Armed-disarmed effect of remote protecting groups on the glycosylation reaction of 2,3-dideoxyglycosyl donors

Article abstract of DOI:10.1016/j.tetlet.2011.02.109

The armed-disarmed effect of remote protecting groups at the C-4 and/or C-6 position(s) on the glycosylation reactions of 2,3-dideoxyglycosyl donors was investigated. It was found that under various glycosylation conditions, 4- or 6-O-Bn 2,3-dideoxyglycos

Full text of DOI:10.1016/j.tetlet.2011.02.109

Tetrahedron Letters 52 (2011) 2399–2403
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Tetrahedron Letters
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Armed–disarmed effect of remote protecting groups on the glycosylation
reaction of 2,3-dideoxyglycosyl donors
⇑
Satoshi Tomono, Shunichi Kusumi, Daisuke Takahashi, Kazunobu Toshima
Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The armed–disarmed effect of remote protecting groups at the C-4 and/or C-6 position(s) on the glyco-
sylation reactions of 2,3-dideoxyglycosyl donors was investigated. It was found that under various glyco-
sylation conditions, 4- or 6-O-Bn 2,3-dideoxyglycosyl donors were much more reactive than the
corresponding 4,6-di-O-Bz 2,3-dideoxyglycosyl donors. Based on these results, an effective and chemose-
lective glycosylation reaction using 4,6-di-O-Bn glycosyl acetate and 4-OH-6-O-Bz glycosyl acetate was
Received 25 January 2011
Revised 18 February 2011
Accepted 25 February 2011
Available online 2 March 2011
realized, producing a 2,3-dideoxydisaccharide in good yield with high a-stereoselectivity.
Keywords:
Ó 2011 Elsevier Ltd. All rights reserved.
2,3-Dideoxyglycosides
Chemoselective glycosylation
Armed–disarmed concept
Remote protecting group effect
Deoxysugars are often found in the oligosaccharide domains of
biologically active natural products, such as antibiotics and anti-
cancer agents, and play important roles in the biological activities
of such products.1 Like many deoxysugars, 2,3-dideoxyglycosides
frequently appear in important natural products, including lando-
mycin A,2 vineomycin B2,3 lactonamycin,4 and PI-080.5 However,
few attempts to construct this structure have been reported to
date6,7 due to the difficulty of preparing highly deoxygenated oli-
gosaccharides. Accordingly, the development of novel and effective
synthetic methods has attracted much attention in the field of syn-
thetic carbohydrate chemistry.8 One of the most attractive strate-
gies for this purpose is chemoselective glycosylation.9 The
‘armed–disarmed’ concept introduced by Fraser-Reid and co-work-
ers has been one of the most influential ideas in this field.10 Using
this method, the reactivity of glycosyl donors that possess a partic-
ular leaving group at the anomeric position can be controlled by
the combinational use of electron-withdrawing and electron-
donating protecting groups at the C-2 position. In addition,
Demchenko and co-workers recently discovered the O-2/O-5 coop-
erative effect,11 which expanded the scope of the classic Fraser-
Reid armed–disarmed concept for chemoselective oligosaccharide
synthesis through the development of a series of building blocks
(superarmed and superdisarmed) for sequential activation.12 How-
ever, these approaches cannot be directly applied to 2-deoxyglyco-
syl donors due to their lack of a C-2 substituent; therefore, only a
few methods for chemoselective glycosylation using 2-deoxy sug-
ars have been reported.8a
For example, we recently reported chemoselective glycosyla-
tion reactions using conformationally modified 2,3-unsaturated
and 2,3-unsaturated 4-keto sugars as novel armed and disarmed
glycosyl donors, respectively.13 However, significantly less infor-
mation about how remote protecting groups affect the reactivity
of 2,3-dideoxyglycosyl donors has been acquired. Herein, we pres-
ent our preliminary study focusing on the armed–disarmed effect
of remote protecting groups on the glycosylation reaction of 2,3-
dideoxyglycosyl donors (Fig. 1).
To investigate how the protecting group at C-4 affects the
reactivity of 2,3-dideoxyglycosyl donors, we first selected 4-O-
Bn-6-O-Bz glycosyl acetate (1) and 4,6-di-O-Bz glycosyl acetate
(2) as 2,3-dideoxyglycosyl donors, and 3 as a glycosyl acceptor,
and then examined competitive glycosylation reactions using sev-
eral activators, such as TMSOTf, TBSOTf, BF3-OEt2, TfOH, and
momorillonite K-10 (MK-10). The results are summarized in Table
1. It was found that the glycosylation of 1 (1.0 equiv) with 3
(1.0 equiv) proceeded smoothly to provide disaccharide 4 in good
to high yields (entries 1–5). Under similar conditions, however,
the glycosylation of 2 (1.0 equiv) with 3 (1.0 equiv) did not provide
disaccharide 5 effectively. In fact, unreacted glycosyl donor 2 was
recovered in moderate to high yields (entries 6–10). The glycosyl
acceptor 3 was stable under the reaction conditions and reasonably
recovered in all cases. Furthermore, it was confirmed that the che-
moselectivity was essentially independent of the activator. These
results clearly indicated that the 4-O-Bn glycosyl donor 1 was
much more reactive than the corresponding 4-O-Bz glycosyl
donor 2.
Next, as shown in Table 2, competitive glycosylations were
carried out using 4-O-Bz-6-O-Bn glycosyl acetate 6 (1.0 equiv) or
⇑
Corresponding author. Tel./fax: +81 45 566 1576.
E-mail address: toshima@applc.keio.ac.jp (K. Toshima).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.02.109

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S. Tomono et al. / Tetrahedron Letters 52 (2011) 2399–2403
a) Glycosylation using 4-O-Bz-6-O-Bn glycosyl donor
BnO
BzO
Armed glycosyl donor
Intermediate
stabilized by
electron
BnO
BzO
BnO
BzO
ROH
Activator
O
O
O
O
O
O
-donating
Glycosylation
group at C-6
LG
OR
OR
OR
b) Glycosylation using 4-O-Bn-6-O-Bz glycosyl donor
Intermediate
stabilized by
electron
BzO
BnO
BzO
BnO
BzO
BnO
Activator
ROH
O
-donating
Glycosylation
group at C-4
Armed glycosyl donor
LG
c) Glycosylation using 4,6-di-O-Bz glycosyl donor
Intermediate
unstabilized
by electron
-withdrawing
groupsat C-4
and 6
BzO
BzO
BzO
BzO
BzO
BzO
ROH
Activator
O
O
Glycosylation
Disarmed glycosyl donor
LG
Figure 1. Armed–disarmed effect of remote protecting groups at the C-4 and/or C-6 position(s) on the glycosylation reaction of 2,3-dideoxyglycosyl donors.
Table 1
Competitive glycosylations using 1 and 2
OBz
O
OBz
O
OBz
OBz
O
OH
BnO
BzO
O
O
Activator
BnO
BzO
BnO
or
O
O
+
or
O
BnO
BnO
CH2Cl2, MS 5A
O
BnO
BnO
BnO
BnO
OAc
OAc
OMe
1
2
3
BnO
BnO
OMe
OMe
4
5
Entrya
Donor
Activator (equiv)
Temp°(C)
Time (h)
Glycoside yieldb (%) (
a
:b ratio)c
Recovery yield of donorb (%)
1
2
3
4
5
6
7
8
9
10
1
1
1
1
1
2
2
2
2
2
TMSOTf (0.3)
TBSOTf (0.3)
TfOH (0.3)
ꢀ50
ꢀ45
ꢀ45
ꢀ50
25
ꢀ50
ꢀ45
ꢀ45
ꢀ50
25
1
1
0.5
18
12
1
4:92 (70:30)
4:91 (69:31)
4:89 (68:32)
4:80 (66:34)
4:73 (69:31)
5:6 (51:49)
5:10 (52:48)
5:9 (54:46)
5:35 (55:45)
5:24 (42:58)
1:0
1:9
1:10
1:11
1:0
2:92
2:84
2:89
2:58
2:30
BF3-OEt2 (2.0)
MK-10d
TMSOTf (0.3)
TBSOTf (0.3)
TfOH (0.3)
1
0.5
18
12
BF3-OEt2 (2.0)
MK-10d
a
b
c
The ratio of glycosyl donor to glycosyl acceptor was 1.0:1.0.
Isolated yields.
a
1
:b ratios were determined by H NMR analysis with aid of coupling constant between H-10 and H-20.
d
100 wt% MK-10 with respect to the donor was used.
Table 2
Competitive glycosylations using 6 and 2
OBn
O
OBz
OBn
OBz
O
OH
O
BzO
BzO
O
O
Activator
BzO
BzO
BnO
or
O
O
BnO
+
or
O
BnO
BnO
CH2Cl2, MS 5A
O
BnO
BnO
OAc
OAc
OMe
BnO
6
2
3
BnO
OMe
BnO
OMe
5
7
Entrya
Donor
Activator (equiv)
Temp (°C)
Time (h)
Glycoside yieldb (%) (
a
:b ratio)c
Recovery yield of donorb (%)
1
2
3
4
5
6
7
8
9
10
6
6
6
6
6
2
2
2
2
2
TMSOTf (0.3)
TBSOTf (0.3)
TfOH (0.3)
ꢀ55
ꢀ45
ꢀ45
ꢀ50
25
ꢀ55
ꢀ45
ꢀ45
ꢀ50
25
1
1
0.5
18
12
1
7:97 (39:61)
7:96 (45:55)
7:86 (45:55)
7:77 (52:48)
7:75 (43:57)
5:0
5:10 (52:48)
5:9 (54:46)
5:35 (55:45)
5:24 (42:58)
6:0
6:0
6:5
6:12
6:0
2:96
2:84
2:89
2:58
2:30
BF3-OEt2 (2.0)
MK-10d
TMSOTf (0.3)
TBSOTf (0.3)
TfOH (0.3)
1
0.5
18
12
BF3-OEt2 (2.0)
MK-10d
a
b
c
The ratio of glycosyl donor to glycosyl acceptor was 1.0:1.0.
Isolated yields.
a
1
:b ratios were determined by H NMR analysis with aid of coupling constant between H-10 and H-20.
d
100 wt% MK10 with respect to the donor was used.

S. Tomono et al. / Tetrahedron Letters 52 (2011) 2399–2403
2401
Table 3
Competitive glycosylations using 8 and 9
OBn
OBz
O
OBn
O
OBz
O
OH
O
BnO
BnO
BnO
TMSOTf
BnO
O
(0.3 equiv.)
BnO
BnO
BnO
BnO
or
BnO
O
BnO
O
or
O
+
BnO
BnO
BnO
CH2Cl2, MS 5A,
8 h
BnO
α β
BnO
O
OAc
OAc
BnO
BnO
BnO
OMe
BnO
8
9 α β
( / = 83:17)
3
(
/
= 80:20)
BnO
BnO
OMe
OMe
10
11
Entrya
Donor
Temp (°C)
Glycoside yieldb (%) (
a
:b ratio)c
Recovery yield of donorb (%)
1
2
3
4
5
6
8
8
8
9
9
9
25
0
ꢀ40
25
0
10:67 (60:40)
10:58 (37:63)
10:27 (19:81)
11:67 (86:14)
11:47 (84:16)
11:5 (80:20)
8:2
8:14
8:59
9:12
9:27
9:79
ꢀ40
a
b
c
The ratio of glycosyl donor to glycosyl acceptor was 1.0:1.0.
Isolated yields.
1
a
:b ratios were determined by H NMR analysis with aid of coupling constant between H-10 and H-20.
2 (1.0 equiv) as donors in order to study the effect of the C-6 pro-
tecting group. It was found that the glycosylation reaction between
6 and 3 provided disaccharide 7 in good to high yields under sev-
eral glycosylation conditions (entries 1–5). In sharp contrast, the
chemical yields for the glycosylation of 2 with 3 were low under
the same reaction conditions; in addition, unreacted glycosyl do-
nor 2 was recovered in moderate to high yields (entries 6–10).
These results clearly indicated that the 6-O-Bn glycosyl donor 6
was much more reactive than the corresponding 6-O-Bz glycosyl
donor 2. It should be noted that the reactivities of 1 and 6 were al-
most the same in these glycosylation reactions; moreover, the
reactivity of the 2,3-dideoxyglycosyl donors could be controlled
based only on the presence of an electron-withdrawing or -donat-
ing protecting group at the C-4 or 6 position.
activator, we examined the glycosylation reaction of 1 and 3 in
the presence of TMSOTf at ꢀ50 °C, using MeOBz (12, 1.0 equiv)
as an additive. As shown in Scheme 1, it was found that chemical
yield of the glycosylation reaction in the presence of 12 was exactly
the same as that of the reaction in the absence of 12 (entry 1, Table
1). This result clearly indicated that the electron-withdrawing ef-
fect of benzoyl group was the major factor in the remote disarming
effect. In addition, the remote arming effect of the benzyl group
was rationalized by comparison of the chemical shifts of H-5 in
the 1H NMR spectra of various glycosyl donors. The O-5 lone pair
can assist in the departure of the leaving group and stabilize the
resulting glycosyl cation. It was found that the H-5 signals of 1, 6
and 17a were more shielded than that of 2 (Table 4).
To demonstrate the versatility of the remote protecting group
effects, we carried out competitive glycosylation reactions using
different glycosyl donors such as thioglycosides and glycosyl fluo-
rides, which are widely used in oligosaccharide synthesis. After
preparing phenylthioglycosides 13 and 15 and glycosyl fluorides
14 and 16, we examined competitive glycosylation reactions, and
the results are shown in Table 5. In the case of phenylthioglyco-
sides, it was found that a sufficient reactivity difference existed be-
tween the armed glycosyl donor 13 (1.0 equiv) and the disarmed
glycosyl donor 15 (1.0 equiv) in the presence of NBS, which is a
mild promoter of thioglycoside activation, and the corresponding
disaccharide 4 was isolated in high yield (entry 1). The formation
We then turned our attention to the armed–disarmed effect of
remote protecting groups on the glycosylation reaction of glucosyl
donors. Accordingly, we carried out competitive glycosylations
using the 6-O-Bn glucosyl donor 8 (a/b = 80:20, 1.0 equiv) or the
6-O-Bz glucosyl donor 9 ( /b = 83:17, 1.0 equiv) and the glycosyl
a
acceptor 3 (1.0 equiv) under several reaction conditions. The re-
sults are summarized in Table 3. When the reaction temperature
was maintained at 0 or 25 °C for 8 h, glycosyl donors 8 and 9
showed similar reactivities, and the corresponding disaccharides
10 and 11, respectively, were generated in moderate yields (entries
1, 2, 4 and 5). A slight difference in reactivity was observed when
the glycosylation reactions were carried out at ꢀ40 °C (entries 3
and 6); however, the chemical yields of 10 and 11 were very
low. These results clearly suggested that the armed–disarmed ef-
fect of remote protecting groups on the glycosylation reaction of
glucosyl donors is insufficient for controlling their reactivity and
chemoselectivity.
Table 4
Comparison of the chemical shift of H-5 in the 1H NMR spectra of glycosyl donors 1, 2,
6 and 17
a
In order to determine whether the major factor in the disarming
effect of the benzoyl group was the electron-withdrawing effect or
the Lewis basicity of the carbonyl oxygen with respect to the
Glycosyl donor
1
2
6
17
a
Chemical shift of H-5 (ppm)
4.07
4.32
4.12
3.83
OBz
O
OBz
O
OH
O
TMSOTf
(0.3 equiv.),
BnO
O
BnO
OMe
BnO
BnO
O
+
+
BnO
OMe
3 (1.0 equiv.)
O
CH2Cl2, MS 5A,
-50 oC, 1 h
BnO
BnO
OAc
1 (1.0 equiv.)
12 (1.0 equiv.)
BnO
OMe
4
92%, α/β = 73:27
Scheme 1. Glycosylation of 1 and 3 in the presence of 12.

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S. Tomono et al. / Tetrahedron Letters 52 (2011) 2399–2403
Table 5
Competitive glycosylation using phenylthioglycosides and glycosyl fluorides
OBz
O
OBz
O
OBz
O
OBz
O
OH
BnO
BzO
O
Activator
BnO
BzO
BnO
or
O
O
+
X
or
O
BnO
BnO
CH2Cl2, MS 5A
O
X
BnO
BnO
BnO
BnO
OMe
BnO
OMe
BnO
13: X = SPh
14: X = F
15: X = SPh
16: X = F
3
OMe
4
5
Entrya
Donor (
a
:b ratio)c
Activator (equiv)
Temp (°C)
Time (h)
Glycoside yieldb (%) ( :b ratio)c
a
1
2
3
4
13 (100:0)
15 (100:0)
14 (85:15)
16 (62:38)
NBS (1.1)
NBS (1.1)
TBSOTf (0.3)
TBSOTf (0.3)
ꢀ45
ꢀ45
ꢀ60
ꢀ60
3
3
0.5
0.5
4:83 (78:22)
5:0
4:83 (71:29)
5:18 (51:49)
a
b
c
The ratio of glycosyl donor to glycosyl acceptor was 1.0:1.0.
Isolated yields.
1
a
:b ratios were determined by H NMR analysis with aid of coupling constant between H-10 and H-20.
OBn
OBn
O
OBz
O
TMSOTf
(0.3 equiv.)
O
BnO
BnO
HO
OBz
+
O
OAc
CH2Cl2, MS 5A,
-60 oC, 1h
O
OAc
18 (1.0 equiv.)
17
19
(2.0 equiv., α/β= 50:50)
OAc
α
64%, only
Scheme 2. Synthesis of disaccharide 19 by chemoselective glycosylation using 2,3-dideoxyglycosyl acetates 17 and 18.
of disaccharide 5 was not observed under the same reaction
conditions (entry 2). In the case of glycosyl fluorides 14 (
b = 85:15, 1.0 equiv) and 16 ( /b = 62:38, 1.0 equiv), the 4-O-Bn
glycosyl donor 14 was found to be much more reactive than the
corresponding 4-O-Bz glycosyl donor 16 (entries 3 and 4). These
results indicated that the armed–disarmed effect of remote pro-
tecting groups on the glycosylation reaction of 2,3-dideoxyglycosyl
donors was essentially independent of the leaving groups.
Finally, we investigated chemoselective glycosylation using 4,6-
di-O-Bn glycosyl acetate (17) and 4-OH-6-O-Bz glycosyl acetate
(18) in the synthesis of 2,3-dideoxydisaccharide 19. As shown in
Scheme 2, chemoselective glycosylation using glycosyl donor 17
Acknowledgments
a/
a
This research was supported by the High-Tech Research Center
Project for Private Universities: Matching Fund Subsidy, 2006–
2011, from the Ministry of Education, Culture, Sports, Science
and Technology of Japan (MEXT).
Supplementary data
Supplementary data (experimental procedures and character-
ization of new compounds) associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.2011.02.109.
(
a
/b = 50:50, 2.0 equiv) and glycosyl acceptor 18 (1.0 equiv) in
the presence of TMSOTf at ꢀ60 °C proceeded smoothly to provide
disaccharide 19 in 64% yield with high -stereoselectivity.
References and notes
a
In conclusion, we studied the armed–disarmed effect of remote
protecting groups at the C-4 and C-6 positions on the glycosylation
reaction of 2,3-dideoxyglycosyl donors. Competitive glycosylation
reactions clearly showed that 4- or 6-O-Bn glycosyl donors 1 and
6 were far more reactive than the corresponding 4,6-di-O-Bz glyco-
syl donor 2. In addition, this tendency was essentially independent
not only of the activator but also of the leaving group at the ano-
meric position. We further showed that chemoselective glycosyla-
tion using 4,6-di-O-Bn glycosyl acetate 17 and 4-OH-6-O-Bz
glycosyl acetate 18 could effectively provide 2,3-dideoxydisaccha-
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