Using In(III) as a promoter for glycosylation

Article abstract of DOI:10.1016/j.carres.2011.10.016

InCl₃, InBr₃, and In(OTf)₃ were tested as promoters in the preparation of glycosides from trichloroacetimidate precursors. A range of protecting groups and of alcohol acceptors were used to determine the versatility of these promoters. Disaccharide formation was demonstrated. In most cases, the In(III) compounds were shown to promote glycosylation better than the widely used promoter BF₃·OEt₂.

Full text of DOI:10.1016/j.carres.2011.10.016

Carbohydrate Research 347 (2012) 142–146
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Note
Using In(III) as a promoter for glycosylation
⇑
Amanda L. Mattson, Anna K. Michel, Mary J. Cloninger
Department of Chemistry and Biochemistry, 103 Chemistry and Biochemistry Building, Montana State University, Bozeman, MT 59717, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
InCl3, InBr3, and In(OTf)3 were tested as promoters in the preparation of glycosides from trichloroacetim-
idate precursors. A range of protecting groups and of alcohol acceptors were used to determine the ver-
satility of these promoters. Disaccharide formation was demonstrated. In most cases, the In(III)
compounds were shown to promote glycosylation better than the widely used promoter BF3ÁOEt2.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 1 February 2011
Received in revised form 3 October 2011
Accepted 10 October 2011
Available online 17 October 2011
Keywords:
Glycosylation
Trichloroacetimidate
Indium(III) promotion
InBr3
InCl3
In(OTf)3
Glycosylation is one of the most widely used processes in carbo-
hydrate chemistry. It is one of the most common post translational
modifications1 and thus is an important reaction in the synthesis
and study of biologically relevant molecules. Synthetic methods
of glycosylation require activation of the carbohydrate donor.2
These reactions have been researched and reviewed exten-
sively,3–8 and promotion of a trichloroacetimidate donor with a Le-
wis acid is common.2,8 Due to the broad scope of glycosylation
reactions, there is a constant need for new promoters to fine tune
the reaction for specific donors and acceptors. Several of these pro-
moters are heavy metal based, which can be impractical due to the
waste generated.9–11 BF3ÁOEt2 and TMSOTf are common alterna-
tives as they avoid heavy metal waste,2,8 however these promoters
are hygroscopic, and BF3ÁOEt2 requires distillation prior to use,
which creates difficulty in utility.
This paper describes the use of indium(III) as an alternative to
current promoters. In(III) is desirable when compared to other Le-
wis acids due to its stability in air and water12,13 and relatively low
toxicity.14 Since it is a weaker Lewis acid than the other heavy me-
tal promoters, In(III) should be compatible with a wider range of
substrates.15 In carbohydrate synthesis, In(III) has been reported
to catalyze peracetylation,16 acetolysis, formation and hydrolysis
of acetals, and formation of thioglycosides.17,18 In addition, InCl3
has been reported as a glycosylation promoter using glycohalide
donors,19,20 as well as peracetylated trichloroacetimidates.21 While
these sources focused on InCl3, this paper broadens the scope to in-
clude other In(III) glycosylation promoters in the presence of a
variety of protecting groups and alcohol acceptors.
To investigate glycosylation reactions with In(III), InBr3, InCl3,
and In(OTf)3 were used as promoters for the glycosylation of three
trichloroacetimidate donors with four alcohol acceptors. The strat-
egy for reaction development is shown in Scheme 1. Acetyl, benzyl,
and acetonide protecting groups were chosen to provide a stability
range that tested the limitations of the In(III) promoters. Primary,
tertiary, and benzylic alcohols were chosen as glycosyl acceptors
to demonstrate the steric range through which the varying In(III)
compounds should be effective. The promoter BF3ÁOEt2 was used
for comparison with In(III) reagents. Overall, this paper describes
a simple reaction procedure for In(III) promoted glycosylation
reactions.
To identify optimal reaction conditions for each promoter, reac-
tions were performed at a variety of different temperatures and
⇑
Corresponding author. Tel.: +1 406 994 3051; fax: +1 406 994 5407.
E-mail address: mcloninger@chemistry.montana.edu (M.J. Cloninger).
Scheme 1. In(III)-promoted glycosylation.
0008-6215/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carres.2011.10.016

A. L. Mattson et al. / Carbohydrate Research 347 (2012) 142–146
143
Table 2
with varying amounts of promoter. Reactions were monitored by
1H NMR for the presence of the trichloroacetimidate starting mate-
rial to determine how long the reactions should be performed. For
acetyl protected mannosyl donor 1, reactions at 0 °C were found to
be optimal. As shown in Table 1, using alcohols 5 and 7 as the
acceptor, 0.5 equiv of InCl3 was necessary with this Lewis acid.
Only 10 mol % of InBr3 was required (with decomposition becom-
ing increasingly significant as larger amounts of InBr3 were added).
A trace amount of In(OTf)3 was sufficient for glycosylation to occur
in good yield. Thus, reactions using trichloroacetimidates 1–3 with
alcohols 4–7 are reported using 0.1 equiv of InBr3, 0.25–0.5 equiv
of InCl3, and 0.05 equiv or a trace amount of In(OTf)3. Additional
optimization reactions are summarized in Table S1 of the Supple-
mentary data.
A series of experiments were performed to compare In(III) pro-
moters against the standard method of BF3ÁOEt2 in glycosylation
promotion, and these studies are summarized in Tables 2–4. Reac-
tions using the peracetylated mannosyl trichloroacetimidate 1 are
reported in Table 2. In general, InBr3 and InCl3 were shown to per-
form comparably to or better than BF3ÁOEt2, with preference to-
ward InBr3 or InCl3 being acceptor dependant. With alcohols 4–6,
In(OTf)3 afforded lower yields of glycosylation products, but this
trend is not observed with trichloroacetimidates 2 and 3 (vide in-
fra). NMR yields with alcohol 4 were so low that no isolated yield
was attempted. Only the alpha product was observed, presumably
due to the neighboring group participation of the acetyl group at
C2 on the mannoside and due to the anomeric effect.
Glycosylation results using peracetylated trichloroacetimidate 1
Product
Promoter (equiv)
Timea (min)
Yield (%)
8
8
8
8
InBr3 (0.1)
InCl3 (0.5)
In(OTf)3 (0.05)
BF3ÁOEt2 (0.2)
30
20
15
30
42
45
6b
59
9
9
9
9
InBr3 (0.1)
InCl3 (0.5)
In(OTf)3 (0.05)
BF3ÁOEt2 (0.2)
30
20
15
30
53
72
31
59
10
10
10
10
InBr3 (0.1)
InCl3 (0.5)
In(OTf)3 (0.05)
BF3ÁOEt2 (0.2)
30
20
15
30
81
42
33
40
11
11
11
11
InBr3 (0.1)
InCl3 (0.5)
In(OTf)3 (0.05)
BF3ÁOEt2 (0.2)
30
20
15
30
60
52
57
43
a
Time to reaction completion.
Yields determined by 1H NMR spiked with a mesitylene standard.
The yields for reactions using the perbenzylated galactosyl tri-
chloroacetimidate 2 are summarized in Table 3. Used in these cat-
alytic amounts, In(III) was shown to be an effective promoter in all
cases tested, with In(OTf)3 giving yields of product formation con-
sistently better than the BF3ÁOEt2 control. Yields for reactions with
InBr3 were generally higher than those for reactions using InCl3,
with the exception of the more hindered alcohol 7, which gave
lower yields overall.
b
Table 3
Glycosylation results using perbenzylated trichloroacetimidate 2
The experiments that are summarized in Table 3 using the perb-
enzylated galactosyl trichloroacetimidate 2 indicate that In(III)
promoters can be catalytically used to effectively promote glyco-
sylation reactions. However, since In(III) has been reported as a
promoter of Friedel–Crafts chemistry,22,23 formation of side prod-
ucts is likely to detrimentally affect reaction yields. Although the
side products were not characterized, the increased complexity
Product
Promoter (equiv)
Timea (min)
Yieldb (%)
a
:b ratiob
12
12
12
12
InBr3 (0.1)
InCl3 (0.5)
In(OTf)3 (0.05)
BF3ÁOEt2 (0.2)
30
20
20
30
57
48
72
65
1:10
1:30
0:1
Table 1
Optimization of reaction conditions using 1 and 5 or 7
0:1
13
13
13
13
InBr3 (0.1)
InCl3 (0.5)
In(OTf)3 (0.05)
BF3ÁOEt2 (0.2)
30
20
20
30
61
53
68
60
0:1
0:1
0:1
0:1
14
14
14
14
InBr3 (0.1)
InCl3 (0.5)
In(OTf)3 (0.05)
BF3ÁOEt2 (0.2)
InBr3 (0.1)
InCl3 (0.5)
In(OTf)3 (0.05)
BF3ÁOEt2 (0.2)
30
20
20
30
57
53
63
61
0:1
0:1
0:1
0:1
Alcohol
Promoter
Time (min)a
Yield (%)b
7
7
7
7
5
5
7
7
7
7
7
7
InBr3 (0.9 equiv)
InBr3 (0.2 equiv)
InBr3 (0.1 equiv)
InBr3 (0.06 equiv)
InCl3 (0.5 equiv)
InCl3 (0.25 equiv)
In(OTf)3 (1.0 equiv)
In(OTf)3 (0.2 equiv)
In(OTf)3 (0.1 equiv)
In(OTf)3 (0.06 equiv)
In(OTf)3 (trace)
None
20
20
30
10
20
50
10
10
10
15
20
60
63
59
75
68
72
59
37
37
52
54
54
0
15
15
15
15
30
20
20
30
30
33
52
24
1:3
2:7
1:1
2:7
a
Time to reaction completion.
Ratios determined by 1H NMR after column purification.
b
of the aromatic region in the 13C and 1H NMR spectra of crude
product mixtures from 2 indicates their formation.
a
Time to reaction completion.
Yields determined by 1H NMR spiked with a mesitylene standard.
For reactions using benzylated donor 2, the b isomer is the ma-
jor product in every case tested, and the most easily isolated, but
b

144
A. L. Mattson et al. / Carbohydrate Research 347 (2012) 142–146
Table 4
Table 5
Glycosylation results using acetonide-protected trichloroacetimidate 3
In(OTf)3 promoted disaccharide formations
Product
Promoter (equiv)
Timea (min)
Yield (%)
Product
Promoter (equiv)
Temp (°C)
Timea (min)
Yield (%)
16
16
16
16
InBr3 (0.1)
InCl3 (0.25)
In(OTf)3 (trace)
BF3ÁOEt2 (0.2)
InBr3 (0.1)
InCl3 (0.25)
In(OTf)3 (trace)
BF3ÁOEt2 (0.2)
25
45
30
30
29
35
92
12
21
22
23
In(OTf)3 (0.05)
In(OTf)3 (0.05)
In(OTf)3 (trace)
0
0
23
20
30
25
78
27b
94
a
Time to reaction completion.
Yield is for a-linked product only.
17
17
17
17
25
45
25
30
37
23
90
20
b
of 3 and required no column chromatography for purification. No
beta product was observed for reactions using acetonide-protected
mannoside 3, presumably due to the anomeric effect in conjunc-
tion with steric hindrance at the C2 position (see Fig. S1 in the Sup-
plementary data for diagrams).
18
18
18
18
InBr3 (0.1)
InCl3 (0.25)
In(OTf)3 (trace)
BF3ÁOEt2 (0.2)
InBr3 (0.1)
InCl3 (0.25)
In(OTf)3 (trace)
BF3ÁOEt2 (0.2)
25
45
25
30
12
0
78
4
19
19
19
19
25
60
25
30
3
9
83
6
Overall, glycosylation of peracetylated mannosyl trichloroace-
timidate 1 with simple alcohols afforded the highest yields using
InBr3 or InCl3. In(OTf)3 was the best promoter for reactions with
perbenzylated galactosyl trichloroacetimidate 2 and acetonide
protected mannosyl trichloroacetimidate 3. Using these results,
disaccharide-forming reactions were explored using 1–3 and
acceptor 20. Initial tests with InBr3 afforded significant amounts
of unreacted 1 even when 20 mol % was used (rather than the
10 mol % that was used with 4–7) and the reaction time was ex-
tended sixfold. In(OTf)3 proved to be the preferred promoter for
all three donors in disaccharide formation with 20, and the results
are shown in Table 5. Acetyl and acetonide protected mannosyl tri-
chloroacetimidates gave high yields of disaccharides (78% and 94%,
a
Time to reaction completion.
a
:b ratios vary depending on the acceptor used. Using In(OTf)3
afforded the best results, but the conditions were optimized based
on overall yield rather than anomeric ratios. It should be noted that
time and strength of the acid promoter (as well as reaction temper-
ature) may have a significant effect on the anomeric ratio of
products. While the
a product gains some anomeric stability, elec-
trostatic interactions in the oxocarbenium ion half-chair interme-
diates should have a strong effect on product formation.24 For 2,
since some unfavorable interactions are likely in both half-chairs,
mixtures of
ion can form, and only the
respectively) with only a-linked products observed and no column
chromatography required in the case of acetonide protected man-
nosyl 3. Benzylated galactosyl trichloroacetimidate 2 afforded a
a
and b products form. For 3, only one oxocarbenium
product is observed (see results below
a
lower yield of
being formed.
a-linked product (27% yield) due to b product also
and Fig. S1 in the Supplementary data for half-chair diagrams).
Products 14 and 15 were independently resubjected to the reac-
tion conditions using 1 equiv of InBr3, InCl3, or BF3ÁOEt2, and inter-
In conclusion, InBr3, InCl3, and In(OTf)3 were effective promot-
ers of glycosylation reactions when used with the variety of pro-
tecting groups tested. With respect to acetylated mannosyl
trichloroacetimidate 1, InBr3 or InCl3 afforded the highest yields.
All three of the In(III) promoters that were tested worked well with
benzylated galactosyl trichloroacetimidate 2, with In(OTf)3 giving
slightly higher yields. When using acetonide protected mannosyl
trichloroacetimidate 3, trace amounts of In(OTf)3 gave the highest
yields and required no column chromatography for purification,
while InBr3 and InCl3 gave lower yields equivalent to BF3ÁOEt2. A
glycoside acceptor was also successfully added to the trichloroace-
timidates using In(OTf)3 as the promoter. Overall, In(III) afforded
good to excellent yields of products without the requirement of
distillation or an inert reaction atmosphere. The ability to vary li-
gands on In(III) also provides a broad range of promotion strength,
enabling optimization of the conditions for desired reactions.
conversion of the
a and b isomers was not observed. The a:b ratio
did change over time for 14 (see Table S3 in the Supplementary
data), but the 1H NMR spectrum clearly indicated decomposition
of 14, and the
was obtained in the reaction (in two trials, the
from rather than toward the reaction ratio). These experiments
indicate that the reaction occurs under kinetic control.
a
:b ratio does not reliably shift toward the ratio that
:b ratio shifts away
a
Glycosylation reactions using acetonide-protected mannoside 3
are summarized in Table 4. Optimization of these reactions re-
quires a balance between the strength and quantity of the pro-
moter that is present and the length of time that is required for
the reaction to achieve completion. Reactions comparing tempera-
tures and times were performed using alcohol 5, and details are
provided in Table S1 in the Supplementary data. Above 20 mol %
for InBr3 and above trace amounts for In(OTf)3, significant product
degradation was observed. The length of reaction time required
when using InCl3 lead to significant product degradation. Removal
of acetonide protecting groups using InCl3 has been reported,25 and
deprotection is apparently a competitive reaction as the length of
reaction and the quantity of promoter are increased, as seen by loss
of protecting group peaks in the NMR spectra. Trace amounts of
In(OTf)3 proved to be the most effective catalyst for glycosylation
1. Experimental
1.1. General methods
To obtain pure products, crude products were purified by
column chromatography on 60 Å silica gel for acetyl protected

A. L. Mattson et al. / Carbohydrate Research 347 (2012) 142–146
145
mannosyls and benzyl protected galactosyls with hexane/ethyl
acetate mobile phase in the indicated ratio. Acetonide protected
mannosyls were purified by column chromatography on 32–
1.5. General procedure for the preparation of acetonide
protected mannopyranoside derivatives using In(OTf)3
63
l
m neutral alumina with hexane/ethyl acetate mobile phase.
A dry 4 mL scintillation vial was charged with the trichloroace-
timidate 3 with a slight excess of alcohol and dissolved in enough
dry CH2Cl2 to obtain a 50 mM solution of the trichloroacetimidate.
Less than 0.1 mg In(OTf)3 was added, and reaction was stirred for
25 min. The reaction was quenched with approximately 0.2 g dry
NaHCO3 for 20 min. The solution was then filtered over a med-
ium-sized plug of alumina using CH2Cl2 as the eluent. The solvent
was removed under reduced pressure affording clean product.
An ice bath was used for reactions run at 0 °C. 13C and 1H NMR
were recorded for purified compounds on a Bruker DRX 500 MHz
Spectrometer using TMS as an internal standard. Mass spectra
were obtained using a high-resolution Bruker micro-TOF system
with electrospray ionization. Yields are either isolated yields or
were calculated from crude 1H NMR spectra using an internal
mesitylene standard as specified. Ratios of alpha to beta diastereo-
mers were determined using integrations of resonances in the 1H
NMR spectra or (for equivalent carbons) in the 13C NMR spectra.26
Trichloroacetimidate starting materials27 and acceptor 2028 were
synthesized using previously described methods. Trace additions
of In(OTf)3 were performed by the addition of a visible amount of
In(OTf)3 that weighed less than 0.1 mg.
1.6. General procedure for preparation of disaccharides
A dry 4 mL scintillation vial was charged with the trichloroace-
timidate starting material with a slight excess of 20 and dissolved
in enough dry CH2Cl2 to obtain a 50 mM solution of the trichloroace-
timidate. The mixture was cooled if specified, and the promoter was
added. The reaction then stirred for the specified time, was
quenched with 2 mL of saturated NaHCO3 for 20 min, diluted with
2 mL CH2Cl2, and filtered. The solvent was removed under reduced
pressure. Chromatographic purification was performed (galactopy-
ranoside product 6:4 hexane/EtOAc; acetyl protected mannopyran-
oside product 4:6 hexane/EtOAc). Acetonide protected man
nopyranoside 23 was prepared via procedure 1.5 above. Exact reac-
tion conditions are provided in Table S4 of the Supplementary data.
1.2. General procedure for optimization of reaction conditions
Duplicate reactions were performed in parallel. A dry 4 mL scin-
tillation vial was charged with the trichloroacetimidate starting
material with a slight excess of alcohol and dissolved in enough
dry CH2Cl2 to obtain a 50 mM solution of the trichloroacetimidate.
The mixture was cooled if specified, and the promoter was added.
BF3ÁOEt2 reactions were run under an argon atmosphere. Aliquots
were taken from vial 1 and evaluated by 1H NMR for reaction com-
pletion. When no starting material was observed, vial 2 was
quenched with approximately 0.2 g dry NaHCO3 for 20 min, di-
luted with 2 mL CH2Cl2, and filtered. The solvent was removed un-
der reduced pressure. Yields of vial 2 were obtained from 1H NMR
spectra using an internal mesitylene standard. Reaction conditions
for vial 2 reactions are provided in Table S1.
Acknowledgment
This work was supported by NIH R01 GM62444.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.carres.2011.10.016.
1.3. General procedure for the preparation of mannopyranoside
and galactopyranoside derivatives
References
A dry 4 mL scintillation vial was charged with the trichloroace-
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