
glycans.[34] Traditional synthesis of the core-6–serine re-
quires different amine protecting functions for the GlcNAc
and GalNAc constituent units. The application of 2-azido-
2-deoxythioglycoside substrates together with the low-con-
centration glycosylation conditions can eliminate such a
Scheme 4. (a) Preparation of 2-azido-2-deoxygalactosylserine ac-
ceptor 37 and protected core-6–serine 38; (b) one-pot synthesis of
protected F1–α-serine 39.
requirement, thereby streamlining the protecting-group ma- high expression in tumorigenic tissue, such O-linked mucins
nipulation. are important targets for antitumor immunological stud-
Convergent synthesis of the protected core-6–serine con- ies.[47] As Gal-β(1Ǟ4)-GlcNAc-β(1Ǟ6)-GalNAc trisaccha-
jugate 38 required 2-azido-2-deoxythioglucoside donor 3 ride constitutes the carbohydrate component of the F1–α-
and 2-azido-2-deoxy-α-galactosylserine acceptor 37. The O-linked mucins, an efficient preparation of this target is
latter was obtained by reductive ring opening of the benzyl- highly desired. Although the F1–α-antigen was previously
idene acetal in known 2-azido-4,6-O-benzylidene-2-deoxy- synthesized,[48–50] only one sequential one-pot glycosylation
α-galactosylserine 36; TMOSTf was again employed as the method has been reported thus far.[50] By applying 2-azido-
acid catalyst for this reaction.[45] With the required building 2-deoxythioglycoside substrates and low-concentration
blocks in hand, the protected core-6–serine 38 was readily glycosylation conditions, an alternative, simpler, one-pot
obtained by glycosylation of 2-azido-2-deoxy-α-galactosyl- glycosylation strategy was realized.
serine 37 with 2-azido-2-deoxythioglucoside donor 3 under
The one-pot synthesis of protected F1–α-antigen 39 em-
low-concentration glycosylation conditions (Scheme 4a). ployed three saccharide building blocks, namely, 2-O-
The β-configuration of the newly formed glycosidic bond benzoyl-protected thiogalactoside 28, 2-azido-2-deoxythio-
in 38 was confirmed by the 13C NMR chemical shift of glucoside 6, and 2-azido-2-deoxy-α-galactosylserine 37.
the signal of the anomeric center at δ = 102.0 ppm and the Glucoside 6 was first glycosylated with thiogalactoside 28
1
corresponding JC–H coupling constant of 160 Hz.[35]
under the conventional glycosylation protocol in CH2Cl2 to
Mucin-related F1–α-antigens are glycoconjugates that obtain thiodisaccharide 32, which served as the disac-
contain Gal-β(1Ǟ4)-GlcNAc-β(1Ǟ6)-GalNAc trisac- charide donor in the subsequent glycosylation. Low-con-
charides α-linked to protein serine or threonine. They are centration conditions were required in the second glycosyl-
an example of aberrant oligosaccharides found on mucins ation step to effect the β-glycosidic bond formation, which
associated with gastric adenocarcinomas.[34,46] Due to their was provided by addition of a CH3CN/EtCN solution of
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7.37–7.23 (m, 13 H, ArH), 7.17–7.14 (m, 2 H, ArH), 5.54 (d, J =
5.4 Hz, 1 H, C-1), 4.90 (d, J = 10.8 Hz, 1 H), 4.81–4.76 (m, 2 H),
4.63–4.50 (m, 4 H), 4.42–4.40 (m, 2 H), 4.32–4.26 (m, 2 H), 4.10–
4.03 (m, 2 H), 3.82–3.60 (m, 4 H), 3.46–3.39 (m, 3 H), 1.54 (s, 3
H, CH3), 1.44 (s, 3 H, CH3), 1.33 (s, 6 H, 2ϫCH3) ppm. 13C NMR
(75 MHz, CDCl3): δ = 138.0, 128.41, 128.38, 128.35, 128.0, 127.8,
127.6, 109.3 [C(CH3)2], 108.7 [C(CH3)2], 102.4 (C-1Ј), 96.3 (C-1),
83.1, 77.7, 75.5, 75.0, 73.5, 71.2, 70.7, 70.5, 68.8, 68.5, 67.6, 66.4,
29.7, 26.0, 25.0, 24.4 ppm.
37 to the mixture. Subsequently, the coupling reaction of
the acceptor 37 and the thiodisaccharide 32 produced the
desired F1–α-serine conjugate 39 in 45% yield as a single
anomer.
Conclusions
2-Azido-2-deoxythioglucoside and 2-azido-2-deoxythio-
galactosides are effective glycosyl donors for 1,2-trans-β-
glycosidic bond formation under low-concentration glyc-
osylation conditions. The suitability of using such thioglyc-
oside substrates for oligosaccharide synthesis was demon-
strated by the synthesis of β-(1Ǟ6)-N-acetylglucosamine
trimers, core-6-unit–serine, and cancer-related F1–α-serine
antigens.
Methyl
2-Azido-3,4,6-tri-O-benzyl-2-deoxy-D-glucopyranosyl-
β(1Ǟ4)-2,3-O-isopropylidene-1-α-
L-rhamnopyranoside (19): Fig-
ure 2; Table 1, Entry 3. Disaccharide 19 was prepared by glycosyl-
ations of 14 (68 mg, 0.22 mmol) with 3 (151 mg, 0.26 mmol) ac-
cording to the general low-concentration glycosylation procedure.
Compound 19 was obtained after chromatography purification
(hexane/EtOAc, 9:1:0 stepwise to 4:1:1, v/v/v) as a lemon-yellow
syrup (145 mg, 60%; α/β = 1:32). [α]3D0 = –33.3 (c = 0.40, CHCl3).
1H NMR (300 MHz, CDCl3): δ = 7.38–7.26 (m, 13 H, ArH), 7.22–
7.21 (d, J = 7.0 Hz, 2 H, ArH), 4.91 (d, J = 11.0 Hz, 1 H), 4.88 (s,
1 H, 1-H), 4.84 (d, J = 8.0 Hz, 1 H, 1Ј-H), 4.82 (dd, J = 8.0,
11.0 Hz, 2 H), 4.63 (d, J = 3 Hz, 1 H), 4.59 (dd, J = 12, 30 Hz, 2
H),4.29 (t, J = 6.0 Hz, 1 H), 4.13 (d, J = 5.5 Hz, 1 H), 3.75 (dd, J
= 4.0, 11.0 Hz, 1 H), 3.70–3.67 (m, 4 H), 3.48 (t, J = 9 Hz, 1 H),
3.41 (br. s, 1 H), 3.40 (s, 3 H, OCH3), 3.37 (d, J = 2.0 Hz, 1 H),
1.48 (s, 3 H, CH3), 1.36 (s, 3 H, CH3), 1.35 (d, J = 5.0 Hz, CH3)
ppm. 13C NMR (75 MHz, CDCl3): δ = 138.07, 138.02, 137.99,
128.37, 128.36, 128.33, 128.0, 127.8, 127.7, 127.6, 127.5, 109.3
[C(CH3)2], 100.2 (JC,H = 163.0 Hz, C-1Ј), 97.9 (C-1), 83.2, 78.4,
78.1, 77.7, 76.0, 75.4, 75.1, 74.9, 73.4, 68.4, 66.4, 63.9, 54.8, 27.8
(CH3), 26.4 (CH3), 17.6 (CH3) ppm. HRMS (ESI): calcd. for
C37H45N3O9 [M + Na]+ 698.3156; found 698.3044.
Experimental Section
General Experimental: See Supporting Information.
General Low-Substrate-Concentration Glycosylation Procedure for
2-Azido-2-deoxythioglycoside Donors 3, 11, and 12:[22] To a suspen-
sion of 2-azido-2-deoxythioglycoside 3, 11, or 12 (1.2 equiv.), ac-
ceptor 7 or 13–17 (1.0 equiv.), flame-dried molecular sieves (4 Å;
ca. 5–10 wt.-% of solvent volume) in CH2Cl2/CH3CN/EtCN (1:2:1,
v/v) were added NIS (1.2 equiv.) and TMSOTf (0.3–1.5 equiv.) at
–70 to –60 °C under N2. The volume (mL) of solvent mixture used
for glycosylation was calculated by referring to the acceptor con-
centration of 10–15 mm. The reaction was monitored by TLC (hex-
ane/EtOAc or hexane/CH2Cl2/EtOAc). After completion of glyco-
sylation, the reaction was quenched with a few drops of NEt3, satd.
aq. NaHCO3 and Na2S2O3 (s), followed by stirring at room temp.
for 20 min. The mixture was diluted with CH2Cl2, filtered to re-
move MS, and concentrated. The residue was dissolved in CH2Cl2
and washed with satd. NaHCO3, H2O, brine, dried (MgSO4), and
concentrated for chromatographic purification.
Methyl
2-Azido-3,4,5-tri-O-benzyl-2-deoxy-
D-glucopyranosyl-
β(1Ǟ4)-2,3,6-tri-O-benzyl-α-
D
-glucopyranoside (20):[19] Table 1, En-
try 6. Prepared by glycosylation of 15 (93 mg, 0.2 mmol) with do-
nor 3 (139 mg, 0.24 mmol) according to the general low-concentra-
tion glycosylation procedure. Disaccharide 20 was obtained
(140 mg, 76%, α/β = 1:33) as a light-yellow syrup after chromato-
graphic purification (hexane/CH2Cl2/EtOAc, 6:1:1, v/v/v). 1H
NMR (500 MHz, CDCl3): δ = 7.38–7.17 (m, 30 H, ArH), 5.03 (d,
J = 11.5 Hz, 1 H), 4.83–4.73 (m, 5 H), 4.67 (d, J = 12.0 Hz, 1 H),
4.60–4.57 (m, 2 H, including 1-H), 4.54 (d, J = 12.0 Hz, 1 H), 4.46
(d, J = 12.0 Hz, 1 H), 4.37 (q, J = 9.5 Hz, 2 H), 4.25 (d, J = 8 Hz,
1 H, 1Ј-H), 3.99–3.3.89 (m, 3 H), 3.79 (d, J = 9.5 Hz, 1 H), 3.70
(d, J = 11.0 Hz, 1 H), 3.60 (dd, J = 11, 15 Hz, 2 H), 3.49 (m, 2 H),
3.37 (s, 3 H, OCH3), 3.32 (t, J = 9.0 Hz, 1 H), 3.24 (t, J = 9 Hz, 1
H), 3.18 (d, J = 8.5 Hz, 1 H) ppm. 13C NMR (125 MHz, CDCl3): δ
= 139.4, 138.23, 138.19, 137.9, 137.8, 137.7, 128.43, 128.36, 128.29,
128.24, 128.17, 128.0, 127.92, 127.89, 127.77, 127.63, 127.28,
Typical Sonication Thioglycosidation Procedure for the Synthesis of
Peracetyl-2-azido-2-deoxythiogalactoside 9:[14,21,25] Peracetyl-2-
azido-2-deoxygalactosyl acetate 8 (31 g, 80 mmol)[31] was dissolved
in dried CH2Cl2 (150 mL), followed by addition of thiocresol (20 g,
160 mmol) and boron trifluoride–diethyl ether (30 mL, 240 mmol).
The reaction mixture was irradiated by ultrasound in a sonication
bath (sonication cleaner: Delta DC300) for 40 min then stirred at
room temp. for 15 min. The sonication cycle was repeated over 5 h
until complete consumption of 8 was observed [reaction analyzed
by TLC; Rf of 8 = 0.3 (hexane/CH2Cl2/EtOAc, 6:1:1)]. The solution
was neutralized and washed with ice-cooled 1 n NaOH (aq.) (4ϫ
200 mL), then washed with H2O and brine, dried with MgSO4, and
concentrated for chromatographic purification. The thioglycosid-
ation product 9 (35 g, 95%; α/β = 1:1) was obtained as white
amorphous solid.
127.47, 127.3, 127.0, 100.8 (JC,H = 161 Hz, C-1Ј), 98.2 (JC,H
=
166 Hz, C-1), 83.2, 80.2, 79.0, 77.8, 76.6, 75.3, 75.2, 75.1, 73.44,
73.39, 73.2, 69.6, 68.5, 68.1, 66.8, 55.2 ppm.
Methyl
2-Azido-3,4,5-tri-O-benzyl-2-deoxy-D-glucopyranosyl-
β(1Ǟ2)-3,4,6-tri-O-benzyl-α-
D-mannopyranoside (21): Table 1, En-
2-Azido-3,4,6-tri-O-benzyl-2-deoxy-β-
D
-glucopyranosyl-(1Ǟ6)-
try 7. A mixture of 3 (140 mg, 0.24 mmol), 16 (93 mg, 0.2 mmol),
and activated molecular sieves (4 Å; 1.5 g) in CH2Cl2/CH3CN/
EtCN (1:2:1, v/v/v; 16 mL) was stirred under N2 at –60 °C for
30 min. NIS (55 mg, 0.24 mmol) and TMSOTf (22 μL, 0.12 mml)
were added, and the mixture was stirred for 4 h. The reaction mix-
ture was worked up as described in the general low-concentration
glycosylation procedure, and the desired disaccharide 21 (100 mg,
55%; α/β = 1:24) was obtained by column chromatographic purifi-
cation (hexane/CH2Cl2/EtOAc, 6.5:1:1, v/v/v). Rf = 0.3 (hexane/
1,2:3,4-di-O-isopropylidene-α-
D
-galactopyranoside (18):[19,22] Fig-
ure 2; Table 1, Entry 1. Disaccharide 18 was prepared by glycosyl-
ations of acceptor 13 (56 mg, 0.22 mmol) with thioglycoside 3
(151 mg, 0.26 mmol) according to the general low-concentration
glycosylation procedure. Compound 18 was obtained as a colorless
syrup (154 mg, 83%; α/β = 1:Ͼ49) after column chromatography
purification (hexane/CH2Cl2/EtOAc, 6:1:0 stepwise to 3:1:1, v/v/v).
1
[α]3D0 = –25.6 (c = 0.14, CHCl3). H NMR (300 MHz, CDCl3): δ =
6
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/KAP1
Date: 03-04-12 15:23:32
Pages: 10
β-Glycoside Formation and Oligosaccharide Synthesis
EtOAc, 1:2.5, v/v). [α]2D0 = –11.8 (c = 0.12, CHCl3). 1H NMR
idene-C), 100.0 (JC,H = 163 Hz, C-1Ј), 97.8 (JC,H = 167 Hz, C-1),
(500 MHz, CDCl3): δ = 7.38–7.22 (m, 26 H, ArH), 7.17–7.14 (m, 78.2, 78.0, 77.5, 76.0, 72.5, 71.7, 69.0, 66.3, 64.0, 62.3, 54.7, 27.8,
4 H, ArH), 4.92–4.78 (m, 6 H), 4.61–4.41 (m, 7 H), 4.37 (d, J =
8.5 Hz, 1 H), 4.27 (s, 1 H), 3.92 (br d, J = 8.5 Hz, 1 H), 3.81–3.64
(m, 6 H), 3.61–3.57 (m, 2 H), 3.49–3.46 (m, 1 H), 3.4 (d, J = 10 Hz,
1 H), 3.39 (s, 1 H, OCH3) ppm. 13C NMR (125 MHz, CDCl3): δ
= 138.9, 138.7, 138.5, 138.3, 138.2, 128.9, 128.83, 128.76, 128.7,
128.47, 128.46, 128.4, 128.34, 128.28, 128.1, 128.01, 127.97, 127.8,
101.3 (JC,H = 158 Hz, C-1Ј), 98.6 (C-1), 83.5, 78.4, 78.0, 75.9, 75.57,
75.54, 75.2, 74.0, 73.8, 73.6, 72.0, 71.2, 70.1, 69.7, 66.6, 55.3 ppm.
HRMS (ESI): calcd. for C55H59N3O10 [M + Na]+ 944.4093; found
944.4053.
26.3, 17.6 ppm. HRMS (ESI): calcd. for C34H39N3O9 [M + Na]+
656.2578; found 656.2632.
Methyl 2-Azido-4,6-O-benzylidene-2-deoxy-3-O-(2-naphthylmethyl)-
D
-galactopyranosyl-β(1Ǟ4)-2,3,6-tri-O-benzyl-α-D-glucopyranoside
(24): Table 1, Entry 10. A mixture of 11 (130 mg, 0.24 mmol), 15
(93 mg, 0.2 mmol), and activated molecular sieves (4 Å; 2 g) in
CH2Cl2/CH3CN/EtCN (1:2:1, v/v/v; 16 mL) was stirred at –60 °C
under N2 for 30 min. NIS (55 mg, 0.24 mmol) and TMSOTf
(14 μL, 0.08 mmL) were added, and the mixture was stirred for 4 h.
The reaction mixture was worked up according to the general low-
concentration glycosylation procedure to give the desired disac-
charide 24 (141 mg, 80%; α/β = 1:33) as a glassy material after
column chromatographic purification (hexane/CH2Cl2/EtOAc,
1:1:4.5, v/v/v). Rf = 0.3 (hexane/CH2Cl2/EtOAc, 1:1:3, v/v/v). [α]2D0
= +34.2 (c = 0.11 CHCl3). 1H NMR (500 MHz, CDCl3): δ = 7.87–
7.80 (m, 4 H, ArH), 7.54 (d, J = 8.5 Hz, 1 H, ArH), 7.49–7.47 (m,
6 H, ArH), 7.32–7.14 (m, 16 H, ArH), 5.42 (s, 1 H, benzylidene-
H), 5.12 (d, J = 10.5 Hz, 1 H), 4.85 (s, 2 H), 4.82–4.77 (m, 2 H),
4.67–4.59 (m, 3 H, including 1-H), 4.37 (d, J = 12.5 Hz, 1 H), 4.17–
4.14 (m, 2 H, including 1Ј-H), 4.03 (d, J = 11 Hz, 1 H), 3.97–3.93
(m, 3 H), 3.79–3.72 (m, 4 H), 3.52 (br. s, 1 H), 3.38 (s, 3 H, OCH3),
3.16 (br. d, J = 10 Hz, 1 H), 2.79 (s, 1 H) ppm. 13C NMR
(125 MHz, CDCl3): δ = 139.6, 138.8, 138.6, 138.2, 135.7, 133.6,
133.5, 129.4, 128.87, 128.80, 128.73, 128.70, 128.58, 128.54, 128.31,
128.21, 128.19, 128.0, 127.7, 126.9, 126.9, 126.7, 126.6, 126.1, 101.8
(JC,H = 160 Hz, C-1Ј), 101.6 (benzylidene-C), 98.7 (C-1), 80.8, 79.7,
78.7, 77.9, 76.3, 74.0, 73.7, 72.6, 71.8, 70.1, 69.3, 68.9, 66.8, 63.0,
55.8 ppm. HRMS (ESI): calcd. for C52H53N3O10 [M + Na]+
902.3623; found 902.3692.
p-Tolyl
2-Azido-3,4,6-tri-O-benzyl-2-deoxy-D-glucopyranosyl-
β(1Ǟ6)-2-azido-3,4-di-O-benzyl-2-deoxy-1-thio-α-
D
-glucopyran-
oside (22): Table 1, Entry 8. Disaccharide 22 was prepared from
glycosylations of 2-azido-2-deoxy-thio-α-d-glucopyranosyl ac-
ceptor 7 (107 mg, 0.22 mmol) with 2-azido-2-deoxy-d-glucopyr-
anosyl phosphate 4 (200 mg, 0.26 mmol). To a suspension of donor
4, acceptor 7, and flame-dried molecular sieves (4 Å; 2 g) in
CH2Cl2/CH3CN/EtCN (1:2:1, v/v/v; 22 mL) was added TMSOTf
(36 μL, 0.21 mmol) at –70 to –65 °C under N2. After completion
of the glycosylation, a few drops of NEt3, satd. aq. NaHCO3, and
Na2S2O3 (s) were added, and the mixture was stirred at room temp.
for 20 min. The mixture was diluted with CH2Cl2, filtered to re-
move molecular sieves, and concentrated as described in the general
low-concentration glycosylation procedure. Disaccharide 22 was
obtained after column chromatographic purification (hexane/
EtOAc/CH2Cl2, 16:1:0 stepwise to 8:1:1, v/v/v) as a lemon-yellow
syrup (172 mg, 77%; α/β = 1:19). [α]3D0 = +74.6 (c = 0.39, CHCl3).
1H NMR (300 MHz, CDCl3): δ = 7.40–7.27 (m, 25 H, ArH), 7.18–
7.15 (m, 2 H, ArH), 7.07 (d, J = 8.1 Hz, 2 H, ArH), 5.53 (d, J =
5.4 Hz, 1 H, 1-H), 4.96–4.89 (m, 3 H), 4.58–4.72 (m, 4 H), 4.58–
4.47 (m, 3 H), 4.4 (d, J = 8.7 Hz, 1 H, 1Ј-H), 4.14–4.06 (m, 2 H),
3.97–3.92 (m, 1 H), 3.87–3.76 (m, 3 H), 3.70–3.52 (m, 3 H), 3.49–
3.34 (m, 3 H), 2.28 (s, 3 H, CH3) ppm. 13C NMR (75 MHz,
CDCl3): δ = 138.1, 138.0, 137.7, 137.6, 132.3, 129.8, 129.7, 128.5,
6-Chlorohexyl 2-Azido-4,6-O-benzylidene-2-deoxy-3-O-(2-naphth-
ylmethyl)-β-D-galactopyranoside (25): Table 1, Entry 11. A suspen-
sion of 11 (100 mg, 0.19 mmol), 17 (37 μL, 0.28 mmol), and acti-
vated molecular sieves (4 Å; 300 mg) in CH2Cl2/CH3CN/EtCN
(1:2:1, v/v/v; 19 mL) was stirred at –70 °C under N2 for 1 h and
then treated with NIS (50 mg, 0.22 mmol) and TMSOTf (9 μL,
0.05 mmol). The mixture was stirred at –70 °C for 2 h. After com-
pletion of the glycosylation, a few drops of NEt3, satd. aq.
NaHCO3 and Na2S2O3 (s) were added, and the mixture was stirred
at room temp. for 20 min. The mixture was diluted with CH2Cl2,
filtered to remove molecular sieves, and concentrated for chromato-
graphic purification (hexane/CH2Cl2/EtOAc, 4:2:1, v/v/v). Glycos-
ide 25 was obtained as a light-yellow oily substance (84 mg, 80%;
128.4, 128.4, 128.3, 128.1, 128.0, 127.9, 127.8, 127.6, 102.0 (JC,H
=
160 Hz, C-1Ј), 87.8 (C-1), 83.2, 81.8, 78.0, 77.7, 75.5, 75.3, 74.9,
74.8, 73.4, 71.3, 68.6, 68.0, 66.3, 63.9, 21.0 (CH3) ppm. HRMS
(ESI): calcd. for C54H56N6O8 [M + Na]+ 965.4558; found 965.4526.
Methyl 2-Azido-4,6-O-benzylidene-2-deoxy-3-O-(2-naphthylmethyl)-
D
-galactopyranosyl-β(1Ǟ4)-2,3-O-isopropylidene-α-L-rhamnopyr-
anoside (23): Table 1, Entry 9. Disaccharide 23 was prepared by
glycosylation of methyl α-l-rhamnopyranoside 14 (50 mg,
0.23 mmol) with the β-anomer of 2-azido-2-deoxy-d-thiogalacto-
pyranoside 11 (148 mg, 0.28 mmol). To a suspension of donor 11,
acceptor 14, and flame-dried molecular sieves (4 Å, AW300; 2 g) in
CH2Cl2/CH3CN/EtCN (1:2:1, v/v/v; 23 mL) were added NIS
(64 mg, 0.29 mmol) and TMSOTf (11 μL, 0.06 mmol) at –70 °C un-
der N2. Compound 23 was obtained after column chromatographic
purification (hexane/EtOAc/CH2Cl2, 3:1:1, v/v/v) as a lemon-yel-
low syrup (102 mg, 70%; α/β = 1:19). [α]2D3 = –2.1 (c = 1.0, CHCl3).
1H NMR (500 MHz, CDCl3): δ = 7.83–7.82 (m, 3 H, ArH), 7.79–
7.78 (d, J = 6 Hz, 1 H, ArH), 7.55–7.52 (m, 3 H, ArH), 7.48–7.35
(m, 5 H, ArH), 5.44 (s, 1 H, benzylidene-H), 4.89 (d, J = 7.5,
12.5 Hz, 2 H), 4.85 (s, 1 H, 1-H), 4.76 (dd, J = 2.0, 8.0 Hz, 1 H,
1Ј-H), 4.27 (s, 1 H), 4.21 (d, J = 12.0 Hz, 1 H), 4.10 (d, J = 5.5 Hz,
1 H), 4.03 (d, J = 3.0 Hz, 1 H), 3.95 (dd, J = 1.5, 12.5 Hz, 1 H),
3.83 (t, J = 8.0 Hz, 1 H), 3.69–3.68 (m, 2 H), 3.42 (dd, J = 3.5,
10.5 Hz, 1 H), 3.36 (s, 3 H, OCH3), 3.21 (s, 1 H), 1.47 (s, 3 H,
α/β = 1:19). Rf = 0.3 (hexane/CH2Cl2/EtOAc, 4:2:1, v/v/v). [α]1D3
=
1
+148.2 (c = 0.34). H NMR (500 MHz, CDCl3): δ = 7.82–7.77 (m,
4 H, ArH), 7.54–7.52 (m, 3 H, ArH), 7.48–7.45 (m, 2 H, ArH),
7.37–7.33 (m, 3 H, ArH), 5.44 (s, 1 H, benzylidene-H), 4.88 (s, 2
H), 4.23 (d, J = 13 Hz, 1 H), 4.20 (d, J = 8.0 Hz, 1 H, 1-H), 4.04
(d, J = 3.0 Hz, 1 H), 3.97–3.92 (m, 2 H), 3.87 (dd, J = 8, 10 Hz, 1
H), 3.51 (t, J = 6.5 Hz, 2 H, CH2), 3.49–3.47 (m, 1 H), 3.36 (dd, J
= 5.0, 11.0 Hz, 1 H), 3.20 (s, 1 H), 1.76 (quint, J = 7.0 Hz, 2 H,
CH2), 1.66–1.62 (m, 2 H, CH2), 1.44–1.40 (m, 2 H, CH2) ppm. 13
C
NMR (125 MHz, CDCl3): δ = 137.6, 135.2, 133.1, 133.0, 128.9,
128.2, 128.1, 127.8, 127.6, 126.5, 126.3, 126.1, 126.0, 125.6, 102.1
(JC,H = 156 Hz, C-1), 101.0 (benzylidene-C), 77.5, 72.4, 71.6, 69.6,
69.0, 66.3, 62.1, 45.0, 32.4, 29.2, 26.5, 25.1 ppm. HRMS (ESI):
calcd. for C30H54ClN3O5 [M + Na]+ 574.2079; found 574.2057.
Methyl 2-Azido-3,4,6-tri-O-benzyl-2-deoxy-
D-galactopyranosyl-
CH3), 1.33 (d, J = 6 Hz, 3 H, CH3), 1.26 (s, 3 H, CH3) ppm. 13C β(1Ǟ4)-2,3,6-tri-O-benzyl-α-
D
-glucopyranoside (26):[19] Table 1, En-
NMR (125 MHz, CDCl3): δ = 137.73, 135.2, 133.1, 133.0, 129.0,
try 12. A mixture of 12 (150 mg, 0.26 mmol), 15 (100 mg,
128.2, 127.7, 126.5, 126.3, 126.1, 126.0, 125.7, 109.1, 101.1 (benzyl-
0.23 mmol), and activated molecular sieves (4 Å; 2 g) in CH2Cl2/
Eur. J. Org. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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7

Job/Unit: O20173
/KAP1
Date: 03-04-12 15:23:32
Pages: 10
K.-K. T. Mong, Y.-F. Yen, W.-C. Hung, Y.-H. Lai, J.-H. Chen
FULL PAPER
CH3CN/EtCN (1:2:1, v/v/v; 20 mL) were stirred at –60 °C under (CH2), 26.5 (CH2), 25.2 (CH2) ppm. HRMS (MALDI-TOF):
N2 for 30 min. NIS (63 mg, 0.28 mmol) and TMSOTf (28 mL, calcd. for C73H82ClN9O13 [M + Na]+ 1350.5613; found 1350.5542.
0.16 mmol) were added, and the mixture was stirred at –60 °C. Pro-
Methyl [2-Azido-3,4,6-tri-O-benzyl-2-deoxy-
D
-glucopyranosyl-
-galactopyranosyl]
-serine Ester 38: Core-6–serine
gress of the reaction was monitored by TLC (hexane/CH2Cl2/
EtOAc, 6:2:1). After 2 h of glycosylation, a few drops of NEt3,
satd. aq. NaHCO3 and Na2S2O3 (s) were added, and the mixture
was stirred at room temp. for 20 min. The mixture was diluted with
CH2Cl2, filtered to remove molecular sieves, and concentrated for
chromatographic purification (hexane/CH2Cl2/EtOAc, 3:1:0.3, v/v/
v) Disaccharide 26 was obtained as a glassy solid (150 mg, 65%;
β(1Ǟ6)-2-azido-3,4-di-O-benzyl-2-deoxy-1-α-
N-[(9-Fluorenylmethoxy)carbonyl]-
D
L
conjugate in Scheme 4b. Prepared from 2-azido-2-deoxythiogluc-
oside 3 (151 mg, 0.26 mmol) and 2-azido-2-deoxygalactosylserine
37 (120 mg, 0.22 mmol) according to the general low-concentration
glycosylation procedure. Protected core-6–serine conjugate 37
(180 mg, 57%) was obtained as a lemon-yellow syrup after chroma-
tographiyc purification (hexane/EtOAc/CH2Cl2, 6:1:0 stepwise to
3:1:1, v/v/v). Rf = 0.4 (hexane/EtOAc, 4:1, v/v). [α]3D2 = +38.3 (c =
0.53, CHCl3). 1H NMR (300 MHz, CDCl3): δ = 7.75 (d, J =
7.5 Hz, 2 H, ArH), 7.64–7.60 (m, 2 H, ArH), 7.38–7.23 (m, 27 H,
ArH), 7.17–7.12 (m, 2 H, ArH), 5.82 (d, J = 8.4 Hz, 1 H, N-H),
4.89–4.65 (m, 7 H), 4.61–4.30 (m, 8 H), 4.23 (t, J = 6.9 Hz, 1 H),
4.10–3.91 (m, 5 H), 3.85–3.52 (m, 8 H), 3.45–3.34 (m, 2 H) ppm.
13C NMR (75 MHz, CDCl3): δ = 170.4 (C=O), 155.9 (C=O), 143.8,
141.3, 138.1, 137.8, 137.3, 128.5, 128.4, 128.3, 128.3, 128.2, 127.9,
127.83, 127.77, 127.72, 127.65, 127.0, 125.1, 119.9, 102.0 (C-1Ј),
99.3 (C-1), 82.9, 75.4, 75.0, 74.8, 73.4, 72.9, 72.0, 70.2, 69.1, 68.4,
67.2, 66.4, 59.5, 54.4, 52.8, 47.1 ppm. HRMS (ESI): calcd. for
C55H58N6O9S [M + Na]+ 1188.4689; found 1188.4911.
α/β = 1:25). Rf = 0.4 (hexane/CH2Cl2/EtOAc, 6:2:1, v/v/v). [α]2D0
=
1
+12.2 (c = 0.12, CHCl3). H NMR (500 MHz, CDCl3): δ = 7.39–
7.38 (m, 3 H, ArH), 7.35–7.20 (m, 24 H, ArH), 7.17–7.13 (m, 3 H,
ArH), 4.96 (d, J = 11.0 Hz, 1 H), 4.88 (d, J = 11.0 Hz, 1 H), 4.76
(dd, J = 11.0, 29.0 Hz, 2 H), 4.67 (dd, J = 5.5, 12.0 Hz, 2 H), 4.61–
4.58 (m, 2 H), 4.576 (d, J = 3.5 Hz, 1 H, 1-H), 4.50 (d, J = 11.5 Hz,
1 H), 4.43 (d, J = 11.0 Hz, 1 H), 4.27 (dd, J = 12.0, 57.0 Hz, 2 H),
4.14 (d, J = 8.0 Hz, 1 H, 1Ј-H), 3.94 (dd, J = 3.0, 10.5 Hz, 1 H),
3.91 (d, J = 9.5 Hz, 1 H), 3.85 (t, J = 9.0 Hz, 1 H), 3.78–3.72 (m,
2 H), 3.69 (dd, J = 1.5, 11.0 Hz, 1 H), 3.50–3.46 (m, 2 H), 3.67 (s,
3 H, OCH3), 3.30 (dd, J = 5.2, 9.1 Hz, 1 H), 3.21 (dd, J = 5.4,
8.3 Hz, 1 H), 3.12 (dd, J = 2.8, 10.3 Hz, 1 H) ppm. 13C NMR
(125 MHz, CDCl3): δ = 139.8, 139.0, 138.8, 138.5, 138.4, 138.1,
128.9, 128.81, 128.80, 128.77, 128.6, 128.5, 128.40, 128.37, 128.33,
128.29, 128.24, 128.22, 128.15, 128.11, 128.07, 127.9, 127.4, 101.6
(C-1Ј, JC,H = 161.3 Hz), 98.8 (C-1, JC,H = 165 Hz), 81.5, 80.6, 79.5,
77.01, 75.8, 75.15, 74.0, 73.84, 73.75, 73.6, 72.5, 70.1, 68.7, 68.3,
64.2, 55.7 ppm.[19]
Methyl [2-O-Benzoyl-3,4,6-tri-O-benzyl-
β(1Ǟ4)-2-azido-3,6-O-dibenzyl-2-deoxy- -glucopyranosyl-β(1Ǟ6)-
2-azido-3,4-dibenzyl-2-deoxy-1-α- -galactopyranosyl] N-[(9-Fluo-
renylmethoxy)carbonyl]- -serine Ester (39): F1–α-serine conjugate
D-galactopyranosyl-
D
D
L
in Scheme 4c. A suspension of 3,4,6-tri-O-benzyl-2-O-benzoyl-thio-
galactoside donor 27 (323 mg, 0.49 mmol), 2-azido-2-deoxythio-
glucoside acceptor 6 (200 mg, 0.40 mmol), and activated molecular
sieves (4 Å, AW300; 500 mg) in CH2Cl2 (8 mL) was stirred at
–70 °C for 20 min. NIS (111 mg, 0.5 mmol) and TMSOTf (22 μL,
0.12 mmol) were added to the mixture. When the glycosylation cou-
pling was complete [assessed by TLC; Rf(disaccharide) = 0.2 (hex-
ane/EtOAc, 4:1, v/v)], 2-azido-2-deoxygalactosylserine 37 (288 mg,
0.40 mmol) in CH3CN (16 mL) and EtCN (8 mL) were added to
the reaction mixture, which was stirred at –70 °C for ca. 30 min and
then treated with NIS (90 mg, 0.40 mmol) and additional TMSOTf
(73 μL, 0.40 mmol). Upon completion of the glycosylation [as-
sessed by TLC; Rf(glycosylation product) = 0.3 (hexane/EtOAc,
2:1, v/v)], a few drops of NEt3, satd. aq. NaHCO3 and Na2S2O3
(s) were added, and the mixture was stirred at room temp. for
20 min. The mixture was diluted with CH2Cl2, filtered to remove
molecular sieves, and concentrated for chromatographic purifica-
tion (hexane/EtOAc/CH2Cl2, 8:1:0.5 stepwise to 3:1:0.5, v/v/v) to
furnish the desired F1–α-serine 39 as a white foam (280 mg, 45%).
[α]3D0 = +38.5 (c = 0.31, CHCl3). 1H NMR (300 MHz, CDCl3): δ =
8.00 (d, J = 7.4 Hz, 1 H), 7.88 (d, J = 7.5 Hz, 4 H), 7.73 (t, J =
6.1 Hz, 5 H), 7.60 (t, J = 7.1 Hz, 7 H), 7.29–7.17 (m, 54 H), 6.14
(t, J = 9.4 Hz, 1 H), 5.61 (dd, J = 18.8, 10.7 Hz, 1 H), 5.08–4.93
(m, 2 H), 4.81 (d, J = 11.0 Hz, 2 H), 4.63–4.55 (m, 8 H), 4.46–4.34
(m, 6 H), 4.25–4.13 (m, 4 H), 3.95 (dd, J = 16.3, 6.5 Hz, 7 H),
3.74–3.65 (m, 7 H), 3.46–3.31 (m, 6 H), 3.10 (d, J = 9.6 Hz, 1 H)
ppm. 13C NMR (75 MHz, CDCl3): δ = 170.3 (C=O), 164.9 (C=O),
155.8 (C=O), 143.8, 141.2, 138.8, 138.2, 138.0, 137.8, 137.6, 137.3,
133.0, 129.7, 128.4, 128.4, 128.3, 128.2, 128.18, 128.15, 128.10,
127.9, 127.84, 127.76, 127.71, 127.63, 127.56, 127.4, 127.2, 127.0,
125.1, 119.9, 101.6 (C-1ЈЈ), 100.3 (C-1Ј), 99.4 (C-1), 81.0, 79.6, 75.4,
74.7, 74.4, 73.4, 73.3, 72.3, 71.9, 71.2, 70.1, 69.2, 67.8, 67.1, 65.7,
59.4, 54.3, 52.7, 47.0 ppm. HRMS (ESI): calcd. for C93H93ClN7O19
[M + Na]+ 1634.6526; found 1635.6451.
6-Chlorohexyl 2-Azido-3,4,6-tri-O-benzyl-2-deoxy-
osyl-β(1Ǟ6)-2-azido-3,4-O-di-O-benzyl-2-deoxy- -glucopyranosyl-
β(1Ǟ6)-3,4-di-O-benzyl-2-deoxy-β- -glucopyranoside (35): N-Acet-
D-glucopyran-
D
D
ylglucosamine trimer in Scheme 3b. A suspension of 2-azido-2-de-
oxyglucosyl phosphate 4 (226 mg, 0.32 mmol), 2-azido-2-deoxy-
thioglucoside 7 (130 mg, 0.27 mmol), and activated molecula sieves
(4 Å; 2 g) in CH2Cl2/CH3CN/EtCN (1:2:1, v/v/v; 26 mL) was
stirred at –70 °C under N2 for ca. 30 min. TMSOTf (48 μL,
0.27 mmol) was then added. Upon completion of the reaction
[monitored by TLC; Rf(thiodisaccharide) = 0.6 (hexane/EtOAc,
3:1, v/v)], reducing-end GlcN3 acceptor 34 (150 mg, 0.32 mmol)
and NIS (78 mg, 0.35 mmol) were added. Upon completion of the
second glycosylation [assessed by TLC; Rf(trisaccharide) = 0.4
(hexane/EtOAc, 3:1, v/v)], a few drops of NEt3, satd. aq. NaHCO3
and Na2S2O3 (s) were added, and the mixture was stirred at room
temp. for 20 min. The mixture was diluted with CH2Cl2, filtered
to remove molecular sieves, and concentrated for chromatographic
purification (hexane/CH2Cl/EtOAc2, 10:1:1 stepwise to 4:1:1, v/v/
v) to furnish the expected trisaccharide 35 (260 mg, 72%) as a pale-
yellow syrup. [α]3D2 = –13.6 (c = 0.17 , CHCl3); Rf = 0.4 (hexane/
1
EtOAc, 3:1, v/v). H NMR (500 MHz, CDCl3): δ = 7.35–7.25 (m,
33 H, ArH), 7.14–7.13 (m, 2 H, ArH), 4.89–4.85 (m, 4 H), 4.82–
4.81 (m, 1 H), 4.79–4.76 (m, 3 H), 4.71 (dd, J = 2.5, 10.5 Hz, 1 H),
4.62–4.58 (m, 2 H), 4.560–4.555 (m, 1 H), 4.50 (br. t, J = 8.5 Hz,
2 H), 4.385 (d, J = 7.5 Hz, 1 H, anomeric-H), 4.379 (d, J = 8.0 Hz,
1 H, anomeric-H), 4.30 (br. d, J = 7.0 Hz, 1 H, anomeric-H), 4.27
(d, J = 3.0, 8.0 Hz, 1 H, anomeric-H), 4.23 (br. d, J = 12 Hz, 1 H),
4.17 (d, J = 11.0 Hz, 1 H), 4.00–3.98 (m, 1 H), 3.75–3.72 (m, 1 H),
3.67–3.50 (m, 19 H), 1.80–1.74 (m, 2 H, CH2), 1.68–1.63 (m, 2 H,
CH2), 1.49–1.40 (m, 4 H, 2ϫCH2) ppm. 13C NMR (125 MHz,
CDCl3): δ = 137.83, 137.78, 137.72, 128.45, 128.42, 128.38, 128.35,
128.0, 127.9, 127.8, 127.7, 127.6, 102.51 (JC,H = 160 Hz, anomeric-
C), 102.47 (JC,H = 160 Hz, anomeric-C), 102.0 (JC,H = 159.0 Hz,
anomeric-C), 83.0, 78.0, 77.7, 77.5, 75.42, 75.39, 74.92, 74.86, 74.6,
73.4, 69.8, 68.6, 68.4, 66.3, 66.2, 66.1, 45.0 (CH2), 32.4 (CH2), 29.3
Supporting Information (see footnote on the first page of this arti-
cle): Preparation of saccharide building blocks 3, 4, 7, 11, 12, 29,
8
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Date: 03-04-12 15:23:32
Pages: 10
β-Glycoside Formation and Oligosaccharide Synthesis
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We thank the National Research Council for financial support
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Received: February 14, 2012
Published Online: ᭿
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9

Job/Unit: O20173
/KAP1
Date: 03-04-12 15:23:32
Pages: 10
K.-K. T. Mong, Y.-F. Yen, W.-C. Hung, Y.-H. Lai, J.-H. Chen
FULL PAPER
Glycosylations
2-Azido-2-deoxyglycosides are commonly
used for 1,2-cis-glycosidic bond formation.
This study demonstrates that such donors
can be used to construct 1,2-trans-β-glycos-
idic bonds through the low-concentration
glycosylation method. Its utility is illus-
trated by the synthesis of a biologically rel-
evant protected β-(1Ǟ6)-N-acetylglucos-
amine trimer, core-6–serine conjugate, and
F1–α-serine conjugate.
K.-K. T. Mong,* Y.-F. Yen, W.-C. Hung,
Y.-H. Lai, J.-H. Chen ..................... 1–10
Application of 2-Azido-2-deoxythioglycos-
ides for β-Glycoside Formation and Oligo-
saccharide Synthesis
Keywords: Azides / Carbohydrates / Glycos-
ylation / Glycopeptides / Regioselectivity
10
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Eur. J. Org. Chem. 0000, 0–0