Activation of glycosyl trichloroacetimidates with perchloric acid on silica (HClO4-SiO2) provides enhanced α-selectivity

Article abstract of DOI:10.1016/j.carres.2010.07.030

Obtaining high stereoselectivity in glycosylation reactions is often challenging in the absence of neighboring group participation. In this study, we demonstrate that activation of glycosyl trichloroacetimidate donors with immobilized perchloric acid on s

Full text of DOI:10.1016/j.carres.2010.07.030

Carbohydrate Research 345 (2010) 2074–2078
Contents lists available at ScienceDirect
Carbohydrate Research
journal homepage: www.elsevier.com/locate/carres
Note
Activation of glycosyl trichloroacetimidates with perchloric acid on silica
(HClO4–SiO2) provides enhanced
a
-selectivity
*
Olaf R. Ludek, Wenlu Gu, Jeffrey C. Gildersleeve
Chemical Biology Laboratory, National Cancer Institute at Frederick, National Institutes of Health, 376 Boyles St., Frederick, MD 21702, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 June 2010
Received in revised form 15 July 2010
Accepted 16 July 2010
Available online 21 July 2010
Obtaining high stereoselectivity in glycosylation reactions is often challenging in the absence of
neighboring group participation. In this study, we demonstrate that activation of glycosyl trichloroace-
timidate donors with immobilized perchloric acid on silica (HClO4–SiO2) provides higher
a-selectivity
than trimethylsilyl triflate (TMSOTf) for reactions that do not involve neighboring group participation.
Published by Elsevier Ltd.
Keywords:
Glycosylation
Trichloroacetimidates
Stereoselectivity
Perchloric acid on silica
Carbohydrates play an important role in a variety of biological
processes and are important constituents of many natural products
and drugs.1 Studies on this class of biopolymers require access to
structurally defined, homogeneous compounds; however, carbo-
hydrates are notoriously difficult to isolate from natural sources
and, when available, are typically only available in small quanti-
ties. Chemical and chemo-enzymatic methods provide a useful
alternative for obtaining carbohydrates.2–4 Both natural and unnat-
ural carbohydrates can be obtained, and much larger quantities of
pure material can be prepared.
rearrangements9,10 and esterifications.11 HClO4–SiO2 is an air-stable
powder that is easily prepared, conveniently handled, and can read-
ily be removed from reaction mixtures by filtration. HClO4–SiO2 has
been examined previously as a catalyst for glycosylation reac-
tions.12,13 In particular, it was used as an alternative to the highly
moisture-sensitive trimethylsilyl triflate (TMSOTf) for the activation
of trichloroacetimidate glycosyl donors. HClO4–SiO2 performed in
all cases as well as TMSOTf for the activation of C-2 acylated donors,
leading to the desired b-products for the Glc/Gal-series and a-prod-
ucts for Man-series compounds withhigh chemical yields. Muchless
was known about the effects of HClO4–SiO2 activation on the stereo-
chemical outcome in the absence of neighboring group participa-
tion. In this study, we demonstrate that activation of glycosyl
trichloroacetimidate donors with HClO4–SiO2 provides enhanced
The key step in the synthesis of carbohydrates is the formation
of the glycosidic linkages between monosaccharide units.2–4 Both
a
and b linkages are possible, and control of stereochemistry is crit-
ical. In some cases, stereochemistry can be controlled by the use of
protecting groups that are capable of neighboring group participa-
tion. For example, esters at the C-2 position of a glycosyl donor typ-
ically provide high selectivity for the 1,2-trans glycoside product.
Stereocontrol in the absence of neighboring group participation
is considerably more challenging.5 Many factors, such as steric hin-
drance of protecting groups, reaction solvent, and temperature, can
affect the stereochemical outcome of a glycosylation reaction, and
these effects are typically difficult to predict for any given donor–
acceptor pair. Better methods to control stereoselectivity in the ab-
sence of neighboring group participation are needed.
a-selectivity relative to TMSOTf in the absence of neighboring group
participation.
Our initial studies focused on the synthesis of glycopeptides. Gly-
copeptides are typically prepared via solid-phase peptide synthesis
with incorporation of suitably protected glyco-amino acids at the
appropriate position(s). For most O-linked glycans, the central con-
nection between the glycan portion and the peptide involves an
linkage between a GalNAc residue and a serine or threonine. There-
fore, stereoselective formation of the requisite -linkage between
a
a
protected GalNAc derivatives and either serine or threonine is
Perchloric acid on silica (HClO4–SiO2) has recently been intro-
duced as a user-friendly acid catalyst for a variety of organic
transformations, such as protection–deprotection reactions,6–8
critical for the synthesis of this family of carbohydrates.
The synthesis of the key glycosidic linkage between GalNAc and
serine/threonine has been studied in detail previously, and a vari-
ety of anomeric leaving groups have been investigated. Most fre-
quently, glycosyl bromides,14–17 trichloroacetimidates (TCA),18–20
phenylsulfides,21,22 sulfoxides,23 and phenylselenides24 have been
* Corresponding author. Tel.: +1 301 846 5699; fax: +1 301 846 6033.
E-mail address: gildersj@mail.nih.gov (J.C. Gildersleeve).
0008-6215/$ - see front matter Published by Elsevier Ltd.
doi:10.1016/j.carres.2010.07.030

O. R. Ludek et al. / Carbohydrate Research 345 (2010) 2074–2078
2075
used as glycosyl donors with adequately protected acceptor amino
acids. In general, many glycosylation methods and conditions will
give good a-selectivity when the secondary alcohol of threonine is
DCM and toluene. However, no selectivity was observed for the
participating solvent THF. Interestingly, when the participating
solvents ether and THF were paired with the non-participating
DCM to form binary solutions, a marked increase in chemical yield
was observed, without affecting the stereoselectivity (entries 2–5).
Due to the high melting point of dioxane, no data could be ob-
tained for the pure solvent at ꢀ30 °C. However, when paired with
used as the acceptor. With primary alcohols, such as serine deriv-
atives, stereoselectivities are typically much lower. For example,
glycosylation of Fmoc-protected serine acceptor 1 with 2-azido-
2-deoxygalactose imidate 225 provided the corresponding glyco-
side 3 with an
a
to b ratio of only 1.4.20 Therefore, better methods
toluene or DCM (entries 8 and 9), the highest
a-selectivities
for preparing these compounds were needed.
(a:b = 2.0) and chemical yields (94% and 97%, respectively) within
NHFmoc
HO
OAllyl
OAc
OAc
OAc
OAc
AcO
O
1
O
O
NHFmoc
OAllyl
A
cO
OAc
NHFmoc
OH
OAc
AcO
N3
O
NH
O
O
NH
CCl3
O
O
3
O
O
N3
glycosylated AA for SPPS
2
To improve the a-selectivity when glycosylating serine derivatives,
we varied a number of reaction parameters. We started by optimiz-
ing the reaction conditions for the standard TMSOTf activated gly-
cosylation of the TCA-donor 2 and serine acceptor 4 with respect
to a-selectivity and overall chemical yield. Because the reaction sol-
vent and temperature can have a dramatic influence, both factors
this set of experiments were reached. Although both solvent sys-
tems behaved quite similar in these experiments, we continued
our study with the mixture of DCM–dioxane, due to the low solu-
bility of certain acceptors and donors in toluene-based solvents,
allowing a more general approach. Raising the reaction tempera-
ture from ꢀ30 to 0 °C (entry 10) and room temperature (entry
were tested systematically for their ability to drive the reaction to
form predominantly the desired
a-product. The reaction media cho-
11) resulted in additional increases in
a-selectivity from 2.0 to
sen in this study were based on solvents and solvent systems that
3.8 and 5.3, respectively, without a significant loss in overall chem-
ical yield.
were reported to successfully lead to
a-glycosylation, regardless
of the anomeric leaving group.26,27 The results of these experiments
are summarized in Table 1. The product ratios were determined by
1H NMR spectroscopy (400 MHz, CDCl3).
Next, we turned our attention to the influence of the catalyst on
the
a/b-product distribution of this reaction. Glycosylations were
performed at 0 °C and room temperature with the same concentra-
tions of donor and acceptor and employing an equal mol % of cat-
alyst as compared to the TMSOTf-catalyzed reaction (Table 2).
Under the same reaction conditions, the use of HClO4–SiO2 con-
Variation of the solvent had a minor influence on the stereose-
lectivity of the glycosylation reaction at ꢀ30 °C (entries 1–9).
ratios ranged from slightly favoring the b-product (toluene, entry
7) to modest -selectivity (toluene–dioxane, entry 8 and DCM–
a:b
a
sistently resulted in higher ratios of
a/b-selectivity for the serine
dioxane, entry 9). In accordance with the literature, reaction media
containing the participating solvents ether or dioxane showed an
increased a-selectivity, compared to the non-participating solvents
acceptor 4, compared to the standard TMSOTf-catalyzed reactions
(entries 1,2: 3.8 to 8.0 and entries 5,6: 5.3 to 24.0). To determine
whether this effect is related solely to the presence of solid SiO2
Table 1
Effects of solvent and temperature on stereoselectivity
OAc
OAc
AcO
OAc
AcO
AcO
NHFmoc
OtBu
O
O
NHFmoc
OtBu
HO
+
O
N3
N3
O
TCA
O
Entry
Solvent
Catalyst
Temperature (°C)
Yielda (%)
Ratiob
(a/b)
1
2
3
4
5
6
7
8
9
DCM
Ether
DCM–ether (1:1)
THF
DCM–THF (1:1)
Nitromethane–ether (1:1)
Toluene
Toluene–dioxane (1:1)
DCM–dioxane (1:1)
DCM–dioxane (1:1)
DCM–dioxane (1:1)
TMSOTf
TMSOTf
TMSOTf
TMSOTf
TMSOTf
TMSOTf
TMSOTf
TMSOTf
TMSOTf
TMSOTf
TMSOTf
ꢀ30
ꢀ30
ꢀ30
ꢀ30
ꢀ30
ꢀ30
ꢀ30
ꢀ30
ꢀ30
0
98
71
97
76
90
77
78
94
97
96
86
1.4
1.9
1.9
1.0
1.0
1.6
0.8
2.0
2.0
3.8
5.3
10
11
rt
a
Combined yield of both anomers after purification.
Determined by 1H NMR spectroscopy.
b

2076
O. R. Ludek et al. / Carbohydrate Research 345 (2010) 2074–2078
Table 2
are sufficiently high for efficient syntheses and were readily ap-
Influence of catalyst and additives on stereoselectivity
plied by us for the multi-gram scale preparation of the Tn-antigen.
In the case of the armed glucose donor 15 and the highly reac-
tive acceptor 11, no difference in glycosylation selectivity was ob-
OAc
NHFmoc
OR
OAc
AcO
HO
O
served for both catalysts (
temperature from 0 °C to room temperature led to a slight increase
in -selectivity to 4.0 (entry 9), as already observed for donor–
acceptor pair 2 and 4. The epimeric mannose donor 18, on the
other hand, showed a marked increase in -selectivity in the glyco-
sylation with acceptor 11 after changing the catalyst from TMSOTf
to HClO4–SiO2. The enhanced -selectivity is especially interesting
a/b = 3.0, entries 7,8). Raising the
4
6
R = tBu:
NHFmoc
OR
O
catalyst
Bn:
N3
O
2
a
7
8
R = tBu:
(1:1)
DCM-dioxane
O
Bn:
a
Entry Acceptor Catalyst
Temperature
Yielda
(%)
Ratiob
(a/
b)
(°C)
a
1
2
3
4
5
6
7
8
4
4
4
4
4
4
6
6
TMSOTf
HClO4–SiO2
0
0
0
0
rt
rt
rt
rt
96
93
91
96
86
42
98
95
3.8
8.0
3.8
3.9
5.3
24.0
3.7
12.0
in light of the fact that activation of mannosyl bromides with silver
silicate, a heterogeneous activator, produces primarily b-mannose
products.36
In summary, we have demonstrated that the heterogeneous cat-
alyst, HClO4–SiO2, provides enhanced a-selectivity in glycosylations
involving trichloroacetimidate donors with non-participating pro-
tecting groups. The chemical yields are comparable to those
achieved by TMSOTf activation, and the catalyst and conditions
are applicable to a range of donor and acceptor pairs. Given the
improved selectivity and ease of handling, HClO4–SiO2 is a useful
alternative to TMSOTf for the activation of trichloroacetimidates.
TMSOTf + SiO2
TMSOTf + LiClO4
TMSOTf
HClO4–SiO2
TMSOTf
HClO4–SiO2
a
Combined yield of both anomers after purification.
Determined by 1H NMR spectroscopy.
b
in the reaction mixture, a control experiment with TMSOTf as
catalyst and additional silica was performed under the optimized
conditions (entry 3). No change of selectivity was observed com-
pared to the reaction without the additive, ruling out an exclusive
effect of the silica. Additionally, we performed a control experi-
ment with TMSOTf as catalyst and 1.5 equiv of LiClO4 present in
the reaction mixture (entry 4). Again, no change in selectivity
1. Experimental
1.1. General methods
All experiments involving water-sensitive compounds were con-
ducted under dry conditions (positive argon pressure) using stan-
dard syringe, cannula, and septa apparatus. Solvents: All solvents
were purchased anhydrous (Aldrich) and stored over activated
molecular sieves. Hexanes, ethyl acetate, methylene chloride, and
MeOH employed in chromatography were purchased HPLC-grade.
Chromatography: Flash chromatography was performed with
Teledyne ISCO CombiFlash Companion. TLC: analytical thin layer
chromatography was performed on Analtech precoated plates (Uni-
was observed, indicating that the enhanced
a-selectivity is not
simply due to the presence of the perchlorate ion. When performed
at room temperature, a significant decrease in overall yield was ob-
served for the HClO4–SiO2-activated glycosylation (entry 6). This
was due to partial cleavage of the acid-labile tert-butyl ester moi-
ety on the amino acid 4. Therefore, this set of experiment was re-
peated with an acid-stable benzyl ester (6), resulting again in a
significant increase in
a-selectivity for the HClO4–SiO2-activated
reaction (entries 7, 8: 3.7 to 12.0), while maintaining very good
chemical yields.
plate, silica gel GHLF, 250 lm) containing a fluorescence indicator;
sugar-containing compounds were visualized with the sugar spray
reagent (5 mL of 4-methoxybenzaldehyde, 90 mL of ethanol, 5 mL
of concentrated sulfuric acid, and 10 mL of glacial acetic acid) by
heating with a heat gun. NMR spectra were recorded using a Varian
Inova 400 MHz spectrometer. The coupling constants are reported
in hertz, and the peak shifts are reported in the delta (ppm) scale;
abbreviations s (singlet), d (doublet), t (triplet), q (quartet), and m
(multiplet). Mass spectra (ESI-MS) were obtained on an Agilent
Technologies multimode ion source (LC/MSD SL) using the loop
injection mode. Optical rotations were measured on a Jasco
P-1010 polarimeter at 589 nm. Infrared spectroscopy data were ob-
tained neat with a Jasco FT-IR/615 spectrometer. Elemental analyses
were performed by Atlantic Microlab, Inc., Norcross, Georgia, 30091.
These results encouraged us to look into a more general appli-
cation of the HClO4–SiO2 catalyst for
a-selective glycosylations.
Therefore, a set of representative and easily accessible trichloro-
acetimidate glycosyl donors, bearing non-participating protecting
groups at C-2, were selected. Besides the previously introduced
disarmed galactosyl donor 2, we also performed glycosylations
with glucose (15)28 and mannose (18)29 donors. The hydroxyl
groups on these donors are protected with benzyl ethers, giving
rise to more reactive, or ‘armed’,30 donors relative to ester-pro-
tected donors. To cover a variety of representative acceptors,
glycosylations were performed with the commercially available
1,2:3,4-di-O-isopropylidene-
a-D-galactopyranose (11) and 1,2:5,
6-di-O-isopropylidene- -glucofuranose (13). To complete our
a-D
investigations on the Tn-antigen, the benzyl-protected threonine
9 was also studied as acceptor. Reactions were performed under
optimized conditions at either 0 °C or room temperature. The re-
sults from these experiments are listed in Table 3.
In all of the studied glycosylation reactions with representative
donors and acceptors, the HClO4–SiO2 performed better, or at least
as well as, the standard activator, TMSOTf, under the same reaction
conditions. As noted before, the less reactive secondary acceptors
1.2. Preparation of perchloric acid immobilized on silica gel
(HClO4–SiO2)
To a suspension of silica gel (20.0 g, SiliaFlash F60, 230–
400 mesh, Silicycle, Quebec City, Canada) in Et2O (100 mL) was
added HClO4 (70% aq solution, 690 lL, 8.00 mmol) and the suspen-
sion was stirred for 2 h at rt. The solvent was removed under re-
duced pressure and the crude catalyst was dried under vacuum
overnight at 80 °C to afford HClO4–SiO2 (0.4 mmol/g) as a free-
flowing colorless powder. The catalyst is light sensitive and should
be stored under argon in the dark to maintain activity.
tended to show a higher selectivity for the a-product, as compared
to the more reactive primary alcohols. Therefore, using threonine-
based acceptors (9) for constructing glycosylated amino acids re-
sulted in better
type (entries 1 and 2). Nonetheless, the
with serine 4 under the presented reaction conditions (vide supra)
a/b-ratio, as compared to acceptors of the serine
a
-selectivities achieved

O. R. Ludek et al. / Carbohydrate Research 345 (2010) 2074–2078
2077
Table 3
Comparisons of
a selectivities for various donor–acceptor pairs
Entry
Donor
Acceptor
NHFmoc
OBn
Product
Conditions
Yielda (%)
Ratiob
(a/b)
OAc
OAc
HO
O
NHFmoc
OBn
AcO
TMSOTf
rt
O
1
2
2
2
O
79
81
8.2
19
N3
9
O
1031
9
10
HClO4–SiO2
rt
OH
O
O
O
OAc
AcO
OAc
O
O
O
TMSOTf
0 °C
O
3
4
2
2
96
94
6.1
8.1
O
O
O
N3
O
O
1232
11
11
12
HClO4–SiO2
0 °C
O
OAc
OAc
O
O
O
O
O
O
HO
O
O
O
AcO
TMSOTf
0 °C
O
O
5
6
2
2
84
86
13
32
N3
13
14
13
11
14
HClO4–SiO2
0 °C
OBn
O
O
OBn
BnO
BnO
O
O
BnO
BnO
TMSOTf
0 °C
O
BnO
7
93
3.0
O
CCl3
NH
O
BnO
O
O
1633
15
8
9
15
15
11
11
16
16
HClO4–SiO2
0 °C
HClO4–SiO2
90
85
3.0
4.0
rt
OBn
O
O
O
O
O
BnO
BnO
TMSOTf
0 °C
10
11
15
13
13
84
88
4.0
9.3
O
O
BnO
1734
15
17
HClO4–SiO2
0 °C
BnO
BnO
O
O
BnO
BnO
BnO
BnO
BnO
BnO
O
O
O
12
13
11
11
TMSOTf
0 °C
85
88
3.0
10
O
CCl3
NH
O
O
O
1935
18
18
19
HClO4–SiO2
0 °C
a
Combined yield of both anomers after purification.
Determined by 1H NMR spectroscopy.
b
1.3. General procedure for TMSOTf-activated glycosylations
sired temperature and TMSOTf (0.075 mmol/mmol donor) was
added dropwise. The reaction was stirred at the indicated temper-
ature until TLC analysis revealed reaction completion. All reac-
tions with the armed donors, 15 and 18, were complete within
0.5 h. Reactions with the disarmed donor, 2, were complete within
0.5 h when carried out at rt and within 1 h when carried out at
0 °C. After neutralization with DIPEA, the solids were filtered off
Trichloroacetimidate donor (1.3 equiv) and glycosyl acceptor
(1.0 equiv) were dissolved in anhyd CH2Cl2 (5.0 mL/mmol donor)
and anhyd dioxane (5.0 mL/mmol donor). Activated molecular
sieves (4 Å, 50 mg/mL solvent) were added and the mixture was
stirred for 0.5 h at rt. The reaction mixture was adjusted to the de-

2078
O. R. Ludek et al. / Carbohydrate Research 345 (2010) 2074–2078
10); 5.20 (dd, 1H, J2 3 11.1 Hz, J3 4 3.7 Hz, H-30); 4.53 (d, 1H, J1,2
3.6 Hz H-2); 4.39 (ddd, 1H, J4,5 8.9 Hz, J5,6a 6.0 Hz, J5,6b 5.2 Hz, H-
5); 4.24 (d, 1H, J3,4 2.7 Hz, H-3); 4.23–4.19 (m, 1H, H-60a); 4.14
(dd, 1H, J6a,6b 8.6 Hz, J5,6a 6.0 Hz, H-6a); 4.10 (s, 1H, H-60b); 4.08
Table 4
0
0
0
0
Diagnostic protons for determining
a
/b product ratios
Compound
Diagnostic protons
d
[ppm]
db [ppm]
a
(d, 1H, J4 5 1.0 Hz, H-50); 4.02 (dd, 1H, J4,5 8.9 Hz, J3,4 2.7 Hz, H-4);
3.94 (dd, 1H, J6a,6b 8.6 Hz, J5,6b 5.2 Hz, H-6b); 3.82 (dd, 1H, J2 3
0
0
7
5.85 (d, 1H, J = 8.5 Hz,
NHFmoc)
2.12 (s, 3H, CH3, OAc)
5.75 (d, 1H, J = 8.5 Hz, NHFmoc)
2.10 (s, 3H, CH3, OAc)
0
0
11.1 Hz, J1 2 3.7 Hz, H-20); 2.12 (s, 3H, OAc); 2.04 (s, 3H, OAc);
2.02 (s, 3H, OAc); 1.47 (s, 3H, CH3, isoprop.); 1.39 (s, 3H, CH3, iso-
prop.); 1.32 (s, 3H, CH3, isoprop.); 1.30 (s, 3H, CH3, isoprop.)
ppm; 13C NMR (100 MHz, CDCl3): 170.49, 169.94, 169.77 (3 ꢂ C@O,
Ac); 112.02, 109.30 (2 ꢂ C(CH3)2, isoprop.); 83.78 (C-2); 81.41 (C-
3); 80.47 (C-4); 71.28 (C-5); 68.51 (C-40); 67.88 (C-30); 67.43 (C-
60); 67.24 (C-50); 61.98 (C-6); 57.93, (C-20); 26.83, 26.77, 26.23,
25.14, 20.59 (CH3, OAc/isoporop.) ppm.; ESI-MS: 596.05 (M+Na+);
Elemental Anal. Calcd for C24H35N3O13: C, 50.26; H, 6.15; N, 7.33.
Found: C, 50.10; H, 6.06; N, 7.32.
0
0
10
3.55 (dd, 1H, J = 11.2, 3.7 H-2) 4.63 (dd, 1H, J = 10.8,
3.2 Hz, H-3)
12
14
16
4.99 (d, 1H, J = 3.5 Hz, H-1)
4.53 (d, 1H, J = 3.6 Hz, H-2)
5.53 (d, 1H, J = 5.0 Hz, H-1)
1.53 (s, 3H, CH3, isoprop.)
5.80 (d, 1H, J = 3.6 Hz, H-1)
5.50 (d, 1H, J = 5.0 Hz, H-1)
1.48 (s, 3H, CH3, isoprop.)
4.50 (d, 1H, J = 8.0 Hz, H-1)
4.55 (d, 1H, J = 3.6 Hz, H-2)
5.57 (d, 1H, J = 5.0 Hz, H-1)
1.50 (s, 3H, CH3, isoprop.)
5.68 (d, 1H, J = 3.6 Hz, H-1)
5.57 (d, 1H, J = 5.0 Hz, H-1)
1.46 (s, 3H, CH3, isoprop.)
17
19
and the solvent was removed under reduced pressure. The crude
was purified by silica-gel flash chromatography (EtOAc in hex-
anes) and product-containing fractions were pooled, concentrated,
a/b-ratio determination.
Additional chromatography provided pure compounds with spec-
Acknowledgment
This research was supported by the Intramural Research Pro-
gram of the NIH, NCI.
and subjected to 1H NMR-analysis for
troscopic data identical to those reported.
References
1. Bertozzi, C. R.; Kiessling, L. L. Science 2001, 291, 2357–2364.
2. Lepenies, B.; Yin, J.; Seeberger, P. H. Curr. Opin. Chem. Biol. 2010, 14, 404–411.
3. Muthana, S.; Cao, H.; Chen, X. Curr. Opin. Chem. Biol. 2009, 13, 573–581.
4. Boltje, T. J.; Buskas, T.; Boons, G. J. Nat. Chem. 2009, 1, 611–622.
5. Demchenko, A. V. Synlett 2003, 1225–1240.
6. Chakraborti, A. K.; Gulhane, R. Chem. Commun. 2003, 9, 1896–1897.
7. Misra, A. K.; Tiwari, P.; Madhusudan, S. K. Carbohydr. Res. 2005, 340, 325–329.
8. Agarwal, A.; Vankar, Y. D. Carbohydr. Res. 2005, 340, 1661–1667.
9. Agarwal, A.; Rani, S.; Vankar, Y. D. J. Org. Chem. 2004, 69, 6137–6140.
10. Misra, A. K.; Tiwari, P.; Agnihotri, G. Synthesis 2005, 260–266.
11. Chakraborti, A. K.; Singh, B.; Chankeshwara, S. V.; Patel, A. R. J. Org. Chem. 2009,
74, 5967–5974.
12. Du, Y.; Wei, G.; Cheng, S.; Hua, Y.; Linhardt, R. J. Tetrahedron Lett. 2006, 47, 307–
310.
13. Mukhopadhyay, B.; Maurer, S. V.; Rudolph, N.; Van Well, R. M.; Russell, D. A.;
Field, R. A. J. Org. Chem. 2005, 70, 9059–9062.
14. Liu, M.; Young, V. G., Jr.; Lohani, S.; Live, D.; Barany, G. Carbohydr. Res. 2005,
340, 1273–1285.
15. Liebe, B.; Kunz, H. Helv. Chim. Acta 1997, 80, 1473–1482.
16. Reipen, T.; Kunz, H. Synthesis 2003, 2487–2502.
1.4. General procedure for HClO4–SiO2-activated glycosylations
Trichloroacetimidate donor (1.3 equiv) and glycosyl acceptor
(1.0 equiv) were dissolved in anhyd CH2Cl2 (5.0 mL/mmol donor)
and anhyd dioxane (5.0 mL/mmol donor). Activated molecular
sieves (4 Å, 50 mg/mL solvent) were added and the mixture was
stirred for 0.5 h at rt. The reaction mixture was adjusted to the de-
sired temperature and HClO4–SiO2 (0.075 mmol/mmol donor) was
added in one portion. The reaction was stirred at the indicated
temperature until TLC analysis revealed reaction completion. The
solids were filtered off and the solvent was removed under reduced
pressure. The crude was purified by silica-gel flash chromatogra-
phy (EtOAc in hexanes) and product-containing fractions were
pooled, concentrated, and subjected to 1H NMR-analysis for
a/b-ra-
tio determination. Additional chromatography provided pure com-
pounds with spectroscopic data identical to those reported.
17. Kuduk, S. D.; Schwarz, J. B.; Chen, X. T.; Glunz, P. W.; Sames, D.; Ragupathi, G.;
Livingston, P. O.; Danishefsky, S. J. J. Am. Chem. Soc. 1998, 120, 12474–12485.
18. Koeller, K. M.; Smith, M. E. B.; Wong, C. H. Bioorg. Med. Chem. 2000, 8, 1017–
1025.
1.5. Determination of the product ratio
19. Gambert, U.; Thiem, J. Carbohydr. Res. 1997, 299, 85–89.
20. Yang, Y. Y.; Ficht, S.; Brik, A.; Wong, C. H. J. Am. Chem. Soc. 2007, 129, 7690–
7701.
21. Cato, D.; Buskas, T.; Boons, G. J. J. Carbohydr. Chem. 2005, 24, 503–516.
22. Miermont, A.; Barnhill, H.; Strable, E.; Lu, X.; Wall, K. A.; Wang, Q.; Finn, M. G.;
Huang, X. Chem. Eur. J. 2008, 14, 4939–4947.
23. Andreotti, A. H.; Kahne, D. J. Am. Chem. Soc. 1993, 115, 3352–3353.
24. Karkkainen, T. S.; Ravindranathan Kartha, K. P.; MacMillan, D.; Field, R. A.
Carbohydr. Res. 2008, 343, 1830–1834.
25. Grundler, G.; Schmidt, R. R. Liebigs Ann. Chem. 1984, 1826–1847.
26. Demchenko, A. V.; Rousson, E.; Boons, G.-J. Tetrahedron Lett. 1999, 40, 6523–
6526.
27. Ishiwata, A.; Munemura, Y.; Ito, Y. Tetrahedron 2008, 64, 92–102.
28. Schmidt, R. R.; Stumpp, M. Liebigs Ann. Chem. 1983, 1249–1256.
29. Schmidt, R. R.; Michel, J.; Roos, M. Liebigs Ann. 1984, 1343–1357.
30. Fraser-Reid, B.; Wu, Z. F.; Udodong, U. E.; Ottosson, H. J. Org. Chem. 1990, 55,
6068–6070.
31. Mathieux, N.; Paulsen, H.; Meldal, M.; Bock, K. J. Chem. Soc., Perkin Trans. 1
1997, 2359–2368.
32. Paulsen, H.; Paal, M. Carbohydr. Res. 1983, 113, 203–218.
33. Adinolfi, M.; Barone, G.; Iadonisi, A.; Schiattarella, M. Org. Lett. 2003, 5, 987–
989.
Pre-purified product (20 mg) was dissolved in 0.75 mL of CDCl3
and the 1H NMR was recorded at 400 MHz at 25 °C. The ratio was
determined by integration of fully isolated peaks of each diastereo-
mer. The diagnostic protons of each compound are listed in Table 4.
1.6. 1,2:5,6-Di-O-isopropylidene-3-O-(30,40,60-tri-O-acetyl-20-
azido-20-deoxy-
a-D-galactopyranosyl)-a-D-glucofuranose (14)
The reaction was carried out according to the general procedure
for HClO4–SiO2-activated glycosylations with 3,4,6-tri-O-acetyl-2-
azido-2-deoxy-
(100 mg, 0.210 mmol), diacetone-
a
-
D
-galactopyranosyl trichloroacetimidate donor 2
-glucose acceptor 13 (42.1 mg,
D
0.162 mmol), activated molecular sieves (4 Å, 100 mg), and immo-
bilized HClO4 on silica (40.0 mg) in CH2Cl2–dioxane (1:1, 2.0 mL) at
0 °C. The crude was purified by chromatography on silica gel
(EtOAc in hexanes 20–40%) to yield the
a
glycosylated disaccharide
14 (79.9 mg, 86%) as a colorless foam. ½a D20
ꢁ
+59.1 (c 0.5, CHCl3); IR
34. Grayson, E. J.; Ward, S. J.; Hall, A. L.; Rendle, P. M.; Gamblin, D. P.; Batsanov, A.
S.; Davis, B. G. J. Org. Chem. 2005, 70, 9740–9754.
35. Singh, G.; Vankayalapati, H. Tetrahedron: Asymmetry 2000, 11, 125–138.
36. Paulsen, H.; Lockhoff, O. Chem. Ber. 1981, 114, 3102–3114.
(neat): 2990, 2109 (N3), 1748, 1372, 1214, 1148, 1031, 847 cmꢀ1
;
1H NMR (400 MHz, CDCl3): d 5.88 (d, 1H, J1,2 3.6 Hz, H-1); 5.42
(dd, 1H, J3 4 3.1 Hz, J4 5 1.0 Hz, H-40); 5.32 (d, 1H, J1 2 3.7 Hz, H-
0
0
0
0
0
0