Significant substituent effect on the anomerization of pyranosides: Mechanism of anomerization and synthesis of a 1,2-cis glucosamine oligomer from the 1,2-trans anomer

Article abstract of DOI:10.1002/chem.201303474

Aminoglycosides containing a 2,3-trans carbamate group easily undergo anomerization from the 1,2-trans glycoside to the 1,2-cis isomer under mild acidic conditions. The N-substituent of the carbamate has a significant effect on the anomerization reaction;

Full text of DOI:10.1002/chem.201303474

DOI: 10.1002/chem.201303474
Full Paper
&
Carbohydrates
Significant Substituent Effect on the Anomerization of
Pyranosides: Mechanism of Anomerization and Synthesis of
a 1,2-cis Glucosamine Oligomer from the 1,2-trans Anomer
Shino Manabe,*[a] Hiroko Satoh,*[b] Jꢀrg Hutter,[c] Hans Peter Lꢀthi,[d] Teodoro Laino,[e] and
Yukishige Ito[a]
Abstract: Aminoglycosides containing a 2,3-trans carbamate
group easily undergo anomerization from the 1,2-trans gly-
coside to the 1,2-cis isomer under mild acidic conditions.
The N-substituent of the carbamate has a significant effect
on the anomerization reaction; in particular, an N-acetyl
group facilitated rapid and complete a-anomerization. The
differences in reactivity due to the various N-substituents
were supported by the results of DFT calculations; the orien-
tation of the acetyl carbonyl group close to the anomeric
position was found to contribute significantly to the direct-
ing of the anomerization reaction. By exploiting this reac-
tion, oligoaminoglycosides with multiple 1,2-cis glycosidic
bonds were generated from 1,2-trans glycosides in a one-
step process.
Introduction
plementary approach to produce homogeneous and well-de-
fined substances. The chemical synthesis of glycosides has con-
tributed greatly to glycobiology and glycotechnology;[4–10] in
particular, the glycosylation reaction is recognized as one of
the most essential reactions in oligosaccharide synthesis. This
reaction, which involves the formation of a glycosidic linkage
between a sugar donor and a glycosyl acceptor, has seen re-
markable developments in recent times with respect to the
wealth of synthetic methods available. Most 1,2-trans glyco-
sides can be easily prepared by exploiting neighboring group
participation of the 2-acyl protecting group. However, 1,2-cis
glycosides are often difficult to synthesize, even with our cur-
rent knowledge. 1,2-cis Aminoglycosides are particularly diffi-
cult to prepare in a completely stereoselective manner,[11–18] al-
though they are found in many biologically active oligosac-
charides such as heparin and antibiotics.[19–24] Since Lemieux
and Paulsen reported a 2-azide-2-deoxy glycosyl donor for 1,2-
cis aminoglycoside preparation, methods toward the synthesis
of 1,2-cis aminoglycosides have not progressed.[25–27] Reasons
for this gap in the literature are attributed to the low yields
and selectivities for 2-azide pyranoside preparation and the
moderate selectivity in glycosylation reactions.[27] Recently, 2,3-
trans carbamates possessing pyranoside moieties have been
developed for 1,2-cis-selective glycosylation.[28–33]
Oligosaccharides and glycoconjugates play important roles in
many biological processes, including cell–cell communication,
bacterial adhesion, masking of immunological epitopes, fertili-
zation, and cell proliferation.[1–3] To carry out precise analyses
of these roles, facile access to glycoconjugate-derived oligosac-
charides is imperative. However, glycoconjugates are usually
heterogeneous, consisting of various glycoforms, and the rep-
ertoire of homogeneous and structurally defined glycans ob-
tainable from natural sources is limited. Hence, chemical syn-
thesis of oligosaccharides would be an extremely useful com-
[a] Dr. S. Manabe, Dr. Y. Ito
RIKEN, Synthetic Cellular Chemistry Laboratory
Hirosawa, Wako, Saitama 351-0198 (Japan)
Fax: (+81)48-462-9430
E-mail: smanabe@riken.jp
[b] Prof. H. Satoh
National Institute of Informatics (NII)
Hitotsubashi, Chiyoda-ku
Tokyo 101-8430 (Japan)
Fax: (+81)3-4212-2120
E-mail: hsatoh@nii.ac.jp
[c] Prof. J. Hutter
Institute of Physical Chemistry
University of Zurich
A typical glycosylation reaction involving glycosyl donors,
such as thioglycosides, glycosylhalides, and imidates, proceeds
by an exocyclic cleavage reaction through a cyclic cation
(Scheme 1).[5–10] The endocyclic cleavage reaction, which gives
an acyclic cation through a bond cleavage between the
anomeric carbon and O5 oxygen, is extremely rare in pyrano-
side cleavage.[34,35] Although glycosyl donors with 2,3-trans car-
bamate or carbonate groups undergo cleavage through the
typical exocyclic reaction mode, several research groups, in-
8057 Zurich (Switzerland)
[d] Dr. H. P. Lꢀthi
Swiss Federal Institute of Technology (ETH)
Laboratory of Physical Chemistry
8093 Zurich (Switzerland)
[e] Dr. T. Laino
IBM Research Zurich
8803 Rꢀschlikon (Switzerland)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201303474.
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tions were carried out in the
presence of BF3·OEt2 in CH2Cl2 at
ꢀ308C for 12 h (Table 1). Benzyl-
substituted
carbamate
1a
(Table 1, entry 1) was smoothly
anomerized to a-anomer 2a in
63% yield. Pyranosides 1b and
1c, containing a carbamate sub-
stituted at nitrogen with a p-me-
thoxybenzyl (electron-donating)
group and an o-nitrobenzyl
Scheme 1. Endocyclic cleavage reaction and exocyclic cleavage reaction.
(electron-withdrawing)
group,
cluding our own, reported that 2,3-trans carbamate- and car-
bonate-containing glycosides are easily cleaved in an endocy-
clic manner and can be anomerized from the 1,2-trans to 1,2-
cis orientation under weak acidic conditions. Experimental evi-
dence of the existence of an endocleavage pathway in pyrano-
sides carrying 2,3-trans carbamate or carbonate moieties was
demonstrated by the identification of adducts of the acyclic
cations produced by the endocyclic cleavage reaction, cap-
tured by reduction, chlorination, and inter- or intramolecular
Friedel–Crafts reactions;[36] however, the substrates failed to
undergo complete conversion, resulting in isomeric mix-
tures.[31,35–38] Quantum-mechanical calculations revealed that
inner strain caused by the fused rings (the pyranoside ring and
the 2,3-trans carbamate group) is the predominant factor for
the enhancement of the endocyclic cleavage reaction.[39] The
excellent agreement between the transition-state energy and
the experimentally observed reactivity indicates that this series
of anomerization reactions is predominantly under kinetic con-
trol. Based on the experiments and the calculations, it was ex-
pected that the substituent on the nitrogen atom of the carba-
mate group would influence
respectively, were also anomerized in favor of the a-anomers
(entries 2 and 3). Carbamates substituted with nitrile (1d) and
ester (1e) groups as intramolecular cation-stabilizing moieties,
did not yield positive results (entries 4 and 5). Interestingly,
however, compounds 1g–l underwent anomerization to pro-
duce solely the a-anomers (entries 7–16). The isolated a-anom-
ers 2g and 2k did not undergo the reverse reaction to the b-
anomer under the same conditions, and 2g and 2k were re-
covered in 80 and 81% yield, respectively. These results sug-
gest that the anomerization reaction is under kinetic control
and that there is no equilibrium between the a- and b-anom-
ers. In addition, these results clearly show that carbamates pos-
sessing N-acyl substitution significantly enhanced the anomeri-
zation reaction in the a-direction and that the anomerization
reaction was irreversible under these conditions. Remarkably,
on reducing the amount of BF3·OEt2 from 2.0 to 0.5 equiva-
lents, the yield of the a-product 2l was increased (entries 12–
14), which was attributed to the suppression of a deacetylation
side reaction at the 6-position caused by the Lewis acidity of
BF3·OEt2. Compound 2m was obtained in entries 12 and 13
both the anomerization ratio
and rate, and that complete
anomerization to the 1,2-cis gly-
coside would be possible by se-
lecting an appropriate substitu-
ent. Here, we report our experi-
mental and computational inves-
tigations performed with the
aim of identifying the optimal
substituent group for the
anomerization reaction, and how
this endocyclic cleavage reaction
can be used to synthesize new
glycosides.
Table 1. Effect of carbamate N-substituents on the anomerization reaction.
Entry
Substrate
R[c]
PG[c]
X
b-Product
Yield [%]
a-Product
Yield [%]
1
2
3
4
5[a]
6
7
8
1a
1b
1c
1d
1e
1 f
1g
1h
1i
1j
1k
1l
1l
1l
Bn
PMB
o-nitrobenzyl
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Ac
Ac
Ac
Ac
Ac
2
2
2
2
2
2
2
2
2
2
2
2
1a
1b
1c
1d
1e
1 f
1g
1h
1i
1j
1k
1l
1l
1l
16
15
26
15
–
69
0
0
0
0
0
0
2a
2b
2c
2d
2e
2 f
2g
2h
2i
2j
2k
2l
2l
2l
63
77
63
71
–
19
80
88
88
88
87
70
87
90
30
89
CH2CN
CH2CO2Me
H
CO2Me
CO2All
CO2Bn
CO2CH2CCl3
Ac
Ac
Ac
Ac
Ac
9
Results and Discussion
10
11
12
13
14
15
16[b]
To investigate substituent effects
at the 2,3-trans carbamate nitro-
gen site on the outcome of the
anomerization reaction, several
glycosides 1a–l were prepared
(see the Supporting Informa-
tion); the anomerization reac-
1
0
0
65
2
0.5
0.1
0.1
1l
1l
1l
1l
2l
2l
Ac
[a] Reaction with 1e yielded a complex mixture. [b] Reaction period was 72 h. [c] PMB=p-methoxybenzyl
ether. PG=protecting group.
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was introduced. These trends in
anomerization due to the differ-
ent substituents were similar to
those for the monosaccharides
(entries 9–13); namely, when an
acetyl group was present as the
carbamate substituent, complete
anomerization of 14a and 15a
was observed (entries 12 and
13). However, the disaccharide
with an unsubstituted carba-
mate 11 a was not anomerized
(entry 9).
Table 2. Scope and limitations of the anomerization reaction.[a]
Entry
Substrate
R1
R2
R3
R4
R5
Products
Yield [%]
1
2
3
4
5
6
7
8
3a
4a
5a
6a
7a
8a
9a
10a
SPh
SPh
SPh
SMP
OMe
OMP
OMP
OMP
H
Ac
H
H
H
OAc
OAc
OAc
OAc
OAc
OAc
OAc
OAc
H
H
H
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
3a/3b
4a/4b
5a/5b
6a/6b
7a/7b
8a/8b
9a/9b
10a/10b
22:66
0:83
0:90
0:92
29:64
98:0
94:0
0:68
Bn
CO2All
Bn
H
Bn
Ac
H
H
As we had previously report-
ed,[41] a significant solvent effect
was observed in the anomeriza-
tion reaction. The anomerization
reaction with compound 17a
was completed in CH2Cl2 within
30 min. In CH3CN, the reaction
was very quick and was com-
pleted within 1 min at ꢀ308C in
the presence of 2 equivalents of
BF3·OEt2. After 1 min in CH3CN,
the a-anomer was obtained in
91% yield and no b-anomer was
obtained (Figure 1).
9
11 a
12a
13a
14a
H
OBn
OAc
OBn
OBn
H
H
H
H
TBDPS
Bn
11a/11b
12a/12b
13a/13b
14a/14b
86:0
90:5
21:73
0:88
10
11
12
Bn
CO2Me
Ac
TBDPS
TBDPS
13
14
15a
16a
Ac
Ac
OBn
OBn
H
H
TBDPS
TBDPS
15a/15b
16a/16b
0:88
0:62
The experimental observation
that the presence of the N-acetyl
group on the carbamate signifi-
cantly accelerated the anomeri-
zation reactions was further in-
vestigated theoretically for pyra-
nosides 1 f, 1g, and 1k by DFT
[a] MP=p-methoxyphenyl; Phth=phthalimide; TBDPS=tert-butyldiphenylsilyl.
(Table 1). In the presence of only 0.1 equivalents of BF3·OEt2,
anomerization did not proceed to completion over the 12 h re-
action time, with the b-anomer 3 remaining in 65% yield
(entry 15); however, after 72 h, the anomerization was nearly
complete, with 89% of the a-product formed (entry 16).
Next, we focused on expanding the scope of substrates for
the anomerization reaction (Table 2). We found that not only
was there an effect of the N-acetyl substituent on the carba-
mate observed for glucosamine derivatives, but also that gal-
actosamine with N-acetylcarbamate 4a and N-benzylcarbamate
5a showed complete anomerization to a-anomer 4b and 5b
(Table 2, entries 2 and 3), whereas the b-anomer 3a remained
in 22% yield for the carbamate without a substituent (entry 1).
The p-methoxy-substituted thioglycoside 6a was also com-
pletely anomerized to a-anomer 6b (entry 4). As with earlier
work, which reported that O-aryl pyranosides do not easily
anomerize,[40] our experiments also showed no anomerization
for either the unsubstituted substrate 8a or the benzyl-substi-
tuted compound 9a (entries 6 and 7). However, the p-meth-
oxyphenyl pyranoside with N-acetylcarbamate 10a underwent
anomerization, giving the a-product in 68% yield (entry 8). Dis-
accharides were also anomerized when a carbamate group
Figure 1. Time-course of anomerization reaction in CH2Cl2 and CH3CN. Red
line: yield of 17a in CH2Cl2, blue line: yield of 17b in CH2Cl2, purple line:
yield of 17a in CH3CN, green line: yield of 17b in CH3CN. The numbers on
the lines are yields of each compound.
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analysis at the B3LYP/6-31G(d,p) level,[42,43] using the Gaussi-
an 03 program.[44] In exploring the potential energy surface for
1 f and 1k in the same manner as previously reported,[39,45] the
transition states were found in the course of the C1ꢀC2 bond
rotation at ffH1-C1-C2-H2=ꢀ35.8 and ꢀ39.88, respectively,
after the bond breaking between C1 and O5 (Figure 2). The
calculations were performed in the gas phase (solid lines in
Figure 2), and the energies in CH2Cl2 solvent were estimated
by using an integral-equation formalism-polarizable continuum
model (IEF-PCM)[46] with the default parameters of Gaussian 03
predominant.[37,47,48] The carbonyl group of an anti-conformer
located near the anomeric position would assist endocyclic
cleavage.
To confirm this hypothesis, we focused on the difference in
reactivity of a pyranoside with 2,3-trans-N-methoxycarbonyl-
carbamate (e.g., compound 1g) and N-acetylcarbamate (e.g.,
compound 1k) substitution. A conformational search was per-
formed upon the rotation of the NꢀC bond of the N-substitu-
ent for 1g and 1k in the gas phase. The geometries of 1g and
1k were optimized to the anti conformer at ffC2-N-C-O=5.5
and ꢀ3.28, respectively (Fig-
ure 4a). The
conformational
search was initiated from the op-
timized geometries by progres-
sively rotating the NꢀC bond
from 0 to 330 (i.e., ꢀ308) in
steps of 308. The minimum
energy of the syn conformer of
1k
was
found
to
be
36.4 kJmolꢀ1 higher than that of
the anti conformer, whereas the
energy of syn conformer of 1g
was only 6.9 kJmolꢀ1 higher
than that of the anti conformer
(Figure 4b). The lowest energy
barrier between the anti and syn
conformer of 1k was found to
be 42.0 kJmolꢀ1, or 16.7 kJmolꢀ1
higher than that of 1g. For both
models, the dipole moment
values of the anti conformers
tended to be lower than those
of the syn conformers, with
Figure 2. Results of the transition state analysis for 1 f and 1k along the C1ꢀC2 bond rotation. Potential energies
of the reactant (b-anomer) and the product (a-anomer) as well as the transition-state energies are represented rel-
ative to the b-anomers. The calculations were performed in the gas phase (solid line). Energies in solution (CH2Cl2)
were estimated using an IEF-PCM approach (dashed line). c: 1 f; c: 1k; a: 1 f in CH2Cl2; a: 1k in
CH2Cl2.
a
maximal difference of 3–
(dashed lines in Figure 2). The transition state energies for 1k,
with an N-acetylcarbamate, were found to be 11–18 kJmolꢀ1
lower than those for 1 f, with an unsubstituted carbamate. The
C1ꢀO5 bond lengths of the transition states for 1 f and 1k
were 2.76 and 2.84 ꢂ, respectively. These results indicate that
1k should undergo the endocyclic cleavage–recyclization reac-
tion more easily than 1 f, through a kinetically predominant re-
action mechanism. The results of the transition-state analysis
are in good agreement with the experimental observations.
We hypothesize that the presence of a carbonyl group as
the N-substituent is responsible for the acceleration of the
anomerization reaction and that the interaction of the carbonyl
oxygen with the anomeric site stabilizes the linear oxacarbeni-
um ion produced by the endo-
4 Debye. These observations suggest that the pyranosides with
N-acetylcarbamate substitution predominantly adopt the anti
conformers, with the carbonyl oxygen oriented toward the
anomeric site, resulting in a lower dipole moment. On the
other hand, the anti and syn conformers of the glycosides with
N-methoxycarbonylcarbamate are almost equally populated,
with only a slight preference for the anti conformer. The theo-
retically estimated ordering of the populations of the anti con-
formers agrees with the experimentally observed ordering of
the reactivity of anomerization. These computations support
the hypothesis that the presence and orientation of the car-
bonyl group at the N-substituent position of the carbamate
groups are important.
cyclic cleavage (Figure 3). As
a result of this effect, the C1ꢀO5
distance is increased, rendering
the subsequent C1ꢀC2 bond-ro-
tation feasible. N-Acyloxazolidi-
nones adopt two predominant
conformers with the carbonyl di-
poles aligned either syn- or anti-
Figure 3. Mechanism of the reaction acceleration. The carbonyl group at the N-substituent position stabilizes the
periplanar, with the latter being cation produced by endocyclic reaction. The anti conformer is assumed to be predominant.
Chem. Eur. J. 2014, 20, 124 – 132 www.chemeurj.org ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
127

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ous anomerization of four 1,2-
trans glycosidic bonds to 1,2-cis
glycosides was thus accom-
plished in a single laboratory op-
eration, in excellent yield.
Conclusion
We have demonstrated that
anomerization through an endo-
cyclic cleavage–recyclization pro-
cess is a powerful strategy to
achieve complete 1,2-cis amino-
glycoside preparation. We found
that the presence of an acetyl
group on the 2,3-trans carba-
mate has a significant positive
effect on the anomerization re-
action. Computational studies
supported the feasibility of the
endocyclic
cleavage-induced
anomerization reaction, and to-
gether with our previous calcula-
Figure 4. DFT analysis of geometries for 1g and 1k in the gas phase. a) Optimized geometries. Pyranosides 1g
and 1k were optimized to the anti-conformer for the NꢀC bond; b) Potential energy and dipole moment plots
along the NꢀC bond rotation. It is suggested that the pyranoside with N-acetylcarbamate (1k) shows a higher-
energy barrier between the anti and the syn conformers and a predominance of the anti conformer, whereas the
pyranoside with N-methoxycarbonylcarbamate (1g) shows a lower barrier between the anti and the syn conform-
ers. The anti conformer exhibits lower dipole moment values for both 1g and 1k.
tions,[39,45] revealed that the
inner strain caused by distorted
carbamate group and the orien-
tation of the carbonyl group
toward the anomeric position
are important factors in the reac-
tion. In the anomerization reac-
Finally, the utility of the endocyclic cleavage reaction was
demonstrated by anomerization of multiple b-glycosyl linkages
in a one-step operation. Chitin and chitosan are polymers con-
sisting of N-acetyl glucosamine/glucosamine units linked by b-
(1,4)-bonds and have many promising applications in water
treatment, food science, cosmetics, and pharmaceutics. Cellu-
lose and amylose are two similar polysaccharides comprised of
1,4-glucose units, but have distinct characteristics because of
the higher-order structure difference imparted by the anomeric
stereochemistry. The unique a-(1,4)-glucosamine polymer is
generally not available because selective 1,2-cis aminoglyco-
side synthesis is difficult. Here, protected chitotetraose 23 was
synthesized in a similar manner reported by Ogawa as shown
in Scheme 2.[49] The glycosylation reaction between 18 and 19
proceeded in high yield. Disaccharide 20 was subsequently
transformed to acceptor 21 and donor 22. Glycosyl fluoride 22
was activated by hafnium(IV) triflate, Hf(OTf)4, by a recently de-
veloped operationally simple method to give tetrasaccharide
23.[50] Subsequent transformation to anomerization precursor
24 through introduction of the N-acetylcarbamate moieties
was accomplished by treatment of aminoalcohol 23 with tri-
phosgene in the presence of NaHCO3 and subsequent acetyla-
tion in the presence of 4-dimethylaminopyridine (DMAP;
Scheme 2). After BF3·OEt2 treatment of 24 (coupling constants
at anomeric position J=8.8 Hz) in CH3CN, the anomerized
compound 25 (coupling constants at anomeric position J=2.4,
2.8, and 3.2 Hz) was obtained in 83% yield.[51] The simultane-
tion, the existing multiple 1,2-trans glycosyl bonds were con-
verted into cis glycosyl bonds, which are difficult to obtain, in
a single operation. Further investigations on extending this
methodology for the synthesis of bioactive oligosaccharides
and high-molecular-weight 1,2-cis chitosan derivatives, a new
previously unknown class of polymers prepared from naturally
occurring chitosan, will be reported in due course.
Experimental Section
General procedures
All commercial reagents were used without further purification.
Analytical TLC was performed on silica gel 60 F254 plates (Merck)
and visualized by UV fluorescence quenching and 12-molybdo(VI)
phosphoric acid acid/phosphoric acid/sulfuric acid staining. Flash
column chromatography was performed on a silica gel 60N (spher-
ical, neutral, 40–100 mm Kanto Co.). Yields refer to chromatographi-
1
cally and spectroscopically pure compounds. H and 13C NMR spec-
tra were recorded on a JEOL EX 400 spectrometer (400 and
100 MHz, respectively) at ambient temperatures (23–248C) in
CDCl3. Chemical shifts (d) are reported in ppm relative to internal
1
TMS (d=0.00 ppm) for H and CDCl3 (d=77.00 ppm) for 13C NMR
spectra. HRMS were measured by quadrupole-TOF mass spectrom-
etry. Optical rotations were measured at room temperature (JASCO
DIP-310). Melting points are not corrected (YANACO micro melting
point apparatus). Compounds 1a,[39] 2a,[39] 1 f,[39]2 f,[39] 1g,[39] 2g,[39]
1k,[39] 2k,[39] 6a,[39] 1l,[52] 3a,[28] 5a,[31] 5b,[41] 7a,[41] 7b,[41] 8a,[40]
9a,[40] 18,[53] 21,[54] and 22[50] were previously reported. Spectro-
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Phenyl
N-4-methoxybenzyl-2-
amino-4-O-acetyl-6-O-benzyl-2,3-
N,O-carbonyl-2-deoxy-1-thio-a-d-
glucopyranoside (2b): [a]D24
122.1 (c=1.0 in CHCl3); 1H NMR
(400 MHz, CDCl3): d=7.39 (d, J=
5.6 Hz, 2H), 7.38–7.21 (m, 8H), 7.11
(d, J=8.4 Hz, 2H), 6.75 (d, J=
8.4 Hz, 2H), 5.34 (d, J=4.4 Hz, 1H),
5.25 (t, J=9.6 Hz, 1H), 4.68 (d, J=
14.4 Hz, 1H), 4.53 (d, J=12.0 Hz,
1H), 4.37–4.43 (m, 2H), 4.24 (m,
1H), 4.07 (d, J=14.4 Hz, 1H), 3.71
(s, 3H), 3.56 (dd, J=12.0, 4.4 Hz,
1H) 3.51 (m, 2H), 1.95 ppm (s, 3H);
13C NMR (100 MHz, CDCl3): d=
169.0, 159.6, 157.9, 137.3, 132.6,
131.7, 130.2, 129.1, 128.3, 127.9,
127.8, 127.7, 126.0, 114.3, 84.9,
76.1, 73.5, 71.6, 68.3, 67.5, 59.7,
55.3, 47.4, 20.7 ppm; HRMS: m/z
=
calcd
572.1713
572.1711.
for
C30H31NO7S+Na+:
[M+Na+];
found:
Time-course
experiment
of
anomerization of 17a: To a solu-
tion of 17a (70 mg-100 mg) in
either CH2Cl2 or CH3CN (substrate
concentration 0.077m), BF3·OEt2
(2 equiv) was added at ꢀ308C
under an Ar atmosphere. After ap-
propriate reaction period, the reac-
tion was quenched with sat.
NaHCO3 and the aqueous layer
was extracted with EtOAc several
times. The combined organic
layers were washed with sat.
NaHCO3 and brine. After drying
the extract over Na2SO4 and filtra-
tion, the mixture was concentrated
in vacuo. The residue was purified
Scheme 2. Synthesis of a new “1,2-cis chitosan” tetrasaccharide. DAST=diethylaminosulfur trifluoride; CAN=ceric
ammonium nitrate.
scopic data of 1c–1e, 1h–1j, 2c–2d, 2h–2j, 3b, 4a, 4b, 6b, 10–
16a, 10–16b are reported in the Supporting Information.
by preparative TLC (hexane/EtOAc, 7:3).
Methyl (N-acetyl-2-amino-2,3-N,O-carbonyl-4-O-acetyl-6-O-benz-
yl-2-deoxy-b-d-glucopyranosyl)-(1!4)-2,3,4-tri-O-benzyl-a-d-glu-
copyranoside (17a): [a]2D4 =ꢀ3.7 (c=0.88 in CHCl3);1H NMR
(400 MHz, CDCl3): d=7.37–7.23 (m, 20H), 5.22 (dd, J=9.2, 4.4 Hz,
1H), 4.99 (d, J=11.2 Hz, 1H), 4.92–4.87 (m, 2H), 4.81 (d, J=11.2 Hz,
1H), 4.75 (d, J=12.4 Hz, 1H), 4.69 (d, J=11.2 Hz, 1H), 4.60 (d, J=
11.6 Hz, 1H), 4.54 (d, J=3.6 Hz, 1H), 4.49 (d, J=12.0 Hz, 1H), 4.45
(d, J=12.0 Hz, 1H), 4.18 (t, J=9.2 Hz, 1H), 4.11–4.06 (m, 2H), 3.99
(t, J=9.6 Hz, 1H), 3.95 (m, 1H), 3.80–3.75 (m, 3H), 3.69–3.64 (m,
2H), 3.51 (dd, J=10.0, 3.6 Hz, 1H), 2.35 (s, 3H), 2.07 ppm (s, 3H);
13C NMR (100 MHz, CDCl3): d=170.1, 169.6, 153.3, 138.8, 138.8,
138.2, 137.7, 129.0, 128.4, 128.3, 128.2, 128.1, 127.9, 127.7, 127.7,
127.5, 100.7, 98.1, 82.2, 79.8, 78.4, 75.6, 75.5, 74.5, 73.4, 73.3, 70.7,
70.1, 69.8, 67.2, 60.1, 55.1, 24.5, 20.7 ppm; HRMS: m/z calcd for
C46H51NO13 +Na+: 848.3253 [M+Na+]; found: 848.3254.
Synthesis
General procedure for anomerization: BF3·OEt2 (2 equiv) was
added to b-glycoside (1 equiv) in CH2Cl2 (substrate concentration:
0.077m) at ꢀ308C. After 12 h, the mixture was quenched with sat.
NaHCO3 and the aqueous layer was extracted with CHCl3 several
times. The combined layers were washed with brine and dried
over Na2SO4. After filtration, the mixture was concentrated in
vacuo, and the residue was purified by preparative TLC.
Phenyl N-4-methoxybenzyl-2-amino-4-O-acetyl-6-O-benzyl-2,3-
N,O-carbonyl-2-deoxy-1-thio-b-d-glucopyranoside (1b): [a]D24
=
1
ꢀ29.0 (c=1.0 in CHCl3); H NMR (400 MHz, CDCl3): d=7.43 (d, J=
6.8 Hz, 2H), 7.34–7.21 (m, 12H), 6.85 (d, J=8.4 Hz, 2H), 5.23 (t, J=
10.0 Hz, 1H), 4.81 (d, J=9.2 Hz, 1H), 4.71 (d, J=15.2 Hz, 1H), 4.60
(d, J=15.2 Hz, 1H), 4.52 (d, J=11.6 Hz, 1H), 4.47 (d, J=11.6 Hz,
1H), 4.11 (t, J=10.8 Hz, 1H), 3.79 (s, 3H), 3.60 (m, 1H), 3.53 (m,
2H), 3.50 ppm (t, J=11.2 Hz, 1H); 13C NMR (100 MHz, CDCl3): d=
158.7, 132.2, 129.9, 129.1, 128.3, 127.8, 127.7, 114.0, 86.9, 79.9, 88.0,
73.6, 68.8, 68.0, 59.9, 55.3, 20.8 ppm; HRMS: m/z calcd for
C30H31NO7S+Na+: 572.1713 [M+Na+]; found: 572.1710.
Methyl (N-acetyl-2-amino-2,3-N,O-carbonyl-4-O-acetyl-6-O-benz-
yl-2-deoxy-a-d-glucopyranosyl)-(1!4)-2,3,4-tri-O-benzyl-a-d-glu-
copyranoside (17b): [a]2D4 =98.9 (c=1.6 in CHCl3); 1H NMR
(400 MHz, CDCl3): d=7.37–7.23 (m, 20H), 5.79 (d, J=2.4 Hz, 1H),
5.36 (t, J=9.6 Hz, 1H), 5.00 (d, J=10.4 Hz, 1H), 4.86 (d, J=11.2 Hz,
1H), 4.80 (d, J=10.8 Hz, 1H), 4.77 (d, J=10.8 Hz, 1H), 4.67 (d, J=
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12.0 Hz, 1H), 4.58–4.55 (m, 2H), 4.40 (d, J=11.6 Hz, 1H), 4.43 (d,
J=12.0 Hz, 1H), 3.98 (t, J=9.6 Hz, 1H), 3.84–3.79 (m, 4H), 3.69 (m,
1H), 3.53–3.46 (m, 3H), 3.35 (s, 3H), 3.35–3.30 (m, 1H), 2.94 (s, 3H),
1.98 ppm (s, 3H); 13C NMR (100 MHz, CDCl3): d=170.9, 169.0,
152.9, 138.7, 138.1, 138.0, 137.5, 128.5, 128.4, 128.4, 128.1, 127.9,
127.9, 127.8, 127.6, 97.8, 95.6, 82.0, 70.8, 75.6, 74.7, 74.3, 73.5, 73.2,
71.2, 69.9, 68.4, 67.4, 66.8, 59.8, 55.1, 23.6, 20.6 ppm; HRMS: m/z
calcd for C46H51NO13 +Na+: 848.3253 [M+Na+]; found: 848.3253.
(d, J=12.0 Hz, 1H), 4.49 (d, J=12.0 Hz, 1H), 4.42–4.27 (m, 3H),
4.14–4.09 (m, 2H), 3.81 (t, J=9.2 Hz, 1H), 3.76 (dd, J=10.0, 4.4 Hz,
1H), 3.69–3.66 (m, 1H), 3.68 (s, 3H), 3.58 (m, 1H), 3.51–3.42 (m,
3H), 1.88 (s, 3H), 1.85 ppm (s, 3H); 13C NMR (100 MHz, CDCl3): d=
171.0, 170.0, 155.5, 150.6, 138.1, 137.3, 134.2, 131.4, 128.5, 128.2,
127.9, 127.7, 127.4, 123.5, 118.9, 114.3, 97.3, 97.2, 74.5, 74.4, 73.6,
73.5, 73.2, 72.7, 71.6, 71.4, 70.0, 67.5, 55.5, 54.9, 54.9, 21.6,
20.6 ppm; HRMS: m/z calcd for C55H52N2O17 +Na+: 1035.3158
[M+Na+]; found: 1035.3168.
p-Methoxyphenyl
phthalimido-b-d-glucopyranosyl)-(1!4)-2-deoxy-3-O-acetyl-6-O-
benzyl-2-phthalimido-b-d-glucopyranoside: BF3·OEt2 (21 mL,
(4,6-O-benzylidene-2-deoxy-3-O-acetyl-2-
Cerium ammonium nitrate (3.2 mL, 3.66 mmol) was added to a so-
lution of p-methoxyphenyl glycoside (1.19 g, 1.18 mmol) in toluene
(14 mL), CH3CN (18 mL) and H2O (14 mL) at 48C. After 2 h, the mix-
ture was diluted with H2O and CHCl3. After separation, the aqueous
layer was extracted with CHCl3 and washed with brine. The com-
bined layers were dried over Na2SO4 and concentrated. The residue
was purified by silica gel column chromatography (toluene/EtOAc)
to give hemiacetal. The hemiacetal was dissolved in CH2Cl2 (20 mL)
and N,N-diethylaminosulfur trifloride (0.31 mL, 2.36 mmol) was
added. After 30 min, the reaction was quenched with sat. NaHCO3.
The aqueous layer was extracted with CHCl3 and the combined
layers were washed with brine. After concentration, the residue
was purified by silica gel column chromatography (toluene/EtOAc,
7:3) to give fluoride 22 (804 mg, 75%).
0.174 mmol) was added to a solution of imidate 18 (0.74 g,
1.27 mmol) and acceptor 19 (0.63 g, 1.16 mmol) in CH2Cl2 (15 mL)
at ꢀ608C. After 12 h, Et3N was added to neutralize the mixture.
The mixture was diluted with CHCl3 and sat. NaHCO3. After extrac-
tion of the aqueous layer with CHCl3, the combined organic layers
were washed with brine. The combined layers were dried over
Na2SO4 and concentrated. The crude material was purified by silica
gel column chromatography (toluene/EtOAc 7:3–1:1) to give disac-
charide 20 (1.03 g, 92%). [a]2D4 =9.4 (c=0.89 in CHCl3); 1H NMR
(400 MHz, CDCl3): d=7.82–7.79 (m, 4H), 7.71–7.58 (m, 4H), 7.42–
7.10 (m, 8H), 6.76 (d, J=9.2 Hz, 2H), 6.63 (d, J=9.2 Hz, 2H), 5.82 (t,
J=9.6 Hz, 1H), 5.76 (d, J=8.8 Hz, 1H), 5.71 (d, J=5.2 Hz, 1H), 5.58
(d, J=8.4 Hz, 1H), 5.47 (s, 1H), 4.39 (t, J=8.8 Hz, 1H), 4.28–4.16 (m,
4H), 4.10 (t, J=8.8 Hz, 1H), 3.71–3.69 (m, 3H), 3.67 (s, 3H), 3.58 (m,
2H), 1.95 (s, 3H), 1.85 ppm (s, 3H); 13C NMR (100 MHz, CDCl3): d=
170.2, 169.7, 167.9, 155.5, 150.5, 137.9, 136.7, 134.3, 131.4, 129.2,
129.0, 128.2, 128.2, 128.2, 127.5, 127.4, 126.2, 126.2, 125.3, 123.6,
119.0, 114.3, 101.7, 98.4, 97.4, 79.0, 75.5, 74.4, 72.7, 72.1, 69.6, 68.6,
67.3, 66.1, 55.7, 55.5, 54.9, 21.4, 20.8, 20.5 ppm; HRMS: m/z calcd
for C53H48N2O16 +Na+: 991.2896 [M+Na+]; found: 991.2908.
p-Methoxyphenyl (6-O-benzyl-2-deoxy-di-3,4-O-acetyl-2-phthali-
mido-b-d-glucopyranosyl)-(1!4)-(2-deoxy-3-O-acetyl-6-O-
benzyl-2-phthalimido-b-d-glucopyranosyl)-(1!4)-(2-deoxy-3-O-
acetyl-6-O-benzyl-2-phthalimido-b-d-glucopyranosyl)-(1!4)-(2-
deoxy-3-O-acetyl-6-O-benzyl-2-phthalimido-b-d-glucopyranoside
(23): Hf(OTf)4 was added to a solution of fluoride 22 (1.22 g,
1.34 mmol) and acceptor 21 (0.97 mg, 1.00 mmol) in CH2Cl2
(20 mL) at ꢀ408C (1.16 g, 1.50 mmol). The mixture was stirred at
ꢀ408C for 1 h, then at ꢀ208C for 1 h, and finally at 08C for 3 h.
The reaction was quenched with sat NaHCO3 and the mixture was
diluted with EtOAc. The mixture was filtered through Celite. The
aqueous layer was extracted with EtOAc and the combined layers
were washed with brine and dried over Na2SO4. After concentra-
tion, the residue was purified by silica gel column chromatography
(toluene/EtOAc, 7:3–1:1) to give tetrasaccharide 23 (1.54 g, 83%).
[a]2D4 =ꢀ3.2 (c=1.2 in CHCl3); 1H NMR (400 MHz, CDCl3): d=7.88–
7.85 (m, 4H), 7.78–7.72 (m, 8H), 7.67–7.65 (m, 4H), 7.33–7.18 (m,
17H), 7.15–7.12 (m, 2H), 7.12–7.10 (m, 2H), 6.90 (m, 1H), 6.74–6.64
(m, 2H), 6.61–6.60 (m, 2H), 5.66–5.61 (m, 3H), 5.46–5.43 (m, 2H),
5.33 (d, J=8.4 Hz, 1H), 5.23–5.13 (m, 3H), 4.50–4.32 (m, 10H),
4.14–3.98 (m, 7H), 3.65 (s, 3H), 3.56–3.46 (m, 4H), 3.40–3.32 (m,
4H), 3.26 (dd, J=10.4, 8.4, Hz, 1H), 3.18 (dd, J=10.4, 8.4 Hz, 1H),
3.00 (d, J=10.0 Hz, 1H), 2.74 (d, J=10.0 Hz, 1H), 1.83 (s, 3H), 1.79
(s, 3H), 1.78 (s, 3H), 1.77 (s, 3H), 1.63 ppm (s, 3H); 13C NMR
(100 MHz, CDCl3): d=170.3, 170.2, 169.4, 168.1, 167.2, 155.4, 150.6,
138.2, 138.1, 137.9, 137.6, 134.3, 131.4, 129.0, 128.3, 128.2, 128.0,
127.9, 127.8, 127.7, 127.4, 127.3, 127.2, 127.1, 126.9, 125.3, 123.6,
123.5, 118.8, 114.3, 97.4, 96.5, 96.4, 95.9, 74.4, 73.9, 73.6, 73.4, 73.3,
72.8, 72.6, 72.5, 72.2, 72.2, 72.1, 70.9, 70.9, 70.7, 69.4, 55.5, 55.3,
55.2, 55.0, 54.8, 21.4, 20.6, 20.5, 20.4 ppm; HRMS: m/z calcd for
C101H94N4O31 +Na+: 1181.5794 [M+Na+]; found: 1881.5810.
p-Methoxyphenyl (6-O-benzyl-2-deoxy-3-O-acetyl-2-phthalimido-
b-d-glucopyranosyl)-(1!4)-2-deoxy-3-O-acetyl-6-O-benzyl-2-
phthalimido-b-d-glucopyranoside (21): Et3SiH (1.2 mL, 7.51 mmol)
and BF3·OEt2 (0.19 mL, 1.33 mmol) was added to a solution of ben-
zylidene compound 20 (0.64 g, 0.67 mmol) in CH2Cl2 (20 mL) at
48C. After 2 h, the mixture was diluted with sat. NaHCO3 and
CHCl3. The aqueous layer was extracted with CHCl3 and the organic
layers were washed with brine. The extract was dried over Na2SO4,
and concentrated. The residue was purified by silica gel column
chromatography (toluene/EtOAc 7:3–1:1) to give alcohol 21
(0.55 g, 82%). [a]2D4 =3.8 (c=0.88 in CHCl3); 1H NMR (400 MHz,
CDCl3): d=7.81 (m, 4H), 7.69 (m, 4H), 7.33–7.16 (m, 10H), 6.76 (d,
J=8.8 Hz, 2H), 6.63 (d, J=8.8 Hz, 2H), 5.72 (t, J=10.0 Hz, 1H), 5.69
(d, J=8.4 Hz, 1H), 5.57 (t, J=10.8 Hz, 1H), 5.46 (d, J=8.4 Hz, 1H),
4.55 (d, J=12.0 Hz, 1H), 4.49 (d, J=12.0 Hz, 1H), 4.42–4.27 (m, 3H),
4.14–4.09 (m, 2H), 3.84–3.74 (m, 2H), 3.77–3.45 (m, 8H), 3.71 (s,
3H), 1.89 (s, 3H), 1.86 ppm (s, 3H); 13C NMR (100 MHz, CDCl3): d=
171.0, 170.0, 155.5, 150.6, 138.1, 137.4, 134.2, 131.5, 128.5, 128.2,
127.9, 127.7, 127.4, 123.5, 118.9, 114.3, 97.4, 97.2, 74.5, 74.4, 73.6,
73.5, 73.2, 72.7, 71.6, 70.0, 67.5, 55.5, 54.9, 20.6 ppm; HRMS: m/z
calcd for C53H50N2O16 +Na+: 993.3053 [M+Na+]; found: 993.3070.
6-O-Benzyl-2-deoxy-di-3,4-O-acetyl-2-phthalimido-b-d-glucopyr-
anosyl-(1!4)-2-deoxy-3-O-acetyl-6-O-benzyl-2-phthalimido-b-d-
glucopyranosyl fluoride (22): Alcohol 21 (0.30 g, 0.309 mmol) was
dissolved in pyridine (3 mL) and Ac2O (1 mL) was added. After stir-
ring at room temperature overnight, the mixture was concentrat-
ed. The residue was purified by silica gel column chromatography
(toluene/EtOAc 4:1) to give the corresponding acetate (0.31 g,
quant.). [a]2D4 =26.0 (c=1.78 in CHCl3); 1H NMR (400 MHz, CDCl3):
d=7.81 (m, 4H), 7.70–7.69 (m, 4H), 7.35–7.17 (m, 10H), 6.76 (d, J=
9.2 Hz, 2H), 6.63 (d, J=9.2 Hz, 2H), 5.73 (t, J=8.8 Hz, 1H), 5.70 (d,
J=8.8 Hz, 1H), 5.57 (t, J=9.2 Hz, 1H), 5.47 (d, J=8.0 Hz, 1H), 4.55
p-Methoxyphenyl (2-deoxy-di-4,6-O-acetyl-2,3-N,O-carbonyl-b-d-
glucopyranosyl)-(1!4)-(6-acetyl-2-deoxy-2,3-N,O-carbonyl-b-d-
glucopyranosyl)-(1!4)-(6-acetyl-2-deoxy-2,3-N,O-carbonyl-b-d-
glucopyranosyl)-(1!4)-(6-acetyl-2-deoxy-2,3-N,O-carbonyl-b-d-
glucopyranoside (24): Phthaloyl-protected tetrasaccharide 23
(2.00 g. 1.08 mmol) was dissolved in DMF (10 mL) and ethylenedi-
amine (5 mL) and stirred at 808C under N2 for 2 d. After concentra-
tion, the crude was purified by Sephadex LH-20 chromatography
(CHCl3/MeOH 1:1) to give tetraamine (1.28 g, quant.). Due to its
Chem. Eur. J. 2014, 20, 124 – 132
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low solubility, the aminoalcohol was used without further purifica-
tion. Triphosgene (362.0 mg, 1.22 mmol) was added to a solution
of tetrasaccharide amino alcohol (430.0 mg, 0.381 mmol) and
NaHCO3 (640 mg, 7.62 mmol) in CH3CN (12 mL) and H2O (3 mL).
After 12 h, additional triphosgene (180.0 mg, 0.61 mmol) and
NaHCO3 (320.0 mg, 3.81 mmol) were added. The mixture was dilut-
ed with CHCl3 and brine, and the extracted with CHCl3. The com-
bined layers were washed with brine and the dried over Na2SO4.
After concentration, the residue was purified by silica gel column
chromatography (CHCl3/MeOH 9:1) to give the carbamate inter-
mediate 24 (303 mg, 64%). [a]2D4 =ꢀ24.5 (c=0.60 in CHCl3);
1H NMR (400 MHz, CDCl3): d=7.31–7.24 (m, 20H), 6.97 (d, J=
8.8 Hz, 2H), 6.80 (d, J=8.8 Hz, 2H), 6.21 (s, 1H), 6.10 (s, 1H), 5.91
(s, 1H), 5.82 (s, 1H), 5.00 (d, J=7.6 Hz, 1H), 4.69–4.43 (m, 11H),
4.21 (m, 2H), 4.20 (m, 2H), 3.98–3.96 (m, 4H), 3.78–3.60 (m, 12H),
3.76 (s, 3H), 3.53–3.45 (m, 4H), 3.31–3.30 (m, 3H), 3.14 ppm (s, 1H);
13C NMR (100 MHz, CDCl3): d=159.2, 159.1, 154.9, 150.0, 138.8,
138.5, 138.4, 128.2, 128.2, 127.5, 118.1, 114.5, 100.2, 100.2, 98.9,
79.2, 78.8, 78.6, 77.1, 76.5, 74.4, 72.6, 72.2, 68.1, 66.7, 58.7, 58.5,
55.4, 55.4 ppm; HRMS: m/z calcd for C63H68N4O22 +Na+: 1255.4217
[M+Na+]; found: 1255.4215. A suspension of benzyl ether (200 mg,
0.162 mmol) and 20% [Pd(OH)2]/C in EtOH (8 mL) and AcOH (8 mL)
was stirred under H2 atmosphere for 2 h. The catalyst was filtered
through filter paper and then washed with AcOH. After evapora-
tion, pentaol was obtained (71.0 mg, 52%). The pentaol-carbamate
(35.3 mg, 0.0404 mmol) and DMAP (1 mg, 0.008 mmol) were dis-
solved in pyridine (0.5 mL), and then Ac2O (0.5 mL) was added. The
mixture was stirred at room temperature overnight. After evapora-
tion, the crude was purified by silica gel column chromatography
to obtain N-acetylcarbamate tetrasaccharide 23 (39.7 mg, 78%).
1H NMR (400 MHz, CDCl3): d=7.08 (d, J=8.8 Hz, 2H), 6.83 (d, J=
8.8 Hz, 2H), 5.64 (d, J=6.4 Hz, 1H), 5.31–5.28 (m, 3H), 5.14 (dd, J=
9.6, 3.2 Hz, 1H), 4.61–4.08 (m, 25H), 3.97 (dd, J=12.8, 6.4 Hz, 1H),
3.90–3.83 (m, 2H), 3.77 (s, 3H), 2.52 (s, 12H), 2.14 (s, 3H), 2.12 ppm
(s, 12H); 13C NMR (100 MHz, CDCl3): d=170.4, 170.4, 169.6, 155.4,
153.0, 152.7, 152.7, 152.6, 150.7, 118.2, 114.5, 98.9, 98.7, 98.2, 98.1,
78.3, 76.2, 76.1, 76.0, 75.9, 75.6, 74.6, 70.1, 64.5, 64.4, 64.3, 60.5,
55.7, 24.3, 21.0, 20.8 ppm; HRMS: m/z calcd for C53H62N4O31 +Na+:
1273.3290 [M+Na+]; found: 1273.3292.
The number of imaginary frequencies of the transition
states
The transition state structures for pyranosides 1 f and 1k were lo-
cated by using the Synchronous Transit-Guided Quasi-Newton
(STQN) method by using the QST3 option and vibrational analysis
of the Gaussian 03 program. The number of imaginary frequencies
of the transition states are 1 f (ꢀ91.7033) and 1k (ꢀ73.7274).
Acknowledgements
S.M. received support through a Grant-in-Aid for Scientific Re-
search (C) (Grant No. 24590041) from the Japan Society for
Promotion of Science, Fund for Seeds of Collaboration Re-
search from RIKEN, Shiseido Female Researcher Science Grant,
Yamada Science Foundation, Takeda Science Foundation, A-
STEP from JST. S.M. and H.S. were supported by a Joint Re-
search Grant from the National Institute of Informatics. H.S.
and J.H. thank the Swiss National Science Foundation for fund-
ing this research with the International Short Visits Program.
H.S. was supported by a Basic Science Research Grant of The
Sumitomo Foundation. S. M. thanks: Dr. Hiroyuki Koshino at
RIKEN for NMR measurements; Dr. Kaori Otsuki, Dr. Masaya
Usui, and Dr. Aya Abe of the Research Resource Center at the
Brain Science Center, RIKEN for help with HRMS measurements,
and; Dr. Yoshiyuki Aihara and Ms. Akemi Takahashi at RIKEN for
their technical assistance. The calculations were performed on
Obelix, a computer cluster at the Competence Center for Com-
putational Chemistry (C4) of the ETH in Zurich, Switzerland.
Keywords: carbohydrates
· cleavage reactions · density
functional calculations · glycosides · isomers
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p-Methoxyphenyl (2-deoxy-di-4,6-O-acetyl-2,3-N,O-carbonyl-a-d-
glucopyranosyl)-(1!4)-(6-acetyl-2-deoxy-2,3-N,O-carbonyl-a-d-
glucopyranosyl)-(1!4)-(6-acetyl-2-deoxy-2,3-N,O-carbonyl-a-d-
glucopyranosyl)-(1!4)-(6-acetyl-2-deoxy-2,3-N,O-carbonyl-a-d-
glucopyranoside (25): BF3·OEt2 (9 mL, 0.0624 mmol) was added to
a solution of 1,2-trans tetrasaccharide 24 (39.0 mg, 0.0312 mmol) in
CH3CN (0.3 mL) at 08C. After the mixture was stirred at 08C for 3 h,
the reaction was quenched with sat. NaHCO3. The aqueous layer
was extracted with CHCl3 and the combined layers were washed
with brine. The organic layers were dried over Na2SO4 and concen-
trated. The residue was purified by silica gel column chromatogra-
phy to give 1,2-cis tetrasaccharide 25 (32.2 mg, 83%). 1H NMR
(400 MHz, CDCl3): d=6.96 (d, J=8.8 Hz, 2H), 6.82 (d, J=8.8 Hz,
2H), 6.17 (d, J=2.4 Hz, 1H), 6.00 (d, J=2.8 Hz, 1H), 5.98 (d, J=
2.4 Hz, 1H), 5.97 (d, J=2.8 Hz, 1H), 5.32 (t, J=9.6 Hz, 1H), 4.79 (t,
J=9.2 Hz, 1H), 4.76–4.40 (m, 6H), 4.28–4.08 (m, 9H), 4.02–3.98 (m,
2H), 3.91–3.37 (m, 4H), 3.87–3.74 (m, 3H), 3.80 (s, 3H), 2.56 (s, 3H),
2.55 (s, 3H), 2.54 (s, 3H), 2.52 (s, 3H), 2.13 (s, 3H), 2.12 (s, 3H), 2.11
(s, 3H), 2.09 ppm (s, 3H); 13C NMR (100 MHz, CDCl3): d=171.9,
171.8, 171.8, 171.1, 170.5, 170.4, 170.3, 169.1, 156.0, 152.4, 152.3,
152.1, 149.7, 118.7, 114.7, 96.7, 96.6, 96.3, 95.4, 76.3, 75.9, 75.5, 75.0,
74.5, 74.4, 73.7, 71.9, 71.9, 71.3, 71.1, 67.7, 62.0, 61.8, 61.8, 61.4,
59.8, 59.8, 59.8, 59.7, 55.7, 23.7, 20.8, 20.7 ppm; HRMS: m/z calcd
for C53H62N4O31 +Na+: 1273.3290 [M+Na+]; found: 1273.3291.
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Received: September 4, 2013
Published online on December 4, 2013
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