Convenient temporary methyl imidate protection of N-acetylglucosamine and glycosylation at O-4

Article abstract of DOI:10.1021/jo801117y

(Chemical Equation Presented) This paper expands on the scope and utility of the temporary conversion of N-acetyl groups to alkyl imidates when attempting to glycosylate at O-4 of N-acetylglucosamine acceptors. The optimized synthesis of alkyl imidate protected glucosamine acceptors at position 4 and carrying various protecting groups at O-3 is described. These imidates were prepared immediately prior to glycosylation by treating the 4-OH acceptors with 0.5 M MeOTf to obtain the corresponding methyl imidates still carrying a free 4-OH group. When preparing these imidates in diethyl ether as the reaction solvent, we observed the unexpected formation of ethyl imidates in addition to the desired methyl imidates. While the 3-O-allyl acceptors were too unstable to be useful in glycosylation reactions, the 3-O-acylated methyl and ethyl imidates of glucosamine were shown to behave well during the glycosylation of the 4-OH with a variety of reaction conditions and various glycosyl donors. Glycosylation of these acceptors was successfully carried out with perbenzylated β-thioethyl rhamnopyranoside under MeOTf promotion, while activation of this donor under NIS/TMSOTf or NIS/TfOH proved less successful. In contrast, activation of the less reactive perbenzylated α-thioethyl and peracetylated β-thioethyl rhamnopyranosides with NIS/TfOH led to successful glycosylations of the 4-OH. Activation of a peracetylated rhamnosyl trichloroacetimidate by TMSOTf at low temperature also gave a high yield of glycosylation. We also report one-pot glycosylation reactions via alkyl imidate protected acceptor intermediates. In all cases the alkyl imidate products were readily converted to their corresponding N-acetyl derivatives under mild conditions.

Full text of DOI:10.1021/jo801117y

Convenient Temporary Methyl Imidate Protection of
N-Acetylglucosamine and Glycosylation at O-4
Anderson Cheng,† Jenifer L. Hendel,† Kimberley Colangelo, Michael Bonin, and
France-Isabelle Auzanneau*
Department of Chemistry, UniVersity of Guelph, Guelph, Ontario, N1G 2W1, Canada
fauzanne@uoguelph.ca
ReceiVed May 22, 2008
This paper expands on the scope and utility of the temporary conversion of N-acetyl groups to alkyl
imidates when attempting to glycosylate at O-4 of N-acetylglucosamine acceptors. The optimized synthesis
of alkyl imidate protected glucosamine acceptors at position 4 and carrying various protecting groups at
O-3 is described. These imidates were prepared immediately prior to glycosylation by treating the 4-OH
acceptors with 0.5 M MeOTf to obtain the corresponding methyl imidates still carrying a free 4-OH
group. When preparing these imidates in diethyl ether as the reaction solvent, we observed the unexpected
formation of ethyl imidates in addition to the desired methyl imidates. While the 3-O-allyl acceptors
were too unstable to be useful in glycosylation reactions, the 3-O-acylated methyl and ethyl imidates of
glucosamine were shown to behave well during the glycosylation of the 4-OH with a variety of reaction
conditions and various glycosyl donors. Glycosylation of these acceptors was successfully carried out
with perbenzylated ꢀ-thioethyl rhamnopyranoside under MeOTf promotion, while activation of this donor
under NIS/TMSOTf or NIS/TfOH proved less successful. In contrast, activation of the less reactive
perbenzylated R-thioethyl and peracetylated ꢀ-thioethyl rhamnopyranosides with NIS/TfOH led to
successful glycosylations of the 4-OH. Activation of a peracetylated rhamnosyl trichloroacetimidate by
TMSOTf at low temperature also gave a high yield of glycosylation. We also report one-pot glycosylation
reactions via alkyl imidate protected acceptor intermediates. In all cases the alkyl imidate products were
readily converted to their corresponding N-acetyl derivatives under mild conditions.
Introduction
reported in high yields when using highly reactive fucosyl
donors,3 glycosylation at this position with less reactive glycosyl
donors often leads to poor yields. The low reactivity of the 4-OH
has generally been attributed to a combination of steric
hindrance2 and hydrogen bonding involving the N-acetylglu-
cosamine amide group.4 Recently, we demonstrated that the
acetamide could act as a competitive nucleophile and react with
glycosyl donors to form a glycosyl imidate side product.5 All
of these factors may account for the observed difficulties in
A large number of naturally occurring oligosaccharides and
polysaccharides, including various tumor-associated carbohy-
drate antigens, contain N-acetylglucosamine residues glycosy-
lated at O-4.1 Since these compounds are biologically important,
it is imperative that there be efficient methods for their chemical
preparation. It has been established that the hydroxyl group at
the C-4 position of the N-acetylglucosamine residue is poorly
nucleophilic.2 Although fucosylation at this position has been
(3) For example, see: (a) Lemieux, R. U.; Bundle, D. R.; Baker, D. A. J. Am.
Chem. Soc. 1975, 97, 4076–4083. (b) Spohr, U.; Lemieux, R. U. Carbohydr.
Res. 1988, 174, 211–237.
(4) Crich, D.; Dudkin, V. J. Am. Chem. Soc. 2001, 123, 6818–6825.
(5) Liao, L.; Auzanneau, F.-I. Org. Lett. 2003, 5, 2607–2610.
† A. Cheng and J. L. Hendel contributed equally to this paper.
(1) For reviews, see: (a) Hakomori, S.-I.; Zhang, Y. Chem. Biol. 1997, 4,
97-104. (b) Hakomori, S.-I. Annu. ReV. Immunol. 1984, 2, 103–126.
(2) Paulsen, H. Angew. Chem., Int. Ed. Engl. 1982, 21, 155–173.
7574 J. Org. Chem. 2008, 73, 7574–7579
10.1021/jo801117y CCC: $40.75 2008 American Chemical Society
Published on Web 09/04/2008

Protection of N-Acetylglucosamine and Glycosylation at O-4
TABLE 1. Synthesis of Alkyl Imidate Protected Acceptors
SCHEME 1
forming a glycosidic linkage at O-4 of N-acetylglucosamine.
To circumvent these problems, analogues of N-acetylglu-
cosamine that mask the acetamide NH are commonly employed.
Indeed, 2-azido-2-deoxy glucosyl derivatives,6 as well as
phthalimido,7 N,N-diacetate,4,5 N-trichlorethoxycarbonyl (N-
Troc),8 and N-acetyl oxazolidinone9 protected glucosamine
acceptors are commonly used for glycosylation at O-4. Studies
have shown that all of these analogues exhibit greater reactivity
than the native acetamide.4 The azido group was found by
competitive glycosylation to be more reactive than the phthal-
imido analogue.4 However, although the 2-azido acceptors are
suitable for glycosylation at O-4, the introduction of this
substituent and its conversion back to an acetamido is nontrivial.
In comparison, N,N-diacetates can be converted back to the
N-acetate by a simple Zemple`n deacetylation, but have to be
synthesized at an early stage in the synthetic scheme and may
be subjected to premature acid hydrolysis.4 Most other amino-
protecting groups must also be introduced early in the synthetic
scheme as, for example, the phthalimido or Troc that are usually
introduced as a first step when the glucosamine residue is fully
unprotected. In a previously reported10 synthesis of the Lea
trisaccharide intermediate 5 (Scheme 1), we discovered that
treatment of the N-acetylglucosamine disaccharide acceptor 1
with 0.5 M MeOTf (25 equiv) in Et2O led to the formation of
a stable methyl imidate (2) with the 4-OH remaining intact. In
turn, glycosylation of this methyl imidate acceptor 2 with the
thioethyl fucosyl donor 3 gave an 85% yield of the trisaccharide
4, which was easily converted to the N-acetylated trisaccharide
5 by treatment with AcOH in Ac2O at 55 °C.10 Thus, we di-
scovered a novel method of temporary protection of the N-acetyl
group in glucosamine glycosyl acceptors that allows glycosy-
lation at O-4. In comparison to the synthetic strategies with
2-azido-2-deoxyglucosylderivatives,6phthalimido,7N,N-diacetate,4,5
N-trichlorethoxycarbonyl (N-Troc),8 and N-acetyl oxazolidinone9
mentioned above, the main advantage of this new strategy
employing an imidate as temporary protection of the N-acetyl
group is that it can be synthesized selectively at the acceptor
stage immediately before glycosylation. In addition, it can be
converted back into the corresponding N-acetyl derivative under
entry
1
substrate
solvent
Et2O
products
R1
yield (%)
65a
6
9a
9b
9a
10a
10b
10a
11a
11b
11a
Me
Et
Me
Me
Et
Me
Me
Et
b
-
2
3
6
7
CH2Cl2
Et2O
69
64a
13a
74
4
5
7
8
CH2Cl2
Et2O
57a
29a
81
6
8
CH2Cl2
Me
a
Estimated from H NMR. b Traces.
1
mild conditions. In this paper, we expand on the broad scope
and utility of this method; we describe the synthesis of imidate
acceptors that carry an allyl, acetyl or benzoyl protecting group
at O-3 and our subsequent attempts to further glycosylate them
at O-4 using either thioglycosides activated with MeOTf, NIS-
TfOH, or NIS-TMSOTf or using a trichloroacetimidate glycosyl
donor activated with TMSOTf.
Results and Discussion
We first attempted to prepare the three methyl imidate
acceptors 9a, 10a, and 11a that carry respectively an allyl, acetyl,
or benzoyl protecting group at O-3. Thus, the known glycosyl
acceptors 6,3b 7,11 and 812 were treated with 0.5 M MeOTf in
Et2O according to the conditions that we have previously
established.10 Interestingly, in these conditions, we isolated in
all three cases the desired methyl imidates (Table 1) 9a-11a
which were contaminated with small amounts of what we
identified later as the corresponding ethyl imidates 9b-11b.
The structures of the major component in each mixture were
confirmed to be the desired methyl imidates by NMR and
infrared spectroscopy. The characteristic NMR signals of methyl
imidates have been well documented in our previous com-
munication.10 Thus, the major products 9a-11a gave, as
expected, 13C NMR signals at 164.0, 164.4, and 164.3 ppm,
respectively, corresponding to the CdN as well as CH3 signals
for the acetimido groups at 15.9, 15.5, and 15.4 ppm, respec-
tively. As expected,10 these signals were upfield from typical
CdO (∼170 ppm) and CH3CdO (∼20 ppm) signals. The 13C
NMR spectra for the methyl imidates 9a-11a also showed
peaks at 52.4, 52.6, and 52.6 ppm, respectively, corresponding
(6) Toepfer, A.; Schmidt, R. R. J. Carbohydr. Chem. 1993, 12, 809–822.
(7) For example, see: Nifant’ev, N. E.; Backinowsky, L. V.; Kochetkov, N. K.
Carbohydr. Res. 1998, 174, 61–72.
(8) For example, see: Gege, C.; Oscarson, S.; Schmidt, R. R. Tetrahedron
Lett. 2001, 42, 377–380.
(9) Crich, D.; Vinod, A. U. Org. Lett. 2003, 5, 1297–1300.
(10) Liao, L.; Auzanneau, F.-I. J. Org. Chem. 2005, 70, 6265–6273.
(11) DeNinno, M. P. Tetrahedron Lett. 1995, 36, 669–672.
(12) Zhang, P.; Appleton, J.; Ling, C.-C.; Bundle, D. R. Can. J. Chem. 2002,
80, 1141–1161.
J. Org. Chem. Vol. 73, No. 19, 2008 7575

Cheng et al.
TABLE 2. Glycosylations with Thioglycoside Donors
to the additional O-methyl group. Finally, infrared spectroscopy
confirmed the presence of the CdN bonds of the imidate
(absorbance peak at ∼1687 cm-1) and the absence of the CdO
bond of the acetamide (absorbance peak typically at ∼1640
cm-1). NMR of the mixtures 9a,b-11a,b showed complicated
systems which seem to be overlaps of two nearly identical
spectra. The absence of the amide proton was observed for both
products present in each mixture. The imidates are rather
unstable when submitted to silica gel chromatography and
separation of the products observed by NMR was possible for
neither 9a,b nor 11a,b. However, careful chromatography of a
large scale reaction leading to 10a,b permitted the isolation of
an analytical sample of 10b, which could be fully characterized
by NMR spectroscopy and HRMS. NMR spectroscopy of
compound 10b showed no NH signal but a quaternary CdN at
163.9 ppm as well as a CH3 signal for an acetimido group at
15.8 ppm. The HMBC spectrum showed long-range correlations
between the CdN signal and both a CH2 quartet found at 3.97
ppm and H-2 found at 3.27 ppm. This new OCH2 signal
correlated in the COSY spectrum with a methyl triplet found
at 1.17 ppm supporting that this compound was ethyl imidate
10b. N-Ethylation was excluded based on the absence of long-
range correlations in the HMBC spectrum between the glu-
cosamine C-2 signal at 62.9 ppm and the new CH2 mentioned
above as well as between the new carbon CH2 found at 60.7
ppm and H-2 of the glucosamine residue at 3.27 ppm. Finally,
HR-ESI mass spectrometry confirmed that 10b was indeed an
ethyl imidate that gave a m/z [M + H]+ peak 14 mass units
larger (found 396.2018) than that given by 10a (found 382.1862).
While no pure analytical samples of 9b and 11b could be
obtained, we confirmed that these products were also ethyl
imidates by HRMS. Indeed in both cases HRMS gave two
smaller signals 14 mass units higher (found 394.2230 and
458.2169 for 9b and 11b, respectively) than the peaks corre-
sponding to the [M + H]+ of the major products 9a and 11a
(found 380.2063 and 444.2026 for 9a and 11a, respectively).
Thus, treating 6, 7, and 8 with 0.5 M MeOTf in Et2O gave
mixtures of the methyl and ethyl imidates 9a,b (68% yield
containing traces of the ethyl imidate), 10a,b (77%, 5:1) and
11a,b (86%, 2:1), respectively (Table 1). Since there was no
reagent containing an ethyl group in these reaction mixtures,
we hypothesized that the ethyl imidates were formed due to
the participation of the solvent: Et2O in the reaction. To test
this hypothesis, compounds 6-8 were treated with 0.5 M
MeOTf while replacing Et2O with dichloromethane as the
reaction solvent. Indeed, under these conditions only the methyl
imidates 9a-11a were isolated in yields that were comparable
to the combined yields of the methyl and ethyl imidates obtained
when the reactions were carried out in Et2O (Table 1, entries 2,
4, and 6). Although the mixtures of methyl and ethyl imidates
make characterization more difficult, we showed, as described
below, that these mixtures can be used directly for glycosylation
of 4-OH. It is important to point out that imidates are acid and
water sensitive and that they must be purified quickly as well
as stored and handled under careful exclusion of water. In fact,
purification of the imidates and silica gel chromatography by
using a mixture of EtOAc and hexanes as the solvent system
led to extensive degradation resulting in low isolated yields,
and addition of Et3N to the solvent systems did not prevent
degradation. However, while the 3-OAll imidate 9a did tend to
degrade and was not obtained in excellent purity, the imidates
10a and 11a could be purified in good isolated yields with silica
donor MeOTf NIS TMSOTf TfOH
entry acceptor (equiv) (M) (equiv) (equiv) (equiv) product (%)
yield
b
b
b
b
b
b
b
b
b
b
b
b
b
1
2
3
4
5
6
10a
12 (3) 0.1a
-
-
-
-
4
-
-
-
-
-
-
-
-
-
15 62c
16 69c
15 56c
16 64c
16 <20f
16 25c
11a,b
12 (3) 0.2a
7 to 10a,b 12 (3) 0.5d
8 to 11a,b 12 (3) 0.5d
b
11a
11a
12 (3)
12 (4)
-
0.1e
b
b
-
4
-
0.1-
0.2g
b
b
7
11a
13 (3)
-
4
0.05-
-
16 63c
0.1e
b
b
b
8
9
11a
11a
13 (5)
14 (5)
-
-
5
5
-
0.5g
0.5h
16 67c
17 50c
b
-
a Reagents and conditions: (i) MeOTf (5-10 equiv), CH2Cl2, MS 4
Å, rt; (ii) AcOH, Ac2O, 55 °C. b Not applicable. c Isolated yield over
multiple steps. d One-pot formation of the methyl/ethyl imidates foll-
owed by glycosylation with 12. Reagents and conditions: (i) 7 or 8,
MeOTf (25 equiv), Et2O, MS 4 Å, rt; (ii) 12 in Et2O, rt; (iii) AcOH,
Ac2O, 55 °C. e Reagents and conditions: (i) Et2O, MS 4 Å, entry 5 -60
°C, entry 7 -70 to -50 °C; (ii) AcOH, Ac2O, 55 °C. f As evaluated
from NMR, the product could not be purified. g Reagents and con-
ditions: (i) Et2O, MS 4 Å, -60 °C; (ii) AcOH, Ac2O, 55 °C. h Reagents
and conditions: (i) Cl(CH2)2Cl, MS 4 Å, 40 to 60 °C; (ii) AcOH, Ac2O,
55 °C.
gel chromatography with use of a mixture of CHCl3 and MeOH
as the solvent system. Thus, we attempted to glycosylate these
imidate acceptors using various types of glycosyl donors and
various promoters and reaction conditions.
The first set of conditions that we tested were based on our
previous report10 and employed the known13 perbenzylated
ꢀ-thioglycoside rhamnosyl donor 12 under activation with 0.1
to 0.2 M MeOTf. In these mild conditions we observed total
degradation of the 3-O-allyl acceptor 9a prior to its glycosylation
and we thus concluded that this acceptor was not suited for use
in glycosylations at O-4 of glucosamine. In contrast, the methyl
imidate 10a as well as the mixture of methyl and ethyl imidates
11a,b were efficiently glycosylated with donor 12 by using
0.1-0.2 M MeOTf at room temperature (Table 2, entries 1 and
2). Since, as mentioned above, the imidates are difficult to purify
without degradation, the resulting disaccharide imidates were
not purified but were directly subjected to treatment with AcOH/
Ac2O to convert the acetimidate to the acetamide. Thus, from
the acceptor 10a and from the mixture of methyl and ethyl
imidates 11a,b, disaccharides 15 and 16 were obtained pure in
62% and 69% yield, respectively (Table 2, entries 1 and 2).
Measurements of the JC-1′,H-1′ coupling constants for each of 15
(1JC-1′,H-1′ ) 168 Hz) and 16 (1JC-1′,H-1′ ) 170 Hz) confirmed14
that the newly formed rhamnosidic bonds were indeed in the R
configuration. The comparable yields obtained for the formation
of disaccharide 15 and 16 from pure 10a and from the mixture
11a,b, respectively, suggest that the glycosylation occurred
equally well on a pure methyl imidate acceptor as on a mixture
of methyl and ethyl imidate acceptors.
(13) Yu, H.; Yu, B.; Wu, X.; Hui, Y.; Han, X. J. Chem. Soc., Perkin Trans.
1 2000, 9, 1445–1453.
(14) Bock, K.; Pedersen, C. J. Chem. Soc., Perkin Trans. 2 1974, 293–297.
7576 J. Org. Chem. Vol. 73, No. 19, 2008

Protection of N-Acetylglucosamine and Glycosylation at O-4
SCHEME 2
Since we could glycosylate the imidate acceptors in adequate
yields, we investigated a one-pot synthetic strategy using the
imidate methodology. We postulated that the 0.5 M MeOTf used
to form the alkyl imidates 10a,b or 11a,b should also allow for
the subsequent activation of thioglycoside donor 12. The
advantage of this one-pot strategy is that it allows for the
formation of the imidate in situ, thereby eliminating the isolation
and purification of the imidate acceptor. The acetamide acceptors
7 and 8 were converted to the corresponding imidates 10a,b
and 11a,b, respectively, and when TLC showed that the starting
materials had been consumed, a solution of thioglycoside 12 in
Et2O was added to the reaction mixture. Subsequent treatment
of the reaction mixtures with AcOH/Ac2O gave disaccharides
15 and 16 in 56% and 64% yield respectively over the 3 steps
(Table 2, entries 3 and 4). Thus, this one-pot synthetic strategy
gave higher yields than those overall yields obtained following
the purification of the imidate donors. Indeed with intermediate
purification of the imidates, the disaccharide 15 and 16 were
obtained in lower yields of 46% and 59% from 7 and 8,
respectively.
conditions, while glycosylations with more reactive donors such
as the perbenzylated ꢀ-thioglycoside 12 give acceptable results
under activation with excess MeOTf at room temperature. We
finally investigated the reaction of the known R-rhamnosyl
trichloroacetimidate glycosyl donor 1815 with methyl imidate
11a under TMSOTf catalysis. Once again the disaccharide
imidate was not isolated but converted directly to the corre-
sponding N-acetylated product. Thus, coupling of the acceptor
11a with donor 18 at low temperature with 0.1 equiv of
TMSOTf gave the desired disaccharide 17 in 80% yield (Scheme
2). This result is particularly interesting, as it was while using
similar conditions that we had previously observed5 the forma-
tion of a rhamnosyl imidate when attempting to glycosylate at
O-4 of an N-acetylated glucosamine disaccharide acceptor.
We also investigated the applicability of the methyl imidate
protection with thioglycoside donors activated with N-iodosuc-
cinimide (NIS) and catalytic trimethylsilyl triflate (TMSOTf)
or trifluoromethane sulfonic acid (TfOH). In these reactions,
we once again decided to convert the imidates back to the
desired N-acetates immediately after glycosylation without
attempting to purify the disaccharide imidates. Thus, starting
with the perbenzylated ꢀ-thioglycoside donor 12, which, as
described above, had been used successfully under MeOTf
activation, we attempted glycosylation of the 3-OBz acceptor
11a using NIS and TMSOTf catalysis at low temperature.
However, under these conditions the desired disaccharide 16
could not be isolated pure and NMR evaluation of the crude
material indicated that it had been formed at best in 20% yield
(Table 2, entry 5). Using a catalytic amount of TfOH instead
of TMSOTf did not significantly increase the yield of the
reaction but in these conditions the disaccharide 16 was isolated
pure in 25% yield (Table 2, entry 6). While the results obtained
with the ꢀ-thioglycoside 12 activated with NIS/TMSOTf (or
TfOH) were disappointing, a significant improvement was
observed when we employed the R-thioglycoside donor 13.13
Indeed, coupling donor 13 with acceptor 11a by using NIS and
TfOH at low temperature gave the disaccharide 16 in 63% yield
(Table 2, entry 7). Similarly, a 67% yield of 16 was obtained
when using NIS and TMSOTf for activation (Table 2, entry 8).
In contrast, glycosylation with the peracetylated ꢀ-thioglycoside
donor 14 proved to be fairly challenging as it was difficult to
activate this donor. Indeed, excess amounts of MeOTf at room
temperature or excess NIS with catalytic amounts of TfOH (or
TMSOTf) at low temperature did not lead to significant reaction.
However, using 1,2-dichloroethane as solvent and heating the
reaction mixture to 60 °C while activating donor 14 with NIS
and 0.5 equiv of TfOH gave disaccharide 17 in 50% yield (Table
2, entry 9). Much to our surprise, TLC analysis of the reaction
mixture indicated that the imidate was relatively stable under
these rather harsh glycosylation conditions. The R configuration
of the newly formed rhamnosidic bond in disaccharide 17 was
confirmed by NMR (1JC-1′,H-1′ ) 171 Hz) as described above
for disaccharides 15 and 16. From the experimental results
described here, it appears that glycosylation of an imidate
acceptor with a less reactive thioglycoside donor (13 or 14)
requires activation under NIS-catalytic TfOH (or TMSOTf)
Conclusion
We have clearly established that the temporary protection of
the N-acetyl group as a methyl or ethyl imidate allows
glycosylation at O-4 of N-acetylglucosamine acceptors. On the
basis of the experimental observations described here, it is clear
that imidate acceptors are compatible with glycosylations with
use of either thioglycoside or trichloroacetimidate donors. By
using 0.5 M MeOTf alkyl imidate protected glucosamine
acceptors can be prepared efficiently and immediately prior to
glycosylation at O-4. Interestingly, when using Et2O as the
solvent, one may observe the formation of mixtures of methyl
and ethyl imidates, whereas reaction in CH2Cl2 only provides
methyl imidate acceptors. The alkyl imidate acceptors can be
used in glycosylations with thioglycoside donors under activa-
tion by either MeOTf or NIS-TMSOTf (or TfOH). Glycosylation
of an imidate acceptor with the more reactive ꢀ-perbenzylated
thiorhamnoside was most effective under MeOTf activation,
whereas NIS/TfOH or NIS/TMSOTf activation at low temper-
ature gave the best results with the corresponding less reactive
perbenzylated R-thiorhamnoside. Interestingly, the relatively
harsh conditions (60 °C, 0.5 equiv of TfOH) required for the
activation of peracetylated ꢀ-thioethyl rhamnoside did not lead
to degradation of the imidate acceptor but allowed the formation
and isolation of the desired disaccharide, albeit in moderate
yield. We also report a successful one-pot synthesis in which
the imidate acceptor was formed in situ, then glycosylated with
a thioglycoside donor. An imidate acceptor was also shown to
be reactive in a glycosylation reaction by using a trichloroace-
timidate donor that was activated with TMSOTf at low
temperature. Together with its ease of introduction and removal
in mild conditions, the imidate has proven to be a versatile
temporary protecting group that allows glycosylation at O-4 of
glucosamine. Indeed, this paper demonstrates that when gly-
(15) Van Steijn, A. M. P.; Kamerling, J. P.; Vliegenthart, J. F. G. Carbohydr.
Res. 1991, 211, 261–277.
J. Org. Chem. Vol. 73, No. 19, 2008 7577

Cheng et al.
3.74-3.63 (m, 1H, H-4); 3.63-3.52 (m, 4H, H-5, OCH3); 3.45 (s,
3H, OCH3); 3.27 (dd, 1H, J ) 7.7, 9.6 Hz, H-2); 2.99 (br s, 1H,
OH); 2.02 (s, 3H, O-acetyl); 1.88 (s, 3H, acetimidate). 13C NMR
(100 MHz, CDCl3) δ 171.5 (CdO); 164.4 (CdN); 137.8 (Ar quat);
128.4, 127.8, 127.7 (Ar); 104.1 (C-1); 78.2 (C-3); 74.4 (C-5); 73.7
(CH2Ph); 70.8 (C-4); 70.3 (C-6); 65.4 (C-2); 57.2, 52.6 (CH3O);
20.9 (CH3CO); 15.5 (acetimidate CH3). HRMS calcd for C19H27NO7
[M + H]+ 382.1866, found 382.1862. IR (NaCl) 1749 cm-1 (CdO
benzoate), 1685 cm-1 (CdN acetimidate).
cosylation at O-4 of an acetamide-containing acceptor does not
proceed as desired, the imidate strategy constitutes a viable
alternative that is compatible with various donors and glyco-
sylation methods.
Experimental Section
General Procedure for the Synthesis of the Methyl and
Ethyl Imidates. A mixture of the acetamide 6, 7, or 8 (20 µmol/
mL) in Et2O (Method A) or CH2Cl2 (Method B) containing 4 Å
molecular sieves (∼25 mg/mL) was stirred at room temperature
for 2 h. MeOTf (25 equiv, 0.5 M) was added and the mixture was
stirred for 18 h at room temperature. The reaction was quenched
with Et3N (25 equiv), the reaction mixture was filtered, and the
solids were washed with CH2Cl2. The combined filtrate and
washings were diluted with CH2Cl2 (50 mL) and washed with
saturated aqueous NaHCO3 (50 mL). The organic layer was dried
and concentrated and column chromatography (CHCl3-MeOH) of
the residue gave the imidates 9-11. When Et2O was used as the
reaction solvent the methyl and ethyl imidates were isolated as a
mixture and the ratio of these two compounds was estimated by
1H NMR. When the reaction was performed in CH2Cl2, the methyl
imidates were isolated as pure compounds.
1
Ethyl Imidate 10b. H NMR (400 MHz, CDCl3) δ 7.32 (m,
5H, Ar); 4.99 (t, 1H, J ) 9.5 Hz, H-3); 4.63-4.54 (m, 2H,
OCH2Ph); 4.30 (d, 1H, J ) 7.6 Hz, H-1); 3.97 (q, 2H, J ) 7.1 Hz,
OCH2CH3); 3.78 (m, 2H, H-6a, H-6b); 3.68 (bt, 1H, J ) 9.4 Hz,
H-4); 3.59-3.52 (m, 1H, H-5); 3.45 (s, 3H, OCH3); 3.27 (dd, 1H,
J ) 7.7, 9.6 Hz, H-2); 2.97 (br s, 1H, OH); 2.02 (s, 3H, O-acetyl);
1.87 (s, 3H, acetimidate); 1.17 (t, 3H, J ) 7.1 Hz, OCH2CH3). 13
C
NMR (100 MHz, CDCl3) δ 171.5 (CdO); 163.8 (CdN); 137.7
(Ar quat); 128.4, 127.8, 127.7 (Ar); 104.2 (C-1); 78.2 (C-3); 74.3
(C-5); 73.7 (CH2Ph); 70.8 (C-4); 70.4 (C-6); 62.9 (C-2); 60.7
(OCH2CH3); 57.3 (CH3O); 20.9 (CH3CO); 15.8 (acetimidate CH3);
14.1 (OCH2CH3). HRMS calcd for C20H29NO7 [M + H]+ 396.2022,
found 396.2018.
Methyl 3-O-Benzoyl-6-O-benzyl-2-deoxy-2-methylacetimido-
ꢀ-D-glucopyranoside (11a) and Methyl 3-O-Benzoyl-6-O-benzyl-
2-deoxy-2-ethylacetimido-ꢀ-D-glucopyranoside (11b). Method
A. From the known12 acetamide 8 (41 mg, 97 µmol) a mixture
(2:1) of the imidates 11a,b (34 mg, 77%) was obtained as colorless
glass after column chromatography (30:1 CHCl3-MeOH). The
imidates could not be separated by chromatography.
Methyl 3-O-Allyl-6-O-benzyl-2-deoxy-2-methylacetimido-ꢀ-
D-glucopyranoside (9a) and Methyl 3-O-Allyl-6-O-benzyl-2-
deoxy-2-ethylacetimido-ꢀ-D-glucopyranoside (9b). Method A.
From the known3b acetamide 6 (24 mg, 67 µmol) the imidate 9a
containing traces of ethyl imidate 9b that could not be separated
was isolated. The mixture of 9a,b (17 mg, 68%) was obtained as
a colorless glass after column chromatography (30:1 CHCl3-
MeOH).
Method B. From the known12 acetamide 8 (30 mg, 70 µmol)
the imidate 11a (25 mg, 81%) was obtained as a colorless glass
after column chromatography (25:1 CHCl3-MeOH).
Method B. From the known3b acetamide 6 (26 mg, 70 µmol)
the imidate 9a (18 mg, 69%) was obtained as a colorless glass after
column chromatography (30:1 CHCl3-MeOH).
Methyl Imidate 11a. 1H NMR (400 MHz, CDCl3) δ 8.02-7.95
(m, 2H, Ar); 7.60-7.51 (m, 1H, Ar); 7.48-7.41 (m, 2H, Ar);
7.39-7.24 (m, 5H, Ar); 5.30 (t, 1H, J ) 9.4 Hz, H-3); 4.76-4.53
(m, 2H, CH2Ph); 4.42 (d, 1H, J ) 7.62 Hz, H-1); 3.87-3.82 (m,
3H, H-4, H-6a, H-6b); 3.71-3.65 (m, 1H, H-5); 3.52-3.45 (m,
7H, H-2, 2 × OCH3); 3.09 (br s, 1H, OH), 1.90 (s, 3H, acetimidate
CH3). 13C NMR (100 MHz, CDCl3) δ 166.9 (CdO); 164.3 (CdN);
137.8, 129.6 (Ar quat); 133.2, 129.7, 128.4, 127.9 (Ar); 104.1 (C-
1); 78.9 (C-3); 74.5 (C-5); 73.7 (CH2Ph); 70.7 (C-4); 70.2 (C-6);
63.2 (C-2); 57.3, 52.6 (OCH3); 15.4 (acetimidate CH3). HRMS calcd
Methyl Imidate 9a. 1H NMR (400 MHz, CDCl3) δ 7.36-7.24
(m, 5H, Ar); 5.90-5.79 (m, 1H, allyl CHdCH2); 5.24-5.09 (m,
2H, CHdCH2); 4.64-4.51 (m, 2H, CH2Ph); 4.25 (d, 1H, J ) 7.7
Hz, H-1); 4.19-4.07 (m, 2H, allyl OCH2); 3.80-3.70 (m, 2H, H-6a,
H-6b); 3.70-3.48 (m, 5H, H-4, H-5, OCH3); 3.43 (s, 3H, OCH3);
3.37 (m, 1H, H-3); 3.21 (dd, 1H, J ) 7.7 Hz, 9.2 Hz, H-2); 2.71
(br s, 1H, OH); 1.91 (s, 3H, acetimidate CH3). 13C NMR (100 MHz,
CDCl3) δ 164.0 (CdN); 137.9 (Ar quat); 135.1 (allyl CHdCH2);
128.3, 127.8, 127.7 (Ar); 117.0 (allyl CHdCH2); 104.3 (C-1); 84.6
(C-3); 74.1 (C-4); 73.9 (CH2Ph); 73.7 (allyl OCH2); 71.9 (C-5);
70.7 (C-6); 64.9 (C-2); 57.2, 52.4 (OCH3); 15.9 (CH3 acetimidate).
HRMS calcd for C20H29NO6 [M + H]+ 380.2073, found 380.2063.
IR (NaCl) 1687 cm-1 (CdN acetimidate).
+
for C24H29NO7 [M + H] 444.2022, found 444.2026. IR (NaCl)
1724 cm-1 (CdO benzoate), 1685 cm-1 (CdN acetimidate).
Ethyl Imidate 11b in the Mixture 11a,b. 1H NMR of the
mixture 11a,b showed a 2:1 ratio of methyl and ethyl imidates 11a
and 11b, respectively (Method A). 1H NMR (400 MHz, CDCl3) of
the mixture 11a,b showed a signal at 1.13 (t, 3H for 11b, J ) 7.0
Hz, OCH2CH3); other signals could not be specifically assigned to
the ethyl imidate 11b. HRMS on the mixture 11a,b showed a minor
molecular signal corresponding to ethyl imidate 11b. HRMS calcd
for C25H31NO7 [M + H]+ 458.2179, found 458.2169.
Ethyl Imidate 9b in the Mixture 9a,b. Only traces of ethyl
imidate 9b were formed when methyl imidate 9a was prepared in
Et2O (Method A). NMR signals could not be specifically assigned
to ethyl imidate 9b. HRMS on the mixture 9a,b showed a minor
molecular signal corresponding to ethyl imidate 9b. HRMS calcd
for 9b C21H31NO6 [M + H]+ 394.2256, found 394.2230.
Methyl 3-O-Acetyl-6-O-benzyl-2-deoxy-2-methylacetimido-
ꢀ-D-glucopyranoside (10a), and methyl 3-O-Acetyl-6-O-benzyl-
2-deoxy-2-ethylacetimido-ꢀ-D-glucopyranoside (10b). Method
A. From the known11 acetamide 7 (20 mg, 56 µmol) a mixture
(5:1) of the imidates 10a,b (19 mg, 86%) was obtained as a colorless
glass after column chromatography (25:1 CHCl3-MeOH). Repeated
chromatographies on a large scale synthesis provided analytical
samples of purified 10a and 10b, which could be fully and
independently characterized by NMR and HRMS.
Methyl 2-Acetamido-3-O-acetyl-6-O-benzyl-4-O-(2,3,4-tri-O-ben-
zyl-r-L-rhamnopyranosyl)-2-deoxy-ꢀ-D-glucopyranoside
(15).
From Isolated Imidate 10a. A solution of imidate 10a (14 mg,
36 µmol) and thioglycoside 12 (55 mg, 115 mmol, 3 equiv) in Et2O
(2 mL) containing 4 Å molecular sieves (167 mg) was stirred at
room temperature for 1 h. MeOTf (20 µL, 179 µmol, 5 equiv, 0.1
M) was added and the mixture was stirred for 18 h at room
temperature. The reaction was quenched with Et3N (29 µL) and
the mixture was filtered over Celite. The solids were washed with
CH2Cl2 (2 × 15 mL) and the combined filtrate and washings were
concentrated. The residue was dissolved in Ac2O (2.0 mL) and
AcOH (0.7 mL) and the solution was stirred for 18 h at 55 °C. The
mixture was coconcentrated with toluene (3 × 40 mL) and flash
chromatography (7:3 EtOAc-hexanes) of the residue gave disac-
charide 15 as a colorless glass (17 mg, 62%).
Method B. From the known11 acetamide 7 (20 mg, 56 µmol)
the imidate 10a (16 mg, 74%) was obtained as a colorless glass
after column chromatography (30:1 CHCl3-MeOH).
Methyl Imidate 10a. 1H NMR (400 MHz, CDCl3) δ 7.35-7.23
(m, 5H, Ar); 5.00 (t, 1H, J ) 9.4 Hz, H-3); 4.63-4.54 (m, 2H,
OCH2Ph); 4.32 (d, 1H, J ) 7.7 Hz, H-1); 3.79 (m, 2H, H-6a, H-6b);
From Imidates 10a,b Generated in Situ. A solution of
acetamide 7 (81 mg, 221 µmol) in Et2O (11 mL) containing 4 Å
7578 J. Org. Chem. Vol. 73, No. 19, 2008

Protection of N-Acetylglucosamine and Glycosylation at O-4
molecular sieves (300 mg) was stirred at room temperature for 1.5 h.
MeOTf (620 µL, 5.48 mmol, 25 equiv, 0.5 M) was added and the
mixture was stirred for 18 h at room temperature. A solution of
thioglycoside donor 12 (330 mg, 690 mmol, 3.1 equiv) dissolved
in Et2O (2.0 mL) was added and stirring was continued for 18 h at
room temperature. The reaction was quenched and worked up, then
the residue was treated with Ac2O and AcOH and worked up as
described immediately above. Flash chromatography (7:3 EtOAc-
hexanes) of the residue gave disaccharide 15 as a colorless glass
(96 mg, 56%). 1H NMR (400 MHz, CDCl3) δ 7.34-7.20 (m, 20H,
Ar); 5.43 (d, 1H, J ) 9.4 Hz, NH); 5.01-4.89 (m, 2H, H-3,
CHHPh); 4.79 (d, 1H, J ) 1.9 Hz, H-1′); 4.70-4.39 (m, 7H,
CHHPh, 3 × CH2Ph); 4.26 (d, 1H, J ) 8.4 Hz, H-1); 3.96 (t, 1H,
J ) 7.4 Hz, H-2); 3.75 (t, 1H, J ) 9.2 Hz, H-4); 3.71-3.58 (m,
2H, H-3′, H-5′); 3.58-3.50 (m, 3H, H-6a, H-2′, H-4′); 3.50-3.38
(m, 4H, H-6b, OCH3); 3.38 - 3.29 (m, 1H, H-5); 2.03 (s, 3H,
CH3CdO); 1.92 (s, 3H, CH3CdO); 1.23 (d, 3H, J ) 6.4 Hz, H-6′).
13C NMR (100 MHz, CDCl3) δ 171.6, 170.2 (CdO); 138.6, 138.4,
138.1, 138.0 (Ar quat); 128.3, 128.3, 127.8, 127.7, 127.7, 127.6,
127.5, 127.4 (Ar); 102.0 (C-1′, 1JC-H ) 168 Hz); 99.6 (C-1); 80.3,
75.4 (C-2′, C-4′); 79.1, 69.0 (C-3′, C-5′); 75.6 (C-4); 75.2, 73.5,
72.7, 72.2 (CH2Ph), 74.8 (C-5); 74.2 (C-3); 68.4 (C-6); 56.4
(OCH3); 54.0 (C-2); 23.3, 21.1 (CH3CdO); 17.8 (C-6′). HRMS
calcd for C45H53NO11 [M + H]+ 784.3697, found 784.3683.
Methyl 2-Acetamido-3-O-benzoyl-6-O-benzyl-4-O-(2,3,4-tri-
O-benzyl-r-L-rhamnopyranosyl)-2-deoxy-ꢀ-D-glucopyranoside
(16). From Isolated Imidates 11a,b. A solution of the imidates
11a,b (22 mg, 50 µmol) and thioglycoside 12 (67 mg, 139 mmol,
2.8 equiv) in Et2O (2 mL) containing 4 Å molecular sieves (50
mg) was stirred at room temperature for 45 min. MeOTf (56 µL,
495 µmol, 10 equiv, 0.2 M) was added and the mixture was stirred
for 18 h at room temperature. The reaction was quenched and
worked up, then the residue was treated with Ac2O and AcOH and
worked up as described above for the synthesis of disaccharide
15. Flash chromatography (2:8 EtOAc-hexanes to 100% EtOAc)
of the residue gave the disaccharide 16 as a colorless glass (29
mg, 69%).
Hz); 80.3, 79.1, 75.4, 75.3 (C-5, C-2′, C-3′, C-4′, C-5′); 75.6 (C-
4); 74.8, 73.6, 72.5, 72.3 (CH2Ph); 74.0 (C-3); 68.4 (C-6); 56.4
(OCH3); 54.3 (C-2); 23.2 (CH3CdO); 17.4 (C-6′). HRMS calcd
for C50H55NO11 [M + H]+ 846.3853, found 846.3818.
Methyl 2-Acetamido-4-O-(2,3,4-tri-O-acetyl-r-L-rhamnopy-
ranosyl)-3-O-benzoyl-6-O-benzyl-2-deoxy-ꢀ-D-glucopyrano-
side (17). From Isolated Imidate 11a and Thioglycoside 14. A
solution of imidate 11a (10 mg, 22 µmol) and thioglycoside 14
(37 mg, 110 µmol, 5 equiv) in Cl(CH2)2Cl (1 mL) containing 4 Å
molecular sieves (57 mg) and NIS (25 mg, 111 µmol, 5 equiv)
was stirred at room temperature for 1 h. TfOH (1 µL, 11 µmol, 0.5
equiv) was then added to the reaction mixture. The reaction was
stirred at room temperature for 1 h and was then stirred at 60 °C
for 18 h. The reaction was quenched and worked up, then the
residue was treated with Ac2O and AcOH and worked up as
described above for the synthesis of disaccharide 15. Flash
chromatography (8:2 EtOAc-hexanes) of the residue gave the
disaccharide 17 as a colorless glass (8 mg, 50%).
From Isolated Imidate 11a and Trichloroacetimidate 18. A
solution of imidate 11a (25 mg, 57 µmol) and the known15
trichloroacetimidate rhamnosyl donor 18 (74.0 mg, 170 mmol, 3.0
equiv) in CH2Cl2 (3 mL) containing 4 Å molecular sieves (150
mg) was stirred at room temperature for 1 h and then cooled to
-70 °C. A freshly prepared 0.11 M solution of TMSOTf in anhyd
CH2Cl2 (50 µL, 5.7 µmol, 0.1 equiv) was added and the mixture
was stirred for 1 h at -70 °C. The reaction was quenched with
NEt3 and worked up, then the residue was treated with Ac2O and
AcOH and worked up as described above for the synthesis of
disaccharide 15. Flash chromatography (4:6 EtOAc-hexanes to
100% EtOAc) of the residue gave disaccharide 17 as a colorless
1
glass (32 mg, 80%). H NMR (400 MHz, CDCl3) δ 8.04-7.21
(m, 10 H, Ar); 5.48 (d, 1H, J ) 9.4 Hz, NH); 5.42 (t, 1H, J ) 9.1
Hz, H-3); 5.18 - 5.02 (m, 2H, H-2′, H-3′); 4.91 (d, 1H, J ) 1.5
Hz, H-1′); 4.85 (t, 1H, J ) 10.0 Hz, H-4′); 4.61 - 4.50 (m, 2H,
CH2Ph); 4.45 (d, 1H, J ) 8.9 Hz, H-1); 4.21 - 4.05 (m, 2H, H-2,
H-4); 3.86-3.74 (m, 2H, H-6a, H-6b); 3.65-3.54 (m, 2H, H-5,
H-5′); 3.49 (s, 3H, OCH3); 2.02, 1.94, 1.90, 1.80 (4 × acetate CH3);
0.59 (d, 3H, J ) 6.2 Hz). 13C NMR (150 MHz, CDCl3) δ 170.2,
170.4, 166.8 (CdO); 133.4, 130.0, 128.5, 128.4, 128.3, 127.9,
127.8, 127.5 (Ar); 131.2, 129.4 (Ar quat); 101.9 (C-1); 98.4 (C-1′,
1JC-H ) 171 Hz); 75.8 (C-4); 74.8, 67.2 (C-5, C-5′); 74.0 (C-3);
73.2 (CH2Ph); 70.6 (C-4); 69.8 (C-4′); 68.9 (C-3′); 68.2 (C-2′);
68.2 (C-6); 56.5 (OCH3); 54.4 (C-2); 20.8, 20.7, 20.7 (CH3 acetates);
16.8 (C-6′). HRMS calcd for C35H43NO14 [M + H]+ 702.2762,
found: 702.2725.
From Imidates 11a,b Generated in Situ. A solution of
acetamide 8 (95 mg, 221 µmol) in Et2O (11 mL) containing 4 Å
molecular sieves (300 mg) was stirred at room temperature for 1.5 h.
MeOTf (620 µL, 5.48 mmol, 25 equiv, 0.5 M) was added and the
mixture was stirred for 18 h at room temperature. A solution of
thioglycoside 12 (332 mg, 695 mmol, 3 equiv) in Et2O (5.5 mL)
was then added to the reaction mixture and stirring was continued
for 18 h at room temperature. The reaction was quenched and
worked up, then the residue was treated with Ac2O and AcOH and
worked up as described above for the synthesis of disaccharide
15. Flash chromatography (7:3 EtOAc-hexanes) of the residue gave
disaccharide 16 as a colorless glass (120 mg, 64%). 1H NMR (400
MHz, CDCl3) δ 8.05-7.94 (m, 2H, Ar); 7.60-7.47 (m, 1H, Ar);
7.46-7.08 (m, 22H, Ar); 5.55 (d, 1H, NH); 5.31 (t, 1H, J ) 9.1
Hz, H-3); 4.86 (d, 1H, J ) 1.6 Hz, H-1′); 4.78 (d, 1H, J ) 11.1
Hz, CHHPh); 4.70-4.36 (m, 8H, CHHPh, 3 × CH2Ph, H-1); 4.15
- 4.07 (m, 1H, H-2); 3.97 (t, 1H, J ) 9.1 Hz, H-4); 3.68-3.34
(m, 10H, H-5, H-6a, H-6b, H-2′, H-3′, H-4′, H-5′, OCH3); 1.78 (s,
3H, N-acetyl); 0.74 (d, 3H, J ) 5.9 Hz, H-6′). 13C NMR (100 MHz,
CDCl3) δ 170.2, 167.0 (CdO); 138.7, 138.5, 138.1, 138.0, 129.4
(Ar quat); 133.3, 129.9, 128.4, 128.3, 128.2, 127.8, 127.7, 127.6,
Acknowledgment. We thank the National Science and
Engineering Research Council of Canada, the Canada Founda-
tion for Innovation, and the Ontario Innovation Trust for
financial support of this work.
Supporting Information Available: General experimental
procedures, 1H and 13C NMR data for compounds 9a, 10a, 10b,
11a, 15, 16, and 17, 1H NMR for 9a,b and 11a,b, as well as IR
data for compounds 9a, 10a, and 11a. This material is available
free of charge via the Internet at http://pubs.acs.org.
1
127.5, 127.5, 127.4 (Ar); 101.9 (C-1); 99.4 (C-1′, JC-H ) 170
JO801117Y
J. Org. Chem. Vol. 73, No. 19, 2008 7579