Automated solid-phase synthesis of protected oligosaccharides containing β-mannosidic linkages

Article abstract of DOI:10.1002/chem.200701864

For automated oligosaccharide synthesis to impact glycobiology, synthetic access to most carbohydrates has to become efficient and routine. Methods to install "difficult" glycosidic linkages have to be established and incorporated into the overall synthet

Full text of DOI:10.1002/chem.200701864

FULL PAPER
DOI: 10.1002/chem.200701864
Automated Solid-Phase Synthesis ofProtected Oligosaccharides Containing
b-Mannosidic Linkages
Jeroen D. C. CodØe,[b] Lenz Krçck,[a] Bastien Castagner,[a] and Peter H. Seeberger*[a]
Abstract: For automated oligosaccha-
ride synthesis to impact glycobiology,
fective b-mannosylation agents and re-
sulted in excellent conversion and good
to moderate selectivities. [(Triisopro-
position of mannose. The desired oligo-
saccharide products were readily sepa-
rated from by-products containing un-
wanted stereoisomers using reverse-
phase HPLC. The methods described
here expand the scope of carbohy-
drates currently accessible by automa-
tion as many oligosaccharides of bio-
logical interest contain b-mannosidic
linkages.
ACHTREUNG
synthetic access to most carbohydrates
has to become efficient and routine.
Methods to install “difficult” glycosidic
linkages have to be established and in-
corporated into the overall synthetic
concept. Described here is the first au-
tomated solid-phase synthesis of oligo-
saccharides containing the challenging
b-mannosidic linkage. Carboxybenzyl
mannoside building blocks proved ef-
pylsilyl)oxy]-methyl
ether
(Tom),
served as an orthogonal, minimally in-
trusive, and readily cleavable protect-
ing group for the elongation of the C3
Keywords: automation · carbohy-
drates · glycosides · oligosaccharide
synthesis · solid-phase synthesis
Introduction
encountered in biologically important carbohydrate motifs
such as N-glycans.
Well-defined oligosaccharides are indispensable tools for
glycobiology. However, the synthesis of complex carbohy-
drates still presents a significant challenge.[1] Automated
solid-phase oligosaccharide synthesis[2] has the potential to
greatly accelerate the routine preparation of carbohydrates.
Although a variety of oligosaccharides have been synthe-
sized in an automated manner,[3–6] some glycosidic linkages
have not been accessed yet using this solid-phase strategy.
Methods for the stereoselective construction of all types of
glycosidic linkages within the automation framework are
crucial to developing a general method for the rapid synthe-
sis of carbohydrates. b-Mannosidic linkages are frequently
Tremendous progress concerning the solution-phase syn-
thesis of b-mannosides has been made.[7] After many “indi-
rect” methods such intramolecular aglycon delivery[8] and
tethering[9] had been developed to overcome the challenges
associated with b-mannoside formation, a more versatile,
“direct” method was introduced by Crich and co-workers.
This approach relies on the use of 4,6-O-benzylidene pro-
tected mannosyl sulfoxides or thiomannosides.[10] This modi-
fication of the sulfoxide method[11] utilizes a mannosyl build-
ing block (A, Scheme 1) that is pre-activated by trifluorome-
thanesulfonic anhydride (Tf2O) to provide the a-mannosyl
triflate E.[11] The incoming nucleophile displaces the anome-
ric triflate in an SN2-like manner to provide the b-mannosi-
dic product (b-F). Crich et al. observed that the a-mannosi-
dic product was preferentially formed when the building
block and nucleophile were mixed prior to activation, so
pre-activation of the building block was found to be neces-
sary for b-selectivity.
[a] L. Krçck, Dr. B. Castagner, Prof. Dr. P. H. Seeberger
Laboratory for Organic Chemistry
Swiss Federal Institute of Technology (ETH) Zürich
Wolfgang-Pauli-Strasse 10, 8093 Zürich (Switzerland)
Fax : (+41)44-633-1235
Subsequent to this breakthrough, modifications of this
general approach with different types of anomeric leaving
group have been introduced, including carboxybenzyl (CB)
mannosides,[12] mannosyl pentenoates,[13] phosphites[14] and
trichloroacetimidates.[15] In contrast to the Crich system, no
adverse effect on the diastereoselectivity were seen when
these building blocks were premixed prior to activation.
E-mail: seeberger@org.chem.ethz.ch
[b] Dr. J. D. C. CodØe
current address: Leiden Institute of Chemistry
Leiden University, PO Box 9502
2300 RA Leiden (The Netherlands)
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
Chem. Eur. J. 2008, 14, 3987 – 3994
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3987

known participating solvent,[18] could play a decisive role in
these condensations leading to the undesired formation of
the a-product via b-etherate C (Scheme 1). Therefore, we
changed the solvent to dichloromethane for our “non pre-
activation” process. Indeed, when mannosyl sulfoxide 1 was
dissolved in dichloromethane together with nucleophiles
such as 2, 4, 6 and 9, cooled to ꢀ788C and then treated with
0.5 equivalents of Tf2O,[11,19a] the b-linked products were
formed in moderate yield, but excellent diastereoselectivity
(Table 1). However, several side products were observed.
Table 1. b-Mannosylations using sulfoxide 1 in CH2Cl2 by a “non pre-ac-
tivation” protocol.
Scheme 1. Solvent effects in b-mannosylations following a “pre-activa-
tion” and “non pre-activation” protocol.
Two approaches to the solid-phase synthesis of b-manno-
sides have been reported thus far.[17] These methods make
the construction of larger oligosaccharides on the solid sup-
port difficult because they either use the donor-bound ap-
proach or result in the cleavage of the b-mannoside from
the resin during coupling. Here, we report the first automat-
ed solid-phase assembly of oligosaccharides containing the
challenging b-mannosidic linkage via the advantageous ac-
ceptor bound approach.
Entry
(a/b)
Nucleophile
Product(s)
Yield [%][a]
Selectivity
AHCTREUNG
1
2
3
4
2
4
6
9
3
5
58
63
50 (31)
50 (30)
b
b
7 (8)
10 (11)
1:13[b]
b
[a] Isolated product. [b] Based on LCMS analysis of the crude reaction
mixture.
Results and Discussion
An ideal building block for automated b-mannosylation
should be suitable for large-scale preparation, shelf-stable
and allow for elongation to larger oligosaccharides. For ease
of automation, no pre-activation of the building block
should be necessary, its activation should be mild and
should not generate electrophiles that are incompatible with
our linker olefin.
The Crich method would be ideally suited for automation
for a number of reasons. First, the method accommodates a
broad range of nucleophiles, and 4,6-O-benzylidene manno-
syl sulfoxides are readily available. Second, the activation
conditions are very mild, even when used in large excess as
it is often the case under the solid-phase paradigm. Howev-
er, mannoside pre-activation at low temperature (typically
ꢀ78 to ꢀ608C) presents a significant technical challenge in
an automated setting. Furthermore, manipulation of the in-
termediate triflate is undesirable as it is both thermally and
hydrolytically labile. Inspired by recent examples demon-
strating the use of different types of 4,6-O-benzylidene man-
nosyl building blocks[12–15] without pre-activation, we rein-
vestigated the mannosyl sulfoxides in a “non pre-activation”
protocol.
Importantly, acceptor-derived phenyl sulfinic ester 8 and 11
were formed in substantial amounts during couplings involv-
ing glucose 6 and glucuronic acid 9. The generation of ac-
ceptor-derived side products is highly undesirable on resin
as it directly reduces the overall yield. Furthermore, activa-
tion of phenyl sulfoxides generates the very electrophilic
phenylsulfenyl triflate (PhSOTf, G, Scheme 1) that can react
with the octenediol linker system commonly employed for
solid-phase assembly. Thus, we abandoned the sulfoxides as
possible b-mannosylating agents for automated solid-phase
synthesis and shifted our attention to the investigation of
carboxybenzyl glycosides.
The carboxybenzyl glycosides (e.g. 12) were introduced as
mannosylating agents[12,16] by the Kim laboratory and can be
activated using Tf2O in combination with a non-nucleophilic
base such as di-tert-butylmethylpyridine (DTBMP) to access
anomeric a-triflates E (Scheme 1), with phthalide as an
inert by-product.
b-Mannosylations using carboxybenzyl glycoside 12 and
the nucleophiles 4, 6 and 13 were performed in solution to
adjust the reaction parameters to suit an automated solid-
phase protocol (Table 2). Increase of the reaction tempera-
ture from ꢀ60 to ꢀ308C and shortening the reaction time to
2h did not influence the selectivity of the coupling (Table 2,
The pre-activation protocol was discovered in the Crich
laboratory during experiments that employed diethyl ether
as the solvent.[10] We reasoned that diethyl ether, a well-
entries 2and 3). Changing the solvent from CH Cl2 to tolu-
2
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Chem. Eur. J. 2008, 14, 3987 – 3994

Automated Oligosaccharide Synthesis
FULL PAPER
Table 2. b-Mannosylations using carboxybenzyl mannoside 12.
reactions were conducted using
two coupling cycles of five
equivalents each of carboxyl
mannoside and Tf2O, and 15
equivalents of DTBMP, that
were added to the cooled reac-
Entry
Conditions
Nucleophile
Product(s)
Yield [%][a]
Selectivity (a/b) tion chamber as a solution in
CH2Cl2.[20] The carboxybenzyl
1
CH2Cl2, ꢀ608C to RT, overnight
2Cl2, ꢀ608C to RT, overnight
CH2Cl2, ꢀ308C (2 h)
4
6
6
6
5
7
7
7
81
85
–
–
b[b]
2CH
1:10[c]
building blocks 12 performed
well on the automated synthe-
sizer. After one coupling cycle,
approximately 80% conversion
was achieved. Repeating this
coupling cycle led to near quan-
titative conversion, as judged
3
4
5
1:10[c]
toluene, ꢀ308C (2 h)
1:11[c]
[c]
CH2Cl2, ꢀ608C to RT, overnight
13
14
821:4.5
[a] Isolated product. [b] Based on NMR of isolated product. [c] Based on LCMS analysis of the crude reaction
mixture.
ene, a mimic for the polystyrene resin environment during
solid-phase assembly (Entry 4), did not affect the diastereo-
selectivity, but resulted in increased self-condensation of the
carboxylic acid 12.[16]
Encouraged by these glycosylations, we adopted the
aforementioned conditions to the automated solid-phase
synthesis of a series of di- and trisaccharides (Table 3). The
by LCMS analysis of the crude reaction mixture after cleav-
age from the resin.
Diastereomeric ratios were usually better than 3:1 in
favor of the desired b-anomer when secondary nucleophiles
were employed. Stereoselectivity as high as 9:1 in the case
of the primary C6 position of glucosamine was observed
(Table 3, entry 4). In contrast to solution phase syntheses,
the stereoselectivity eroded when toluene was used as a sol-
vent (Table 3, entry 2). The glycosylation selectivities of
building block 12 are lower on solid-support than in solu-
tion, but since all mixtures could be separated by column
chromatography or preparative HPLC, access to pure carbo-
hydrates in good overall yields was possible. A representa-
tive coupling cycle for the automated b-mannosylation of
compound 19 is summarized in Table 4.
Table 3. Automated b-mannosylations using carboxybenzyl mannoside
12.
Table 4. Coupling cycle for glycosylation of compound 19.
Step Function
Reagents
t
[min]
1
wash
THF
2Cl2
5
10
120
2wash
3
CH
glycosylation 5 equiv building block 12, 5 equiv Tf2O,
15 equiv DTBMP, CH2Cl2, ꢀ308C
4
5
wash
CH2Cl2, ꢀ308C
10
120
glycosylation 5 equiv building block 12, 5 equiv Tf2O,
15 equiv DTBMP, CH2Cl2, ꢀ308C
6
7
8
9
10
11
wash
wash
wash
wash
wash
wash
CH2Cl2, ꢀ308C
5
2
5
18
5
20% piperidine, DMF
CH2Cl2
alternating CH2Cl2 and methanol
CH2Cl2
THF
5
Entry
Nucleophile
Cleaved
Yield [%]
Selectivity (a/b)[b]
product[a]
1
2
3
4
15
15
17
19
16
16
18
20
77[c]
–
1:3.5
1:2.5[d]
1:3.5
1:9
After conditions to install b-mannosides on solid support
had been established, the synthesis of oligosaccharides by
elongation at the C3 hydroxyl was explored.[21] The choice
of protecting groups for the C3 position of b-mannosylation
agents is limited. Acyl groups have been shown to efficiently
participate in the condensation reaction of the benzylidene
mannoside building blocks to provide the undesired a-
linked saccharides,[22,10e] and allyl, p-methoxybenzyl or naph-
thylmethyl ethers require deprotection conditions that are
–
47[c]
AHCTREUNG
5
21
22
1:3.7
AHCTREUNG
[a] Products were cleaved from the resin by transesterification (NaOMe
in CH2Cl2/MeOH) or cross-metathesis (the Grubbs 1st generation cata-
lyst, CH2=CH2). [b] Based on LCMS analysis of the crude reaction mix-
ture. [c] Based on a/b mixture. [d] Toluene was used as
[e] Based on pure b anomer.
a solvent.
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3989

P. H. Seeberger et al.
Table 5. Condensations of C3-O-Tom protected mannosides.
incompatible with the solid-phase format. Silyl ethers are at-
tractive protecting groups as they are orthogonal to most
other protecting groups employed during oligosaccharide as-
sembly and are readily cleaved. However, silyl ethers are
sterically demanding and can erode the selectivity of the b-
mannosylation reactions.[23,24] To retain the attractive fea-
tures of silyl ethers, but reduce the steric bulk, we employed
the [(triisopropylsilyl)oxy]methyl (Tom) ether that is rou-
tinely used for RNA synthesis.[25]
Various building blocks were prepared to evaluate the
C3-O-Tom protecting group in solution phase glycosylations
(Scheme 2). The regioselective alkylation of the 2,3-O-stan-
nylidene of diol 23 with triisopropylsilyloxymethyl chloride
(Tom-Cl) afforded Tom-protected mannose 24. Benzylation,
and subsequent allyl cleavage gave mannose 27. Placement
of the Tom group on carboxybenzyl mannoside 28 proceed-
ed uneventfully[26] before benzylation of the C2-hydroxyl
and generation of the carboxylic acid furnished mannosyl
building block 31.
Entry
Building
block[a]
Nucleophile
Product
Yield [%]
Selectivity
(a/b)
ACHTREUNG
1
2
3
4
5
6
7
32
33
27
31
31
31
31
4
4
4
4
9
37
39
5
81[b]
92[b]
90
81
73
(b)
2:3
(b)
(b)
34
35
35
36
38
40
1:15[c]
1:9[d]
1:7[c]
94
85
[a] Coupling conditions for dehydrative condensations: 1-OH mannosides
(1.5 equiv) were pre-activated using Ph2SO (3 equiv) and Tf2O
(1.5 equiv) in the presence of TTBP (5 equiv) at ꢀ408C to ꢀ258C over
1.5 h. The nucleophile was added and the mixture stirred overnight while
warming to room temperature before the reaction was quenched by the
addition of triethylamine. Condensation of the CB-mannosides were ac-
complished with the described “non pre-activation” protocol. [b] Taken
from ref. [23b]. [c] Determined by LCMS analysis. [d] Determined by
NMR analysis.
ty and yield. Investigation of the fluoride mediated depro-
tection of the Tom-ketal on allyl mannoside 25 (Scheme 2)
revealed that 1.5 equivalents of tetrabutylammonium fluo-
ride in wet THF sufficed to cleave the Tom group within
five minutes in solution. Cleavage under buffered condi-
tions, in the presence of two equivalents acetic acid, re-
quired 3.5 h. The Tom protecting group thus proved a good
alternative as C3 protecting group in b-mannosylations,
giving excellent selectivities and deprotection conditions or-
thogonal to typical carbohydrate synthesis.
With this information in hand, CB-mannoside building
block 31 was used for the automated synthesis of a series of
b-mannosides (Table 6). We were gratified to find that the
selectivities of couplings involving building block 31 general-
ly increased compared to the 3-O-benzyl mannoside 12
counterpart. Coupling of 31 and nucleophile 17 gave prod-
uct in 1:6.5 a/b selectivity (Table 6, entry 1) compared to
1:3.5 obtained with building block 12 (Table 3, entry 3). The
selectivity for nucleophile 21 increased from 1:3.7 (Table 3,
entry 5) to 1:6.5 (Table 6, entry 3). Tom-cleavage at the tri-
saccharide stage of the automated synthesis using five equiv-
alents of TBAF in THF required 15 min and was repeated
once.[27] Trisaccharide 45 was isolated in 41% yield over
Scheme 2. Synthesis of the C3-O-Tom protected mannosyl building
blocks 27 and 31. a) Bu2SnCl2, DIPEA, ClCH2CH2Cl, then Tom-Cl, 24:
91%, 29: 85%; b) BnBr, NaH, DMF, 25: quant., 30: 96%; c) PdCl2,
HOAc, H2O, NaOAc, EtOAc, 74%; d) H2, Pd/C, EtOAc, MeOH, 96%;
e) TBAF, THF (94%) or TBAF, HOAc, THF (88%).
The 3-O-Tom-protected mannose building block per-
formed well in the construction of b-mannosidic linkages in
solution (Table 5). The Tom protecting group is compared
to other C3 protecting groups. (Table 5, entries 1–3). The
previously published 3-O-tert-butyldimethylsilyl ether build-
ing block 33 caused complete loss of selectivity when com-
pared to the 3-O-benzyl ether in 32.[23b] However, when the
oxymethyl bridged silyl ether building block 31 was used,
the desired b-linked product was formed with excellent se-
lectivity (Table 5, entry 3). The 3-O-Tom protected carboxy-
benzyl mannoside 31 gave good results with other nucleo-
philes (Table 5, entries 4–6) using the “non-pre-activation”
protocol. The condensation with glucosamine-derived nucle-
ophile 39 (Table 5, entry 7) also proceeded in good selectivi-
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Automated Oligosaccharide Synthesis
FULL PAPER
Table 6. Automated solid-phase b-mannosylations using C3-O-Tom pro-
tected carboxybenzyl mannoside 31.
Scheme 3. Synthesis of trimannoside 52 in solution. a) Tf2O, DTBMP,
CH2Cl2, ꢀ308C, 94% (a/b 1:9); b) TBAF, THF, 79%; c) Tf2O, DTBMP,
CH2Cl2, ꢀ308C, 74% (a/b 1:15).
Entry
Nucleophile
Cleaved
Yield [%]
Selectivity
product[a]
ACHTREUNG
1
2
3
17
19
21
42
44
45
51[c] (4 steps)
58[e] (3 steps)
41[d] (7 steps)
ACHTREUNG
AHCTREUNG
AHCTREUNG
[a] Products were cleaved from the resin by cross-metathesis (the Grubbs
1st generation catalyst, CH2=CH2). [b] Based on LCMS analysis of the
crude reaction mixture. [c] After TBAF-mediated deprotection of the
Tom group. [d] Based on pure b anomer. [e] Based on a/b mixture.
[f] Determined by LCMS analysis of the crude Tom-deprotected disac-
charide.
Scheme 4. Synthesis of trimannoside 52 on solid support. a) 47
(4.5 equiv), TMSOTf (0.5 equiv), toluene, CH2Cl2, 15 min, repeated once;
b) NaOMe (10 equiv), MeOH, CH2Cl2, 0 min, repeated once; c) building
block (5 equiv), Tf2O (5 equiv), DTBMP (15 equiv), CH2Cl2, ꢀ308C, 2h,
repeated once; d) TBAF (5 equiv), THF, 20 min, repeated once; e) the
Grubbs 1st generation catalyst, ethylene atmosphere, CH2Cl2, overnight,
50% from resin 46 (of a mixture of anomers (8:1.3:1 in favor of the de-
sired product 52).
seven steps. For couplings involving glucosamine nucleo-
phile 19 the selectivity decreased from 1:9 to 1:5.
To demonstrate that multiple b-mannosidic linkages can
be installed in the same molecule, trisaccharide 52 was as-
sembled in solution (Scheme 3) and by automated solid-
phase synthesis (Scheme 4). Using the automated synthesiz-
er, octenediol-functionalized resin 46 was treated with man-
nosyl trichloroacetimidate 47 followed by sodium methoxide
mediated acetate removal. Coupling of Tom-protected CB-
mannoside building block 31 was followed by cleavage of
the Tom group by subjecting the resin manually to TBAF
(2”5 equiv, 15 min each) in THF. Final mannosylation using
dibenzyl mannoside building block 12 by automation gave
the immobilized trisaccharide. Cleavage of the oligosaccha-
steps). Pure trisaccharide 52, containing the two desired b-
mannosidic linkages was isolated using preparative HPLC.
Conclusion
We report the development of synthetic protocols for the in-
corporation of b-mannosidic linkages by automated solid-
phase synthesis using stable CB glycosides. The formation of
the challenging b-mannosidic bond was achieved for the
first time by automated solid-phase synthesis. The Tom
ether group served as an efficient protecting group to mask
the C3 position of mannose, allowing for stereoselective
coupling and mild, orthogonal deprotection. The Tom group
should prove valuable as a sterically minimally intrusive
protecting group in oligosaccharide synthesis. The scope of
oligosaccharide structures accessible by automated solid-
phase synthesis using this b-
A
ieved using the Grubbs first generation pre-catalyst under
an ethylene atmosphere. Trimannoside 52 was isolated in
50% yield (over six steps) as a mixture of anomers with the
desired anomer as the major product (8:1:1.3 in favor of the
desired product (Figure 1)). This result compares favorably
to the solution-phase synthesis (37!52, 55% over three
mannosylation technique has
been increased significantly.
Currently, we are applying this
methodology to prepare
series of N-linked oligosaccha-
rides.
a
Figure 1. Crude HPLC trace. Product 52 (tR =8.4 min) and its anomers (tR =8.8 and 9.1 min).
ACHTREUNG
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P. H. Seeberger et al.
was diluted with CH2Cl2 (150 mL) and washed with saturated aqueous
NaHCO3 (100 mL). The water layer was extracted three times with
CH2Cl2, after which the organic layers were combined, filtered over
Celite, dried (MgSO4) and concentrated. The crude product was purified
by flash silica gel column chromatography (cyclohexane/EtOAc 2:1) to
give the pure title compound as a yellow oil (34.51 g, 9.1 mmol, 91%).
Experimental Section
All chemicals used were reagent grade and used as supplied. Tf2O was
purified by drying over P2O5 for 4 h, followed by distillation. All reac-
tions were performed in oven-dried glassware under an inert argon at-
mosphere unless noted otherwise. Reagent grade dichloromethane
(CH2Cl2), was passed through activated neutral alumina column prior to
use. Triethylamine was distilled over CaH2 and stored over KOH. Analyt-
ical thin-layer chromatography (TLC) was performed on Merck silica gel
60 F254 plates (0.25 mm). Compounds were visualized by UV irradiation
or dipping the plate in a cerium sulfate/ammonium molybdate solution.
Flash column chromatography was carried out using forced flow of the
indicated solvent on Fluka Kieselgel 60 (230–400 mesh). LCMS analysis
was performed on an Agilent 1100 Series LC/MSD instrument on a
Waters Symmetry C18 5 mm column (3.9”150 mm), using solvent systems
A (20% isopropanol and 0.1% TFA in H2O) and B (20% isopropanol
and 0.1% TFA in acetonitrile), at a flow rate of 1 mLminꢀ1.1H and 13C
and NMR spectra were recorded on a Varian Mercury 300 (300 and
75 MHz, respectively), spectrometer in CDCl3 unless specified otherwise,
with chemical shifts referenced to internal standards CDCl3 (7.26 ppm
1H, 77.0 ppm 13C). High-resolution mass spectral (HRMS) analyses were
performed by the MS-service at the Laboratory for Organic Chemistry at
ETH Zurich. ESI-MS and MALDI-MS were run on an IonSpec Ultra in-
strument. The automated synthesis was performed on an ABI 431 A pep-
tide synthesizer with a custom-made jacketed glass reaction vessel. IR
spectra were recorded on a Perkin–Elmer 1600 FTIR spectrometer. Opti-
cal rotations were measured at room temperature using a Perkin Elmer
241 polarimeter. Recycling HPLC system (GPC: LC-9101 Japan Analyti-
cal Industry Co. Ltd., JAIGEL-2H and 2.5H, mobile phase: CHCl3) was
used for size exclusion chromatography. Preparative HPLC was per-
formed using a Waters 1525 pump and Waters 2487 detector on a Waters
Sunfire prep C8 reversed-phase column (10”150 mm). The stereochemis-
try of b-mannosides was confirmed by the characteristic chemical shift of
H5.[10c]
1
Rf =0.60 (hexanes/EtOAc 2:1); [a]2D0 =+74.1 (c=1.0 in CHCl3); H NMR
(300 MHz, CDCl3): d=7.59–7.34 (m, 5H), 5.98–5.84 (m, 1H), 5.57 (s,
1H), 5.36–5.19 (m, 2H), 5.17 (d, 1H, J=4.8 Hz), 5.05 (d, 1H, J=5.1 Hz),
4.94 (d, 1H, J=1.2 Hz), 4.29–3.81 (m, 8H), 2.97 (d, 1H, J=1.5 Hz),
1.15 ppm (m, 21H); 13C NMR (75 MHz, CDCl3): d=138.3, 137.6, 133.5,
128.8, 128.3, 128.0, 127.8, 127.6, 126.1, 116.9, 101.6, 98.7, 90.1, 78.8, 78.2,
73.8, 73.3, 68.8, 67.9, 64.3, 17.9, 12.0 ppm; IR: n˜ =2944, 1464, 1044 cmꢀ1
HR-MS: m/z: calcd for C26H42O7Si+Na+: 517.2598; found: 517.2583.
;
Allyl 2-O-benzyl-4,6-O-benzylidene-3-O-(triisopropylsilyloxymethyl)-a-
d-mannopyranoside (25): Allyl mannoside 24 (3.42g, 6.90 mmol) and
benzyl bromide (1.07 mL, 8.97 mmol) were dissolved in DMF (30 mL)
and cooled to 08C. Sodium hydride (317 mg, 7.94 mmol, 60% in mineral
oil) was added in small portions. The mixture was allowed to reach room
temperature and stirred for 1 h. Methanol (0.5 mL) was added to quench
the reaction, after which the mixture was diluted with diethyl ether and
washed with water. The aqueous phase was extracted three times with di-
ethyl ether and the combined organic layers were dried (MgSO4), filtered
and concentrated. Purification by flash silica gel chromatography (0% to
10% EtOAc in hexanes) yielded 25 as a colorless oil (4.03 g, 6.90 mmol,
100%). Rf =0.65 (hexanes/EtOAc 3:1); [a]2D0 =+54.3 (c=1.0 in CHCl3);
1H NMR (300 MHz, CDCl3): d=7.59–7.34 (m, 10H), 5.98–5.84 (m, 1H),
5.66 (s, 1H), 5.35–5.17 (m, 4H), 4.98 (d, 1H, J=12.0 Hz), 4.91 (d, 1H,
J=1.2Hz), 4.78 (d, 1H, J=12.0 Hz), 4.41 (dd, 1H, J=3.3, 10.2Hz),
4.32–4.24 (m, 2H), 4.21 (m, 1H), 3.92 (m, 4H), 1.15 ppm (m, 21H);
13C NMR (75 MHz, CDCl3): d=138.6, 137.9, 133.8, 129.1, 128.6, 128.4,
128.1, 127.9, 126.4, 117.2, 101.9, 99.0, 90.4, 79.0, 78.5, 74.1, 73.6, 69.1, 68.2,
64.6, 18.2, 12.3 ppm; IR: n˜ =2943, 1463, 1044 cmꢀ1; HR-MS: m/z: calcd
for C33H48O7Si+Na+: 607.3067; found: 607.3049.
2-O-Benzyl-4,6-O-benzylidene-3-O-(triisopropylsilyloxymethyl)-a-d-man-
nopyranoside (27): Allyl mannoside 25 (507 mg, 0.969 mmol) was dis-
solved in EtOAc (6 mL). Aqueous acetic acid (90%, 20 mL) was added,
followed by NaOAc (476 mg, 5.80 mmol) and PdCl2 (258 mg, 1.46 mmol)
and the mixture was stirred overnight. After filtration over Celite, the
mixture was diluted with EtOAc and washed with water. The aqueous
phase was extracted once with EtOAc and the combined organic layers
were washed with saturated aqueous NaHCO3 until they reached neutral
pH. The aqueous phase was extracted three times with EtOAc. The com-
bined organic layers were dried (MgSO4), filtered and concentrated. Pu-
rification by flash column chromatography (0!17.5% EtOAc in hex-
anes) gave the title compound as a colorless oil (389 mg, 0.714 mmol,
General procedure A: Condensations by using S-phenyl 2,3-di-O-benzyl-
4,6-O-benzylidene-1-deoxy-1-thia-a-d-mannopyranoside sulfoxide (1): A
mixture of 1 (1.25 equiv), acceptor (2, 4, 6 or 9; 1.0 equiv, ꢁ0.15 mmol,
0.04m) and di-tert-butylmethylpyridine (DTBMP, 2.5 equiv) in CH2Cl2
was stirred over freshly activated powdered molecular sieves for 30 min
and cooled to ꢀ788C. Trifluoromethanesulfonic anhydride (Tf2O,
0.65 equiv) was added and the reaction mixture was allowed to very
slowly warm to room temperature overnight. The reaction was quenched
by the addition of triethylamine (ꢁ5 equiv), filtered over Celite in a mix-
ture of aqueous Na2S2O3 (1m) and saturated aqueous NaHCO3. The
layers were separated, and the aqueous phase was extracted with di-
chloromethane. The combined organic phases were washed with saturat-
ed aqueous NaCl, dried (NaSO4) and concentrated. The crude product
was purified by flash silica gel column chromatography (EtOAc in hex-
anes or toluene) to provide the title compound.
1
74%). Rf =0.45 (hexanes/EtOAc 3:1); major isomer: H NMR (300 MHz,
CDCl3): d=7.53–7.30 (m, 10H), 5.61 (s, 1H), 5.12(m, 3H), 4.89 (d, 1H,
J=12.0 Hz), 4.69 (d, 1H, J=12.0 Hz), 4.35 (1H, m), 4.02–4.62 (m, 4H),
3.82–3.92 (m, 2H), 3.46 (brs, 1H), 1.10 ppm (m, 21H); 13C NMR
(75 MHz, CDCl3): d=138.3, 137.5, 128.7, 128.3, 128.2, 128.0, 127.8, 127.5,
126.12, 126.06, 101.6, 94.0, 90.1, 78.8, 78.3, 73.7, 72.9, 68.8, 64.1, 17.8,
General procedure B: Condensations by using carboxybenzyl donors: A
mixture of 12[12] (1.25 equiv), acceptor (4, 6 or 13; 1.0 equiv, ꢁ0.12mmol,
0.04m) and di-tert-butylmethylpyridine (DTBMP, 3.0 equiv) in CH2Cl2
was stirred over freshly activated powdered molecular sieves for 30 min
and cooled to ꢀ608C. Trifluoromethanesulfonic anhydride (Tf2O,
1.25 equiv) was added and the reaction mixture was allowed to very
slowly warm to room temperature overnight. The reaction was quenched
by the addition of triethylamine (ꢁ5 equiv), filtered over Celite and con-
centrated. The crude product was purified by flash silica gel column chro-
matography (EtOAc in hexanes or toluene) to provide the title com-
pound.
11.9 ppm; IR: n˜ =2944, 1464, 1095 cmꢀ1
C30H44O7Si+Na+: 567.2754; found: 567.2737.
; HR-MS: m/z: calcd for
4,6-Benzyloxycarbonylbenzyl-O-benzylidene-3-O-(triisopropylsilyloxy-
methyl)-a-d-mannopyranoside (29): Benzyloxycarbonylbenzyl-4,6-O-ben-
zylidene-a-d-mannopyranoside (28)[12] (2.10 g, 4.27 mmol) in 1,2-dichloro-
ethane (22 mL), was treated with dibutyltindichloride (1.43 g, 4.70 mmol)
and DIPEA (2.97 mL, 17.1 mmol), until all starting material was dis-
solved, and the resulting solution was stirred for an additional 2h. Triiso-
propylsilyloxymethylchloride[25] (1.05 mL, ꢁ4.7 mmol) was added and the
reaction mixture was stirred overnight. The mixture was diluted with
CH2Cl2 (150 mL) and washed with saturated aqueous NaHCO3 (100 mL).
The water layer was extracted three times with CH2Cl2, after which the
organic layers were combined, washed with aqueous saturated NaCl,
dried over MgSO4 and concentrated. The crude product was purified by
flash silica gel column chromatography (0!15% EtOAc in hexanes) to
give the pure title compound as a colorless oil (2.27 g, 3.34 mmol, 79%).
Allyl 4,6-O-benzylidene-3-O-(triisopropylsilyloxymethyl)-a-d-mannopyr-
anoside (24): Allyl mannoside 23 (3.08 g, 10.0 mmol) in 1,2-dichloro-
ethane (40 mL), was treated with dibutyltindichloride (3.04 g, 10.0 mmol)
and N,N-diisopropylethylamine (6.3 mL, 36 mmol), until all starting ma-
terial was dissolved, and the resulting solution was stirred for an addi-
tional 1.5 h, after which the mixture was brought to 808C. Triisopropylsi-
lyloxymethylchloride[25] (2.9 g, 13 mmol) was added and the reaction mix-
ture was stirred for 15 min, after which it was cooled to RT. The mixture
3992
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 3987 – 3994

Automated Oligosaccharide Synthesis
FULL PAPER
1
Rf =0.45 (hexanes/EtOAc 3:1); [a]2D0 =+70.9 (c=1.0 in CHCl3); H NMR
(300 MHz, CDCl3): d=8.07 (m, 1H), 7.68 (m, 1H), 7.59–7.36 (m, 12H),
5.61 (s, 1H), 5.37 (s, 2H), 5.25–5.21 (m, 2H), 5.13–5.08 (m, 2H), 5.01 (d,
1H, J=14.4 Hz), 4.31–4.23 (m, 3H), 4.13 (t, 1H, J=9.0 Hz), 3.98 (m,
1H), 3.88 (t, 1H, J=9.9 Hz), 2.99 (brs, 1H), 1.11 ppm (m, 21H);
13C NMR (75 MHz, CDCl3): d=166.2, 139.6, 137.3, 135.6, 130.5, 128.7,
128.4, 128.1, 128.0, 127.9, 127.7, 127.2, 127.0, 126.0, 101.8, 99.8, 89.9, 77.8,
74.6, 70.9, 68.8, 67.4, 66.7, 63.7, 17.9, 11.9 ppm; IR: n˜ =2934, 1713, 1256,
1046 cmꢀ1; HR-MS: m/z: calcd for C30H50O9SiNa+: 701.3116; found:
701.3102.
(1m in THF, 0.1 mL) was added. The mixture was stirred for 10 min after
which the mixture was diluted with EtOAc and washed with water and a
saturated aqueous solution of NaCl. The organic phase was dried over
MgSO4, and concentrated. Flash silica gel column chromatography (5!
20% EtOAc in hexanes) gave the title compound (55 mg, 0.064 mmol,
79%). Rs =0.50 (hexanes/EtOAc 2:1); [a]2D0 =ꢀ11.1 (c=0.5 in CHCl3);
1H NMR (300 MHz, CDCl3): d=7.43–7.27 (m, 20H), 5.83 (m, 1H), 5.41
(s, 1H), 5.37 (m, 1H), 5.10–4.97 (m, 2H), 4.93 (d, 1H, J=11.7 Hz), 4.84
(d, 1H, J=2.1 Hz), 4.75 (d, 1H, J=12.0 Hz), 4.67 (s, 2H), 4.61–4.55 (m,
3H), 4.48 (d, 1H, J=12.07 Hz), 4.23 (t, 1H, J=9.6 Hz), 3.87–3.65 (m,
6H), 3.59–3.42 (m, 3H), 2.95 (m, 1H), 2.34 (brs, 1H), 2.14 (m, 2H), 1.73
(m, 2H), 1.22ppm (s, 9H); 13C NMR (75 MHz, CDCl3): d=177.3, 138.4,
138.04, 137.95, 137.7, 137.1, 128.9, 128.3, 128.1, 128.0, 127.8, 127.7, 127.4,
127.2, 126.8, 126.1, 115.0, 101.8, 101.3, 97.7, 79.1, 78.9, 75.6, 75.4, 74.9,
73.3, 71.3, 70.9, 70.8, 68.7, 68.6, 68.3, 67.3, 66.7, 39.0, 30.3, 28.6, 27.2 ppm;
Benzyloxycarbonylbenzyl-2-O-benzyl-4,6-O-benzylidene-3-O-(triisopro-
pylsilyloxymethyl)-a-d-mannopyranoside (30): Mannopyranoside 29
(4.55 g, 6.70 mmol) and benzyl bromide (1.03 mL, 8.66 mmol) were dis-
solved in DMF (35 mL) and cooled to 08C. Sodium hydride (308 mg,
7.70 mmol, 60% in mineral oil) was added in small portions. The mixture
was allowed to reach room temperature and stirred for 1.25 h. Saturated
aqueous NH4Cl was added to quench the reaction, after which the mix-
ture was diluted with diethyl ether and washed with water. The aqueous
phase was extracted three times with diethyl ether and the combined or-
ganic layers were dried (MgSO4), filtered and concentrated. Purification
by flash silica gel chromatography (0!10% EtOAc in hexanes) yielded
30 (4.97 g, 96%). Rf =0.45 (hexanes/EtOAc 3:1); [a]2D0 =+68.8 (c=1.0 in
CHCl3); 1H NMR (300 MHz, CDCl3): d=8.03 (d, 1H, J=8.1 Hz), 7.59–
7.36 (m, 19), 5.59 (s, 1H), 5.31 (s, 2H), 5.16–5.09 (m, 3H), 5.87–4.95 (m,
3H), 4.72(d, 1H, J=11.7 Hz), 4.38 (dd, 1H, J=3.3, 9.9 Hz), 4.22 (d,
2H), 4.00 (m, 1H), 3.88 (m, 2H), 1.13 ppm (m, 21H); 13C NMR
(75 MHz, CDCl3): d=177.7, 140.3, 138.5, 137.9, 136.0, 132.7, 130.9, 129.0,
128.9, 128.5, 128.4, 128.3, 128.1, 127.9, 127.8, 127.3, 127.2, 126.4, 101.8,
99.5, 90.3, 79.0, 78.3, 73.9, 73.5, 69.0, 67.6, 66.9, 64.8, 18.1, 12.2 ppm; IR:
n˜ =2933, 1713, 1256, 1041 cmꢀ1; HR-MS: m/z: calcd for C45H56O9SiNa+
791.3591; found: 791.3586.
IR: n˜ =3054, 1729, 1421, 695 cmꢀ1
C50H60O12+Na+: 875.3983; found: 875.3988.
;
HR-MS: m/z: calcd for
n-Pentenyl 3,6-di-O-benzyl-4-O-[2-O-benzyl-4,6-O-benzylidene-3-O-(2,3-
di-O-benzyl-4,6-O-benzylidene-b-d-mannopyranosyl)-b-d-mannopyrano-
syl]-2-O-pivaloyl-a-d-mannopyranoside (52): Using general procedure B,
52 was prepared on a 0.053 mmol (acceptor glycoside) scale. LCMS anal-
ysis (85% B in A: 2min; linear gradient to 95% B in A in 30 min) of
the reaction mixture revealed the a/b ratio to be 12:1; b-52: tR =
4.83 min; a-52; tR =5.85 min. Flash silica gel column chromatography
(0!25% EtOAc in hexanes) gave the title compound (50 mg,
0.039 mmol, 74%). Rf =0.60 (hexanes/EtOAc 2:1); [a]2D0 =ꢀ44.0 (c=1.0
1
in CHCl3); H NMR (300 MHz, CDCl3): d=7.50–7.10 (m, 35H), 5.81 (m,
1H), 5.51 (s, 1H), 5.49 (s, 1H), 5.37 (dd, 1H, J=1.8, 3.0 Hz), 4.97–5.07
(m, 2H), 4.88 (d, 1H, J=11.7 Hz), 4.80–4.84 (m, 2H), 4.74 (s, 1H), 4.70
(s, 1H), 4.54–4.67 (m, 7H), 4.48 (d, 1H, J=12.0 Hz), 4.25 (t, 1H, J=
9.6 Hz), 3.95–4.18 (m, 5H), 3.61–3.90 (m, 9H), 3.46 (m, 1H), 3.27 (dd,
1H, J=3.0, 9.6 Hz), 3.10 (m, 2H), 2.13 (m, 2H), 1.70 (m, 2H), 1.20 ppm
(s, 9H); 13C NMR (75 MHz, CDCl3): d=177.4, 138.5, 138.4, 138.2, 138.12,
138.07, 137.7, 137.4, 137.3, 128.8, 128.7, 128.26, 128.21, 128.1, 128.0, 127.9,
127.64, 127.56, 127.4, 127.34, 127.27, 127.2, 126.1, 125.9, 115.0, 101.6,
101.2, 101.1, 97.8, 97.7, 78.4, 77.2, 75.6, 75.0, 74.7, 74.6, 74.5, 74.2, 73.4,
72.0, 71.2, 71.1, 69.2, 68.5, 68.4, 67.7, 67.39, 67.33, 39.0, 30.3, 28.7,
27.2 ppm; IR: n˜ =2672, 1730, 1453, 1091 cmꢀ1; HR-MS: m/z: calcd for
C77H87O17+Na+: 1305.576; found: 1305.576.
Carbonylbenzyl 2-O-benzyl-4,6-O-benzylidene-3-O-(triisopropylsilyloxy-
methyl)-a-d-mannopyranoside (31): Mannopyranoside 30 (1.0 g,
1.30 mmol) and NH4OAc (300 mg, 3.9 mmol) were dissolved in EtOAc/
methanol 1:4 (50 mL). The mixture was degassed three times and then
Pd/C (94 mg) was added. The suspension was degassed once more. H2
was bubbled through the mixture for 1 min, after which the solution was
stirred for 2h under an H 2 atmosphere. The catalyst was filtered off and
the solvents were evaporated in vacuo. The crude product was purified
by flash silica gel column chromatography (30!80% EtOAc in hexanes,
containing 1% AcOH) to provide the title compound as a white foam
(869 mg, 1.28 mmol, 98%). Rf =0.50 (hexanes/EtOAc 1:1, 5% AcOH);
[a]2D0 =+66.1 (c=1.0 in CHCl3); 1H NMR (300 MHz, CDCl3): d=10.6
(brs, 1H), 8.12(d, 1H, J=7.5 Hz), 7.59–7.36 (m, 14), 5.61 (s, 1H), 5.22–
5.13 (m, 3H), 5.02–4.89 (m, 3H), 4.74 (d, 1H, J=12.0 Hz), 4.43 (dd, 1H,
J=3.0, 10.2Hz), 4.27 (d, 2H), 4.05 (m, 1H), 3.91 (m, 2H), 1.12ppm (m,
21H); 13C NMR (75 MHz, CDCl3): d=172.7, 140.9, 138.5, 137.8, 133.5,
131.8, 129.0, 128.6, 128.3, 128.0, 127.9, 127.4, 127.3, 126.4, 101.8, 99.5,
90.3, 79.0, 78.4, 74.0, 73.4, 69.0, 67.7, 64.9, 18.1, 12.2 ppm; IR: n˜ =2944,
General procedure for the automated solid-phase synthesis of b-manno-
side-containing oligosaccharides
Automated module A: The resin is washed with CH2Cl2 for 15 s followed
by hexanes for 10 s; repeated six times.
Automated module B: The resin is washed six times with CH2Cl2 for 15 s
each.
Automated module C: The building block (5 equiv, 0.125 mmol) and
DTBMP (15 equiv, 0.375 mmol) in CH2Cl2 (2.5 mL) is delivered to the
reaction vessel containing the resin. The mixture is allowed to cool for
3 min (with vortex for 30 s followed by standing for 30 s). Tf2O (5 equiv,
0.125 mmol, in 0.5 mL CH2Cl2) is added to the reaction vessel with
vortex. The reaction mixture is then left for 120 min (with vortex for 30 s
followed by standing for 30 s). After that time, the solution is drained
and the resin is washed once with CH2Cl2.
1713, 1046 cmꢀ1
found: 701.3104.
;
HR-MS: m/z: calcd for C38H50O9SiNa+: 701.3116;
n-Pentenyl 3,6-di-O-benzyl-4-O-[2-O-benzyl-4,6-O-benzylidene-3-O-tri-
isopropyl-silyloxymethyl-b-d-mannopyranosyl]-2-O-pivaloyl-a-d-manno-
ACHTREUNG
pyranoside (38): Using general procedure B, 38 was prepared on a
0.096 mmol (acceptor glycoside) scale in 94% yield (a/b 1:9). [a]2D0 =ꢀ1.5
(c=0.5 in CHCl3); 1H NMR (300 MHz, CDCl3): d=7.43–7.27 (m, 20H),
5.83 (m, 1H), 5.44 (s, 1H), 5.36 (m, 1H), 5.10–4.98 (m, 4H), 4.88–4.78
(m, 3H), 4.73–4.63 (m, 4H), 4.48 (d, 1H, J=11.7 Hz), 4.22 (t, 1H, J=
9.6 Hz), 4.03–3.68 (m, 9H), 3.57 (t, 1H, J=9.9 Hz), 3.46 (m, 1H), 3.03
(m, 1H), 2.15 (m, 2H), 1.72 (m, 2H), 1.20 (s, 9H), 1.08 ppm (s, 21H);
13C NMR (75 MHz, CDCl3): d=177.4, 138.7, 138.6, 138.2, 137.8, 137.5,
128.7, 128.2, 128.0, 127.4, 127.3, 127.1, 126.1, 115.0, 101.5, 101.3, 97.7,
89.5, 78.5, 78.4, 78.1, 75.6, 75.2, 73.3, 71.5, 71.1, 69.0, 68.9, 68.5, 67.3, 39.0,
Automated module D: The resin is washed six times with THF for 15 s
each.
Automated module E: The resin is washed six times with MeOH/CH2Cl2
1:9 for 15 s each.
Automated module F: The resin is washed is treated with TBAF (5 equiv,
0.125 mmol) in 1.6 mL THF for 15 min (with vortex for 30 s followed by
standing for 30 s). After that time, the solution is drained and the resin is
washed once with THF; repeated once.
Automated module G: The resin is treated with 1.5 mL 20% piperidine/
DMF for 35 s, then washed once with CH2Cl2.
30.4, 28.7, 27.2, 12.2, 12.1 ppm; IR: n˜ =2943, 2871, 1730, 1456, 1093 cmꢀ1
HR-MS: m/z: calcd for C60H82O13Si+Na+ 1061.542; found: 1061.542.
;
Automated module H: The resin is washed with CH2Cl2 for 15 sec fol-
lowed by MeOH for 15 s; repeated six times.
n-Pentenyl 3,6-di-O-benzyl-4-O-[2-O-benzyl-4,6-O-benzylidene-b-d-man-
nopyranosyl]-2-O-pivaloyl-a-d-mannopyranoside (42): Mannopyranoside
38 (85 mg, 0.085 mmol) was dissolved in THF (1 mL), after which TBAF
Automated module I: The resin is washed six times with acetic acid (0.2m
in THF) for 15 s each.
Chem. Eur. J. 2008, 14, 3987 – 3994
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3993

P. H. Seeberger et al.
Automated module J: The resin is washed six times with DMF for 15 s
each.
[4] K. R. Love, P. H. Seeberger, Angew. Chem. 2004, 116, 612; Angew.
Chem. Int. Ed. 2004, 43, 602.
[5] D. M. Ratner, E. R. Swanson, P. H. Seeberger, Org. Lett. 2003, 5,
4717.
[6] D. B. Werz, B. Castagner, P. H. Seeberger, J. Am. Chem. Soc. 2007,
129, 2770.
[7] a) J. J. Gridley, H. M. I. Osborn, J. Chem. Soc. Perkin Trans. 1 2000,
1471; b) A. V. Demchenko, Synlett 2003, 1225.
[8] a) F. Barresi, O. Hindsgaul, J. Am. Chem. Soc. 1991, 113, 9376; b) Y.
Ito, T. Ogawa, Angew. Chem. 1994, 106, 1843; Angew. Chem. Int.
Ed. Engl. 1994, 33, 1765.
[9] a) G. Stork, G. Kim, J. Am. Chem. Soc. 1992, 114, 1087; b) G. Stork,
J. J. LaClair, J. Am. Chem. Soc. 1996, 118, 247.
[10] a) D. Crich, S. Sun, J. Org. Chem. 1996, 61, 4506; b) D. Crich, S.
Sun, J. Org. Chem. 1997, 62, 1198; c) D. Crich, S. Sun, Tetrahedron
1998, 54, 8321; d) D. Crich, S. Sun, J. Am. Chem. Soc. 1998, 120,
435; e) D. Crich, J. Carbohydr. Chem. 2002, 21, 667.
[11] L. Yan, D. Kahne, J. Am. Chem. Soc. 1996, 118, 9239.
[12] K. S. Kim, J. H. Kim, Y. J. Lee, Y. J. Lee, J. Park, J. Am. Chem. Soc.
2001, 123, 8477.
[13] J. Y. Baek, T. J. Choi, H. B. Jeon, K. S. Kim, Angew. Chem. 2006,
118, 7596; Angew. Chem. Int. Ed. 2006, 45, 7436.
Automated module K: The resin is submitted to piperidine (20% v/v in
DMF, 2mL) for 5 min (with vortex for 30 s, followed by standing for
30 s). After that time, the solution is drained and the resin is submitted
to the same conditions twice.
n-Pentenyl 3,6-di-O-benzyl-4-O-[2-O-benzyl-4,6-O-benzylidene-3-O-(2,3-
di-O-benzyl-4,6-O-benzylidene-b-d-mannopyranosyl)-b-d-mannopyrano-
syl]-2-O-pivaloyl-a-d-mannopyranoside (52): Resin 46 (0.22 mmolgꢀ1
,
113 mg, 0.025 mmol) was loaded in the reaction vessel of the synthesizer.
Modules D, A, B were performed. The resin was glycosylated twice with
building block 47 (4.5 equiv, 0.112mmol) in toluene (2mL) and TMSOTf
(0.5 equiv, 0.012mmol in 0.5 mL CH 2Cl2) for 15 min with one module B
in between. Modules B, D, B, were performed. The acetate protecting
group was removed by treating the resin twice with NaOMe (10 equiv,
0.25 mmol) in MeOH (0.4 mL) and CH2Cl2 (3.5 mL) for 30 min. Modules
E, D, I, D, A, B, D, A and B were executed. The reaction vessel was
cooled to ꢀ308C. Module C was then performed twice with building
block 31 with two modules B in between. The resin was subjected to
module B and G. The reaction vessel was warmed to room temperature,
and modules B, H, and B were performed. The resin was then treated
twice manually with TBAF (5 equiv, 0.125 mmol) in THF (1.5 mL) for
20 min each, before washing with THF and CH2Cl2 several times. The
resin was charged in the synthesizer reaction vessel. Module D, A, and B
were performed. The reaction vessel was then cooled to ꢀ308C. Module
C was performed twice with building block 12 with two modules B in be-
tween. The resin was subjected to module B and G. The reaction vessel
was warmed to room temperature, and modules B, H, and B were per-
formed. The resin was charged in a 10 mL round-bottom flask and swell-
ed with CH2Cl2 (2mL) under an atmosphere of argon. The Grubbs 1st
generation catalyst (2mg) was added and the flask was put under ethyl-
ene atmosphere and stirred overnight. The resin was washed eight times
[14] T. Tsuda, R. Arihara, S. Sato, M. Koshiba, S. Nakamura, S. Hashi-
moto, Tetrahedron 2005, 61, 10719.
[15] a) For glycosyl trichloroacetimidates see: A. A.-H. Abdel-Rahman,
S. Jonke, E. S. H. El Ashry, R. R. Schmidt, Angew. Chem. 2002, 114,
3100; Angew. Chem. Int. Ed. 2002, 41, 2973; b) for trifluoro-N-phe-
nylacetimidates see: S. Tanaka, M. Takahashi, H. Tokimoto, Y. Fuji-
moto, K. Tanaka, K. Fukase, Synlett 2005, 2325.
[16] a) K. S. Kim, S. S. Kang, Y. S. Seo, H. J. Kim, Y. L. Lee, K.-S. Jeong,
Synlett 2003, 459; b) Y. J. Lee, J. Y. Baek, B.-Y. Lee, S. S. Kang, H.-S.
Park, H. B. Jeon, K. S. Kim, Carbohydr. Res. 2006, 341, 1708.
[17] a) For “donor-bound” approach, see: D. Crich, M. Smith, J. Am.
Chem. Soc. 2002, 124, 8867; b) For an intermolecular aglycon deliv-
ery (IAD) method, see: Y. Ito, T. Ogawa, J. Am. Chem. Soc. 1997,
119, 5562.
[18] a) G. Wulff, G. Rçhle, Angew. Chem. 1974, 86, 173; Angew. Chem.
Int. Ed. Engl. 1974, 13, 157; b) A. Demchencko, T. Stauch, G.-J.
Boons, Synlett 1997, 818.
[19] a) D. Crich, S. Sun, J. Am. Chem. Soc. 1997, 119, 11217; b) D. Crich,
N. S. Chandrasekera, Angew. Chem. 2002, 114, 4664; Angew. Chem.
Int. Ed. 2002, 41, 4489.
with CH2Cl2. The resulting solution was filtered through
a pipette
column with 3 cm silica, eluting with ethyl acetate. The eluted solution
was concentrated under vacuum. The crude residue was purified by
column chromatography to afford the title compound 52 (16 mg,
12.5 mmol, 50% yield from resin 46) as a mixture of anomers. Anomeri-
cally pure 52 was obtained by preparative HPLC. The spectroscopic data
of this compound was in perfect agreements with the compound prepared
in solution.
[20] The use of Tf2O posed no problem and the stock solution was stable
for several days on the synthesizer.
[21] Elongation of the saccharide can also be accomplished by acid hy-
drolysis of the benzylidene acetal and ensuing regioselective glyco-
sylation. This option has not been investigated yet.
Acknowledgements
We thank the ETH Zürich, the Swiss National Science Foundation (SNF
Grant 200121-101593), the Roche Research Foundation (fellowship to
L.K.), and the Netherlands Organization for Scientific Research (NWO)
(fellowship to J.D.C.C.) for financial support.
[22] D. Crich, W. Cai, Z. Dai, J. Org. Chem. 2000, 65, 1291.
[23] a) D. Crich, V. Dudkin, Tetrahedron Lett. 2000, 41, 5643; b) J. D. C.
CodØe, L. H. Hossain, P. H. Seeberger, Org. Lett. 2005, 7, 3251.
[24] The Crich laboratory recently employed the 2-O-propargyl ether as
a minimally intrusive protecting group to overcome this erosion in
selectivity: a) D. Crich, P. Jayalath, Org. Lett. 2005, 7, 2 2 77; b) D.
Crich, P. Jayalath, T. K. J. Hutton, Org. Chem. 2006, 71, 3064.
[25] a) S. Pitsch, P. A. Weiss, X. Wu, D. Ackermann, T. Honegger, Helv.
Chim. Acta 1999, 82, 1753; b) S. Pitsch, P. A. Weiss, L. Jenny, A.
Stutz, X. Wu, Helv. Chim. Acta 2001, 84, 3773.
[26] The regioselectivity of the alkylation was unambiguously established
by acetylation of the C2hydroxyl. 1H NMR of the 2-O-acetylated
product clearly showed a downfield shift for H2.
[27] The use of TBAF on the ABI peptide synthesizer might have result-
ed in a selenoid valve failure, so we refrained from performing fur-
ther Tom deprotection by automation. This technical difficulty will
be overcome by the next generation of synthesizer developed in our
laboratory.
[1] For recent reviews on oligosaccharide synthesis see: a) Carbohy-
drates in Chemistryand Biology, Vol. 1 (Eds.: B. Ernst, G. W. Hart,
P. Sinaþ), Wiley-VCH, Weinheim, 2000; b) S. Hanessian, Preparative
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Marel, Chem. Soc. Rev. 2005, 34, 769.
[2] a) O. J. Plante, E. R. Palmacci, P. H. Seeberger, Science 2001, 291,
1523; b) P. H. Seeberger, W. C. Haase, Chem. Rev. 2000, 100, 4339;
c) Solid Support Oligosaccharide Synthesis and Combinatorial Car-
bohydrate Libraries (Ed.: P. H. Seeberger), Wiley, New York, 2001.
[3] E. R. Palmacci, O. J. Plante, M. C. Hewitt, P. H. Seeberger, Helv.
Chim. Acta 2003, 86, 3975.
Received: November 26, 2007
Published online: March 17, 2008
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