N-Trifluoromethylthiosaccharin/TMSOTf: A New Mild Promoter System for Thioglycoside Activation

Article abstract of DOI:10.1002/ejoc.201900265

For the first time, N-trifluoromethylthiosaccharin was used to activate thioglycosides in the presence of catalytic amounts of TMSOTf. The results show that the activated thioglycosides undergo glycosidation reactions with various glycosyl acceptors to gi

Full text of DOI:10.1002/ejoc.201900265

A Journal of
Accepted Article
Title: N-Trifluoromethylthiosaccharin/TMSOTf: A New Mild Promoter
System for Thioglycoside Activation
Authors: Cian Mc Carthy, Matthias Tacke, and Xiangming Zhu
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201900265
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10.1002/ejoc.201900265
European Journal of Organic Chemistry
N-Trifluoromethylthiosaccharin/TMSOTf: A New Mild Promoter System for
Thioglycoside Activation
Cian Mc Carthy, Prof. Matthias Tacke, Prof. Xiangming Zhu*
Centre for Synthesis and Chemical Biology, UCD School of Chemistry,
University College Dublin, Belfield, Dublin 4, Ireland
Tel: +353 17162386; Fax: +353 17162501; e-mail: Xiangming.Zhu@ucd.ie
https://people.ucd.ie/xiangming.zhu
Abstract: For the first time, N-trifluoromethylthiosaccharin was used to activate thioglycosides
in the presence of catalytic amounts of TMSOTf. The results show that the activated
thioglycosides undergo glycosidation reactions with various glycosyl acceptors to give the
corresponding glycosides in moderate to excellent yields. In addition, orthogonal activation of
ethyl thioglycoside in the presence of phenyl thioglycoside was achieved under the above
conditions.
Introduction
The first report of thioglycoside dates back more than a century ago[1] but new preparative
and activation procedures are still emerging. The great interest in thioglycosides stems from their
high stability under a wide range of conditions for protecting group manipulation and their
survival under various activation conditions for other glycosyl donors. Anomeric thioether
groups can thus act themselves as temporary protecting groups and therefore, are often used as
glycosyl acceptors in glycosylation strategies. The compatibility of thioether with other leaving
groups makes thioglycosides key building blocks in many oligosaccharide syntheses and greatly
improves synthetic efficiency.[2] Such dependence on the stability of thioglycosides continues to
be demonstrated in recent years. For example, the use of thioglycosides was central to a recent
one-pot synthesis of a hexasaccharide intermediate, which was then triply glycosylated with a
thiofucoside donor in one step to furnish the cancer-associated nonasaccharide KH-1 antigen.[3]
Tumor-associated carbohydrate antigen RM2 derived from prostate cancer was also synthesized
efficiently in one-pot manner with a series of thioglycosides as important building units.[4]
1
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10.1002/ejoc.201900265
European Journal of Organic Chemistry
Undoubtedly, thioglycosides have proven and will continue to be one of the most versatile and
frequently employed classes of sugar building blocks.[5] Albeit high stability, thioglycosides can
be activated effectively under the action of different promoters, and as such serve widely as
glycosyl donors. In general, thioglycosides can be activated by two types of promoters: one is via
a halonium system and the other is via an organosulfur-based system. The latter has become a
valuable promoter for thioglycoside activation in the past decade. Several sulfinates in
combination with Tf2O have been developed as thioglycoside activators.[6] For example, S-(4-
methoxyphenyl) benzenethiosulfinate (MPBT)/Tf2O was used to promote the formation of β-
mannosidic linkage in excellent yield and stereoselectivity.[6a] Both 1-benzenesulfinylpiperidine
(BSP)/Tf2O[6b] and Ph2SO/Tf2O[6c] also proved to be very powerful thiophilic promoter systems
and could both activate disarmed thioglycosides. An important feature of these sulfinyl systems
is their capacity to preactivate thioglycoside donors at low temperatures,[7] which allows a
thioglycosyl donor to be activated prior to the addition of another thioglycosyl acceptor,
regardless of their reactivity.[8] The success of this preactivation strategy has thus been utilized to
design efficient one-pot synthesis strategies.[6d],[9] Numerous other organosulfur systems, such as
Me2S2/Tf2O,[10]
O,O-dimethylthiophosphonosulfenyl
bromide
(DMTPSB)/AgOTf,[11]
EtSNPhth/TrB(C6F5)4,[12] N-(phenylthio)-ε-caprolactam/Tf2O[13] and N-(p-methylphenylthio)-ε-
caprolactam/TMSOTf[14] have also been developed for the activation of thioglycosides. At
present, thioglycoside activation is a focus point in carbohydrate chemistry,[5] however, there is
still a drive for finding more amenable and compatible promoters for oligosaccharide assembly.
Recently, we reported an effective thioglycoside activation system, in which an organosulfur
thioperoxide reagent in combination with TMSOTf was found to be able to activate both armed
and disarmed thioglycoside donors.[15] A plausible reaction mechanism was also proposed for the
activation process. As an extension of this work and part of our ongoing project on thioglycoside
chemistry,[16] a more readily available organosulfur reagent was also developed as a
thioglycoside promoter and reported here. Recently, a new electrophilic trifluoromethylthiolating
reagent, N-trifluoromethylthiosaccharin 1 (Figure 1), was reported by Shen and his coworkers for
the direct trifluoromethylthiolation of various nucleophiles such as alcohols, amines, thiols, and
electron rich arenes.[17] This reagent is moisture and air stable and can be synthesized on a large
scale.[18] We became curious whether this compound could also be used to activate thioglycoside
donors.
2
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10.1002/ejoc.201900265
European Journal of Organic Chemistry
Figure 1. Structure of N-Trifluoromethylthiosaccharin 1
O
O
S
N
SCF3
O
1
N-Trifluoromethylthiosaccharin
Hence, investigations towards thioglycoside activation with compound 1 were initiated. The
results show that compound 1 is a mild and effective promoter for thioglycoside activation. Its
usefulness is illustrated by a successful orthogonal activation of S-ethyl donors in the presence of
S-phenyl donors, enabling the design of sequential glycosidation strategies in the future.
Results and Discussion
Based on our previous work,[15] compound 1 alone would unlikely activate thioglycosides.
Indeed, when thioglucoside donor 2 was treated with 1 in the presence of 1,2:3,4-di-O-
isopropylidiene galactopyranose 3, no reaction was observed and 2 was not affected even after a
prolonged reaction time (Table 1, entry 1). Hence, several common acids were tested in order to
initiate the reaction, as shown in Table 1. Upon addition of Lewis acid BF3·OEt2, the desired
disaccharide 4 was produced but in relatively low yields (Table 1, entry 2). This result reassured
that an additional acidic catalyst was needed to initiate the reaction. Brønsted acid TfOH was
also used to initiate the reaction (Table 1, entry 3). As anticipated, the reaction proceeded well to
give disaccharide 4 in much higher yield (67%). However, the reaction was undermined by
heavy decomposition of thioglycoside 2 as detected by TLC. TMSOTf being less acidic than
TfOH was then used together with 1 to promote the reaction, and to our delight, the
glycosidation proceeded smoothly at -30 °C to generate 4 in a very high yield (Table 1, entry 4).
Table 1. Acid effect on 1-mediated activation of thioglycoside
3
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10.1002/ejoc.201900265
European Journal of Organic Chemistry
OBz
OH
OBz
O
O
O
O
1, acid
CH2Cl2
BzO
BzO
O
O
O
BzO
O
O
SEt +
BzO
O
BzO
-30 oC to rt
OBz
O
O
2
3
4
O
Entry
Acid
Yield (%)
1
2
3
4
-
-
.
BF3 Et2O
TfOH
46
67
74
TMSOTf
With optimal conditions established, the next step was to assess the ability of N-
trifluoromethylthiosaccharin 1 to activate a series of thioglucoside donors. Recently the
reactivity of a series of α- and β-thioglucosyl donors bearing different protecting groups has been
investigated in our laboratory and their reactivity differences have been exploited to design novel
one-pot oligosaccharide synthesis strategies.[19] Thioglucosides 2 and 5-8 with disparity in
reactivity from this series were then chosen to assess the capability of 1 to activate thioglycoside
donors. The results are summarized in Table 2. All the reactions were cooled down to a low
temperature and warmed up gradually with a strategy being to initiate the reaction at the lowest
possible temperature to assess the donor reactivities under the present conditions. It is worth
mentioning here that reaction conditions could have a great impact on reactivity of thioglycoside
donors.[20] The reactions were monitored carefully by TLC and the temperature, at which the
reactions were initiated, was recorded. Once initiated, the reaction mixture was slowly warmed
up to the point where no acceptor remained, as determined by TLC, and was kept at that
temperature for 30 minutes. The reactions were then ceased upon quenching by triethylamine. As
expected, activation of donor 2 occurred smoothly under the above conditions to give
disaccharide 10 in 74% yield in the presence of acceptor 9 (Table 2, entry 1). It is evident that
the complete β-stereoselectivity in this glycosidation resulted from the assistance of the
participating benzoyl group at the C-2 position of the donor 2.
4
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10.1002/ejoc.201900265
European Journal of Organic Chemistry
Table 2. Activation of donors 2 and 5-8 with N-Trifluoromethylthiosaccharin 1/TMSOTf[a]
OH
1, TMSOTf
O
BzO
10-12
Donors 2 or 5-8
+
CH2Cl2
-40 oC to rt
BzO
BzO
OMe
9
Entry
1
Donor
Product
Temp
Yield
(%)[b]
α/β
(ºC)
ratio[c]
OBz
O
BzO
OBz
O
-30
74
89
90
86
0
β only
BzO
O
BzO
O
BzO
BzO
SEt
SEt
BzO
BzO
OBz
BzO
10
OMe
OMe
OMe
OMe
2
OBn
OBn
O
O
BnO
BnO
BnO
BnO
2
3
4
5
-40
-40
-40
-30
1:2
O
OBn
BnO
O
5
BzO
BzO
BzO
11
OBn
OBn
O
O
BnO
BnO
BnO
BnO
SEt
O
β only
OAc
AcO
O
6
BzO
BzO
BzO
12
OBn
OBn
O
O
BnO
BnO
BnO
BnO
1:1
O
BnO
BnO
O
SEt
7
8
BzO
BzO
BzO
11
OBz
O
OBz
O
BzO
BzO
BzO
BzO
O
-
BzO
O
BzO
SEt
BzO
BzO
BzO
10
OMe
[a] Reaction conditions: donor (1.1-1.2 equiv), 9 (1.2 equiv), 1 (1.2 equiv), TMSOTf (0.6 equiv),
under a N2 atmosphere. [b] Isolated yield. [c] Determined by integration of the proton signals in
the 1H NMR spectrum after chromatographic purification.
5
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10.1002/ejoc.201900265
European Journal of Organic Chemistry
The armed β-thioglucoside 5 being more reactive than donor 2, was activated at a lower
temperature (-40 °C) and coupled to acceptor 9 to generate the desired disaccharide 11 in 89%
yield. All the glycosidation reactions in Table 2 were carried to completion and were clean with
trace hydrolysis, which proceeded efficiently to form disaccharides in yields that are comparable
to those in the literature.[10-14] Examination of the α/β composition of 11 gave an unexpected
result, with the β-anomer being the predominant component of the mixture. We surmised that the
influence of promoter 1 was at play. Investigation into the reaction mechanism in the future will
be needed to explain the stereoselectivity. Next, a superarmed donor was prepared to
demonstrate that it could undergo glycosidation under the new activation conditions. The 3,4,6-
tri-O-benzyl-2-O-acetyl β-thioglucoside 6 was found to be a more reactive donor than the
perbenzylated derivatives.[19],[21] This increase in reactivity was attributed to the O-2/O-5
cooperative effect.[22] Donor 6 was thus coupled with 9 under the action of 1 and catalytic
amounts of TMSOTf. As anticipated, the reaction was initiated at -40 °C, and the desired
disaccharide 12 was produced in excellent yield (90%) and β-only selectivity.
Following these promising results, the compatibility of this new system with α-thioglucoside
donors was then tested. The fully benzylated α-donor 7, being slightly less reactive than its
counterpart 5,[19] was successively activated under the same conditions at -40°C and coupled
with 9 to produce disaccharide 13 in very high yield (86%) and α/β 1:1 selectivity (Table 2, entry
4). The result has indicated that anomeric configuration of thioglycosides can also have a bearing
on reaction stereoselectivity. Apparently, these glycosidation reactions did not undergo simple
SN1 mechanism. To test the effectiveness of the new promoter system, the most unreactive α-
thioglucoside 8 of the series was also subjected to the above conditions; surprisingly, no
activation of this α-donor was observed (Table 2, entry 5). Donor 8 having an ester protecting
group at 2-position and α-configured leaving group was predicted to have a negating effect on its
reactivity,[19] but we had expected this disarmed donor to be activated by the new promoter.
Despite no activation, this exceptionally low reactivity of 8 could potentially be exploited to
design one-pot oligosaccharide synthesis where we could activate selectively its counterpart
donor 2 in the presence of its derived thioglycosyl acceptor using the present new promoter
system.
6
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10.1002/ejoc.201900265
European Journal of Organic Chemistry
To further explore the scope and generality of the new promoter system for the activation of
thioglycosides, we proceeded to use a wide range of thioglycosyl donors and other acceptors, as
shown in Table 3. Based on the above results, all reactions were cooled down at -30 °C and
slowly brought up to -10 °C and stirred at this temperature for 30 min. Under such conditions,
the compatibility of the new system with sugars carrying different protecting groups was
demonstrated. Acceptors with secondary hydroxyl groups were also used for the glycosylation
reactions. Hence, acceptor 17 having a free hydroxyl group at the 2-position was glycosylated
with donor 2 to give disaccharide 26 in 71% yield (Table 3, entry 1). Similarly, acceptor 18
having a secondary hydroxyl group was also reacted with α-donor 7 under the same conditions to
furnish disaccharide 27 in very high yield (Table 3, entry 2). In both cases, the acid sensitive
benzylidene group survived under the reaction conditions indicating the mildness of this new
activation system. The mannosyl acceptor 19 was also coupled to donor 5 under the conditions to
produce disaccharide 28 as an α/β mixture in 68% yield.
Table 3. Glycosidation reactions promoted by 1/TMSOTf
Entry
Donor
Acceptor
Product
Yield α/β
(%) ratio
OBz
O
O
O
BnO
O
O
O
Ph
Ph
O
BzO
BzO
SEt
BnO
HO
17
1
71
β
O
OMe
OBz
BzO
O
OMe
2
BzO
BzO
only
OBz
26
OBn
O
O
O
O
Ph
Ph
O
O
O
BnO
BnO
O
HO
BnO
18
O
BnO
BnO
BnO
2
3
87
68
1:1
BnO
BnO
OMe
OMe
SEt
7
OBn
27
OBn
OH
O
OBn
BnO
BnO
BnO
O
O
BnO
BnO
BnO
BnO
SEt
1.5:1
BnO
OBn
OAll
19
O
O
5
BnO
BnO
BnO
OAll
28
OBz
OBn
OBn
β
O
O
O
BnO
BnO
BnO
BnO
BzO
BzO
OAll
OAll
SEt
4
74
only
OBz
OH
O
BzO
BzO
O
20
2
BzO
OBz
29
7
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10.1002/ejoc.201900265
European Journal of Organic Chemistry
OBz
OBn
O
OBn
O
BnO
BnO
HO
BnO
BnO
5
β
O
BzO
SEt
BzO
BzO
O
O
OBz
OAc
BzO
70
48
only
OBz
OMe
OMe
21
2
BzO
30
OAc
OH
O
6
β
O
O
O
O
O
AcO
SEt
AcO
AcO
O
AcO
OAc
13
O
O
O
only
OAc
O
O
O
3
31
O
OBz
OBz
OH
BzO
BzO
BzO
O
7
β
O
O
O
O
O
SEt
O
BzO
O
O
69
only
OBz
BzO
14
O
3
O
32
O
OPiv
OBz
OPiv
BzO
OBz
BzO
BzO
8
β
O
O
O
O
HO
BnO
O
BnO
SEt
BzO
BnO
77
75
only
OBz
BnO
BzO
OMe
OMe
OMe
OMe
14
33
22
O
O
OH
9
BnO
BnO
BnO
SEt
O
BnO
OBn
15
O
BnO
BnO
BnO
O
1:2
-
BnO
BnO
BnO
23
BnO
34
OBz
OH
OBz
O
10
O
O
BzO
SPh
BzO
BzO
BzO
BzO
O
BzO
OBz
16
0
BzO
O
BzO
OMe
BzO
BzO
9
BzO
10
OMe
SPh
OBz
BzO
OBz
OH
BzO
BzO
β
O
O
O
BzO
BzO
O
SEt
SPh
BzO
11
73
72
only
OBz
BzO
BzO
35
O
OBz
14
24
BzO
BzO
OBz
BzO
OBz
O
OBz
BzO
BzO
HO
BzO
BzO
β
O
O
SEt
OBz
O
O
BzO
12
only
OBz
BzO
BzO
BzO
36
14
SPh
25
SPh
The β-configurated glucosyl acceptor 20, together with donor 2, was also subjected to the
above reaction conditions, and expectedly, disaccharide 29 was produced smoothly in 74% yield
(Table 3, entry 4). Another secondary alcohol 21 was also glycosylated with donor 2 under the
same conditions to give the desired disaccharide 30 in 70% yield. The peracetylated
thioglucoside 13 was also tested against the new promoter, and coupled with acceptor 3 (Table 3,
8
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10.1002/ejoc.201900265
European Journal of Organic Chemistry
entry 6). Unfortunately, the desired disaccharide 31 was produced in low yield (48%), and the
reaction was undermined by donor hydrolysis as detected by TLC. This indicated peracetylated
thioglycosides would not be suitable for this glycosidation protocol. This result, however,
conforms to the literature where peracetylated thioglycoside donors often preformed less
efficiently, compared to perbenzoylated donors.[23] As such, the fully benzoylated thiogalactoside
14 was then used as donor and subjected to the glycosidation reactions with acceptors 3 and 22,
respectively, under the above activation conditions; as expected, the corresponding disaccharides
32 and 33 were generated in very good yields (Table 3, entries 7 and 8). Subsequently, the
xylose-derived thioglycoside 15 was treated with the new promoter system in the presence of
acceptor 23, again the reaction proceeded smoothly to give the product 34 as an α/β mixture in
75% yield (Table 3, entry 9).
In addition to ethyl thioglycosides, we applied the glycosidation protocol to phenyl
thioglycoside donors. Work by Lahmann et al.[24] quantified the reactivity difference between
thioglycoside donors differing only in the aglycon moiety. An ethyl thioglycoside was reported
to be more reactive than its corresponding phenyl thioglycoside, however, the reactivity
difference between these donors was rarely exploited for one-pot strategies.[5i] That said, phenyl
thioglucoside 16 was chosen to couple with acceptor 9 (Table 3, entry 10). To our surprise, the
reaction failed to give any product under the same conditions and 16 was not affected even after
a prolonged reaction time, i.e. no activation of donor 16 was observed. This was in sharp contrast
with our previous organosulfur-mediated glycosidation reactions, in which phenyl thioglycosides
could be readily activated with a thioperoxide in the presence of catalytic amounts of
TMSOTf.[15] In spite of the extremely low reactivity of phenyl thioglycoside under the present
conditions, we felt that we could turn it into an advantage. Hence, phenyl thioglucosyl acceptor
24[25] was prepared and exposed, together with ethyl thiogalactoside 14, to the above activation
conditions; as expected, the orthogonal activation of ethylthio group in the presence of
phenylthio group took place smoothly and gave rise to the disaccharide 35 in 73% yield (Table 3,
entry 11). Obviously, 35 could be used immediately as glycosyl donor for further glycosidation
reactions if necessary. Likewise, phenyl thiomannoside 25 remained also inactive under the
present activation conditions, as shown in entry 12 in Table 3. The selective activation of ethyl
thioglycoside 14 was readily attained in the presence of 25, and disaccharide 36 was generated in
9
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10.1002/ejoc.201900265
European Journal of Organic Chemistry
high yield, which could also serve as donor for further glycosidation reactions. The ready
formation of 35 and 36 offered a preliminary suggestion that the new organosulfur system may
well be employed to design thioglycoside-based one-pot oligosaccharide synthesis.
As could be summarized from the above results, N-trifluoromethylthiosaccharin 1 plus
TMSOTf is a very mild and effective promoter and holds great potential in selective activation of
thioglycosides with different reactivity. Although a thorough mechanistic investigation is
pending, based on our previous work,[15] we proposed that the above glycosidation reactions
proceed via the pathway outlined in Scheme 1. It is TMSOTf that enables thiosaccharin 1 to
become a promoter as 1 alone cannot initiate the glycosidation reaction. The electrophilicity of 1
is enhanced in the presence of TMSOTf so that it can trifluoromethylthiolate the anomeric sulfur.
The thioglycoside donor is then transformed into the sulfonium ion intermediate 37, which
subsequently collapses into an oxacarbenium ion 38 and ethyltrifluoromethyl disulphide
(EtSSCF3). Nucleophilic attack by an alcohol at the anomeric centre of 38 or other reactive
Me3Si
O
OTf
O
S
O
SEt
CF3S
N
RO
O
1
O
O
OTMS
O
S
S
N
NH
SCF3
40
O
O
41
O
S+
-OTf
Et
RO
RO
37
H+
O
Tf
EtSSCF3
O+
O
-OTf
OR'
R'OH
RO
39
38
Scheme 1. Proposed mechanistic pathway for 1/TMSOTf-promoted thioglycoside activation
10
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10.1002/ejoc.201900265
European Journal of Organic Chemistry
intermediates, such as glycosyl triflate or acyloxonium ion in the case of neighbouring group
participation, then gives the glycosidation product 39. Meanwhile, thiosaccharin 1 is converted
into saccharin after loss of SCF3 group.
Conclusions
In conclusion, we have identified N-trifluoromethylthiosaccharin 1, in the presence of
catalytic amounts of TMSOTf, as a powerful reagent to activate a range of ethyl thiogylcosides
with varying reactivity. Upon activation, these thioglycosides underwent glycosidation reactions
with various glycosyl acceptors to give the desired disaccharides in good to very high yields.
More interestingly, this new promoter system could selectively activate ethyl thioglycoside in the
presence of phenyl thioglycoside with the same protecting group pattern. This orthogonal
activation, in combination with our previous thioperoxide promoter system, can be readily
utilized to design iterative one-pot oligosaccharide assembly, which is currently underway.
Further investigation on the use of this new promoter system to achieve orthogonality within
thioglycosides bearing different aglycon substituents is also in progress.
Acknowledgments
The authors thank University College Dublin for financial support. C. Mc C. is grateful for a
demonstratorship from the UCD School of Chemistry.
Keywords: Thioglycoside activation; N-trifluoromethylthiosaccharin; Glycosidation reaction;
Chemoselective glycosidation; Oligosaccharide
Supporting information and ORCID(s) from the author(s) for this article are available on the
WWW under http://doi.org/
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11
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