Chemoselectivity in Self-Promoted Glycosylation: N- vs. O-Glycosylation

Article abstract of DOI:10.1002/ejoc.202000526

Self-promoted glycosylation using trichloroacetimidates and sulfonamides have recently been developed. In this communication, we study the parameters controlling the chemoselectivity between a nucleophilic sulfonamide nitrogen and an alcohol, both contain

Full text of DOI:10.1002/ejoc.202000526

European Journal of Organic Chemistry
Accepted Article
Accepted Article
Title: Chemoselectivity in Self-Promoted Glycosylation: N- versus O-
Glycosylation.
Authors: Alessandro Pinna and Christian Marcus Pedersen
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.202000526
Link to VoR: https://doi.org/10.1002/ejoc.202000526
01/2020

10.1002/ejoc.202000526
European Journal of Organic Chemistry
COMMUNICATION
Chemoselectivity in Self-Promoted Glycosylation: N- versus O-
Glycosylation.
Alessandro Pinna[b], Christian Marcus Pedersen*[a]
[a]
Prof. C. M. Pedersen
Department of Chemistry, University of Copenhagen
Universitetsparken 5, 2100 Copenhagen O
Denmark.
E-mail: cmp@chem.ku.dk
[b]
A. Pinna
Department of Chemistry and Industrial Chemistry
Università di Genova
Italy
Supporting information for this article is given via a link at the end of the document.
Abstract: Self-promoted glycosylation using trichloroacetimidates
and sulfonamides have recently been developed. In this
communication, we study the parameters controlling the
chemoselectivity between a nucleophilic sulfonamide nitrogen and an
alcohol, both contained in the same molecule. The influence of solvent
polarity and concentrations have been studied, and it has been
revealed how the chemoselectivity, and to some extend the
stereoselectivity, can be controlled. The experimental results
furthermore give insight into the reaction mechanism of this self-
promoted glycosylation and the mechanism of glycosylation reactions
in general.
Recently, we reported that sulfonamides react with
trichloroacetimidates (TCAs) in
a stereospecific and self-
promoted manner forming N-glycosides (Figure 1). 19 The acidity
The glycosylation reaction is the most studied reaction in
carbohydrate chemistry.1-4 It involves a glycosyl donor, which
must be activated and acts as electrophile, and a glycosyl
acceptor which plays the role as the nucleophile.5 Based on the
kind of acceptor involved, different types of glycosyl bonds can be
formed.6-11 Among them, the C-O bond and the C-N bond are of
particular interest, as they are ubiquitous in Nature and hence
have received much attention.12-14 O-glycosides are omnipresent
with different functions and relevance from a biological point of
view.5 N-glycosylation is crucial in post-translational modifications,
where monosaccharides are connected to proteins via a glycosyl
amide bond linkage.13,15 Other biologically active N-glycosides,
like N-glycosyl sulfonamides, have recently gained attention.
These can be obtained via a palette of reactions e.g. synthetized
from addition of sulfamides to acetyl-protected glycals (via Ferrier
rearrangement) or from monosaccharides, both in the presence
of boron trifluoride etherate.16,17 Some of these compounds have
been found to be carbonic anhydrase (CA) inhibitors.17-19 The
same biological activity has been shown by O-glycosides
Figure 1 Self-promoted glycosylations. Previous work was focused on self-
promoted N-glycosylation.19 This work studies the chemoselectivity when
having two nucleophiles in the same molecule – high-lighted in red and blue.
of the sulfonamide functionality makes them able to activate TCA
donors with a consecutive nucleophilic attack to the glycosyl
cation (Scheme 1) i.e. they act as both catalysts and nucleophiles
(glycosyl acceptors). The reactions have been found to be highly
stereospecific, which is very useful as TCAs often can be
synthesized in a stereoselective manner.23 One can thereby
determine the N-glycoside stereochemistry already when
synthesizing the glycosyl donor. In our preceding work, it was also
realized that the degree of stereospecificity was dependent on the
anomeric configuration. Axial TCAs often resulted in high
equatorial selectivity, whereas the opposite was not always the
case. This difference in selectivity, when using the preferred
solvent for glycosylations (DCM), was explained by two
mechanistic pathways (Scheme 1). The activation of the glycosyl
donor can either lead to the formation of an associated or
dissociated ion pair. When the TCA is axially oriented, the leaving
group departure and nucleophilic attack are more aligned and
follows a more associated IP (Ion Pair) mechanism resulting in
shielding one site and hence nucleophilic attack from the opposite
site of the glycosyl cation. When β-TCAs are activated and
containing aromatic sulfonamide residues.20-22
A different
synthetic pathway was developed for this type of compounds
where a 1,3-dipolar cycloaddition reaction was used in order to
generate
1,4-disubstituted
1,2,3-triazole
glycoconjugate
sulfonamides from alkyne-substituted sugars and azido aromatic
sulfonamides.20,21 Due to the general importance of N-glycosides,
the interest in studying the “Glycome” as well as drug properties
of small molecules, glycosyl sulfonamides and the development
of new simple, selective and mild methods are highly important.
1
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10.1002/ejoc.202000526
European Journal of Organic Chemistry
COMMUNICATION
substituted a more dissociative reaction path is followed and the
nucleophilic
attack is sterically hindered by the leaving group, as well as the
substituents of the pyranose ring (Scheme 1). Consequently, this
can lead to in situ anomerisation (Scheme 1 in green and red),
resulting in formation of the α-TCA, with a consecutive loss of
stereochemical information from the glycosyl donor and hence a
mixture of products.19 Contact and solvent-separated ion pairs are
on the borderline between a SN1 and a SN2 mechanism. Small
changes in the reaction condition can therefore shift the balance
to either site. Important parameters in this is e.g. the polarity of
the solvent and its ability to promote a charge separation.3,24,25
The dissociative reaction path allows two competing reaction
pathways resulting in low anomeric selectivity. In this work, we
study the reaction mechanism in self-promoted glycosylations
and how it is influenced by the reaction conditions.
Trichloroacetimidates are one of the most used glycosyl donor
types. They are prepared under basic conditions, starting from the
hemiacetal. Depending on the conditions, mainly the α- or β-
anomer can be obtained.19,23 This study primary involves the
thermodynamically more stable axial TCA (α) as this will give raise
to the equatorial product in the glycosylation and has a greater
stability.
As a model glycosyl donor the perbenzylated glucopyranosyl
trichloroacetimidate 1 was synthesized and as a model glycosyl
acceptor 2-hydroxyethyl tosylcarbamate 2.19 This acceptor
contains both a primary alcohol and a sulfonamide, but only the
later can activate the TCA. Our hypothesis is that a mechanism
proceeding through an associative mechanism leads primary to
the N-glycoside with high stereospecificity, whereas a more
dissociative pathway gives more of the O-glycoside and this with
lower anomeric selectivity. As the reaction is self-promoted only
the two reactants and solvent are present, hence ruling out effects
of metals ions, counter ions and consequently reduce the
complexity significantly.
Initially, the reactions were performed in DCM as this is the most
commonly used solvent for glycosylations. This was done on a
preparative scale in order to separate the closely eluting reaction
products, which were to be used as references for the
determination of product ratios from crude-NMR. The isolated
yields (34% for N-glycoside and 40% for O-glycoside) were found
to be in accordance with the ratio obtained from crude NMR.
Glycosylating 2 gave, as expected, a mixture of O- and N-
glycosides. The N-glycosylation was very selective towards the β-
product 3 in line with earlier work (associative mechanism).19 The
O-glycosides 4 on the other hand were found to be a 1:1 mixture
suggesting a reaction under diffusion control and presumably
through
a
highly reactive glycosyl cation (dissociative
mechanism). With DCM as a reference point, other solvents were
then used to study the effects of polarity and solvent properties.
Interestingly, a change in the chemoselectivity was observed
going from the least polar solvent in the study, toluene (entry 1,
Table 1) to the most polar MeNO2 (entry 4, Table 1). The
difference in ratio is going from very N-selective to O-selective,
but without a change in the anomeric selectivity of the O-
Scheme 1 Proposed mechanism for self-promoted N-glycosylation reactions.
Axial TCAs results in a more associative mechanism and hence inversion of
stereochemistry. Equatorial TCAs on the other hand follows a more dissociative
resulting in less stereospecificity and anomerisation of the TCA.
glycosides, which also seems unaffected when using
a
participating solvent like THF (See SI for details). Using MeCN
(not shown) results in the formation of a N-glycosyl acetimidate
product, confirming the participation of acetonitrile and in line with
earlier work.19
2
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10.1002/ejoc.202000526
European Journal of Organic Chemistry
COMMUNICATION
Table 1. Effect of the increasing polarity of the solvent.
glycosylation (entry 1 and 2, Table 3). Because of the lower
solvent polarity the ion pairs are tighter and the reaction with the
alcohol is less competitive. The slow conversion of the
sulfonamide to its dissociated form combined with the activation
mode, based on a pre-complexation, ensures the presence of 2
upon activation, but also an intermolecular competition from the
free alcohol giving the O-glycosylation (Scheme 1), even if it is not
predominant (entry 2 and 3, Table 2). Sub-stoichiometric amounts
of the acceptor is therefore required in order to get a more
selective N-glycosylation as a bimolecular (H-bond complex)
reaction is favored and a termolecular reaction unlikely (entry 1,
Table 2).
A rearrangement of α-TCA donor can compete in the reaction
(Scheme 1), producing the glucosyl trichloroacetamide as side
product, both with high and low concentration of 2 (see SI).
The participation of a second molecule 2 was also supported by
the results obtained when increasing the concentration of the
reaction both in toluene and DCM (Table 3), where more of the
O-glycoside formed.
Entry
1 (M)
2 (M)
Solv.
N- vs O-
glycosides
1[a]
2
0.073
0.073
0.073
0.073
0.110
0.110
0.110
0.110
PhMe
DCM
87:13
46:54
56:44
0:100
3
THF
4
MeNO2
The donor and acceptor were dissolved in the solvent at rt. and the reaction
progress was followed by TLC [a] Performed at 40˚C. See SI for more detail and
anomeric ratios of the O-glycosides.
When increasing the charge separation by using more polar
solvents, the hydroxyl group competes with the amide being the
negatively charged counter-ion. In more polar solvents, the
reaction is consequently shifted towards the O-glycosylation
(dissociative mechanism). In less polar solvent like DCM there is
less charge separation and the reaction results in both N- and O-
glycosides.
Table 3. Effect of changing the concentration of the reaction keeping the ratios.
With the reaction on the borderline between mechanistic
pathways it was expected that the concentration would also
influence the reaction outcome. Therefore, the change of
concentration of the sulfonamide was studied in both toluene and
DCM (Table 2). Increasing the concentration of the acceptor
without changing the concentration of 1 resulted in a decrease in
N-selectivity in DCM (table 2). This change in reaction outcome
can be explained by a second molecule of 2 attacking the
dissociated IP resulting in more O-glycoside. This also explains
why only a small excess of 2 is sufficient to get a change in the
chemoselectivity (entry 7 and 8, Table 2).
Entry
1 (M)
2 (M)
Solv.
N- vs O-
glycosides
1[a]
2[a]
3[a]
4
0.146
0.073
0.0146
0.146
0.073
0.0146
0.220
0.110
0.022
0.220
0.110
0.022
PhMe
PhMe
PhMe
DCM
DCM
DCM
83:17
87:13
75:25
27:73
46:54
56:44
5
6
Table 2. Effect of changing the concentration of sulphonamide 2.
The donor and acceptor were dissolved in the solvent at rt. and the reaction
progress was followed by TLC [a] Performed at 40˚C. See SI for more detail and
anomeric ratios of the O-glycosides.
Entry
1 (M)
2 (M)
Solv.
N- vs O-
glycosides
1[a]
2[a]
3[a]
4[a]
5
0.073
0.073
0.073
0.073
0.073
0.073
0.073
0.073
0.073
0.0146
0.049
0.110
0.367
0.0146
0.049
0.091
0.110
0.367
PhMe
PhMe
PhMe
PhMe
DCM
DCM
DCM
DCM
DCM
100:0
85:15
87:13
47:53
70:30
70:30
60:40
46:54
14:86
To study the effect of other counter ions, in the self-promoted
glycosylation, lithium salts were added. It has earlier been
demonstrated that counter-ions influences the outcome of
glycosylations and also that the Lewis acidity of Li+ is enough to
activate a TCA. 26,27 Using 1, 20 mol% of TfOLi or Tf2NLi and 2 in
DCM completely changed the chemoselectivity and only the O-
glycoside was observed (Table 4). Thus, the salt can change the
reaction course of the self-promoted chemoselective
6
glycosylation resembling
a
common acid catalyzed O-
glycosylation. 26 The O-glycoside 4 was obtained as a 1:1 mixture
as the major product suggesting a dissociated mechanism under
diffusion control.
7
8
When performing the same reaction, without sulfonamide present,
but with EtOH as the glycosyl acceptor, a similar stereoselectivity
was obtained, suggesting that the sulfonamide is not participating
in the activation when lithium salts are present (Table 5).
As described above, β-TCA donors often gives rise to a mixture
of anomers. When mixing 5 and 2 in DCM, a 40:60 mixture of α-
and β-N-glycosides was obtained confirming that the reaction
proceeds through a less associated reaction pathway and
therefore gives lower selectivity (Scheme 1). Furthermore, it was
noticed that the α-N-glycosides decomposed on the silica column
biasing the product ratio and yield.
9
The donor and acceptor were dissolved in the solvent at rt. and the reaction
progress was followed by TLC [a] Performed at 40˚C. See SI for more detail and
anomeric ratios of the O-glycosides.
A different trend was observed in toluene (entry 3, Table 2). The
low polarity of the solvent resulted in a decrease of the rate of
reaction and therefore the reaction temperature was increased to
40 °C in order to be complete within 48 hours. The increase of
temperature has no effect on the ratio between N- and O-
3
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10.1002/ejoc.202000526
European Journal of Organic Chemistry
COMMUNICATION
Table 4 Effect of adding litium salts.
(2) S.van der Vorm, T. Hansen, J. M. van Hengst, H. S. Overkleeft,
G. A.van der Marel, J. D.Codée, Chem. Soc. Rev. 2019, 48, 4688.
(3) L. Bohé, D. Crich, C. R. Chim. 2011, 14, 3.
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2017, 13, 93.
Entry
Cat.
Acceptor
N-
vs
O-
glycosides
(5) S. Chatterjee, S. Moon, F. Hentschel, K. Gilmore, P. H.
Seeberger, J. Am. Chem. Soc. 2018, 140, 11942.
(6) J. D. Codée, R. E. Litjens, L. J. van den Bos, H. S. Overkleeft,
G. A. van der Marel, Chem. Soc. Rev. 2005, 34, 769.
(7) C. M. Pedersen, L. U. Nordstrøm, M. Bols, J. Am. Chem.
Soc.2007, 129, 9222.
1
2
TfOLi
2
2
0:100
Tf2NLi
0:100
The donor 1 (0.073 M) and acceptor (0.110 M) were dissolved in the solvent at
rt. and the reaction progress was followed by TLC. See SI for more detail and
anomeric ratios of the O-glycosides.
(8) E. Leclerc, X. Pannecoucke, M. Ethève-Quelquejeu, M.
Sollogoub, Chem. Soc. Rev. 2013, 42, 4270.
(9) R. Miethchen, J. F. chem. 2004, 125, 895.
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4479.
Table 5 Effect of adding litium salts.
Entry
Cat.
TfOLi
Tf2NLi
/
Acceptor
EtOH
α : β product
3
4
5
56:44
25:75
/
(14) Q. Zhang, J. Sun, Y. Zhu, F. Zhang, B. Yu, Angew. Chem. Int.
Ed. 2011, 50, 4933.
EtOH
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A. Colinas, C. T. Supuran, Bioorg. med. chem. let. 2011, 21,
4447.
EtOH
The donor 1 (0.073 M) and acceptor (0.110 M) were dissolved in the solvent at
rt. and the reaction progress was followed by TLC.
(18) R. Crespo, M. G. de Bravo, P. A. Colinas, R. D Bravo,. Bioorg.
med. chem. let. 2010, 20, 6469.
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1651.
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29, 419.
Scheme 3 Self-promoted glycosylation using the -TCA 5 and 2. A mixture of
anomers of N-glycosides was obtained and the -N-glycosides was found to be
unstable upon purification.
(22) M. Singer, M. Lopez, L. F. Bornaghi, A. Innocenti, D. Vullo, C.
T. Supuran, S.-A. Poulsen, Bioorg. med. chem. let. 2009, 19,
2273.
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In conclusion, we have demonstrated that the self-promoted N-
glycosylation of sulfonamides takes place in a concerted manner
and presumably through an activated H-bond complex. When
increasing the ratio of the sulfonamide a competing reaction
pathway involving another molecule of the sulfonamide results in
the formation of O-glycosides. The same effect can be obtained
by adding lithium salts, which acts as both acid and nucleophilic
catalysts, forming an intermediate glycosyl triflate. Consequently,
all stereochemical information is lost. As the reaction is on the
borderline between mechanisms, solvents influence the outcome
dramatically. Apolar solvents favors tight ion pair leading to an
associative mechanism, i.e. SNi-type, whereas polar solvents
leads to a dissociative mechanism closer to the SN1 mechanism.
By tuning the parameters one can control the chemoselectivity
and to some extend the stereoselectivity.
Acknowledgements
EU is thanked for an Erasmus+ stipend to AP.
Keywords: Glycosylation • Self-promoted • Chemoselectivity •
Stereoselectivity • N-Glycosides
(1) T. G. Frihed,. M.Bols, C. M. Pedersen, Chem. rev., 2015, 115,
4963.
4
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10.1002/ejoc.202000526
European Journal of Organic Chemistry
COMMUNICATION
Entry for the Table of Contents
The chemo- and stereoselectivity of a self-promoted glycosylation is studied by changing solvents, molarity and ratio between the
glycosyl donor and acceptor. The outcome of the reaction is found to be strongly dependent on these parameters and can to a large
extend be controlled. SNi and SN1 are suggested as to be the competing mechanistic pathways.
Key topic: Glycosylation • Self-promoted • Chemoselectivity • Stereoselectivity • N-Glycosides
5
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