Facile glycosylation strategy with two-stage activation of allyl glycosyl donors. Application to concise synthesis of Shigella flexneri serotype y O-antigen

Article abstract of DOI:10.1039/c002865g

A practical, useful glycosylation method employing only allyl glycoside building blocks has been developed. The donor's glycosylation reactivity is turned on via isomerization of its anomeric allyl protecting group into the corresponding prop-1-enyl moiet

Full text of DOI:10.1039/c002865g

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www.rsc.org/obc | Organic & Biomolecular Chemistry
Facile glycosylation strategy with two-stage activation of allyl glycosyl donors.
Application to concise synthesis of Shigella flexneri serotype Y O-antigen†
Yun Wang, Xin Zhang and Pengfei Wang*
Received 12th February 2010, Accepted 21st June 2010
DOI: 10.1039/c002865g
A practical, useful glycosylation method employing only allyl glycoside building blocks has been
developed. The donor’s glycosylation reactivity is turned on via isomerization of its anomeric allyl
protecting group into the corresponding prop-1-enyl moiety. Subsequent chemoselective activation with
NIS/TfOH achieves high yields in glycosidic bond construction at room temperature. The efficacy and
efficiency of this approach in carbohydrate synthesis is demonstrated in the concise synthesis of the
fully protected Shigella flexneri serotype Y O-antigen.
Introduction
Ready access to structurally well-defined carbohydrates is im-
portant to both basic and applied research in the fields of
glycobiology, biochemistry, immunology and related biomedical
sciences. Organic synthesis has clearly emerged as a key force
in providing pure natural/unnatural carbohydrates. Considerable
effort has been devoted to the development of simple, facile,
and cost-effective glycosylation methods over the past several
decades.1 Despite progress, the efficient synthesis of complex
carbohydrates is still not a trivial task. Continuing to develop
new glycosylation methods remains a major focus in synthetic
carbohydrate chemistry.
We have been pursuing a glycosylation approach which would
Scheme 1 Glycosylation with allyl glycoside building blocks.
use readily accessible building blocks, a set of simple and flexible
reaction conditions that can be repeatedly applied in iterative
synthesis, and activation reagents that are inexpensive and en-
vironmentally benign. Moreover, to improve the overall synthetic
efficiency, manipulation of the anomeric group and protecting
groups should be minimized.
Sinay¨ and his coworkers were among the pioneers who explored
glycosylation with vinyl glycosyl donors.4 They reported the first
and only example of glycosylation with a prop-1-enyl glycosyl
donor 7 (Scheme 2) derived from the corresponding allyl glycoside
prior to our recent work.2 They demonstrated that glycosylation
reactions of the donor 7 with different acceptors could lead to
formation of the disaccharides 8 and 9 in modest yields (Scheme 2).
To search for necessary improvements, they turned their attention
to isopropenyl donors.4a Later, Chenault and coworkers studied
glycosylation with isopropenyl donors in more detail,5 while Boons
and coworkers focused their attention on 2-buten-2-yl glycosyl
donors.6 Boons et al. integrated their glycosylation method in a
latent-active strategy7 for iterative oligosaccharides synthesis.8
Mindful of these criteria, we have looked into the potential
of widely used allyl glycosides as building blocks for glycosidic
bond construction.2 We envision that an anomeric allyl group can
serve a dual role. In addition to its traditional role merely as a
protecting group, frequently as an anomeric protecting group in
carbohydrate synthesis, it can also be readily converted into a
good leaving group (i.e. a prop-1-enyl group) for glycosylation.
A number of convenient and high-yield isomerization procedures
are available.3 Thus, the allyl glycoside 1 is first isomerized to the
corresponding prop-1-enyl glycoside 2 (Scheme 1). Subsequent
chemoselective activation of the anomeric enol ether moiety of 2
with a proper electrophile (E+) in the presence of the allyl glycosyl
acceptor 3 passes through the reactive intermediates 4 then 5,
leading to the new allyl glycoside 6, along with the a-substituted
propanal. The key step is to activate vinyl glycoside 2 for glycosidic
bond formation.
Department of Chemistry, University of Alabama at Birmingham, Birming-
ham, AL 35294, USA. E-mail: wangp@uab.edu
† Electronic supplementary information (ESI) available: Experimental
details, spectroscopic data of 12a, 12b, 16a, 16b, 20, 21, 22, 25a, 25b,
26, 29a, 29b, and 30b, 1H and 13C NMR spectra of 12a, 12b, 16a, 16b, 20,
21, 22, 26, 27, 30a, 30b, 31, 38 and 39. See DOI: 10.1039/c002865g
Scheme 2 Sinay¨’s pioneering work.4a
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Instead of pursuing specially designed vinyl glycosyl donors to
improve glycosylation outcomes, our focus remains on prop-1-enyl
glycosyl donors that appeal to us with their distinct advantages.
Firstly, prop-1-enyl glycosides can be directly isomerized from allyl
glycosides without time-consuming anomeric group replacement
and intermediate purification. Secondly, a variety of facile and
highly effective methods are available for allyl to prop-1-enyl
isomerization.3 Thirdly, allyl glycosides have widespread use in
carbohydrate synthesis. The anomeric allyl group is typically
installed onto unprotected free monosaccharides at the very first
step of building block preparation via a Fischer glycosylation
process, greatly simplifying building block preparation. Finally,
with much simplified building block preparation, versatile isomer-
ization methods, and simple reaction conditions, this approach
should significantly advance the latent-active strategy for iterative
oligosaccharide synthesis. This strategy features no requirement
for any building block reactivity tuning as necessary for the
armed/disarmed approach employing thio and n-pentenyl build-
ing blocks in iterative syntheses.9–11 The reactivity tuning pro-
cess typically involves extensive protecting group manipulations
and/or anomeric group modifications.11,12
from the intermediacy of a a-glycosylacetonitrilium ion4a was not
observed under our reaction conditions. Reactions conducted in
other solvents (e.g. diethyl ether, dioxane, chloroform, and tert-
butanol) or solvent combinations did not improve the yields or
stereoselectivity.
When the new protocol was applied to the donor 10 isomerized
from 11 and the acceptors 1314 and 15,15 the desired disaccharides
14 and 16 were obtained in 96% yield (b/a = 70 : 30), and 86%
yield (b/a = 45 : 55), respectively (Table 1, entries 1 and 2). High
yields were also achieved with the prop-1-enyl donor 7 isomerized
from 17 (b/a = 25 : 75) 4a and the acceptors 13 and 15. The desired
disaccharide 18 and 19 were obtained in 96% yield (b/a = 77 : 23),
and 83% yield (b/a = 74 : 26), respectively (Table 1, entries 3 and
4). The yields of disaccharides 14, 18 and 19 obtained with this
modified protocol are much higher than the yields obtained from
the reactions activated with only NIS (i.e. 85%, 62%, and 65%,
respectively).2
Anomeric stereochemistry
As in other glycosylation strategies, the anomeric stereoselectivity
in the absence of proper directing effects varies under different
conditions. In addition to the solvent effect and the influence of
the structure of the glycosylation partner, we observed that the
b/a ratio of the newly formed glycosidic bond was affected by the
amount of TfOH used. For example, in the glycosylation reactions
between 11 and 2-propanol in acetonitrile, the product 12 was ob-
tained with the b/a ratio ranging from 58 : 42 (NIS/TfOH 100 : 5)
to 38 : 62 (NIS/TfOH 100 : 15); with the acceptor 13, the b/a ratio
of the disaccharide 14 changed from 70 : 30 (NIS/TfOH 100 : 1) to
52 : 48 (NIS/TfOH 100 : 15). Presumably, an acid-catalyzed b to a
isomerization was accelerated under the reaction conditions with
an increase of acid. Indeed, the isolated disaccharide 16b provided
a mixture of two anomers (b/a = 60 : 40) upon treatment with
TfOH (5%) in acetonitrile for 30 min at room temperature.
The anomeric stereochemical outcome is also slightly affected
by the reaction temperature. In MeCN, the reaction between 11
and 13 (NIS/TfOH 100 : 1) provided 14 in 83% (b/a = 72 : 28) at
-10 ◦C, 96% (b/a = 70 : 30) at room temperature and 93% (b/a =
64 : 36) at 70 ◦C, respectively. However, the ratio of donor/acceptor
has no perceptible influence on production yields and anomeric
stereoselectivity.
For proof of feasibility, we have recently demonstrated that
prop-1-enyl xylosides underwent glycosidic bond formation in
high yields upon activation with a stoichiometric amount of
N-iodosuccinimide (NIS) in acetonitrile at room temperature.2
Under these conditions, the prop-1-enyl glucosyl donor 7 pro-
duced disaccharides in yields (62–68%) comparable to those of
Sinay¨ (Scheme 2).4a These results encouraged us to pursue further
improvements and to expand the scope.
Results and discussion
Activator and solvent effect
In the glycosylation method illustrated in Scheme 1, the activating
reagent (E+) plays a crucial role. Herein we report a new protocol
using a stoichiometric amount of NIS and a catalytic amount
of triflic acid (TfOH) as the activating reagent combination.
We first examined the glycosylation reaction between the simple
glycosyl acceptor 2-propanol and the prop-1-enyl glycosyl donor
10 isomerized from allyl 2,3,4-tri-O-benzyl-D-xylopyranoside 11
(b/a = 25 : 75) with potassium tert-butoxide (Scheme 3).3,13 Upon
activation of the donor with NIS/TfOH (100 : 15) in the presence
To control anomeric stereochemistry, the traditional
neighbouring-participating-group strategy can be utilized.
For instance, the glycosyl donor 20, having a 2-O directing
pivalolyl group, underwent the one-pot isomerization catalyzed
by [Ir(COD)(PMePh2)2]PF6 2,16 and activation with NIS/TfOH to
produce the desired disaccharides 21 in 79% yield and 22 in 84%
yield with the anticipated b linkage (Table 1, entries 5 and 6).
◦
of 2-propanol at 23 C, the corresponding isopropyl 2,3,4-tri-O-
benzyl-D-xylopyranoside 12 was obtained in 95% yield (b/a =
38 : 62) in acetonitrile, 80% yield (b/a = 45 : 55) in THF, and
69% yield (b/a = 42 : 58) in CH2Cl2, respectively. These results
illustrate the solvent effect on product yields and anomeric stere-
oselectivity. Acetonitrile appeared to be the most suitable solvent.
Interestingly, in acetonitrile, the expected b-selectivity originating
Glycosylation versus undesired addition reaction
It is noteworthy that activation of disarmed prop-1-enyl glycosides
with NIS alone would not generate new glycosidic bonds. Instead,
NIS promotes addition of the acceptor to the C C bond of the
anomeric enol ether moiety. Upon treatment with NIS/TfOH,
both glycosylation and addition reactions completed rapidly.
However, the addition reaction products can be diminished by
increasing the amount of TfOH and/or diluting the reaction
Scheme 3 Glycosylation in different solvents.
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Table 1 Glycosylation of allyl glycosidesa
Entry Donor
1
Acceptor
Product
Yield (b/a)b
Activator: NIS/TfOH(cat.)c
96% (70 : 30)d
82% (52 : 48)e
TfOH (1%)
TfOH (15%)
13
14
11
2
11
86% (45 : 55)d
TfOH (15%)
15
13
16
18
3
4
96% (77 : 23)d
83% (74 : 26)d
TfOH (5%)
TfOH (5%)
17
17
15
19
21
5
6
13
15
79% (b)e
84% (b)e
TfOH (25%)
TfOH (25%)
20
20
22
25
7
8
24a:BnOH
25a:95% (a)ef
25b:95% (a)ef
TfOH (10%)
TfOH (10%)
24b:2-propanol
23
23
13
15
89% (a)e
88% (a)ef
26
27
9
23
TfOH (10%)
TfOH (50%)
10
24a:BnOH
24b:2-propanol
29a:85% (a)df
29b:89% (a)df
29
28
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Table 1 (Contd.)
Entry Donor
Acceptor
Product
Yield (b/a)b
Activator: NIS/TfOH(cat.)c
11
28
15
82% (a)d
TfOH (25%)
TfOH (5%)
TfOH (5%)
86% (25 : 75)d
85% (25 : 75)df
30
a Glycosylation reactions were preformed with 1.5 equiv of the donor and NIS, 1 equiv of the acceptor and a catalytic amount of TfOH in MeCN at 23 ◦C
1
unless indicated otherwise. b The anomeric ratio was determined from H NMR and confirmed by the isolated yield of each anomer. c The percentage
of TfOH was calculated on the base of each donor. d Isomerization was carried out with tBuOK. e Isomerization was carried out with H2-activated
[Ir(COD)(PMePh2)2]PF6 (2%). f The donor/NIS/acceptor ratio was 1 : 1 : 1.5.
solution. Presumably, the addition reaction involves the acceptor
in the rate-determining step, while the acceptor-consuming step in
the glycosylation pathway en route to 6 is not a rate-limiting step
(i.e. 5 + 3 → 6 in Scheme 1). One plausible route to the addition
reaction product could be trapping intermediate 4 (Scheme 1) with
the acceptor,17 in competition with its fragmentation to 5 leading to
new glycosidic bond formation. Therefore, dilution of the reaction
solutions slows down the addition reactions but affects little the
glycosylation reactions. In agreement with this postulation, we
Scheme 4 Retrosynthesis of the tetrasaccharide 31.
observed that increasing the donor’s concentration only increased
the amount of the addition reaction product.
and more than one million annual deaths. Rapid and cost-effective
The combination of NIS/TfOH is also required for acti-
access to these antigens is important to the development of
vation of prop-1-enyl mannosides and rhamnosides in order
diagnostic tools and multivalent vaccines that can provide a full
to avoid addition of acceptors to the anomeric C C bond.
coverage against the infection.24,25
The synthesis began with the preparation of mono-saccharide
With NIS/TfOH, rhamnoside 2318 reacted with 24 to afford the
corresponding glycosides 2519 in high yields (Table 1, entry 7). With
building blocks 23, and 32–34 (Scheme 4 and 5). Thus, benzyli-
the monosaccharide acceptors 13 and 15, the desired disaccharides
denation of the allyl rhamnoside 3518 (prepared from L-rhamnose
26 and 27 were obtained in 89%, and 88% yields, respectively
and allyl alcohol in 84% yield) followed by benzylation afforded
(Table 1, entries 8 and 9). Similar results were also obtained with
36 as a mixture of two diastereomers (endo/exo = ca. 1 : 1).
the mannoside donor 28.20 Thus, the expected new glycosides 2921
and 30 were obtained in high yields (Table 1, entries 10 and 11).
Interestingly, in the case of synthesizing 30, decreasing TfOH from
25% to 5% led to a mixture of anomers (i.e. b/a = 25 : 75 with 5%
TfOH vs. a only with 25% TfOH). The anomeric stereochemistry
of 30 was confirmed by the C1–H1 coupling constants, i.e. 1JC1–H1
26
Stereospecific ring opening of 36 with LiAlH4–AlCl3 produced
a mixture of 3227 and 3328 (90%, 32/33 = ca.1 : 1). The latter was
converted to 37.29 The third rhamnoside building block 23 was also
obtained from 35 in 84% yield. The known allyl 2-acetamido-4,6-
O-benzylidene-2-deoxy-a-D-glucopyranoside (34)30 was prepared
in two steps from N-acetyl-D-glucosamine.
1
(a) = 172 Hz and JC1–H1 (b) = 154 Hz.22 However, the reaction
yields were not influenced significantly. Again, we assumed that
the unstable b mannoside, probably formed via a competing SN2-
like mechanism, could undergo the acid-catalyzed isomerization
to the more stable a anomer. We indeed observed that treatment of
30bwith TfOH (5%) in acetonitrile for 30 min at room temperature
led to a mixture containing both anomers (b/a = 64 : 36).
Application: Concise synthesis of fully protected Shigella flexneri
serotype Y O-antigen
To illustrate the efficacy and efficiency of the new protocol in
oligosaccharide synthesis, we applied it to the synthesis of the
fully protected Shigella flexneri serotype Y O-antigen 31 (Scheme
4).23 This is one of the 15 known Shigella surface carbohydrate
antigens.24 Shigella infections cause ca. 163 million illness episodes
Scheme 5 Preparation of allyl rhamnoside building blocks for the
synthesis of S. flexneri serotype Y O-antigen.
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With all the building blocks in hand, we began to construct
glycosidic bonds with the two-stage, one-pot protocol (Scheme
6). Thus, the allyl glycosyl donor 37 was isomerized to its
corresponding prop-1-enyl glycoside with the iridium catalyst.
Subsequent activation with NIS/TfOH in the presence of the
allyl glycosyl acceptor 34 led to the disaccharide 38 in 82% yield
after alkaline workup (Scheme 6). In the same manner, the other
disaccharide 39 was obtained from the glycosylation reaction
of the donor 23 and the acceptor 32 in 83% yield; coupling of
the two disaccharides (i.e. the donor 39 and the acceptor 38)
produced the desired fully protected tetrasaccharide 31 in 74%
yield. The anomeric stereochemistry of newly formed glycosidic
bonds in 38, 39 and 31 was confirmed by measurement of the C1–
H1 coupling constants. In a short synthesis, the fully protected
tetrasaccharide was rapidly prepared from L-rhamnose and N-
acetyl-D-glucosamine.
To the reaction solution, NIS (11 mg, 0.05 mmol) was added,
followed immediately by TfOH (5 mmol) in 44 mL of acetonitrile
at room temperature. The reaction was stirred for 40 min and
quenched with triethyl amine. The solution was then concentrated
and purified by flash column chromatography on silica gel (eluted
with petroleum ether/ethyl acetate gradient 7 : 1 to 3 : 1) to afford
27 (40 mg, 88% yield). Rf 0.3 (petroleum ether/ethyl acetate 5 : 1);
[a]D = +2.84 (c = 0.67, CHCl3); 1H NMR (400 MHz, CDCl3) d
24
7.35–7.15 (m, 30 H, 6¥Ph), 5.9 (m, 1 H, –OCH2CH CH2), 5.26
(dq, J = 17.2, 1.6 Hz, 1 H, –OCH2CH CH2), 5.17 (ddt, J = 10.3,
1,7, 1.1 Hz, 1 H, –OCH2CH CH2), 4.98 (AB1, J = 10.9 Hz, 1 H,
Bn), 4.92 (AB2, J = 10.9 Hz, 1 H, Bn), 4.81–4.57 (m, 9 H), 4.71 (d,
J = 1.8 Hz, 1 H, Rha H-1), 4.66 (d, J = 3.2 Hz, 1 H, Glu H-1), 4.39
(AB3, J = 11 Hz, 1 H, Bn), 4.08 (ABMX2, J = 12.9, 5.2, 1.4 Hz,
1 H, –OCH2CH CH2), 3.98 (t, J = 9.3 Hz, 1 H, Glu H-3), 3.92
(ABMX2, J = 12.9, 6.5, 1.2 Hz, 1 H, –OCH2CH CH2), 3.82 (dd,
J = 10.6, 1.8 Hz, Glu H-6), 3.82 (dd, J = 9.3, 3.0 Hz, 1 H, Rha
H-3), 3.75 (ddd, J = 10.0, 4.8, 1.7 Hz, 1 H, Glu H-5), 3.70 (m, 1
H, Rha H-5), 3.6 (t, J = 9.3 Hz, 1 H, Rha H-4), 3.48 (dd, J = 9.6,
3.6 Hz, 1 H, Glu H-2), 3.46 (dd, J = 10.8, 5.3 Hz, 1 H, Glu H-6),
3.36 (dd, J = 10.0, 9.0 Hz, 1 H, Glu H-4), 1.28 (d, J = 6.2 Hz,
3 H, CH3); 13C NMR (101 MHz, CDCl3) d 138.7, 138.5, 138.4,
138.2, 138.1, 133.6, 128.43, 128.40, 128.36, 128.28, 128.08, 128.04,
128.00, 127.88, 127.81, 127.78, 127.76, 127.69, 127.65, 127.62,
127.58, 118.3, 98.2 (1JCH = 170 Hz), 95.3 (1JCH = 170 Hz), 81.9, 80.5,
79.88, 79.84, 77.8, 75.8, 75.5, 75.0, 74.8, 73.1, 72.7. 72.3, 69.9, 68.1,
68.0, 66.0, 17.9; FTIR (neat film) 3088, 3064, 3030, 2921, 2873,
1498, 1454, 1362, 1261, 1090, 1073, 1042, 1028 cm-1; HRMS (ESI)
m/z: calcd for C57H62O10Na (M+Na) 929.4241, found 929.4247.
Scheme 6 Glycosidic bond construction.
Representative glycosylation procedure of the prop-1-enyl glycosyl
donor obtained from isomerization of the allyl glycoside with
potassium tert-butoxide
Conclusions
A simple glycosylation protocol employing only allyl glycosides as
building blocks has been developed. The anomeric allyl protecting
group of glycosyl donors is converted to the more reactive
prop-1-enyl leaving group. Thus time-consuming anomeric group
removal/installation and intermediate purification are avoided.
Moreover, iterative use of allyl glycosides for oligosaccharide
synthesis does not require any armed/disarmed property tuning,
significantly simplifying building block preparation, and improv-
ing the overall synthetic efficiency. The efficacy and efficiency of
this method has been illustrated in the concise synthesis of the
fully protected Shigella flexneri serotype Y O-antigen.
(Synthesis of disaccharide 30): Potassium tert-butoxide (553 mg,
4.93 mmol) was added to allyl 2,3,4,6-tetra-O-benzyl-D-
mannopyranoside 28 (1.39 g, 2.24 mmol) in 2 mL of DMSO and
the reaction mixture was stirred at room temperature until the
reaction completed. The reaction was worked up with ethyl acetate
and water. The organic layer was dried over anhydrous sodium
sulfate, filtered, and concentrated to provide the corresponding
prop-1-enyl mannoside in quantitative yield.
Prop-1-enyl 2,3,4,6-tetra-O-benzyl-D-mannoside (26 mg,
0.045 mmol) and the glycosyl acceptor 15 (15 mg, 0.03 mmol)
were dried together by azeotropic removal of water with toluene.
The glycosyl partners were dissolved in 1 mL of acetonitrile. To the
reaction solution was added NIS (10 mg, 0.045 mmol) followed
immediately by triflic acid (11.3 mmol) in 5 mL of acetonitrile. The
reaction was stirred at room temperature for 1 min and quenched
with triethylamine. The reaction mixture was concentrated under
reduced pressure and purified by column chromatography (eluted
with dichloromethane/diethyl ether gradient 60 : 1 to 10 : 1) to
afford the disaccharide 30a (25 mg, 82% yield). Rf = 0.35
Experimental
Representative procedure of the one-pot glycosylation reaction
(Synthesis of 27): A solution of [Ir(COD)(PMePh2)2]PF6 (0.85 mg,
1 mmol) in 0.3 mL of THF was degassed and stirred under
a hydrogen atmosphere for 15 min at room temperature. The
obtained clear solution of the activated catalyst was injected
to allyl 2,3,4-tri-O-benzyl-a-L-rhamnopyranoside 23 (24 mg,
0.05 mmol) in 0.5 mL of THF. The reaction mixture was stirred
for 30 min, and THF was removed. The residue and allyl 2,3,4-
tri-O-benzyl-a-D-glucopyranoside 15 (37 mg, 0.075 mmol) were
dried together by azeotropic removal of water with toluene. The
glycosylation partners were then dissolved in 1 mL of acetonitrile.
24
(methylene chloride/diethyl ether 60 : 1); [a]D = +46.8 (c = 0.56,
CHCl3); 1H NMR (400 MHz, CDCl3) d 7.37–7.13 (m, 35 H,
7¥Ph), 5.9 (m, 1 H, –OCH2CH CH2), 5.3 (dq, J = 17.2, 1.5 Hz,
1 H, –OCH2CH CH2), 5.18 (ddt, J = 10.3, 1.5, 1.1 Hz, 1 H,
–OCH2CH CH2), 4.98 (AB1, J = 10.9 Hz, 1 H, Bn), 4.95 (d,
J = 1.8 Hz, 1 H, Man H-1), 4.86 (AB2, J = 10.9 Hz, 1H, Bn), 4.84
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(AB3, J = 10.9 Hz, 1 H, Bn), 4.79 (AB1, J = 10.9 Hz, 1 H, Bn),
4.76 (d, J = 3.7 Hz, 1 H, Glu H-1), 4.73 (AB4, J = 10.9 Hz, 1 H,
Bn), 4.72 (AB5, J = 10.9 Hz, 1 H, Bn), 4.68 (AB5, J = 10.9 Hz,
1 H, Bn), 4.66 (AB4, J = 10.9 Hz, 1 H, Bn), 4.61 (s, 2 H, Bn),
4.59 (AB6, J = 10.9 Hz, 1 H, Bn), 4.5 (AB2+3, J = 10.9 Hz, 2 H,
Bn), 4.45 (AB6, J = 10.9 Hz, 1 H, Bn), 4.1 (ABMX2, J = 12.9,
5.3, 1.4 Hz, 1 H, –OCH2CH CH2), 4.2–3.93 (m, 3 H), 3.89–3.58
(m, 8 H), 3.45 (dd, J = 9.6, 3.6 Hz, 1 H, Glu H-2), 3.4 (dd, J =
9.8, 9.1 Hz, 1H); 13C NMR (101 MHz, CDCl3) d 138.7, 138.6,
138.4, 138.34, 138.3, 138.12, 138.11, 133.5, 128.4, 128.38, 128.45,
128.24, 128.21, 128.18, 128.0, 127.9, 127.86, 127.8, 127.71, 127.70,
127.6, 127.58,127.5, 27.4, 127.38, 118.5, 98.2 (1JCH = 169 Hz), 95.1
(1JCH = 168 Hz), 82.07, 79.9, 79.6, 77.6, 75.8, 75.0, 74.9, 74.8, 74.5,
73.2, 73.0, 72.4, 72.0, 71.8, 70.0, 69.0, 68.0, 65.7; FTIR (neat film)
3088, 3063, 3030, 2919, 2869, 1497, 1454, 1362, 1261, 1208, 1093,
1073, 1043, 1028 cm-1; HRMS (ESI) m/z: Calcd for C64H68O11Na
(M+Na) 1035.4659, found 1035.4655.
Allyl (2,4-di-O-benzyl-a-L-rhamnopyranosyl)-(1→3)-2-deoxy-
2-acetamido-4,6-O-benzylidene-a-D-glucopyranoside
(38).
A
solution of [Ir(COD)(PMePh2)2]PF6 (1.2 mg,
1 mmol) in
1 mL of THF was degassed and stirred under hydrogen
atmosphere for 5 min at room temperature. The clear solution
obtained was injected into allyl 3-O-acetyl-2,4-di-O-benzyl-a-L-
rhamnopyranoside 37 (44 mg, 0.1 mmol) in 0.7 mL of THF. The
reaction mixture was stirred for 25 min, and THF was removed.
The residue and 34 (27 mg, 0.077 mmol) were dried by azeotropic
removal of water with toluene. The mixture of glycosylation
partners were dissolved in 6 mL of acetonitrile and treated with
NIS (23 mg, 0.1mmol), immediately followed by TfOH (0.18
mL, 2 mmol) in 100 mL of acetonitrile. The reaction mixture was
stirred at room temperature for 70 min and then was hydrolyzed
by adding saturated sodium methoxide in methanol (0.3 mL,
0.9 mmol). After 5 min, the reaction mixture was acidified by
acetic acid and concentrated. The residue was purified by flash
column chromatography (petroleum ether/ethyl acetate 1 : 2 Rf
24
0.4) to afford 38 (43 mg, 82% yield) as a white solid. [a]D
=
1
+11.3 (c = 0.21, CHCl3); H NMR (400 MHz, CDCl3) d 7.46
(d, J = 7.6 Hz, 2 H, Ph), 7.38–7.2 (m, 13 H, Ph), 5.9 (m, 1 H,
–OCH2CH CH2), 5.79 (d, J = 9.8 Hz, 1 H, AcNH–), 5.52 (s,
1 H, PhCH), 5.35–5.25 (m, 2 H, –OCH2CH CH2), 5.05 (s, 1
H, Rha H-1), 4.76 (AB1, J = 11.0 Hz, 1 H, Bn), 4.76 (dd, J =
3.9 Hz, 1 H, Glu H-1), 4.65 (AB2, J = 11.7 Hz, 1 H, Bn), 4.53
(AB1, J = 11.3 Hz, 1 H, Bn), 4.52 (AB2, J = 11.6 Hz, 1 H, Bn),
4.4 (td, J = 9.7, 3.6 Hz, 1 H, Glu H-2), 4.28 (dd, J = 10.3, 4.8 Hz,
1 H, Glu H-6), 4.21 (m, 1 H, –OCH2CH CH2), 4.05–4.00 (m,
1 H, Rha H-5), 4.01(t, J = 9.6 Hz, 1 H, Rha H-4), 4.01 (m, 1
H, –OCH2CH CH2), 3.96 (dd, J = 9.2, 3.2 Hz, 1 H, Rha H-3),
3.91 (td, J = 9.9, 4.8 Hz, 1 H, Glu H-5), 3.78 (d, J = 2.6 Hz, 1
H, Rha H-2), 3.76 (t, J = 10.3 Hz, 1 H, Glu H-6), 3.6 (t, J =
9.5 Hz, 1 H), 3.21 (t, J = 9.5 Hz, 1 H), 2.2 (d, J = 8.4 Hz, 1 H,
Rha –OH), 1.95 (s, 3 H, CH3C(O)NH), 0.92 (d, J = 6.2 Hz, 3 H,
Rha CH3); 13C NMR (101 MHz, CDCl3) d 169.5, 138.7, 137.8,
136.9, 133.1, 128.9, 128.5, 128.2, 128.1, 127.9, 127.6, 127.5, 126.2,
118.5, 101.8, 97.3 (1JCH = 174 Hz), 97.31 (1JCH = 170 Hz), 82.0,
80.0, 79.2, 74.4, 73.7, 72.9, 71.0, 68.8, 68.4, 67.2, 63.0, 53.3, 23.5,
17.4; FTIR (neat film) cm-1 3065, 3029, 2922, 2868, 1646, 1558,
155, 13781261, 1184, 1125, 1092, 1077, 1062, 1049, 1029; HRMS
(ESI) m/z: Calcd for C38H45NO10 (M) 675.3043, found 675.3029.
Synthesis of fully protected tetrasaccharide 31
Allyl (2,3,4-tri-O-benzyl-a-L-rhamnopyranosyl)-(1→2)-3,4-di-
O-benzyl-a-L-rhamnopyranoside (39). A solution of [Ir(COD)-
(PMePh2)2]PF6 (5.5 mg, 6.5 mmol) in 1.0 mL of THF was degassed
and stirred under hydrogen atmosphere for 30 min at room
temperature. The clear solution was injected into allyl 2,3,4-tri-O-
benzyl-a-L-rhamnopyranoside 23 (308 mg, 0.65 mmol) in 1.0 mL
of THF. The reaction mixture was stirred for overnight, and
THF was removed. The residue and allyl 3,4-di-O-benzyl-a-L-
rhamnoside 32 (162 mg, 0.42 mmol) were dried by azeotropic
removal of water with toluene. To the mixture was added 12 mL
of acetonitrile, followed by 1.0 mL of an acetonitrile solution
containing premixed NIS (146 mg, 0.65 mmol) and TfOH (5.8 mL,
65 mmol) at room temperature. The reaction was stirred for
15 min and quenched with triethyl amine. The solution was
then concentrated and purified by flash column chromatography
on silica gel (eluted with hexane/ethyl acetate 5 : 1, Rf 0.4) to
afford the desired disaccharide 39 (274 mg, 82% yield) as a
colorless syrup. [a]D = -19.4 (c = 0.16, CHCl3); 1H NMR
25
(400 MHz, CDCl3) d 7.36–7.21 (m, 25 H, 5¥Ph), 5.85 (m, 1 H,
–OCH2CH CH2), 5.25 (dq, J = 17.2, 1.5 Hz –OCH2CH CH2),
5.17 (ddt, J = 10.4, 1.3, 1.1 Hz, 1 H, –OCH2CH CH2), 5.12 (d,
J = 1.6 Hz, 1 H, H-1), 4.94 (AB1, J = 10.8 Hz, 1 H, Bn), 4.86 (AB2,
J = 10.8 Hz, 1 H, Bn), 4.71 (d, J = 1.6 Hz, 1 H, H-1), 4.64–4.57 (m,
5 H), 4.56 (AB3, J = 12.3 Hz, 1 H, Bn), 4.49 (AB3, J = 12.5 Hz, 1 H,
Bn), 4.1 (ABMX2, J = 13.0, 5.0, 1.5 Hz, 1 H, –OCH2CH CH2),
4.05 (m, 1 H, H-2), 3.92 (ABMX2, J = 13.0, 6.1, 1.1 Hz, 1 H,
–OCH2CH CH2), 3.86 (dd, J = 9.8, 3.0 Hz, 1 H, H-3), 3.79 (dd,
J = 2.9, 2.0, 1 H, H-2), 3.76 (m, 1H, H-5), 3.66 (m, 1 H, H-5),
3.60 (t, J = 9.4 Hz, 1 H, H-4), 3.30 (t, J = 9.4 Hz, 1 H, H-4),
1.3 (d, J = 6.2 Hz, 3 H, CH3), 1.26 (d, J = 6.2 Hz, 3 H, CH3);
13C NMR (101 MHz, CDCl3) d 138.5, 138.4, 138.2, 138.18, 133.7,
128.4, 128.37, 128.32, 128.25, 128.1, 128.0, 127.9, 127.88, 127.8,
127.72, 127.70, 127.6, 127.5, 117.2, 99.1 (1JCH = 173 Hz), 97.9
(1JCH = 172 Hz), 80.5, 80.2, 80.1, 79.1, 75.3, 75.27, 74.7, 73.9, 72.4,
72.1, 72.0, 68.4, 67.9, 67.6, 18.0, 17.9; FTIR (neat film) cm-1 3088,
3064, 3031, 2973, 2917, 1497, 1454, 1383, 1363, 1309, 1286, 1261,
1208, 1115, 1074, 1028; HRMS (ESI) m/z: Calcd for C50H56O9Na
(M+Na) 823.3822, found 823.3806.
Allyl (2,3,4-tri-O-benzyl-a-L-rhamnopyranosyl)-(1→2)-(3,
4-di-O-benzyl-a-L-rhamnopyranosyl)-(1→3)-(2,4-di-O-benzyl-a-
L-rhamnopyranosyl)-(1→3)-2-deoxy-2-acetamido-4,6-O-benzyl-
idene-a-D-glucopyranoside (31).
A solution of [Ir(COD)-
(PMePh2)2]PF6 (0.6 mg, 0.7 mmol) in 1 mL of THF was degassed
and stirred under hydrogen atmosphere for 5 min at room
temperature. The obtained clear solution was injected into the
solution of the disaccharide 39 (28 mg, 0.036 mmol) in 1 mL
of THF. The reaction mixture was stirred for 2 h, and THF
was removed. The residue and the disaccharide 38 (16 mg,
0.024 mmol) were dried by azeotropic removal of water with
toluene. The glycosyl donor and acceptor were dissolved in 1.5 mL
of acetonitrile after briefly heating, and were treated with NIS
(8 mg, 0.036 mmol), immediately followed by TfOH (0.063mL,
0.7 mmol) in 16 mL of acetonitrile. The reaction mixture was stirred
at room temperature for 60 min and quenched by triethyl amine.
The reaction solution was concentrated and purified by flash
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column chromatography on silica gel (eluted with a benzene/ethyl
Chem. Soc., Perkin Trans. 1, 1997, 3357; (c) M. Johnson, C. Aries and
G.-J. Boons, Tetrahedron Lett., 1998, 39, 9801.
9 D. R. Mootoo, P. Konradsson, U. Udodong and B. Fraser-Reid, J. Am.
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acetate gradient 9 : 1 to 3 : 1) to afford 31 (25 mg, 74% yield) as a
25
white solid. Rf 0.6 (benzene/ethyl acetate 3 : 1); [a]D = -9.0 (c =
0.16, CHCl3). 1H NMR (400 MHz, CDCl3) d 7.5–7.15 (m, 40 H,
8¥Ph), 5.85 (m, 1 H, –OCH2CH CH2), 5.66 (d, J = 9.9 Hz, 1
H), 5.57 (s, 1 H), 5.3–5.2 (m, 2 H, –OCH2CH CH2), 5.08 (d, J =
1.6 Hz, 1 H), 5.0 (s, 2 H), 4.9 (d, J = 11 Hz, 1 H), 4.84 (d, J =
11.2 Hz, 1 H), 4.75–4.7 (m, 2 H), 4.6–4.35 (m, 11 H), 4.28 (m, 1 H),
4.18 (m, 1 H), 4.02–3.92 (m, 5 H), 3.89–3.62 (m, 10 H), 3.53 (t, J =
9.2 Hz, 1 H), 3.42 (t, J = 9.4 Hz, 1 H), 3.29 (t, J = 9.4 Hz, 1 H), 1.85
(s, 3 H), 1.26 (d, J = 6.2 Hz, 3 H), 1.08 (d, J = 6.2 Hz, 3 H), 0.81
(d, J = 6.2 Hz, 3 H); 13C NMR (101 MHz, CDCl3) d 169.3, 138.7,
138.6, 138.3, 138.29, 138.17, 137.0, 133.2, 128.8, 128.38, 128.33,
128.32, 128.28, 128.26, 128.20, 128.14, 128.07, 127.90, 127.89,
127.83, 127.82, 127.81, 127.76, 127.75, 127.72, 127.57, 127.52,
127.51, 127.46, 127.45, 127.44, 127.38, 127.31, 126.1, 118.4, 101.6,
101.0 (1JCH = 176 Hz), 99.0 (1JCH = 175 Hz), 97.6 (1JCH = 173 Hz),
97.4 (1JCH = 176 Hz), 80.6, 80.5, 80.3, 80.1, 79.9, 79.1, 75.1, 74.93,
74.90, 74.4, 73.4, 72.7, 72.4, 72.1, 72.0, 68.8, 68.6, 68.5, 68.2, 63.0,
53.2, 23.4, 17.93, 17.85, 17.37; FTIR (neat film) cm-1 3065, 3032,
2974, 2930, 1652, 1497, 1454, 1377, 1090, 1059, 1029, 1001; HRMS
(ESI) m/z: Calcd for C85H95NO18Na (M+Na) 1440.6447, found
1440.6470.
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4328 | Org. Biomol. Chem., 2010, 8, 4322–4328
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