Bismuth(V)-mediated thioglycoside activation

Article abstract of DOI:10.1002/anie.201304099

A straightforward method utilizing a bismuth(V) compound was developed for the activation of thiopropylglycosides for coupling to various acceptors; good to excellent yields were obtained without applying additional additives/co-promoters. The method does

Full text of DOI:10.1002/anie.201304099

Angewandte
Chemie
DOI: 10.1002/anie.201304099
Glycosylation
Bismuth(V)-Mediated Thioglycoside Activation**
Manibarsha Goswami, Arkady Ellern, and Nicola L. B. Pohl*
Chemical glycosylation[1,2] is a crucial step in any oligosac-
charide[3] synthesis.[4,5] Among the different classes of com-
monly used glycosyl donors,[2] thioglycosides[6,7] offer distinct
advantages. Thioglycoside donors are relatively simple to
prepare, are stable under various reactions for protecting-
group manipulations, and offer orthogonality in their activa-
tion in the presence of other glycosyl donors.[8] As a result,
a wide variety of promoters have been developed for
activation of these donors in the past 20 years:[2] from heavy
metal-cation-based promoters (HgII sulfate),[9] to the current
halonium-based reagents [e.g., N-iodosuccinimide/trifluoro-
methane sulfonic acid (NIS/TfOH),[10] N-bromosuccinimide
(NBS),[11] ICl or IBr/AgOTf,[12] etc.], alkylating reagents
[methyl triflate (MeOTf)],[13] and organosulfur-based pro-
promoter systems, we herein report a straightforward method
for the activation of thiopropylglycosides for coupling to
various acceptors in good to excellent yields by utilizing
a bismuth(V) compound without additional additives/co-
promoters.
In lieu of the available promoters based on heavy-metal
cations, bismuth presents interesting possibilities. Bismuth is
a post-transition metal and like its neighboring metals such as
mercury and lead, it is considered thiophilic as well as soft
Lewis acidic. However, unlike Hg and Pb compounds,
bismuth[23] is not only inexpensive, but also nontoxic.
Unfortunately, despite its popularity as a treatment for
digestive problems,[24] the synthetic utility of bismuth com-
pounds remains relatively unexplored. However, the chemis-
try of this element[25] has gained considerable interest[26,27]
over the past decade.[28] Various bismuth(III) compounds
have been developed that play crucial roles in different
functional-group transformations. In carbohydrate chemistry,
bismuth(III) triflate combined with NBS[29] was reported as
a promoter for the activation of thio- and selenoglycosides.
Moreover, Bi(OTf)3 by itself can also be used for selective
activation[30] of an S-benzoxazolyl (SBox) sialyl donor over
a galactosyl acceptor equipped with a thioethyl anomeric
moiety. However, in both cases, it was observed that Bi(OTf)3
was not only used in excess amounts, but owing to its
insolubility in the organic solvents often used for glycosyla-
tions, it has to be used in the presence of co-solvents like 1,4-
dioxane and tetrahydrofuran that play a significant role[31] in
the diastereoselectivity of the glycosylation products. Ideally,
a method for thioglycoside activation using bismuth chemistry
could be developed that avoided the use of additives, co-
solvents, low temperatures, and even the requirement for
excess amounts of promoter.
Though a variety of bismuth(III) reagents have been used
in oxidation, phenyl addition, and glycosylation reactions,
applications of bismuth(V)[32] compounds remain relatively
unexplored. A recent report[33] demonstrates the use of BiV
salts and ylides in carbon–carbon and carbon–heteroatom
bond-forming reactions. We were curious as to the thiophi-
licity of BiV complexes as well as the possibility of the addition
of solubilizing ligands on BiIII, but soon we discovered the
challenges of trying to synthesize and characterize new
bismuth-containing compounds.
Given the current limitations in the definitive character-
ization[22] of new bismuth compounds in solution, we next
sought a complex that was amenable to crystallization. We
chose a pentavalent bismuth compound containing three
phenyl and two triflate groups, namely triphenyl bismuth
ditriflate (2). Ph3Bi(OTf)2 was synthesized in two steps
(Scheme 1) starting from relatively inexpensive triphenyl
bismuth, which was first oxidized to triphenyl bismuth
diacetate (1) and then later converted to the desired
moters
[e.g.,
dimethyl(thiomethyl)sulfonium
triflate
(DMTST),[14] methylsulfenyl triflate (MeSOTf),[15] dimethyl
disulfide/triflic anhydride (Me2S/Tf2O),[16] benzenesulfinylpi-
peridine/triflic anhydride (BSP/TTBP),[17] N-(phenylthio)-e-
caprolactam-Tf2O,[18] etc.]. A recent method applies single-
electron transfer using ruthenium or iridium-containing
catalysts that are active under visible light[19] to activate
thioglycosides. Although these methods have been effective
in carrying out a range of glycosylations, most of these still
have a limited scope. Generally, these activations need excess
amounts of promoters,[2,9–18] or require a co-promoter to form
the reactive intermediates. Moreover, present methods often
require extremely low temperatures (< ꢀ208C) as a result of
generating reactive intermediates. Some of the popular
halonium-based promoters are challenging to use in the
presence of alkenes,[20] because they tend to give various
addition by-products, thereby ultimately resulting in the
cleavage of the alkenyl moiety. These issues with solubility,
undesired by-products, stability, or reagent handling are
particularly problematic in the context of the development
of robust automated protocols[21,22] for oligosaccharide syn-
thesis. To circumvent some of these issues with current
[*] Prof. N. L. B. Pohl
Department of Chemistry, Indiana University
Bloomington, IN 47405 (USA)
E-mail: npohl@indiana.edu
Homepage: http://www.indiana.edu/~pohllab/index.html
M. Goswami, Dr. A. Ellern, Prof. N. L. B. Pohl
Department of Chemistry, Iowa State University
Ames, IA 50010 (USA)
[**] This work was supported in part by the U.S. National Science
Foundation under CHE-0911123/1261046. Purchase of the AVIII-
600 NMR spectrometer used to obtain results included herein was
supported by the National Science Foundation under Grant No.
MRI 1040098. N.P. acknowledges the Joan and Marvin Carmack
fund. We thank Prof. Jason Chen for helpful discussions.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201304099.
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peratures (08C–reflux), the glycosylations were found to be
best (without degradation of starting material) when carried
out at ambient temperature. These propanethiol-modified
sugars were then subjected to these optimized reaction
conditions with a range of glycosyl acceptors in presence of
2 (Scheme 2 and Table 1).
Scheme 1. Preparation of a bismuth(V) promoter.
compound 2 in an 80% overall yield. Compound 2 is
a colorless solid that crystallizes[34] in a highly disordered
orthorhombic space group with trigonal bipyramidal coordi-
nation geometry around the bismuth metal center; this
geometry is similar[35] to reported BiV-containing compounds.
More importantly, the compound surprisingly proved to have
activity in an initial glycosylation reaction screen.
Table 1: Reaction of glycosyl donors and acceptors.[a]
Further exploration of this BiV reagent revealed several
advantages. Solubility has been a major drawback with most
promoters, thereby complicating their employment in auto-
mation platforms (solution[21] or solid phase[22]) that carry out
iterative oligosaccharide synthesis. Interestingly, compound 2
was found to be readily soluble in most organic solvents,
particularly dichloromethane and toluene, which are desir-
able nonparticipating solvents in glycosylation. Furthermore,
the promoter was also found to be stable in an oxygen
atmosphere and under irradiation with light. No degradation
or decomposition was seen when kept under anhydrous
conditions for months. So, unlike many thioglycoside pro-
moters, complex 2 does not need to be synthesized just prior
to the activation reaction, but can be made in batches and
stored. In addition, the promoter 2 does not require a co-
promoter like NBS/NIS to first make a soft electrophilic
halonium species to attach to the soft nucleophilic sulfur.
Finally, the activation does not require extreme low temper-
atures (ꢀ20 to ꢀ788C) primarily to control side reactions or
unwanted by-products.
To test the scope of the developed methodology, a range
of thiopropyl analogues of glucosyl (3a,b), galactosyl (4a,b),
and fucosyl (5a) donors were prepared (Scheme 2). A
[a] Reaction conditions: donor (1 equiv), acceptor (0.9 equiv), 2
(1 equiv), CH2Cl2, RT, 0.5m. [b] Yield of isolated products after silica gel
chromatography, [c] calculated by NMR spectroscopy.
Scheme 2. Activation of thiopropylglycosides with 2.
number of alkyl- and aryl-containing thiols were examined
for the preparation of thioglycoside donors. Thiols in general
are difficult to handle owing to their unpleasant odor. We
therefore eschewed the more common volatile methane/
ethanethiols and the highly pungent, more toxic aryl thiols
and settled on n-propylthiol,[36] a compound that is safe
enough to be approved as a food additive for its savory onion-
like smell. Next, the solvent chosen for the activation was
dichloromethane, since it is relatively inert, easy to handle,
has negligible solvent effects,[31] and completely solubilizes
the donor/acceptor/promoter. After examining various tem-
The study of thioglycoside activation with our model
promoter 2 was started with a simple acceptor: allyl alcohol
6a. The presence of double bonds is remarkable, as they are
generally avoided in donor/acceptor compounds, since they
tend to compete as a potential soft nucleophilic center with
sulfur. Though some controlled activation[37] protocols can
avoid these issues, alkenyl protecting groups always have the
potential to be cleaved.[38] Fortunately, the “armed”[39]
perbenzylated galactosyl donor 3b and the disarmed perace-
tylated galactosyl donor 3a gave the desired O-allyl galacto-
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sides in high yields (Table 1, entries 1–2). The armed glucosyl
donor 4b could also be activated (Table 1, entry 3) to give the
O-allyl glucoside in good yield. The alkenyl system remained
intact throughout these reactions, and formation of an
addition side product was not observed.
The glycosylation was observed to be very fast, resulting in
the formation of the 1!4-linked fucose–glucosamine disac-
charide in very high yield, favoring the a-anomer. Comparing
all the entries in Table 1, we can conclude that the glyco-
sylation time depends on the nature of the donor (armed or
disarmed) more so than on the incoming acceptor.
Another noteworthy limitation with available thioglyco-
side activators has been the amounts of promoter/co-pro-
moter needed for thioglycoside activation. To our knowledge,
none of the available methods to date require less than
stoichiometric amounts of promoter. Considering this and the
above successful glycosylations, experiments were then
designed to probe the amounts needed for full consumption
of the glycosyl donor (Table 2) and isolation of the desired
With our initial success with acceptor 6a, the method was
applied to various glycosyl acceptors containing a wide range
of functional groups. The glucosyl acceptor 6b was selected to
test the method for the formation of 1!6-linked disacchar-
ides as well as glycosylation with a primary hydroxy acceptor.
Both the galactosyl donors 3a and 3b (Table 1, entries 4–5)
gave the disaccharides in excellent yields. As predicted, the
1,2-trans-glycosides were favored for the disarmed thioglyco-
sides, and a low stereoselectivity was observed in the case of
armed thioglycosides. Similarly, the armed glucosyl donor 4b
(Table 1, entry 6) was also activated to give the 1!6
disaccharide in high yield with a slight preference for the a-
anomer.
Table 2: Promoter equivalence studies with model glycosylation of donor
3b and acceptor 6a.[a]
Next, from common sugars like d-glucose and d-galac-
tose, we moved to less common sugars. An armed l-fucosyl
donor 5a was chosen for this purpose and was synthesized
from its acetate analogue (see the Supporting Information).
The activations were investigated with a-thiopropyl l-fuco-
side donors; these are more-stable anomers for l-fucose.
Interestingly, the initial glycosylation with the glucosyl
acceptor 6b (Table 1, entry 7) was extremely fast, since the
donor was consumed in 12 min to give the fucose 1!6-linked
glycoside in good yield. The formation of the a-anomer of the
disaccharide was favored, as generally seen with fucose
analogues.[40]
Entry
Promoter equivalence
Yield [%][b]
1
2
3
4
1
86
82
76
68
0.7
0.5
0.3
[a] Reaction conditions: donor, acceptor (1 equiv), RT, CH2Cl2, 1 h.
[b] Yield of isolated product after silica gel chromatography.
To explore another acceptor containing a commonly used
protecting group, acceptor 6c was selected. Unlike the benzyl
groups (OBn) on 6b, the benzoate groups (OBz) on 6c lead to
glycosides. Here, the coupling of benzylated galactoside
donor 3b to allyl acceptor 6a in the presence of promoter 2
was considered as our model thioglycoside activation reac-
tion. Moreover, to make accurate and consistent comparison
of the differential loading, we quenched the reactions
(Table 2) after one hour, since the total reaction time was
already determined for the same reaction in Table 1, entry 2.
Fortunately, very high to good conversion rates were still
seen as the amount of promoter was steadily decreased
(Table 2). Only a modest decrease in yields (86% to 68%)
was observed as the loading was decreased from 100% to
30%. Nevertheless, a high yield of 76% was obtained even
when decreasing the amount of promoter to 50% of that
previously used. This result is particularly remarkable, since
none of the reported thioglycoside promoters have been able
to effect such activations with less than stoichiometric
amounts of promoter without other additives or co-promot-
ers.
a
deactivated acceptor. This acceptor was particularly
selected, because previously[41] it was observed that coupling
similarly deactivated acceptors with reactive donors in the
presence of NIS resulted in the formation of irreversible N-
succinimide glycosides of the donors as major products.
However, when using promoter 2, the required disaccharides
were obtained in high to good yields with both reactive
donors (Table 1, entries 9–10) and a deactivated donor
(entry 8). Interestingly, the rate of glycosylation did not
differ much with alterations in the electronics of the acceptor
(Table 1, entries 5 and 9, 6 and 8), but a change from disarmed
to armed donors (entries 8,9) had a significant impact on the
reaction times.
To extend the method to amino sugars, a glucosamine
acceptor 6d containing a variety of functional groups,
including benzyl and allyl as alcohol protecting groups and
phthalimido (Phth) as amine protecting group, was chosen.
This acceptor with a free 4-hydroxy group would also validate
our promoter for making 1!4-linked disaccharides, which in
general are difficult to construct owing to the low reactivity[42]
of the C-4 hydroxy group. On reaction of acceptor 6d with the
perbenzylated galactosyl donor 3b (Table 1, entry 11) using
promoter 2, the 1!4-linked galactose–glucosamine disac-
charide was obtained in 72% yield without any interference
with the other functional groups. Coupling of acceptor 6d to
fucosyl donor 5a (Table 1, entry 12) met with similar success.
To further authenticate as well as to confirm our finding
with another thioglycoside activation, we chose the glycosy-
lation of fucosyl donor 5a and glucosamine acceptor 6d
(Scheme 3) with only half the amount of promoter previously
used. The activation was achieved in similar times (Table 1,
entry 12). The donor was completely consumed and the yield
was also comparable to the earlier trial. These trials show that
less than a stoichiometric amount of promoter 2 (ꢁ 0.7 equiv)
is sufficient for complete activation.
The two common oxidation states for Bi are BiIII and BiV;
the + 3 state is to date better known. For a preliminary
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tion, the b-galactosyl donor was noticed to be anomerizing to
the a-anomer, yet still no product was detected. However, the
donor was not seen to be hydrolyzed in this case. Considering
the results above, we can assume that BiV is responsible for
the activation of thioglycosides rather than BiIII. Previously
BiV was used[32] in oxidation and some addition reactions,
particularly phenylation. David and Thieffry also tried to
selectively oxidize carbohydrate alcohols with Ph3Bi(OAc)2,
which incidentally is the first example[44] of BiV in carbohy-
drate chemistry. Nonetheless, formation of such addition
products was not observed with the thioglycosyl donors or
glycosyl acceptors under consideration.
In conclusion, the first demonstration of the catalytic
utility of pentavalent bismuth compounds, specifically Ph3Bi-
(OTf)2, has been shown in the context of a thioglycoside
activation reaction. This new promoter has shown distinct
advantages over most current thioglycoside activators,
namely high solubility, air/light stability, and a long shelf
life. Most importantly, this promoter can activate thioglyco-
sides with as little as 0.5 equivalent and at room temperatures.
The scope of reactivity was studied with a wide variety of
sugar donors carrying diverse protecting groups, and products
were seen to form in good to excellent yields. The diastereo-
selectivity of the reactions seem to follow reported trends.
Unexpectedly, the activation was found to be uniquely related
to pentavalent and not trivalent bismuth, a fact that should
spur additional work in developing the chemistry of this
relatively nontoxic metal.
Scheme 3. Glycosylation with donor 5a and acceptor 6d.
investigation into the activity of the Bi species, a set of control
experiments (Scheme 4) was designed. For these studies, we
went back to the model glycosylation of donor 3b and
acceptor 6a, which was also selected for the loading experi-
ments. To compare the reactivity with that observed in the
Scheme 4. Control studies with model glycosylation of donor 3b and
acceptor 6a. Reaction conditions: donor, acceptor, promoter (1 equiv),
a–c) CH2Cl2, RT, d) CH3CN, RT.
Received: May 14, 2013
Published online: June 28, 2013
previous study (Table 2), the reaction was also analyzed after
one hour. At first, a trial glycosylation using only triphenyl
bismuth (Scheme 4a) as a promoter was performed. How-
ever, no change in the reaction mixture or formation of
product after 1 h, or even after reaction overnight, was
observed. Next, a 1:2 mixture of Ph3Bi and triflic acid was
used as an activator (Scheme 4b), which resembles the
composition of the promoter Ph3Bi(OTf)2. Product formation
again was not observed, although a slow anomerization of the
b-galactosyl donor was seen, likely owing to the presence of
the strong acid, TfOH. When examining the reaction over
time, the donor was hydrolyzed completely without any
formation of desired product. This result led to the inference
that Bi in the BiV state is necessary; it is not a mixture of Ph3Bi
and TfOH that performs the activation.
Next, the activity of TfOH (Scheme 4c) as a promoter in
thioglycoside activation was tested. The donor anomerized
slightly faster, thereby indicating that previously the reactivity
of TfOH was slowed perhaps in association with Ph3Bi. The
hydrolyzed donor was also found to be the major product
after an overnight reaction. Finally, another BiIII compound
(Ph2BiOTf)[43] was checked for its reactivity, since it resembles
the promoter Ph3Bi(OTf)2. The mechanistic pathway of the
activation is still unclear (NMR experiments were not
definitive), yet the soft Lewis acidity of the Bi compound
can be imagined to play a pivotal role. The glycosylation
(Scheme 4d) was closely monitored, but no formation of our
desired product was seen in 1 h. After an overnight observa-
Keywords: bismuth · carbohydrates · glycosylation ·
Lewis acids · synthetic methods
.
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[34] Crystal data: C20H19BiF6O6S2, Mr = 780.45; orthorhombic,
Cmcm; a = 10.7232(9), b = 18.469(2), c = 14.1481(11) ꢄ; V=
2802.0(4) ꢄ3; Z = 4; Dcalcd = 1.850 gcmꢀ3; F(000) = 1496; T=
173 K; R = 0.0388; Rw = 0.1184 for 2315 observed data. Inten-
sity data were collected on a Bruker APEX 2 diffractometer
with Mo Ka radiation (l = 0.71073 ꢄ). The molecule occupies
a special crystallographic position m2m and it is disordered by
[44] S. David, A. Thieffry, J. Org. Chem. 1983, 48, 441 – 447.
Angew. Chem. Int. Ed. 2013, 52, 8441 –8445
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8445