Iterative one-pot α-glycosylation strategy: Application to oligosaccharide synthesis

Article abstract of DOI:10.1002/adsc.201200396

Based on the combined use of dimethylformamide (DMF) modulation and neighboring group participation, three iterative one-pot α-glycosylation methods, i.e., one-pot (α,α)-, one-pot (β,α)-, and one-pot (α,β)-glycosylations, were developed. These methods are

Full text of DOI:10.1002/adsc.201200396

FULL PAPERS
DOI: 10.1002/adsc.201200396
Iterative One-Pot a-Glycosylation Strategy: Application to
Oligosaccharide Synthesis
Chih-Yueh Ivan Liu,a Shaheen Mulani,a and Kwok-Kong Tony Monga,*
a
Applied Chemistry Department, Science Building II, National Chiao Tung University, 1001 Ta Hsueh Road, Hsinchu,
Taiwan 300, ROC
Fax : (+886)-3-572-3764; e-mail: tmong@mail.nctu.edu.tw
Received: May 4, 2012; Revised: July 18, 2012; Published online: November 13, 2012
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200396.
Abstract: Based on the combined use of dimethyl- serine conjugate, the d-Gal-a
formamide (DMF) modulation and neighboring l-Rha trisaccharide unit of the cell wall component
group participation, three iterative one-pot a-glyco- in Streptococcus pneumoniae, and the d-Glc-a
(1!2)-
sylation methods, i.e., one-pot (a,a)-, one-pot (b,a)-, d-Glc-a
(1!3)-d-Glc trisaccharide terminus of the
A
U
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
and one-pot (a,b)-glycosylations, were developed. N-linked glycan precursor. Confirmation of the
These methods are applicable to a range of thiogly- anomeric configurations of these oligosaccharides is
cosyl donors, confer stereocontrol in a-/b-glycosidic evidenced by 1H, 13C, 13C-non-proton decoupling,
bond formation, and thus provide for rapid access to and heteronuclear correlation 2D NMR experiments.
oligosaccharides with various permutations of Global deprotection of selected oligosaccharide tar-
anomeric configurations. The utility of these one-pot gets is illustrated.
glycosylation methods is demonstrated in the synthe-
sis of eight non-natural and natural oligosaccharide Keywords: DMF; modulation; oligosaccharides; one-
targets, including the core 1 serine conjugate, core 8 pot glycosylation; stereoselectivity
Introduction
bond to be constructed is often present at the non-re-
ducing end of the target oligosaccharide.[17] Kobayashi
Natural oligosaccharides contain monosaccharide et al. recently reported a one-pot a-glycosylation
units joined by a- and b-glycosidic bonds.[1,2] Because method for d-galactosyl and d-glucosyl donors, but se-
of their structural diversity, the chemical synthesis of lectivity obtained was moderate.[18] Thus, developing
oligosaccharides and glycoconjugates involves long a more general and practical one-pot a-glycosylation
synthetic routes and requires tedious purification strategy is both highly desirable and important for the
methods. Advanced technology for oligosaccharide advancement of glycosylation chemistry.
synthesis involves one-pot glycosylation strategies and
We recently reported a stereoselective glycosylation
automated solid phase synthesis.[3–6] One-pot glycosy- method based on the dimethylformamide (DMF)
lation refers to sequential coupling of glycosyl build- modulation concept. With this modulated glycosyla-
ing blocks to produce an oligosaccharide in the same tion method, DMF is employed to entrap the cationic
reaction pot, obviating the need for reptitive inter- glycosyl oxacarbenium ion as a glycosl imidate.[19,20]
mediate isolation and transformation of the anomeric Further reaction of the imidate with an acceptor pro-
group.
duces the desired cis a-glycoside in high selectivity.
Numerous one-pot glycosylation methods have Because the modulation method includes a pre-acti-
been developed; they are classified as orthogonal,[7,8] vation step, it is further elaborated to iterative glyco-
chemoselective,[9,10] and iterative strategies.[11–16] In sylation for thioglycosyl substrates.
general, these methods work well for the synthesis of
Herein, we propose that the coupling of the glyco-
oligosaccharides invoking b-glycosidic bonds. Howev- syl imidate with a thioglycosyl acceptor produces a thi-
er, the scope of one-pot glycosylation with respect to odisaccharide and regenerates the DMF (Scheme 1,
cis a-glycoside formation is somewhat limited. Previ- a). As a result, the thiodisaccharide and regenerated
ous examples are restricted mainly to the use of d-gal- DMF may be engaged in another modulated glycosy-
actosyl and/or l-fucosyl donors, and the a-glycosidic lation cycle. Under such conditions, a trisaccharide
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Scheme 1. a) Iterative one-pot (a,a)-glycosylation. b) Iterative one-pot (b,a)-glycosylation. c) Iterative one-pot (a,b)-glyco-
sylation.
with two contiguous cis a-glycosidic bonds can be pating group reacts with an acceptor under the con-
produced in one pot. We coin this “iterative one-pot ventional glycosylation procedure to afford a trisac-
(a,a)-glycosylation.” Furthermore, by combined use charide with (a,b)-anomeric configuration.
of the DMF modulation concept and neighboring
Because the selectivity of DMF-modulated glycosy-
group participation (NGP), two one-pot glycosylation lation has been described in a previous publication,
procedures were developed.[21,22] These new proce- the present study focuses mainly on the isolated yield
dures provide access to oligosaccharides with two and anomeric purity of the desired product in the fol-
other permutations of b- and a-anomeric configura- lowing one-pot reactions.[20]
tions. Accordingly, these procedures are named “iter-
ative one-pot (b,a)-glycosylation” and “ACTHUNRTGNEUNG(b,a)-glycosy-
lation.”
Results and Discussion
In iterative one-pot (b,a)-glycosylation, a participat-
ing thioglycosyl donor is reacted with a non-partici- Iterative One-Pot (b,a)- and (b,a)-Glycosylations
pating thioglycosyl acceptor by means of a convention-
al glycosylation method to furnish a b-linked thiodi- At the outset, one-pot (b,a)-glycosylation was investi-
saccharide (Scheme 1, b). Because this thiodisacchar- gated. Protected d-Gal-b
N
U
ide is non-participating, it can couple with an acceptor Gal trisaccharide 4 was selected as the synthetic
under the DMF modulation procedure to produce target. The synthesis of 4 required 2-O-benzoyl thio-
a trisaccharide with (b,a)-anomeric configuration. In galactoside 1, 3-hydroxy unprotected 2-azido-2-deoxy-
iterative one-pot (a,b)-glycosylation, the DMF modu- thiogalactoside 2, and galactose acceptor 3 (Scheme 2,
lation procedure is first applied to couple a non-par- a).[23] N-Iodosuccinimide (NIS) and trimethylsilyl tri-
ticipating glycosyl donor with a participating glycosyl fluoromethanesulfonate (TMSOTf) were employed as
acceptor to furnish an a-linked thiodisaccharide the promoters.[24] Due to the presence of the azido
(Scheme 1, c). This thiodisaccharide bearing a partici- function at the C-2 position, 2-azido-2-deoxy thioga-
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Iterative One-Pot a-Glycosylation Strategy: Application to Oligosaccharide Synthesis
Scheme 2. a) Building blocks 1, 2, and 3. b) One-pot synthe-
sis of trisaccharide 4.
lactoside 2 is less reactive than 2-O-benzoyl thiogalac-
toside 1.[25,26] Therefore, 2 was reacted with 1 under
reactivity-based glycosylation conditions to furnish
the expected thiodisaccharide (Scheme 2, b).[27] With-
out isolation of the thiodisaccharide, the DMF modu-
lator and a supplementary amount of NIS/TMSOTf
were added, kicking off the modulated glycosylation
and converting the thiodisaccharide to a disaccharide
imidate. Subsequent addition of 3 concluded the one-
pot reaction, producing the desired trisaccharide 4 in
43% isolated yield with excellent anomeric purity.
Scheme 3. a) Building blocks 5, 6, and 7. b) Synthesis of thi-
odisaccharide 8a. c) Glycosylation of 7 with 8a in the pres-
ence of DMF using a conventional procedure. d) One-pot
The (b,a)-configurations of the anomeric centers in 4
are confirmed by (i) the anomeric carbon signals at
103.1 (for C-1’’), 99.3 (for C-1’), and 96.7 (for C-1) (a,b)-glycosylation for the synthesis of 8.
1
ppm and (ii) their corresponding JC,H coupling con-
stants of 159, 171, and 178 Hz, respectively.[28]
After achieving one-pot (b,a)-glycosylation, we di- Because the acceptor 6 contains two electron-with-
rected our effort to one-pot (a,b)-glycosylation. Pro- drawing benzoyl (Bz) functions, it is presumably less
tected d-Gal-a
N
U
ide 8 was selected as the synthetic target. In a one-pot reduce the equivalent amount of DMF for the DMF-
synthesis of 8, 4,6-O-benzylidene thiogalactoside 5, 6- modulated glycosylation of 6; 1 equiv. of DMF (with
hydroxy unprotected thioglucoside 6, and a reducing respect to donor) was applied in glycosylation of 6
end rhamnoside 7 were needed (Scheme 3, a).[29] Be- with 5 (Scheme 3, b). The modulated reaction pro-
cause the cis a-glycosidic bond was to be constructed duced the desired thiodisaccharide 8a in 70% yield
under the DMF modulation conditions, it was thought with an excellent >20:1 a/b-anomeric ratio.
that the regenerated DMF might interfere with the b-
directing effect of the participating donor in the (with respect to 8a) on the b-selectivity of the glycosy-
second glycosylation step. lation of 3 with 8a using the conventional glycosyla-
Next, we assessed the effect of 1.2 equiv. of DMF
In previous studies, 4 equiv. of DMF (with respect tion procedure (Scheme 3, c). The target trisaccharide
to donor) were used to provide the optimal selectivity 8 was obtained in 65% yield as a single anomer.
for a reactive acceptor.[20] According to experimental Building on these preliminary studies, the stage was
observations and published reports, the a-selectivity set for the one-pot synthesis of the trisaccharide 8
of glycosylation for a less reactive acceptor is general- (Scheme 3, d). Thus, 1.2 equiv. of thiogalactoside 5
ly higher than that for a more reactive acceptor.[30] and 1.2 equiv. of DMF were treated with NIS and
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TMSOTf to yield a putative imidate. Subsequent re- trisaccharide 15 were selected as the synthetic targets
action of the imidate with thioglucoside 6 produced for iterative one-pot (a,a)-glycosylation.
the thiodisaccharide 8a. Without isolation, 8a was
The synthesis of the trisaccharide 11 required per-
coupled with the rhamnoside 7 under the convention- O-benzyl thiogalactoside 9, 3-hydroxy unprotected
al glycosylation procedure to afford the target trisac- thiogalactoside 10, and 4-hydroxy unprotected rham-
charide 8 in 47% isolated yield with excellent anome- noside 7 (Scheme 4, a).[32] Prior to the one-pot reac-
ric purity. The (a,b)-configurations of the anomeric tion, the selectivity of the glycosylation of 10 with 9
centers in 8 are supported by (i) the anomeric carbon was evaluated (Scheme 4, b). Thiogalactoside 10 was
signals at 99.0 (for C-1’’), 100.8 (for C-1’), and 98.3 coupled with 9 under the modulated glycosylation
1
(for C-1) ppm and (ii) their corresponding JC,H cou- procedure to furnish the desired disaccharide 11a in
pling constants of 170, 163, and 168 Hz, respective- 62% yield with an excellent >20:1 a/b-anomeric
ly.[28]
ratio. After confirming the selectivity of the glycosyla-
tion, we proceeded to the one-pot operation.
In the one-pot synthesis of 11, the thiogalactoside 9
was subjected to the DMF modulation procedure to
react with the acceptor 10, producing the desired thio-
Iterative One-Pot (a,a)-Glycosylation
Motivated by the results of the aforementioned one- disaccharide 11a (Scheme 4, c). Without isolation, 11a
pot (b,a)- and (a,b)-glycosylations, we investigated was reacted with the rhamnoside acceptor 7 under the
one-pot (a,a)-glycosylation. It should be mentioned DMF modulation procedure to furnish the target tri-
that only a few reports in the literature describe one- saccharide 11 in 52% isolated yield with high anome-
pot construction of contiguous cis a-glycosidic ric purity. The cis (a,a)-configuration of the anomeric
bonds,[17e,18,31] and some of them are lacking in ade- centers in 11 are evidenced by (i) the anomeric
quate selectivity for practical application.[17e,18] To 13C NMR signals observed at 93.0 (for C-1’’), 99.6 (for
demonstrate the practicability of the present methods, C-1’), and 98.2 (for C-1) ppm, and (ii) the correspond-
1
protected d-Gal-a
charide 11 and l-Rha-a
G
E
G
(1!2)-d-Glc respectively.[28]
Scheme 4. a) Building blocks 9, 10, and 7. b) Glycosylationof
10 with 9 using the DMF modulation procedure. c) One-pot
synthesis of trisaccharide 11.
Scheme 5. a) Building blocks 12, 13, and 14. b) Glycosylatio-
nof 13 with 12 using the DMF modulation procedure. c)
One-pot synthesis of trisaccharide 15.
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Iterative One-Pot a-Glycosylation Strategy: Application to Oligosaccharide Synthesis
For the synthesis of the l-Rha-a
(1!2)-d-Glc trisaccharide 15, three monosaccharide reoselectivity.
building blocks were prepared, namely per-O-benzyl The synthesis of 19 required the 2-O-benzoyl thio-
A
ACHTUNGTRENNUNG
thiorhamnoside 12, 3-hydroxy unprotected thiorham- galactoside 1, 2-azido-2-deoxy thiogalactoside 2, and
noside 13, and 2-hydroxy unprotected glucoside 14 serine ester 16 (Scheme 6, a).[42] In the one-pot reac-
(Scheme 5, a).[33] The selectivity of the glycosylation tion, the glycosylation of 2 with 1 produced a b-linked
of 13 with 12 was assessed before carrying out the thiodisaccharide under conventional glycosylation
one-pot reaction (Scheme 5, b). The glycosylation
generated the expected disaccharide 15a in 60% iso-
lated yield with excellent a-selectivity.
In the one-pot synthesis of 15, thiorhamnoside 12
was subjected to the DMF modulation procedure for
coupling with the thiorhamnoside acceptor 13, fur-
nishing the desired thiodisaccharide 15a (Scheme 5,
c). Without intermediate isolation, 15a was subjected
to DMF-modulated glycosylation to react with the
glucoside acceptor 14. As noted for the glycosylation
of thiorhamnoside 13, optimization of the reaction
temperature was required to improve the coupling ef-
ficiency. Nevertheless, the expected trisaccahride 15
was obtained in 41% isolated yield with high anome-
ric purity. The trans (a,a)-configuration of the anome-
ric centers in 15 are supported by (i) the anomeric
carbon signals at 100.4 (for C-1’’), 98.3 (for C-1’), and
1
102.3 (for C-1) ppm, and (ii) the corresponding JC.H
coupling constants of 169, 170, and 158 Hz, respec-
tively.[28]
Iterative One-Pot Synthesis of Natural
Oligosaccharides
In the aforementioned Scheme 1, Scheme 2,
Scheme 3, and Scheme 4, the utility of the present
methods for the preparation of non-natural oligosac-
charides was demonstrated. At this stage, we evaluat-
ed their practicability for synthesis of natural oligo-
saccharides. Thus, the protected a-linked core
1 serine 19,[34,35] the a-linked core 8 serine 20,[34,35] the
d-Gal-a
structural unit in the cell wall of Streptococcus pneu-
moniae 21,[36,37] and the d-Glc-a
A
E
trisaccharide
A
U
d-Glc trisaccharide terminus of N-linked glycan pre-
cursor 28 were selected as synthetic targets because
no one-pot glycosylation methods have been reported
for most of these targets (20, 21, and 28).[35,38]
The core 1 serine conjugate 19 contains a d-Gal-b-
(1!3)-d-GalNAc disaccharide conjugating to a serine
via a cis a-glycosidic bond. This disaccharide conju-
gate is known as T antigen and is found to be over-ex-
pressed in mucosal cells under tumorigenic condi-
tions.[39,40] The chief obstacle of the one-pot synthesis
of 19 is the construction of the cis a-glycosidic bond.
A one-pot synthesis of the protected core 1 serine has
been previously reported by Takahashi, but the ste-
reocontrol achieved was moderate.[41] By using the
Scheme 6. a) Building blocks 1, 2, and 16 for the synthesis
of 19 and 20. b) One-pot synthesis of 19. c) Glycosylation of
2 with 9 using the DMF modulation procedure. d) One-pot
iterative one-pot (b,a)-glycosylation method, we synthesis of 20.
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conditions (Scheme 6, b). Without isolation, the treat- 6B of Streptococcus pneumoniae is composed of a d-
ment of the thiodisaccharide with DMF and NIS/ Gal-a
N
N
TMSOTf promoters produced the putative disacchar- a-linked to d-ribitol.[51] Synthesis of this trisaccharide
ide imidate to react with serine ester 16. This one-pot unit was previously achieved with a laborious step-
reaction furnished the desired core 1-serine conjugate wise glycosylation approach.[52] Use of the present gly-
19 in 42% isolated yield with excellent anomeric cosylation method should make the first one-pot syn-
purity.
Unlike the core 1 serine 19, the core 8 serine 20
thesis of the trisaccharide target possible.
The trisaccharide 21 was prepared from per-O-
contains two cis a-glycosidic bonds. One of these gly- benzyl thiogalactoside 9, 3-hydroxy unprotected thio-
cosidic bonds derives from a galactosyl donor (Gal) glucoside 17, and 3-hydroxy unprotected rhamnoside
and the other from a 2-azido-2-deoxy galactosyl 18 (Scheme 7, a).[53] As was done earlier, the selectivi-
donor (GalN3). Hence, the synthesis of 20 requires an ty of the glycosylation of 17 with 9 was first evaluated
a-glycosylation method effective for Gal and GalN3 under the DMF modulation procedure (Scheme 7, b).
glycosyl donor. In this respect, the DMF-modulated The glycosylation produced the desired a-linked thio-
glycosylation is more appropriate than other meth- disaccharide 21a in 51% isolated yield with an excel-
ods[17e,18,31,43–45] because it can apply to a range of gly- lent a/b ratio of >20:1. After evaluating the selectivi-
cosyl substrates including Gal and GalN3.
ty of the reaction, we attempted the one-pot synthesis
The synthesis of 20 required per-O-benzyl thioga- of 21.
lactoside 9, 3-hydroxy unprotected 2-azido-2-deoxy
Thus, the thiogalactoside 9 was subjected to the
thiogalactoside 2, and serine ester 16 (Scheme 6, a). DMF-modulated glycosylation procedure for reaction
Prior to the one-pot operation, selectivity of the gly- with the thioglucoside acceptor 17, furnishing the de-
cosylation of 9 with 2 was evaluated (Scheme 6, c). sired a-linked thiodisaccharide 21a (Scheme 7, c).
With the DMF-modulated glycosylation procedure, Without isolation, the thiodisaccharide 21a and regen-
the glycosylation furnished the desired thiodisacchar- erated DMF were engaged in the second modulated
ide 20a in a good 64% yield with a high 12:1 a/b- glycosylation cycle to react with the rhamnoside 18,
anomeric ratio. Worthy of note, the a- and b-amoners affording the trisaccharide 21 in 42% isolated yield
of 20a could be separated by standard chromatogra-
phy.
In the one-pot synthesis of 20, the per-O-benzyl thi-
ogalactoside 9 was reacted with the 2-azido-2-deoxy
thiogalactoside 2 under the DMF-modulated glycosy-
lation method to furnish a-linked thiodisaccharide
20a (Scheme 6, d). Without isolation, 20a underwent
the second DMF modulation procedure to couple
with the serine ester 16 affording the desired anomer
of the core 8 serine20. Subsequent analysis by HPLC
revealed that a trace amount of the (a,b)-isomer of 20
in a 16:1 (a,a)/ACHTUNGTRENNUNG(a,b) isomer ratio was present, but
other (b,a) and (b,b)-isomers remained undetecta-
ble.[46] This might be attributed to the relatively small
scale of the reaction (0.33 mmol with respect to 9)
rendering the detection of the isomers difficult. Nev-
ertheless, the desired (a,a)-isomer of 20 was obtained
in a practical 40% yield with high anomeric purity.
The cis (a,a)-configurations of the anomeric centers
in 20 are evidenced by (i) the anomeric carbon signals
at 94.4 (for C-1’) and 100.1 (for C-1) ppm and (ii) the
1
corresponding JC,H coupling constants of 170.0 and
168.0 Hz, respectively.[28]
Streptococcus pneumoniae is a leading cause of
fatal infections such as pneumonia, septicemia, and
meningitis.[47] Streptococcus pneumoniae bacteria are
covered with a polysaccharide capsule; thus, develop-
ing carbohydrate vaccines based on this capsule con-
stitutes a promising therapeutic strategy in the pre-
vention of such infectious diseases.[48–50] The carbohy-
Scheme 7. a) Building blocks 9, 17, and 18 for the synthesis
of trisaccharide 21. b) Glycosylation of 17 with 9 under
DMF modulation conditions. c) One-pot synthesis of trisac-
drate constituent of the capsule in serotypes 6A and charide 21.
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Iterative One-Pot a-Glycosylation Strategy: Application to Oligosaccharide Synthesis
with high anomeric purity. The cis (a,a)-configura-
tions of the anomeric centers in 21 are confirmed by
(i) the anomeric carbon signals at 98.0 (for C-1’’), 94.1
(for C-1’), and 99.4 (for C-1) ppm; and (ii) the corre-
1
sponding JC,H coupling constants of 170, 166, and
164 Hz, respectively.[28]
Tetradecasaccharide Glc3Man9GlcNAc2 is the car-
bohydrate component of N-linked glycan, constituting
a common precursor in the biosynthesis of high man-
nose-, complex-, and hybrid-type N-linked glycans.
This carbohydrate precursor contains a d-Glc-a
2)-d-Glc-a
N
G
Such a terminus is presumably the recognition
domain for oligosaccharyl-transferase (OGT)[54] and is
believed to play some roles in protein folding.[55] Be-
cause of the diverse biological properties, chemical
syntheses of the tetradecasaccharide N-glycan and its
key components have been actively pursued.
The glucosyl units in the Glc3 terminus are joined
by cis a-glycosidic bonds at C-2 and C-3 positions.
Due to the configuration and hindered position of the
constituent glycosidic bonds, synthesis of the Glc3 ter-
minus is formidable.[56] In the past, several synthetic
approaches, including intramolecular aglycone-deliv-
ery glycosylation[57] and high-throughput optimization
of glycosylation, were explored to obtain this target,
but both required tedious stepwise glycosylations.[58]
With the present one-pot a-glycosylation method at
our disposal, we envisioned a straightforward synthet-
ic strategy for the Glc3 terminus.
Synthesis of the protected Glc3 terminus 28 re-
quired per-O-benzyl thioglucoside 22, 2-hydroxy un-
protected thioglucoside 23a, and 3-hydroxy unprotect-
ed thioglucoside 17 (Scheme 8, a). Considering the
hindered positions of the cis a-glycosidic bonds in the
Glc3 target, the selectivity and yield of each DMF-
modulated glycosylation was first optimized before
the one-pot reaction. Thus, thioglucoside 23a was re-
acted with thioglucoside 22 and thioglucoside 17 was
reacted with thioglucoside 23b by means of the DMF-
modulated glycosylation method (Scheme 8, b and c).
In the glycosylation of 23a with 22, 5 equiv. of DMF
(with respect to donor 22) were required to subdue
the thio-transfer reaction.[59] Due to the hindered po-
sitions of the hydroxy functions in 17 and 23a, the
completion of glycosylations took ca. 1.5 to 2 days.
Nevertheless, the desired a-linked thiodisaccharides
26 and 27 were obtained in 60% and 57% yields, re-
spectively, with high anomeric purity. Notably we
were unable to detect or isolate the b-linked anomers
of 26 and 27. Thus, a conservative estimated >20:1 a/
b ratio was given to these products.
Scheme 8. a) Building blocks 22, 23a, 23b, and 17 for the
synthesis of the Glc3 trisaccharide 28. b) Glycosylation of
23a with 22 under DMF modulation conditions. c) Glycosy-
lation of 17 with 23b under DMF modulation conditions. d)
Convergent synthesis of Glc3 trisaccharide 28. e) One-pot
synthesis of Glc3 trisaccharide 28.
tion factor. Fortunately, the glycosylation of 17 with
In addition to the thioglucosyl donors 22 and 23b, 26 furnished the desired trisaccharide 28 in good 61%
we evaluated the glycosylation of thioglucoside 17 isolated yield (Scheme 8, d).
with thiodisaccharide 26 because the behavior of a di-
saccharide donor would be different from that of tion step, the stage was set for the one-pot synthesis
a monosaccharide counterpart due to the conforma- of 28 (Scheme 8, e). The glycosylation of thiogluco-
Having confirmed the efficiency of each glycosyla-
ACHTUNGTRENNUNG
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ent one-pot a-glycosylation methods should prove to
be useful for their synthesis.
Experimental Section
2-O-Benzoyl-3,4,6-tri-O-benzyl-b-d-galactopyranosyl-
(1!3)-2-azido-4,6-O-benzylidene-2-deoxy-a-d-
galactopyranosyl-(1!6)-1,2:3,4-di-O-isopropylidene-
a-d-galactopyranose (4)
2-O-Benzoyl-3,4,6-tri-O-benzyl thio-b-d-galactopyranoside
1 (100 mg, 0.15 mmol), 2-azido-4,6-O-benzylidene-2-deoxy
thio-b-d-galacto-pyranoside
2 (50 mg, 0.125 mmol), and
flame-dried molecular sieve (AW300) were suspended in
dried CH2Cl2 (50 mM with respect to acceptor). The result-
ing mixture was stirred at room temperature for 10 min and
then in a ꢀ658C cooling bath. Subsequently, NIS (34 mg,
0.15 mmol) and TMSOTf (12 mL, 0.0625 mmol) were added.
After 2.5 h, the formation of desired thiodisaccharide was
complete as judged by TLC examination (hexane/EtOAc,
2:1, Rf =0.3). The reaction mixture was then stirred at room
temperature for 10 min and followed by cooling to ꢀ108C
Scheme 9. Global deprotection and acetylation of trisacchar-
ides 15 and 21.
side acceptor 23a with per-O-benzyl thioglucoside
donor 22 under the DMF-modulation procedure
yielded the a-linked thiodisaccharide 26. Without iso-
lation, thiodisaccharide 26 was employed as a donor
for glycosylation of thioglucoside 17 under the DMF-
modulation conditions to afford the target trisacchar-
ide 28 in 35% isolated yield with good anomeric
purity. The cis (a,a)-anomeric configuration of the
anomeric centers in 28 are supported by (i) the
for further 10 min in
a cooling bath. DMF (46 mL,
0.6 mmol), NIS (28 mg, 0.125 mmol) and TMSOTf (29 mL,
0.19 mmol) were added to the mixture and produced the dis-
accharide imidate ester. After 2.5 h of modulation at
ꢀ108C, galactosyl acceptor 3 (39 mg, 0.15 mmol) was added,
anomeric carbon signals at 95.7 (for C-1’’), 96.2 (for and the mixture was stirred at ꢀ108C for addtional 2.5 h.
Upon completion of reaction, saturated NaHCO3 and small
lumps of solid Na2S2O3 were added to the mixture, followed
by vigorous stirring until the color of the reaction mixture
changed from deep red to pale yellow. The resulting mixture
was dried over MgSO4, filtered, and concentrated for flash
chromatography purification over silica gel to furnish de-
sired trisaccharide 4, which was obtained as a pale-yellow
glassy solid after column chromatography purification
(CH2Cl2/hexane/Et2O 20:1.5:1, v/v/v); yield: 58 mg (43%).
For trisaccharide 4: Rf =0.28 (CH2Cl2/hexane/Et2O 20:1:1, v/
C-1’), and 88.5 (for C-1) ppm; and (ii) the correspond-
1
ing JC,H coupling constants of 169, 168, and 153 Hz,
respectively.[28]
After the synthesis of oligosaccharides 4, 8, 11, 15,
19, 20, 21, and 28, protected trisaccharides 15 and 21
were selected for global deprotection. Because of the
simplicity of the protecting group pattern, the depro-
tection of 15 and 21 was simply achieved by a single
Pd(OH)2-catalyzed hydrogenolysis (Scheme 9). For
characterization, unprotected oligosaccharides were v/v); [a]2D8: +20.2 (c 3.00, CHCl3); 1H NMR (500 MHz,
CDCl3): d=8.04 (d, J=8.5 Hz, 2H, ArH), 7.54 (t, J=
7.45 Hz, 1H, ArH), 7.47–7.22 (m, 18H, ArH), 7.21–7.10 (m,
4H, ArH), 5.70 (dd, J=8.0, 23.0 Hz, 1H), 5.50 (d, J=
5.0 Hz, 1H, H-1), 5.45 (s, 1H, benzylidene-H), 5.00 (d, J=
11.5 Hz, 1H), 4.99 (d, J=3.5 Hz, 1H, H-1’), 4.80 (d, J=
8 Hz, 1H, H-1’’), 4.63 (d, J=11.5 Hz, 2H), 4.59, (dd, J=2.5,
7.5 Hz, 1H), 4.49 (d, J=12.5 Hz, 1H), 4.44 (d, J=2 Hz,
2H), 4.37 (d, J=3.3 Hz, 1H), 4.31 (dd, J=2.5, 5.0 Hz, 1H),
4.23 (dd, J=1.9, 8.0 Hz, 1H), 4.15 (d, J=12.4 Hz, 1H), 4.09
(dd, J=3.3, 11.0 Hz, 1H), 3.99 (d, J=5 Hz, 1H), 3.96 (t, J=
acetylated to give per-O-acetyl trisaccharides 29 and
30.
Conclusions
By combining the DMF modulation concept and
neighboring group participation, one-pot (a,a)-,
(b,a)-, and (a,b)-glycoyslation methods were devel-
oped, providing rapid access to oligosaccharides with 8 Hz, 1H), 3.87 (d, J=10.7 Hz, 1H), 3.81–3.77 (m, 1H),
3.70–3.60 (m, 7H), 1.50 (s, 3H), 1.41 (s, 3H), 1.33 (s, 3H),
1.30 (s, 3H); 13C NMR (125 MHz, CDCl3): d=165.8 (C=O),
138.9, 138.3, 138.2, 138.1, 133.3, 130.7, 130.3, 129.03, 128.97,
128.91, 128.86, 128.81, 128.78, 128.76, 128.73, 128.64, 128.49,
128.43, 128.40, 128.38, 128.3, 128.2, 128.1, 126.7, 109.8, 109.0,
103.1 (C-1’’, JC,H =158.8 Hz), 100.9 (benzylidene-C), 99.3 (C-
1’, JC,H =171.3 Hz), 96.7 (C-1, JC,H =177.5 Hz), 80.6, 76.3,
74.92, 74.86, 74.3, 74.0, 73.1, 72.4, 72.1, 71.4, 71.03, 71.00,
69.51, 69.49, 67.6, 67.1, 63.6, 59.2, 26.5, 26.4, 25.43, 24.81;
HR-MS (ESI): m/z=1094.4194, calcd. for C59H65N3NaO16
different permutations of a- and b-anomeric configu-
rations. The utility of these methods was demonstrat-
ed in the synthesis of eight non-natural and natural
oligosaccharide targets, including the protected core
1 serine conjugate 19, the core 8 serine conjugate 20,
the trisaccharide unit of cell wall component in Strep-
tococcus pneumoniae 21, and the glucan trisaccharide
terminus of N-linked glycan precursor 28. Because
natural oligosaccharides contain multiple glycosidic
bonds with diverse anomeric configurations, the pres- [M+Na]+: 1094.4257.
3306
asc.wiley-vch.de
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2012, 354, 3299 – 3310

Iterative One-Pot a-Glycosylation Strategy: Application to Oligosaccharide Synthesis
10 min. NIS (14 mg, 0.33 mmol) and TMSOTf (59.2 mL)
were added. Upon completion of the activation of 9, 2-
azido-2-deoxy thio-b-d-galactopyranoside (100 mg,
Methyl 4,6-O-Benzylidene-2,3-di-O-benzyl-a-d-
galacto pyranosyl-(1!6)-2,3-di-O-benzoyl-4-O-
benzyl-b-d-glucopyranosyl-(1!4)-2,3-O-
isopropylidene-a-l-rhamnopyranoside (8)
2
0.25 mmol) was added to the mixture and stirred at 08C.
After stirring at 08C for ca. 3 h and desired disaccharide 20a
was detected by TLC (Rf 0.28, CH2Cl2/hexane/EtOAc,
1:3:1). Then, the reaction mixture was stirred at room tem-
perature for 10 min (making sure of complete consumption
of the acceptor) and followed by cooling to ꢀ108C . Subse-
quently, NIS (62 mg, 0.275 mmol) and TMSOTf (59.2 mL)
were added to the mixture for activation of 20a. After the
activation was complete (ca. 1 h), serine ester 16 (111 mg,
0.33 mmol) was added to the reaction mixture. The resulting
mixture was stirred at 08C to effect the coupling. Upon
completion of the reaction (ca. 18 h), the general work-up
procedure – as described for compounds 4 and 8 – was fol-
lowed to give the crude reaction product. After concentra-
tion and flash chromatography purification (CH2Cl2/hexane/
EtOAc 1:3:1, v/v/v), the desired core 8 serine conjugate 20
was obtained as a white glassy solid; yield: 115 mg (40%).
For core 8 serine conjugate 20: Rf =00.28 (CH2Cl2/hexane/
EtOAc, 1:3:1); [a]2D8: +80.0 (c 3, CHCl3); 1H NMR
(500 MHz, CDCl3): d=7.76 (d, J=7.6 Hz, 2H, ArH), 7.58
(d, J=7.4 Hz, 2H, ArH), 7.48 (d, J=6.7 Hz, 2H, ArH), 7.39
(t, J=7.5 Hz, 2H, ArH), 7.36–7.07 (m, 25H, ArH), 5.95 (d,
J=8.1 Hz, 1H), 5.39 (s, 1H), 5.26 (d, J=2.9, 1H, H-1’), 4.99
(d, J=3.2, 1H, H-1), 4.93 (d, J=11.4 Hz, 1H), 4.82 (d, J=
11.8 Hz, 1H), 4.70 (d, J=11.8 Hz, 1H), 4.59 (s, 2H), 4.55 (d,
J=11.5 Hz, 1H), 4.53–4.38 (m, 4H), 4.37 (d, J=2.7 Hz,
1H), 4.32 (dd, J=7.4, 10.6 Hz, 1H), 4.21–3.93 (m, 10H),
3.87 (d, J=12.2 Hz, 1H), 3.76 (s, 3H, CH3), 3.61 (s, 1H),
3.59–3.48 (m, 2H); 13C NMR (125 MHz, CDCl3,): d=170.8
(ester C=O), 156.4 (carbamate C=O), 144.20, 144.16, 141.8,
141.7, 139.3, 139.1, 139.0, 138.5, 137.9, 129.5, 128.82, 128.78,
128.70, 128.68, 128.66, 128.61, 128.58, 128.54, 128.26, 128.24,
128.19, 128.14, 128.12, 128.04, 127.98, 127.90, 27.82, 127.62,
127.58, 126.6, 125.6, 125.5, 120.5, 101.3 (benzylidene-C),
100.1 (C-1, JC,H =170 Hz), 94.4 (C-1’, JC,H =167.5 Hz), 79.2,
76.1, 75.5, 75.3, 73.79, 73.77, 72.8, 72.1, 70.74, 70.67, 69.9,
69.7, 69.3, 67.72, 63.67, 59.0, 54.9, 53.3, 47.5; HR-MS (ESI):
4,6-O-Benzylidene-3,6-di-O-benzyl thio-b-d-galactopyrano-
side 5 (83 mg, 0.15 mmol), DMF (12 mL, 0.15 mmol), and
flame-dried molecular sieve (AW300) were suspended in
dried CH2Cl2 (50 mM). The resulting mixture was stirred at
room temperature for 10 min and in a ꢀ108C cooling bath
for further 10 min. Subsequently, NIS (34 mg, 0.15 mmol)
and TMSOTf (29 mL, 0.16 mmol) were added. Upon com-
pletion of the DMF-modulation (ca. 2 h), 2,3-di-O-benzoyl-
4-O-benzyl thio-b-d-glucopyranoside 6 (73 mg, 0.13 mmol)
was added to the mixture and stirring continued at 08C. The
first glycosylation coupling took 3 h at 08C and produced
the desired disaccharide 8a, and the reaction was monitored
by TLC (Rf =0.3, CH2Cl2/hexane/EtOAc 5:3:1). Subsequent-
ly, NIS (28 mg, 0.13 mmol), TMSOTf (29 mL, 0.16 mmol)
and methyl a-l-rhamnopyranoside 7 (35 mg, 0.16 mmol)
were added. The reaction mixture was stirred for 4.0 h at
08C and the progress of the reaction was monitored by TLC
(Rf =0.5, CH2Cl2/hexane/EtOAc 5:3:1). After that, saturated
NaHCO3 and small lumps of solid Na2S2O3 were added to
the mixture, followed by vigorous stirring until the color of
the reaction mixture changed to pale yellow. The resulting
mixture was then dried over MgSO4, filtered, and concen-
trated for flash chromatography purification over silica gel
to furnish desired trisaccharide (CH2Cl2/hexane/EtOAc
5:3:1, v/v/v). The trisaccharide 8 was obtained as a white
amorphous powder; yield: 68 mg (47%) For trisaccharide 8:
Rf =0.3 (CH2Cl2/hexane/EtOAc 5:3:1); [a]2D8: +91.6 (c 3.00,
1
CHCl3); H NMR (500 MHz, CDCl3): d=8.0 (d, J=6.9 Hz,
2H, ArH), 7.85 (d, J=7.0 Hz, 2H, ArH), 7.56–7.03 (m,
26H, ArH), 5.70 (t, J=9.5 Hz, 1H), 5.47 (s, 1H), 5.32 (dd,
J=8.0, 9.8 Hz, 1H), 5.186 (d, J=3.3 Hz, 1H, H-1’’), 5.184
(d, J=8.1 Hz, 1H, H-1’), 4.86–4.75 (m, 5H, including H-1),
4.54 (d, J=11.3 Hz, 1H), 4.47 (d, J=11.4 Hz, 1H), 4.19 (d,
J=10.9 Hz, 1H), 4.14 (d, J=3.6 Hz, 1H), 4.12 (d, J=3.4 Hz,
1H), 4.03–3.83 (m, 7H), 3.69 (board d, J=9.6 Hz, 1H),
3.59–3.49 (m, 3H), 3.29 (s, 3H), 1.52 (s, 3H), 1.26 (s, 6H);
13C NMR (125 MHz, CDCl3): d=166.1 (C=O), 165.8 (C=O),
139.2, 139.1, 138.3, 138.0, 133.5, 133.4, 130.38, 130.35, 130.2,
130.1, 129.9, 129.3, 129.0, 128.76, 128.75, 128.73, 128.68,
128.64, 128.61, 128.59, 128.48, 128.35, 128.30, 128.25, 128.23,
128.19, 128.16, 128.06, 128.00, 126.8, 109.7, 101.5 (benzyli-
+
m/z=1161.4413, calcd. for C66H66N4NaO14 [M+Na]+:
1161.4468.
Methyl 2,3,4,6-tetra-O-benzyl-a-d-galactopyranosyl-
(1!3)-2,4,6-tri-O-benzyl-a-d-glucopyranosyl-(1!3)-
2,4-di-O-benzyl-a-l-rhamnopyranoside (21)
dene-C), 100.8 (C-1’, JC,H =162.5 Hz), 99.0 (C-1’’, JC,H
=
170 Hz), 98.3 (C-1, JC,H =167.5 Hz), 80.6, 78.5, 76.8, 76.4,
76.2, 75.8, 75.7, 75.6, 75.2, 75.0, 73.5, 72.9, 72.5, 69.8, 66.1,
64.1, 63.1, 55.2, 28.5, 26.6, 18.2; HR-MS (ESI): m/z=
1131.4293, calcd. for C64H68NaO17 [M+Na]+: 1131.4349.
Per-O-benzyl
thio-b-d-galactopyranoside
9
(65.0 mg,
0.1 mmol), DMF (31 mL, 0.4 mmol), and activated molecular
sieve (AW300) were suspended in dried CH2Cl2 (2 mL). The
resulting mixture was stirred at room temperature for
10 min and in a ꢀ108C cooling bath for further 10 min. Sub-
sequently, NIS (23 mg, 0.101 mmol) and TMSOTf (20 mL)
were added. Upon completion of pre-activation of 9, 2,4,6-
Methyl N-(Fluorenylmethoxycarbonyl)-[2,3,4,6-tetra-
O-benzyl-a-d-galactopyranosyl-(1!3)-2-azido-4,6-O-
benzylidene-2-deoxy-a-d-galactopyranosyl]-l-serine
Ester (20)
tri-O-benzyl
thio-b-d-glucopyranoside
17
(43.0 mg,
0.077 mmol) was added to the mixture at 08C. After the
coupling reaction was complete (ca. 4.0 h), the desired thio-
disaccharide 21a was detected by TLC (Rf 0.45, EtOAc/
hexane 4:1). Then, the reaction mixture was continuously
stirred at room temperature for 10 min and followed by
cooling to ꢀ108C. Subsequently, NIS (18 mg, 0.079 mmol)
and TMSOTf (23 mL) were added to the mixture for pre-ac-
Per-O-benzyl
thio-b-d-galactopyranoside
9
(210 mg,
0.33 mmol), DMF (78 mL, 1.0 mmol), and activated molecu-
lar sieve (AW300) were suspended in dried CH2Cl2
(6.5 mL). The resulting mixture was stirred at room temper-
ature for 10 min and in a ꢀ108C cooling bath for additional
Adv. Synth. Catal. 2012, 354, 3299 – 3310
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
3307

Chih-Yueh Ivan Liu et al.
FULL PAPERS
tivation of 21a. After completion of the pre-activation (ca.
2 h), methyl a-l-rhamnopyranoside 18 (36.0 mg, 0.1 mmol)
was added to the reaction mixture. The reaction tempera-
ture was raised to ca. 208C to effect the coupling reaction
(note: no coupling occurred when the temperature was
below ca. 108C). Upon completion of the reaction (ca.
18 h), the general work-up procedure – as described for the
one-pot synthesis of 4 and 8 – was followed to obtain the
crude reaction mixture, which was concentrated for flash
chromatography purification (toluene/Et2O, 15:1, v/v) to fur-
nish the desired trisaccharide 21 as a white glassy solid;
yield: 55 mg (42%). For trisaccharide 21: Rf =0.28 (toluene/
Et2O, 15:1); [a]D28: +37.1 (c 3.00, CHCl3); 1H NMR
(500 MHz, CDCl3): d=7.33–7.08 (m, 41H, ArH), 7.05 (t, J=
7.3 Hz, 2H, ArH), 6.95 (dd, J=2.6, 7.4 Hz, 2H, ArH), 5.63
(d, J=3.5 Hz, 1H, H-1’’), 5.22 (d, J=3.5 Hz, 1H, H-1’), 4.87
(d, J=11.6 Hz, 1H), 4.84 (d, J=11.3 Hz, 1H), 4.80 (d, J=
4.5 Hz, 1H), 4.79 (d, J=5.6 Hz, 1H), 4.73–4.43 (m, 12H),
4.35 (d, J=11.6 Hz, 1H), 4.32 (d, J=10.6 Hz, 1H), 4.29 (d,
J=12 Hz, 1H), 4.22 (dd, J=12.2, 15.7 Hz, 2H), 4.07 (dd, J=
2.9, 9.3 Hz, 1H), 4.04 (dd, J=3.5, 10.3 Hz, 1H), 3.97 (dd, J=
2.5, 10.5 Hz, 2H), 3.89 (t, J=2.4 Hz, 1H), 3.86 (t, J=9.9 Hz,
1H), 3.83 (s, 1H), 3.69 (dd, J=3.5, 10.0 Hz, 1H), 3.66–3.62
(m, 1H), 3.57 (t, J=9.5 Hz, 1H), 3.49 (dd, J=2.5, 10.9 Hz,
1H), 3.44 (dd, J=1.7, 10.8 Hz, 1H), 3.40 (d, J=3.3 Hz, 1H),
3.39 (d, J=1.6 Hz, 1H), 3.27 (s, 3H, OCH3), 1.32 (d, J=
6.0 Hz, 3H, CH3); 13C NMR (125 MHz, CDCl3): d 139.2,
139.1, 138.92, 138.90, 138.8, 138.7, 138.44, 138.39, 138.2,
128.82, 128.78, 128.76, 128.73, 128.72, 128.71, 128.69, 128.61,
128.60, 128.56, 128.54, 128.34, 128.16, 128.03, 128.00, 127.98,
127.95, 127.91, 127.89, 127.87, 127.85, 127.80, 127.6, 127.3,
99.4 (C-1, JC,H =164.0 Hz), 98.0 (C-1’’, JC,H =170.0 Hz), 94.1
(C-1’, JC,H =166.0 Hz), 80.2, 79.7, 79.5, 79.1, 76.1, 75.90,
48 h), the general work-up procedure – as described for the
one-pot synthesis of trisaccharides 4 and 8 – was followed to
give the crude reaction mixture, which was concentrated for
flash chromatography purification (CH2Cl2/hexane/EtOAc,
1:10:1, v/v/v) to furnish the desired trisaccharide 28 as
a pale-yellow amorphous solid; yield: 96 mg (35%). For tri-
saccharide 29: Rf =0.45 (hexane/CH2Cl2/Et2O 5:3:1); [a]2D8:
+16.4 (c 3.00, CHCl3); 1H NMR (500 MHz, CDCl3,): d=
7.47 (d, J=7.6 Hz, 2H, ArH), 7.44 (d, J=8.0 Hz, 2H, ArH),
7.30–7.04 (m, 48H, ArH), 7.01 (d, J=8.0 Hz, 2H, ArH),
5.79 (d, J=3.15 Hz, 1H, H-1’’), 5.10 (d, J=3.25 Hz, 1H, H-
1’), 4.93 (d, J=11.5 Hz, 1H), 4.88 (t, J=7.0 Hz, 1H), 4.84–
4.75 (m, 6H), 4.70 (d, J=12.1 Hz, 1H), 4.64 (d, J=11.2 Hz,
1H), 4.61 (d, J=12.2 Hz, 1H), 4.54–4.46 (m, 7H), 4.42 (d,
J=10.4 Hz, 1H), 4.40 (d, J=9.0 Hz, 1H), 4.38 (d, J=
11.8 Hz, 1H), 4.26 (d, J=12.1 Hz, 1H), 4.18 (d, J=6.7 Hz,
1H), 4.14 (d, J=8.8 Hz, 1H), 4.09–4.04 (m, 2H), 3.99 (d, J=
10.0 Hz, 1H), 3.79 (dd, J=10.0, 3.2 Hz, 1H), 3.69–3.64 (m,
4H), 3.60 (dd, J=10.9, 3.3 Hz, 1H), 3.56–3.52 (m, 3H), 3.46
(d, J=9.5 Hz, 1H), 3.44–3.43 (m, 1H), 3.39 (dd, J=9.8,
4.1 Hz, 1H), 3.32 (d, J=10.6 Hz, 1H), 2.28 (s, 1H, CH3);
13C NMR (125 MHz, CDCl3): d=139.3, 139.2, 138.9, 138.8,
138.7, 138.6, 138.54, 138.46, 137.9, 132.6, 130.8, 130.1, 129.3,
128.89, 128.87, 128.77, 128.73, 128.67, 128.61, 128.42, 128.34,
128.30, 128.27, 128.22, 128.19, 128.13, 128.06, 128.01, 127.97,
127.90, 127.87, 127.82, 96.2 (C-1’, JC,H =167.5 Hz), 95.7 (C-
1’’, JC,H =168.8 Hz), 88.5, 82.5, 81.2, 80.7, 79.7, 78.90, 78.85,
78.5, 78.1, 76.2, 75.8, 75.6, 75.5, 75.2, 73.82, 73.77, 73.73, 73.0,
71.03, 70.98, 69.6, 69.0, 68.5, 30.2, 21.6; HR-MS (ESI): m/z=
1533.6523, calcd. for C95H98NaO15S+ [M+Na]+: 1533.6519.
Methyl 2,3,4,6-Tetra-O-acetyl-a-d-galactopyranosyl-
75.87, 75.6, 75.5, 75.3, 74.9, 74.3, 73.8, 73.7, 73.62, 73.56, 73.4, (1!3)-2,4,6-tri-O-acetyl-a-d-glucopyranosyl-(1!3)-
72.9, 70.7, 69.1, 69.0, 68.6, 68.5, 55.1 (OCH3), 18.4 (CH3);
2,4-di-O-acetyl-a-l-rhamnopyranoside (30)
+
HRMS (ESI): m/z=1335.5943, calcd. for C82H88NaO15
[M+Na]+: 1335.6015.
Trisaccharide 21 (200 mg) was dissolved in a solution of 5/1/
1/3 v/v THF/MeOH/H2O/acetic acid (2 mL). To the reaction
mixture, Pd(OH)2/C (ca. 200 mg) was added under N2. Then
the mixture was purged with H2 at 1 atm for 5 min (H2 was
kept in a balloon and bubbling through the mixture via
a 20-G needle). The above reaction mixture was stirred at
room temperature for overnight under H2 at 1 atm and the
progress of the hydrogenolysis was monitored by TLC (Rf =
0.5, CH2Cl2/MeOH, 2:1). Upon completion of the hydroge-
nolysis, the reaction mixture was filtered over celite and
concentrated at <408C on a rotary evaporator to give the
crude unprotected trisaccharide. The crude unprotected tri-
saccharide obtained from preceding step was dissolved in
pyridine (2 mL); to which, acetic anhydride (Ac2O) (1 mL)
was added and the reaction mixture was stirred from 08C to
rookm termperature for ca. 4–6 h. Progress of the reaction
was monitored by TLC. Upon completion of the acetylation,
the reaction mixture was cooled to 08C, followed by addi-
tion of water (1 mL) and stirred for 10 min. Then 5% aque-
ous HCl (3 mL) was added to the mixture, which was stirred
for further 15 min. Then reaction mixture was then diluted
with EtOAc (10 mL) and water (10 mL). The organic phase
was separated. The aqueous phase underwent back extrac-
tion with EtOAc (10 mLꢁ2). All organic phases were
pooled and washed with 5% aqueous HCl to remove the
pyridine and subsequently washed with water. The EtOAc
solution was dried (over MgSO4) and concentrated for flash
p-Tolyl 2,3,4,6-Tetra-O-benzyl-a-d-glucopyranosyl-
(1!2)-3,4,6-tri-O-benzyl-a-d-glucopyranosyl-(1!3)-
2,4,6-tri-O-benzyl-thio-b-d-glucopyranoside (28)
Per-O-benzyl thio-b-d-glucopyranoside 22 (139.0 mg,
0.22 mmol), DMF (85 mL, 1.08 mmol), and flame-dried mo-
lecular sieve (AW300) were suspended in dried CH2Cl2
(3.6 mL). The resulting mixture was stirred at room temper-
ature for 10 min and in a ꢀ108C cooling bath for further
10 min. Subsequently, NIS (49 mg, 0.22 mmol) and TMSOTf
(49 mL, 0.27 mmol) were added to the mixture. Upon com-
pletion of pre-activation of 22 (ca. 2 h), 3,4,6-tri-O-benzyl
thio-b-d-glucopyranoside 23a (100 mg, 0.18 mmol) was
added and the resulting mixture was stirred at ꢀ108C. After
completion of the coupling reaction (ca. 38 to 40 h), thiodi-
saccharide 26 was detected by TLC (Rf =0.4, hex-
ane/CH2Cl2/Et2O, 10:6:1, developed twice). Subsequently,
NIS (40 mg, 0.18 mmol) and TMSOTf (55 mL) were added
to the reaction mixture for pre-activation of 26 forming pu-
tative disaccharide imidate. After the activation was com-
plete (ca. 4 h), 2,4,6-tri-O-benzyl thio-b-d-glucopyranoside
17 (100.0 mg, 0.18 mmol) was added to the reaction mixture.
The reaction temperature was raised to ꢀ58C to effect the
coupling reaction. Upon completion of reaction (ca. 36–
3308
asc.wiley-vch.de
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2012, 354, 3299 – 3310

Iterative One-Pot a-Glycosylation Strategy: Application to Oligosaccharide Synthesis
chromatography purification over silica gel to afford the de-
sired per-O-acetyl trisaccharide 30 as a white foamy sub-
stance by column chromatography (hexane/EtOAc, 1:1, v/
v); yield: 76 mg (60% over two steps). For trisaccharide 30:
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1
Rf =0.35 (hexane/EtOAc, 1:1); H NMR (500 MHz, CDCl3):
d=5.40 (d, J=2.4 Hz, 1H), 5.28 (d, J=3.6 Hz, 1H, H-1’’),
5.23 (m, 1H), 5.21 (dd, J=3.0, 11.2 Hz, 1H), 5.15 (d, J=
3.15 Hz, 1H, H-1’), 5.13 (d, J=10.0 Hz, 1H), 5.10–5.02 (m,
2H), 4.85 (dd, J=3.4, 10.1 Hz, 1H), 4.61 (d, J=1.6 Hz, 1H,
H-1), 4.39 (t, J=7 Hz, 1H), 4.20 (dd, J=7.5, 11.1 Hz, 1H),
4.11 (d, J=3.4 Hz, 2H), 4.10–4.07 (m, 1H), 4.06 (t, J=
3.1 Hz, 1H), 4.02 (dd, J=3.3, 9.9 Hz, 1H), 3.90 (dt, J=3.4,
10.3 Hz, 1H), 3.76–3.69 (m,1H), 3.36 (s, 3H, OCH3), 2.16 (s,
3H, CH3C=O), 2.12 (s, 3H, CH3C=O), 2.11 (s, 3H, CH3C=
O), 2.10 (s, 3H, CH3C=O), 2.09 (s, 3H, CH3C=O), 2.08 (s,
3H, CH3C=O), 2.060 (s, 3H, CH3C=O), 2.056 (s, 3H,
CH3C=O), 1.97 (s, 3H, CH3C=O), 1.21 (d, J=6.3 Hz, 3H,
CH3); 13C NMR (125 MHz, CDCl3): d=171.1 (C=O), 171.0
(C=O), 170.7 (C=O), 170.5 (C=O), 170.3 (C=O), 170.13 (C=
O), 170.08 (C=O), 169.98 (C=O), 169.7 (C=O), 98.9 (C-1,
J
J
C,H =170.0 Hz), 96.5 (C-1’’,
C,H =172.5 Hz), 73.6, 72.6, 72.4, 72.0, 70.3, 68.5, 68.4, 68.3,
JC,H =173.8 Hz), 93.9 (C-1’,
68.1, 67.2, 67.1, 66.9, 62.0, 61.2, 55.5 (OCH3), 21.3 (CH3C=
Oꢁ2), 21.2 (CH3C=O), 21.12 (CH3C=O), 21.07 (CH3C=O),
21.00 (CH3C=Oꢁ2), 20.99 (CH3C=Oꢁ2), 17.9 (CH3); HR-
MS (ESI): m/z=903.2819, calcd. for C37H52NaO24 [M+Na]+:
903.2741.
[18] T. Shirahata, A. Kojima, S. Teruya, J. Matsuo, M. Yo-
Acknowledgements
koyama, S. Unagiike, T. Sunazuka, K. Makino, E. Kaji,
¯
S. Omura, Y. Kobayashi, Tetrahedron 2011, 67, 6482–
Mr. Mulani contributed significantly in the one-pot syntheses
of trisaccharides 20 and 28. We thank Mr. Shao-Yu Ryan Lu
and Dr. Chin-Sheng Chao for initial studies of this work.
Thanks are given to the National Science Council of Taiwan
(NSC 99-2113M-009-009) and EEC of NCTU for financial
support.
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