Indolylthio glycosides as effective building blocks for chemical glycosylation

Article abstract of DOI:10.1021/acs.joc.0c00943

The S-indolyl (SIn) anomeric moiety was investigated as a new leaving group that can be activated for chemical glycosylation under a variety of conditions including thiophilic and metal-assisted pathways. Understanding of the reaction pathways for the SIn moiety activation was achieved via the extended mechanistic study. Also reported is how the new SIn donors fit into selective activation strategies for oligosaccharide synthesis.

Full text of DOI:10.1021/acs.joc.0c00943

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Article
Indolylthio glycosides as effective building blocks for chemical glycosylation
Ganesh Shrestha, Matteo Panza, Yashapal Singh, Nigam P. Rath, and Alexei V. Demchenko
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.0c00943 • Publication Date (Web): 06 Jul 2020
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Indolylthio glycosides as effective building blocks for chemical glycosylation
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Ganesh Shrestha, Matteo Panza, Yashapal Singh, Nigam P. Rath, and Alexei V. Demchenko*
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Department of Chemistry and Biochemistry, University of Missouri – St. Louis, One University
Boulevard, St. Louis, Missouri 63121, USA
Need excess NIS to proceed
ROH
OBx
O
H
BxO
BxO
N
Activation A:
NIS/TfOH
S
BxO
I
OBx
OBx
H
These conditions are
orthogonal to thioglycosides
and/or thioimidates
O
BxO
BxO
O
N
BxO
S
BxO
BxO
OR
BxO
Bx = Bn or Bz
OBx
O
Activation B:
TMSOTf/Ag2CO3
BxO
BxO
N
S
ROH
BxO
Need excess TMSOTf to tautomerize
ABSTRACT: S-Indolyl (SIn) anomeric moiety was investigated as a new leaving group that can
be activated for chemical glycosylation under a variety of conditions including thiophilic and
metal-assisted pathways. Understanding of the reaction pathways for the SIn moiety activation
was achieved via the extended mechanistic study. Also reported is how the new SIn donors fit
into selective activation strategies for oligosaccharide synthesis.
Introduction
Chemical synthesis of glycan sequences of even moderate complexity still remains a
considerable challenge. As such, the development of efficient strategies for the oligosaccharide
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and glycoconjugate synthesis has been a demanding area of research effort worldwide. A vast
majority of glycan syntheses is nowadays accomplished using thioglycosides1-4 and O-
trichloroacetimidates (TCAI).5-7 As a part of the ongoing research effort in our laboratory to
develop versatile methods for chemical glycosylation and expeditious oligosaccharide synthesis,
we became interested in glycosyl thioimidates, glycosyl donors equipped with SCR1=NR2
leaving group.8 Among a variety of thioimidates studied by us and others, S-benzoxazolyl
(SBox),9 S-thiazolinyl (STaz),10 and S-benzimidazolyl (SBiz)11 moieties were found to be
excellent building blocks for oligosaccharide synthesis. We also determined that thioimidates fit
into practically all existing expeditious strategies for oligosaccharide synthesis due to the unique
reaction conditions associated with their activation.12 In addition, the glycosyl thioimidates led us
to the development of conceptually new strategies for oligosaccharide synthesis: the inverse
armed-disarmed strategy, temporary deactivation concept, O-2/O-5 cooperative effect,
coordination-assisted glycosylation, and Surface-Tethered Iterative Carbohydrate Synthesis
(STICS).12 At the core of the study presented herein is the development of a new method for
chemical glycosylation and expeditious oligosaccharide synthesis based on (1H-indol-2-yl)thio
(S-indolyl, SIn) glycosides. These compounds are similar to thioimidates developed previously,
but the study reported herein unexpectedly revealed that SIn glycosyl donors act neither as
thioimidates nor as conventional alkyl/aryl thioglycosides in glycosylation.
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Results and Discussion
The thioindole (HSIn) aglycone was obtained from commercially available oxindole (2-
indolone) by means of thionation with P2S5 in tetrahydrofuran (THF).13 HSIn was then reacted
with a variety of bromide precursors in the presence of potassium carbonate and 18-crown-6 to
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afford the respective SIn glycosides. Thus, as depicted in Scheme 1, per-benzoylated glucosyl
bromide 114 was converted into SIn glycoside 2 in 63% yield. Per-benzylated glucosyl 3 and
galactosyl bromide 5, freshly prepared from the corresponding ethyl thioglycosides,15-18 were
converted into SIn glycosides 4 and 6 in 60 and 52% yield, respectively. We acknowledge that
these yields are far from excellent, but would like to note that the yields listed herein are after
crystallization. For X-ray structure determination of glucose derivative 2 and galactose derivative
6, crystals of appropriate dimensions were obtained by slow evaporation of ethyl acetate/hexane
solutions. Refer to the experimental section and the SI for complete details of X-ray structure
determination of compounds 2 and 6.
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Scheme 1. Synthesis of SIn glycosyl donors 2, 4 and 6
H
H
P2S5 (1.22 equiv.)
N
N
O
S
NaHCO3 (2 equiv.)
THF,4h
oxindole
thioindole (HSIn)
OBz
Thioindole (1.1 equiv)
K2CO3 (1.1 equiv)
OBz
H
N
O
O
BzO
BzO
BzO
S
BzO
BzO
BzO
18-C-6 (0.2 equiv)
Acetone, 3 h
Br
2
1
63%
SIn
OBn
O
OBn
O
Thioindole (1.1equiv)
K2CO3 (1.1 equiv)
BnO
BnO
BnO
BnO
BnO
SIn
BnO
Br
18-C-6 (0.2 equiv)
CH2Cl2, 8 h
4
3
60%
BnO
BnO
BnO
OBn
O
OBn
O
Thioindole (1.1 equiv)
K2CO3 (1.1 equiv)
BnO
BnO
SIn
BnO
Br
18-C-6 (0.2 equiv)
CH2Cl2, 17 h
5
6
52%
With novel SIn glycosides 2, 4 and 6 in hand, we endeavored for investigation of their properties
as glycosyl donors. To study the preliminary scope of this approach, we chose model glycosyl
acceptors 7-1019 depicted in Figure 1. As possible activators, we decided to rely on the reaction
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conditions that worked well with thioimidates (AgOTf or AgBF4), and thioglycosides
(NIS/TfOH). To our great surprise, no activation took place when we attempted to react SIn
donor 2 with reactive primary acceptor 7 in the presence of AgOTf at rt. Even large excess of 3.0
equiv and prolonged reaction time (24 h) produced no desired disaccharide 1120 (Table 1, entry
1). For comparison, the SBox LG could be smoothly and rapidly activated with 2.0 equiv of
AgOTf.9
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Figure 1. Standard glycosyl acceptors used in this study
OH
O
OBn
O
BnO
BnO
HO
BnO
BnO
BnO
OMe
OMe
7
8
OBn
O
OBn
O
BnO
BnO
HO
BnO
HO
BnO
OMe
OMe
9
10
A drastic change was noted when AgBF4, another effective activator for thioimidates,21 was
employed as the promoter for SIn donor 2. Although this reaction required 5.0 equiv of AgBF4, it
could be smoothly driven to a completion in 5 h. As a result, disaccharide 11 was produced in a
good yield of 85% (entry 2). In our prior experience, silver triflate and tetrafluoroborate act
similarly,21 and to explain this discrepancy in application to the SIn glycosides we came up with
the following hypothesis. We thought that this could be due to SIn glycosides being particularly
sensitive to the marginal difference in the availability of Ag+ ions between these two promoters.
Therefore, we looked at alternative sources of silver, and silver salt/Lewis acid cooperative
promoter system recently developed by us for the activation of bromides22 and chlorides23
appealed to us as a possible solution. However, when donor 2 was coupled with standard
acceptor 7 in the presence of Ag2CO3 (1.0 equiv) and TMSOTf (2.0 equiv) disaccharide 11 was
produced only in a poor yield of 20% in 19 h (entry 3). The reaction was significantly
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accelerated in the presence of Ag2CO3 and a larger excess of TMSOTf (4.0 equiv), and
disaccharide 11 was obtained in 87% yield in 2 h (entry 4). Being generally encouraged by this
outcome, albeit realizing that large excess of TMSOTf is undesirable, we continued the search of
alternative promoter systems.
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Table 1. Optimization of glycosidation of SIn donor 2 with 6-OH acceptor 7
OBz
O
H
BzO
BzO
OBz
O
N
S
BzO
BzO
BzO
O
2
BzO
BnO
Activators
O
BnO
MS 3Å, 1,2-DCE
OH
BnO
O
OMe
BnO
BnO
11
BnO
OMe
(0.90 equiv)
7
Entry
Conditions
Yield of 11
1
2
AgOTf (3.0 equiv), rt, 24 h
AgBF4 (5.0 equiv), rt, 5 h
NR
85%
20%
87%
36%
60%
72%
NR
3
Ag2CO3 (1.0 equiv), TMSOTf (2.0 equiv), rt, 19 h
Ag2CO3 (1.0 equiv), TMSOTf (4.0 equiv), rt, 2 h
NIS (2.0 equiv), TfOH (0.2 equiv), 0 °C-rt, 24 h
NIS (4.0 equiv), TfOH (0.8 equiv), rt, 1 h
4
5
6
7
NIS (4.0 equiv), TfOH (1.0 equiv), rt, 1 h
8
NIS (2.0 equiv), TMSOTf (0.2 equiv), 0 °C-rt, 24 h
NIS (4.0 equiv), TMSOTf (1.0 equiv), rt, 16 h
NIS (4.5 equiv), TMSOTf (1.0 equiv), rt, 1.5 h
9
57%
77%
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Glycosidation of SIn donor 2 with acceptor 7 in presence of 2.0 equiv of NIS and 0.2 equiv of
TfOH, a common promoter system used for the activation of alkyl/aryl thioglycosides,24 in 1,2-
dichloroethane (1,2-DCE) afforded disaccharide 11 only in a very modest yield of 36% (entry 5).
Moreover, the reaction was very sluggish, appeared as something is hindering the activation of
the SIn leaving group, and required 24 h to get to this stage. Increasing the amount of promoters,
NIS to 4.0 equiv and TfOH to 0.8 equiv, again appealed as a possible solution. Under these
reaction conditions, disaccharide 11 was obtained rather swiftly, in 1 h, in a significantly
improved, albeit still unremarkable yield of 60% (entry 6). Further increase in the amount of
TfOH to 1.0 equiv turned out to be the most advantageous for this promoter system. As shown in
entry 7, disaccharide 11 was obtained in 72% yield. In a further search of the promoters, we
replaced TfOH with TMSOTf. While somewhat similar, this modified promoter system allowed
to achieve disaccharide 11 in 77% yield in 1.5 h. A gradual increase in the amounts of promoters
(entries 8-10) showed that 4.5 equiv of NIS and 1.0 equiv of TMSOTf are required to achieve
this result.
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To expand the scope of this reaction, we extended our study to secondary glycosyl acceptors 8-
10 (Figure 1). Glycosylation reaction of SIn glucosyl donor 2 with the secondary glycosyl
acceptors 8-10 in presence of Ag2CO3 (1.0 equiv) and TMSOTf (4.0 equiv), conditions that
worked best for the primary acceptor 7 (see Table 1) were also successful herein. As shown in
Table 2, the corresponding disaccharides 12,20 13,19 and 1425 were obtained in commendable
yields ranging between 69-79% (entries 1-3). Subsequently, we extended our study to
glycosylation of per-benzylated donors 4 and 6 with primary and secondary acceptors 7-10. This
study included both silver-and iodonium-mediated pathways, and the reaction conditions were
adjusted to account for differences in reactivity among the donor and acceptor combinations.
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Surprisingly, only slightly reduced amounts of promoters were permitted for the activation of
supposedly much more reactive (armed) per-benzylated glycosyl donors.26 Selected key results
of this study are summarized in Table 2.
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Table 2. Expanding the scope of glycosylation with SIn donors: synthesis of disaccharides
12-22
2,4,6
Glycosyl donors
(Scheme 1)
BxO
BxO
OBx
O
Activators
OR
BxO
MS 3Å, Solvent
Bx = Bn or Bz
ROH
Glycosyl acceptors
12-22
Disaccharides
(see Table)
7-10
(Figure 1)
Donor + Acceptor,
conditions
2 + 8, Ag2CO3 (1.0 equiv),
TMSOTf (4.0 equiv),
Product
Entry
(yield, α/β ratio)
OBn
O
BzO
BzO
BzO
O
BnO
O
OBz
1
BnO
OMe
0 °C-rt, 20 h, 1,2-DCE
12 (69%, β only)
OBn
OBz
O
2 + 9, Ag2CO3 (1.0 equiv),
TMSOTf (4.0 equiv),
0 °C-rt, 4 h, 1,2-DCE
O
BnO
BzO
BzO
O
2
3
BnO
OMe
BzO
13 (78%, β only)
B
n
n
O
O
2 + 10, Ag2CO3 (1.0
equiv), TMSOTf (4.0
equiv),
B
B
n
O
O
BzO
BzO
O
O
BzO
O
M
e
BzO
0 °C-rt, 4 h, 1,2-DCE
14 (79%, β only)
OBn
O
BnO
BnO
O
4 + 7, Ag2CO3 (1.0 equiv),
TMSOTf (3.0 equiv),
0 °C-rt, 2 h, DCM
O
BnO
BnO
BnO
4
5
6
BnO
OMe
15 (91%, 1.1/1)
15 (76%, 1/1.2)
4 + 7, NIS (4.0 equiv),
TfOH (1.0 equiv),
rt, 0.5 h, DCM
OBn
O
BnO
BnO
OBn
O
4 + 8, Ag2CO3 (1.0 equiv),
TMSOTf (4.0 equiv),
-70 °C-rt, 3 h, DCM
BnO
O
BnO
BnO
OMe
16 (68%, 1/1.1)
OBn
O
BnO
BnO
4 + 9, Ag2CO3 (1.0 equiv),
TMSOTf (4.0 equiv),
-70 °C-rt, 3 h, DCM
O
BnO
BnO
BnO
OMe
7
O
OBn
17 (70%, 1.4/1)
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OBn
O
BnO
BnO
4 + 10, Ag2CO3 (1.0
equiv), TMSOTf (4.0
equiv),
-70 °C-rt, 3 h, DCM
O
BnO
OMe
8
BnO
BnO
O
OBn
18 (81%, 1.2/1)
BnO
OBn
O
6 + 7, Ag2CO3 (1.0 equiv),
TMSOTf (4.0 equiv),
rt, 0.5 h, DCM
BnO
O
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BnO
O
BnO
BnO
9
BnO
OMe
19 (70%, 1/1.3)
19 (80%, 1/1.2)
6 + 7, Ag2CO3 (1.0 equiv),
TMSOTf (3.0 equiv),
0 °C-rt, 3 h, DCM
10
11
BnO
BnO
OBn
O
6 + 8, Ag2CO3 (1.0 equiv),
TMSOTf (4.0 equiv),
-70 °C-rt, 2 h, DCM
OBn
O
BnO
O
BnO
BnO
OMe
20 (54%, 6.0/1)
OBn
O
BnO
BnO
O
6 + 9, Ag2CO3 (1.0 equiv),
TMSOTf (4.0 equiv),
-70 °C-rt, 2 h, DCM
BnO
BnO
BnO
OMe
12
13
O
OBn
21 (62%, 1.7/1)
OBn
O
BnO
6 + 10, Ag2CO3 (1.0
equiv), TMSOTf (4.0
equiv), -70 °C-rt, 2 h,
DCM
BnO
O
BnO
OMe
BnO
O
OBn
BnO
22 (75%, 2.0/1)
Being marginally more reactive than its per-benzoylated counterpart, benzylated donor 4 could
be smoothly activated for the reaction with the primary acceptor 7 with Ag2CO3 (1.0 equiv) and
the reduced amount of TMSOTf (3.0 equiv). As a result, disaccharide 1527 was obtained in 91%
in 2 h (α/β = 1.1/1, entry 4). This series of experiments was performed in dichloromethane as the
reaction solvent that would allow to start reactions at a low temperature (vide infra). Benzylated
donor 4 could also be activated with NIS (4.0 equiv) and TfOH (1.0 equiv), essentially the same
conditions used for the activation of the per-benzoylated SIn donor. As a result, disaccharide 15
was obtained in 76% yield in 30 min (α/β = 1/1.2, entry 5). We then investigated glycosidation of
donor 4 with secondary glycosyl acceptors 8-10 in the presence of Ag2CO3 and TMSOTf. These
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reactions were slower, and to achieve faster reactions, we increased the amount of TMSOTf back
to 4.0 equiv. To achieve higher yields, these reactions were started at -70 oC, and then allowed to
slowly warm to rt. This series of experiments produced the corresponding disaccharides 16,28
17,29 and 1830 in good yields of 68-81% albeit unremarkable stereoselectivity (α/β from 1.4/1 to
1/1.1, entries 6-8).
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After that, we investigated glycosylations with SIn galactosyl donor 6 with the same series of
glycosyl acceptors 7-10. Glycosylation reaction of donor 6 with primary acceptor 7 in the
presence of Ag2CO3 (1.0 equiv) and TMSOTf (4.0 equiv) afforded disaccharide 1931 in 70%
yield in 30 min (α/β = 1/1.3, entry 9). Decreasing the amount of TMSOTf to 3.0 equiv afforded
disaccharide 19 in an increased yield of 80% was observed with practically no change in
stereoselectivity (entry 10). This reaction, however, required 3 h to complete, and to achieve
practical rates with the secondary acceptors we increased the amount to TMSOTf to 4.0 equiv.
Like in case of glucosyl donor 4, glycosylations of galactosyl donor 6 with the secondary
glycosyl acceptors 8-10 were started at -70 oC, and then allowed to slowly warm to rt. This series
of experiments produced the respective disaccharides 20,32 21,33 and 2234 in yields ranging
between 54-75% with preferential α-stereoselectivity (α/β = 1.7-6/1, entries 11-13).
After completing these series of experiments, we were still puzzled by the fairly low reactivity of
SIn glycosides that required greater excess reagents than even stable alkyl/aryl thioglycosides.
The requirement for using excess reagent resulted in unusually prominent side reactions that
accompanied many glycosylations. Side products observed included silylated acceptors,
hemiacetals, 11-linked disaccharides, and 1,6-anhydro sugars. To gain a better understanding
of this glycosylation reaction and uncover its mechanism we turned our attention to performing a
set of experiments monitored by NMR. First, we investigated the activation with NIS. For this
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purpose, SIn donor 2 was dissolved in CDCl3, the solution was transferred into a standard 5 mm
5
6
1
NMR tube, and H NMR spectrum was recorded (Scheme 2A). NIS (1.0 equiv) was added and
7
8
9
1H NMR spectrum was recorded in 15 min showing the formation of 23 as the only product
(Scheme 2B).
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Scheme 2. NMR experiments with SIn donor 2 in the presence of NIS (1-4 equiv) to
observe the formation of 23 and 24
While somewhat unanticipated, the formation of such intermediate is not surprising since the 3-
position of the indole ring system is strongly nucleophilic and known to react easily with
electrophiles of different kinds.35,36 Compound 23 was sufficiently stable to be isolated through
column chromatography and fully characterized (see the experimental section for details). The
NH signal in 23 was noted to shift downfield, and the identity of this proton was confirmed by a
separate deuterium exchange experiment that indicated the remanence of an exchangeable
hydrogen on the N atom of 23 (see the SI for further details). Upon addition of the second equiv
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of NIS, H NMR spectrum was recorded in 25 min showing the formation of a mixture of
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compounds (Scheme 2C). As suggested by the H NMR spectrum, a partial conversion of
7
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compound 23 to a new compound, presumably 24, took place. Only when a total 4.0 equiv of
1
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NIS was added to the NMR tube, H NMR spectrum recorded after 1 h showed the complete
formation of intermediate 24 with no traces of 23 remaining (Scheme 2D). This explains the
need for multiple equivalents of NIS to activate the SIn leaving group. The formation of the
activated sulfonium intermediate 24 is supported by a  1.4 ppm downfield chemical shift of
the H-1 signal, as indicated in Scheme 2C.
Based on the extended NMR monitoring our current understanding of the NIS-promoted reaction
mechanism is as follows. Upon addition of 4.0 equiv of NIS and TfOH to the reaction mixture
containing SIn donor 2 in 1,2-DCE the first equivalent of NIS gets consumed by the electrophilic
addition of the iodonium ion at the remote C-3 position of the SIn aglycone to afford a stable
intermediate 23 (Scheme 3). The presence of TfOH (or TMSOTf) facilitates the formation of the
iodonium ion from NIS. The additional NIS is needed to convert 23 into the activated species 24
that results in the leaving group departure followed by glycosylation. While glycosidation of
benzoylated SIn donor 2 proceeds via the intermediacy of the acyloxonium intermediate leading
to complete stereoselectivity, glycosidations of benzylated SIn donors 4 and 6 proceed via
oxacarbenium ion intermediate. The formation of the latter intermediate explains poor
stereoselectivity of many uncontrolled glycosylations taking place without the assistance of the
neighboring group at C-2.37
Scheme 3. Plausible mechanism of the NIS/TfOH-promoted reaction
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OBz
OBz
O
I+
O
H
H
BzO
BzO
BzO
BzO
N
N
S
S
BzO
BzO
23
I
2
I+ (excess)
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OBz
OBz
O
I
I
R'OH
O
BzO
BzO
BzO
BzO
N
S
OR'
BzO
BzO
24
I
To uncover the mechanism of glycosylation reactions proceeding under the cooperative silver
salt-Lewis acid catalysis, we set up a series of experiments to monitor the interaction of SIn
donor 2 with the promoter system by NMR. For this purpose, donor 2 was dissolved in CDCl3,
1
the solution was transferred into a standard 5 mm NMR tube, and H NMR spectrum was
recorded (Scheme 4A). TMSOTf (1.0 equiv) was added and 1H NMR spectrum was recorded in
10 min showing the formation of thioimidate 25 (Scheme 4B). Formation of such a tautomer was
somewhat unexpected. The same product was formed with other amounts of TMSOTf (up to 4.0
equiv). Thioimidate 25 was found to be stable in solution for at least one hour, it survives
conventional aqueous work-up and evaporation. However, compound 25 tautomerizes back into
SIn glycoside 2 during column chromatography on silica gel.
Scheme 4. NMR experiments with SIn donor 2 in the presence of TMSOTf and Ag2CO3 to
study the mechanism
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When Ag2CO3 (1.0 equiv) was added to the NMR tube containing 25 produced from 2 with 1
equiv of TMSOTf, compound 25 quickly tautomerizes back to compound 2. Shown in Scheme
4C is a silver complex of 2 wherein the complexation has presumably occurred via the N-atom of
indole. This mode of complexation is supported by a  0.7 ppm downfield chemical shift of the
NH signal with practically no other changes observed in comparison to the uncomplexed 2
(Scheme 4A). The analogy is found in the mode of complexation taking place with other
thioimidates. Thus, it was previously found that non-aromatic thioimidates, such as S-thiazolinyl
(STaz),10 complex silver and other metals via the nitrogen atom.38 This mode of complexation is
known to result in the remote activation with silver39,40 or temporary deactivation with platinum
or palladium salts.41 Conversely, aromatic thioimidates such as S-benzoxazolyl (SBox)9 and S-
benzimidazolyl (SBiz)11 complex silver strictly via the anomeric sulfur.42 This differential
activation, remote versus direct, has allowed us to devise a thioimidate-only orthogonal strategy
for oligosaccharide synthesis.43
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When Ag2CO3 (1.0 equiv) was added to the NMR tube containing 25 produced from 2 with 2.0
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or 4.0 equiv of TMSOTf, imidate 25 remained intact. H NMR spectrum recorded in 10 min
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showed the presence of compound 25 only, presumably as a complex with silver due to minor
changes in chemical shifts observed (Scheme 2D). Since no glycosylation takes place until
multiple equiv of TMSOTf are added, we believe that tautomerization of 2 to 25 needs to have
occurred prior to glycosylation under these reaction conditions.
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Based on extended NMR monitoring our current understanding of the reaction mechanism of
glycosidation of 2 in the presence of TMSOTf and Ag2CO3 is as follows. In presence of excess
of TMSOTf, glycosyl donor 2 is first tautomerized into its thioimidate counterpart 25. We
believe that the occurrence of this intermediate is the key for the successful activation to take
place under these reaction conditions. While the formation of 2-Ag complex is possible, there is
no feasible pathway for the SIn leaving group activation, neither remotely nor directly (Scheme
5). Silver carbonate then complexes with thioimidate 25 to produces complex 25-Ag that is not
yet sufficiently ionized to cause the leaving group departure. As proposed in our previous study
of the cooperative silver/acid catalysis with glycosyl halides, after the initial interaction of the
donor with the silver salt, a strongly ionized species A is produced due to the interaction with an
acid. This intermediate will then cause the remote SIn leaving group departure. Depending on the
protecting group at C-2, the subsequent glycosylation proceeds via an acyloxonium (SIn donor 2)
or oxacarbenium (SIn donors 4 and 6) ion. Differently from our previous studies with glycosyl
halides, wherein the glycosylations were driven by a strong affinity of silver to halogens and the
irreversible formation of the Ag-X bond, the activation of the SIn donors proceeding via the
interaction of silver with the N-atom is significantly slower. It is also possible that silver
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carbonate will react with excess TMSOTf first to generate a loosely bound silver cation species,
which are then attacked by the N-atom of thioimidate 25 to produce the ionized intermediate.
Scheme 5. Plausible mechanism of the Ag2CO3/TMSOTf-promoted reaction
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OBz
O
OBz
O
H
N
BzO
BzO
BzO
BzO
TMSOTf
N
S
S
BzO
BzO
2
25
TMSOTf
and Ag2CO3
1.0 equiv
1.0 equiv
2.0-4.0 equiv
1.0 equiv
TMSOTf
and Ag2CO3
AgOCO2Ag
AgOCO2Ag
OBz
O
OBz
H
O
BzO
BzO
BzO
BzO
N
N
S
S
BzO
BzO
2-Ag
25-Ag
R'OH
TMSOTf
TMS
Ag OCO2Ag
OBz
O
OBz
O
R'OH
BzO
BzO
BzO
BzO
N
OTf
S
OR'
BzO
BzO
A
Given the specific set of reaction conditions needed for the activation of this new leaving group,
the orthogonality of SIn donors in respect to other popular leaving groups, thioglycoside 2615 and
thioimidate 27,42 was then investigated. For this purpose, we set up a series of comparative
competition experiments wherein 1.0 equiv of SIn donor 4 was set to compete with 1.0 equiv
other glycosyl donors for 2.0 equiv of acceptor 7 (Scheme 6). All reactions were stopped after 30
min, and all reaction components were isolated and characterized. First, we set up a comparative
competition experiment wherein SIn donor 4 and ethylthio glycoside 26 were set to compete for
acceptor 7 in the presence of NIS (1.2 equiv) and TfOH (0.2 equiv). These are typical reaction
conditions for the activation of thioglycosides. Indeed, SEt donor 26 reacted entirely, whereas
SIn donor 4 was recovered in the form of iodine and SEt adducts 28 and 29 that were isolated in
the combined yield of 90% (Scheme 6A). This experiment ultimately confirmed slow and
incomplete activation of the SIn glycosides under the reaction conditions common for the
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activation of alkyl/aryl thioglycosides. Adduct 28 can be readily activated for subsequent
glycosylation because it is an intermediate formed as the first step of the interaction of the SIn
leaving group with the iodonium promoter. In the competition experiment, it acts as a trap for
excess I+ remaining after the activation of the SEt leaving group. Upon this, adduct 29 was
formed as a result of trapping the SEt group, and this compound can also be glycosidated under
typical conditions for the SIn leaving group activation.
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We then set up a comparative competition experiment wherein SIn donor 4 and ethylthio
glycoside 26 were set to compete for acceptor 7 in the presence of Ag2CO3 and TMSOTf, newly
developed reaction conditions for the activation of SIn glycosides. As a result, SIn donor 4
reacted completely, whereas SEt donor 26 was recovered nearly quantitatively (97% yield,
Scheme 6B). This experiment ultimately confirmed that common thioglycosides cannot be
activated under the reaction conditions that have been devised for the activation of SIn
glycosides.
Scheme 6. Competition experiments of SIn glycoside 4 vs SEt and SBox donors 26 and 27
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A
5
6
7
8
15
(98%)
Disaccharide
4
26
Donors and
Also recovered
7
Acceptor
NIS (1.2 equiv)
TfOH (0.2 equiv)
OBn
O
BnO
9
BnO
30 min
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H
N
BnO
S
1,2-DCE
28
29
: R = I, 78%
: R = SEt, 12%
R
B
4
26
Donors and
15
(96%)
Disaccharide
7
Acceptor
Also recovered
Ag2CO3 (1.0 equiv)
TMSOTf (4.0 equiv)
26
97% recovery
30 min
1,2-DCE
C
15
4
27
(96%)
Donors and
Disaccharide
7
Acceptor
AgOTf (2.0 equiv)
Also recovered
4
90% recovery
30 min
1,2-DCE
OBn
OBn
O
OBn
O
O
BnO
BnO
BnO
BnO
BnO
BnO
SEt
SBox
BnO
BnO
BnO
SIn
26
4
27
glycosyl donor B
glycosyl donor A
glycosyl donor C
OH
O
OBn
O
BnO
BnO
BnO
BnO
BnO
O
BnO
BnO
BnO
O
OMe
7
15
glycosyl acceptor
BnO
OMe
Finally, we set up a comparative competition experiment wherein SIn donor 4 and SBox imidate
2742 were set to compete for acceptor 7 in the presence of 2 equiv of AgOTf. These are common
reaction conditions of the SBox leaving group activation9 that were found practically ineffective
with SIn glycosides, as determined over the course of the preliminary screening (vide supra,
Table 1). As a result of this competition experiment, SBox donor 27 reacted completely, whereas
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SIn donor 4 was recovered in 90% yield (Scheme 6B). This series of experiments demonstrated
that SIn glycosides possess different glycosyl donor properties than both thioglycosides and
thioimidates to which they are structurally related to.
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In summary, we developed a new class of glycosyl donors, indolylthio glycosides. This
new glycosyl donor can be activated using a range of activators. Although the activation of the
SIn leaving group requires excess reagents, their reactivity profile is interesting. The activation
process was studied by NMR and the increased understanding of the mechanism led to a
discovery of orthogonality of the SIn leaving group versus thioglycosides and thioimidates.
Further investigation of this leaving group in oligosaccharide synthesis, as well as structural and
mechanistic studies are currently underway in our laboratory.
Experimental
General. The reactions were performed using commercial reagents and the ACS grade solvents
used for reactions were purified and dried in accordance with standard procedures. AgOTf was
co-evaporated at least twice with dry toluene, dried in vacuo and stored under dark for better
yield of glycosylated product. Column chromatography was performed on silica gel 60 (70-230
mesh), reactions were monitored by TLC on Kieselgel 60 F254. The compounds were detected
by examination under UV light and by charring with 5% sulfuric acid in methanol. Solvents were
removed under reduced pressure at <40 °C. CH2Cl2 and ClCH2CH2Cl (1,2-DCE) were distilled
from CaH2 directly prior to application. Molecular sieves (3 Å), used for reactions, were crushed
and activated in vacuo at 390 °C for 8 h in the first instance and then for 2-3 h at 390 °C directly
1
prior to application. Optical rotations were measured at ‘Jasco P-2000’ polarimeter. H NMR
13
spectra were recorded in CDCl3 at 300 MHz, C{1H} NMR spectra were recorded at 75 MHz.
1
The H NMR chemical shifts are referenced to tetramethyl silane (TMS, δH = 0 ppm) or CDCl3
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(δH = 7.26 ppm) for H NMR spectra for solutions in CDCl3. The C NMR chemical shifts are
referenced to the central signal of CDCl3 (δC = 77.00 ppm) for solutions in CDCl3. Anomeric
purity or anomeric ratios were accessed or calculated by comparing of the integration intensities
1
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of the relevant signals in their H NMR spectra. Accurate mass spectrometry determinations
were performed using Agilent 6230 ESI TOF LCMS mass spectrometer.
1,3-Dihydro-2H-indole-2-thione (indoline-2-thione, HSIn) was obtained from oxindole and
P2S5 following the previously described protocol13 that was modified as follows. A mixture
containing oxindole (500 mg, 3.76 mmol) and P2S5 (1.0 g, 4.6 mmol) in dry THF (25 mL) was
stirred under argon for 10 min at rt. Sodium bicarbonate (631 mg, 7.5 mmol) was added
portionwise, and the resulting mixture was stirred for 4 h at rt. After that, the volatiles were
removed under reduced pressure. The residue was dissolved in ethyl acetate (~200 mL) and
washed with water (4 x 50 mL). The organic phase was separated, dried over Na2SO4, and
concentrated under reduced pressure. The residue was dried in vacuo for 1 h then dissolved in
hot methanol (ca 50 °C). The resulting solution was allowed to slowly cool to rt to afford the title
compound as a yellow crystalline solid in 64% yield. Analytical data for HSIn were in
accordance with that previously reported.13
1H-Indol-2-yl 2,3,4,6-tetra-O-benzoyl-1-thio-β-D-glucopyranoside (2). HSIn (0.45 g, 3.34
mmol), anhydrous K2CO3 (0.46 g, 3.33 mmol) and 18-crown-6 (0.16 g, 0.61 mmol) were added
to a solution of 2,3,4,6-tetra-O-benzoyl-α-D-glucopyranosyl bromide14 (1, 2.0 g, 3.03 mmol) in
dry acetone (50 mL), and the resulting mixture was stirred under argon at 50 °C for 3 h. After
that, the solids were filtered off through a pad of Celite, rinsed successively with CH2Cl2 and the
combined filtrate was concentrated under reduced pressure. The residue was dissolved in CH2Cl2
(~150 mL), washed with 10% aq. NaHCO3 (25 mL) and water (2 x 25 mL). The organic phase
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was separated, dried over Na2SO4, and concentrated under reduced pressure. The residue was
purified by column chromatography on silica gel (toluene - ethyl acetate 1% gradient elution)
followed by crystallization from diethyl ether-hexanes to afford the title compound as colorless
crystals in 63% yield (1.39 g, 1.904 mmol). Analytical data for 2: Rf 0.60 (ethyl acetate/hexane,
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3/7, v/v); m.p. 204-205 °C (diethyl ether-hexanes); [α]D22 -81.8 (c = 1.0, CHCl3); H NMR (300
MHz): δ 4.12-4.18 (m, 1H, J5,6a = 4.8, J5,6b = 2.6 Hz, H-5), 4.52 (dd, 1H, J6a,6b = 12.4 Hz, H-6a),
4.74 (dd, 1H, H-6b), 4.85 (d, 1H, J1,2 = 9.8 Hz, H-1), 5.45 (dd, 1H, J2,3 = 9.6 Hz, H-2), 5.54 (dd,
1H, J4,5 = 9.8 Hz, H-4), 5.91 (dd, 1H, J3,4 = 9.5 Hz, H-3), 6.76 (s, 1H, =CH), 7.10-8.08 (m, 24H,
aromatic), 9.08 (s, 1H, NH) ppm; C{1H} NMR (75 MHz): δ 62.3, 68.6, 70.6, 76.6, 76.9, 77.4,
13
84.1, 111.1, 112.3, 119.8, 120.6, 121.5, 123.0, 127.7, 128.2 (x2), 128.3 (x2), 128.4 (x2), 128.5
(x2), 128.6 (x2), 128.9, 129.3, 129.6 (x2), 129.7 (x2), 129.8 (x2), 129.9 (x2), 133.2, 133.3, 133.5,
137.9, 164.9, 165.2, 165.7, 166.6 ppm; ESI-TOF [M + Na]+: calcd for [C42H33NO9SNa]+
750.1774; found: 750.1763.
1H-Indol-2-yl 2,3,4,6-tetra-O-benzyl-1-thio-α-D-glucopyranoside (4). A mixture containing
ethyl 2,3,4,6-tetra-O-benzyl-1-thio-β-D-glucopyranoside (26,15 5.0 g, 8.55 mmol) and freshly
activated molecular sieves (3 Å, 1.0 g) in CH2Cl2 (50 mL) was stirred under argon for 1 h at rt.
Bromine (0.53 mL, 10.26 mmol) was added and the resulting mixture was stirred for 20 min at
rt. After that, the volatiles were removed under reduced pressure, the residue was co-evaporated
with dry toluene (2 x 20 mL) and dried in vacuo for 2 h. The resulting residue containing crude
2,3,4,6-tetra-O-benzyl-α-D-glucopyranosyl bromide 3 was dissolved in CH2Cl2 (60 mL), HSIn
(1.4 g, 9.41 mmol), anhydrous K2CO3 (1.30 g, 9.41 mmol), and 18-crown-6 (0.45 g, 1.75 mmol)
were added, and the resulting mixture was stirred under argon for 16 h at rt. The solids were
filtered off through a pad of Celite and rinsed successively with CH2Cl2. The combined filtrate
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(~200 mL) as washed with water (50 mL), 10% aq. NaHCO3 (250 mL), and water (2 x 50 mL).
The organic phase was separated, dried over Na2SO4, and concentrated under reduced pressure.
The residue was purified by column chromatography on silica gel (ethyl acetate-hexane 5%
gradient elution) followed by crystallization from diethyl ether -hexanes to afford the title
compound as colorless crystals in 60% yield (3.44 g, 9.27 mmol). Analytical data for 4: Rf 0.55
(ethyl acetate/hexane, 1/4, v/v); m.p. 116-117 °C (diethyl ether-hexanes); [α]D22 +159.8 (c = 1.0,
CHCl3); 1H NMR (300 MHz): δ 3.29 (dd, 1H, J6a,6b = 10.2 Hz, H-6a), 3.48 (dd, 1H, J4,5 = 8.8 Hz,
H-4), 3.76-3.90 (m, 3H, H-2, 3, 6b), 4.44 (d, 1H, 2J = 11.0 Hz, CHPh), 4.56-4.82 (m, 7H, H-5, 3
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x CH2Ph, J5,6a = 8.3 Hz), 4.92 (d, 1H, J = 10.8 Hz, CHPh), 5.47 (d, 1H, J1,2 = 5.2 Hz, H-1),
13
6.47-7.45 (m, 24H, aromatic), 6.60 (dd, 1H, =CH), 9.39 (s, 1H, NH) ppm; C{1H} NMR (75
MHz): δ 69.6, 70.7, 72.2, 73.6, 75.0, 75.8, 77.9, 79.2, 82.3, 86.9, 108.0, 110.6, 119.5, 119.7,
122.0, 126.4, 127.4, 127.6, 127.8, 127.9 (x5), 128.0 (x5), 128.3 (x2), 128.4 (x4), 128.6 (x2),
137.2, 137.3, 137.7, 137.8, 138.4 ppm; ESI-TOF [M + H]+: calcd for [C42H42NO5S]+ 672.2784;
found: 672.2792.
1H-Indol-2-yl 2,3,4,6-tetra-O-benzyl-1-thio-α-D-galactopyranoside (6). Ethyl 2,3,4,6-tetra-O-
benzyl-1-thio-β-D-galactopyranoside16 (5.0 g, 8.55 mmol) and freshly activated molecular sieves
(3Å, 1.0 g) in CH2Cl2 (50 mL) were stirred under argon for 1 h at rt. Bromine (0.53 mL, 10.26
mmol) was added and the resulting mixture was stirred for 20 min at rt.44 After that, the volatiles
were removed under reduced pressure, the residue was co-evaporated with dry toluene (2 x 20
mL), and dried in vacuo for 2 h. The residue containing crude 2,3,4,6-tetra-O-benzyl-α-D-
galactopyranosyl bromide 5 was dissolved in dichloromethane (60 mL), HSIn (1.40 g, 9.41
mmol), anhydrous K2CO3 (1.30 g, 9.41 mmol), and 18-crown-6 (0.45 g, 1.75 mmol) were added,
and the resulting mixture was stirred under argon for 10 h at rt. The solids were filtered off
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through a pad of Celite and rinsed successively with CH2Cl2. The combined filtrate (~200 mL) as
washed with water (50 mL), 10% aq. NaHCO3 (250 mL), and water (2 x 50 mL). The organic
phase was separated, dried over Na2SO4, and concentrated under reduced pressure. The residue
was purified by column chromatography on silica gel (ethyl acetate-hexane 5% gradient elution)
followed by crystallization from diethyl ether -hexanes to afford the title compound as colorless
crystals in 52% yield. Analytical data for 6: Rf 0.40 (ethyl acetate/hexane, 1/4, v/v); m.p. 187-
188 °C (diethyl ether-hexanes); [α]D22 +108.3 (c = 1.0, CHCl3); 1H NMR (300 MHz): δ 3.24 (dd,
1H, J6a,6b = 10.0 Hz, H-6a), 3.80-3.90 (m, 3H, H-3, 4, 6b), 4.40 (dd, 1H, J2,3 = 5.5 Hz, H-2),
4.32-4.96 (m, 9H, J5,6a = 2.3 Hz, H-5, 4 x CH2Ph), 5.67 (d, 1H, J1,2 = 5.5 Hz, H-1), 6.67-7.51 (m,
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24H, aromatic), 6.67 (dd, 1H, =CH), 9.57 (s, 1H, NH) ppm; C{1H} NMR (75 MHz): δ 70.7,
72.5, 73.6, 73.7, 74.4, 74.8, 76.1, 79.2, 87.6, 108.1, 110.6, 119.4, 119.7, 121.9, 126.7, 127.4,
127.6 (x2), 127.7, 127.8 (x2), 127.9 (x5), 128.0, 128.3 (x4), 128.4 (x4), 128.7 (x2), 137.1, 137.8,
137.9 (x2), 138.4 ppm; ESI-TOF [M + H]+: calcd for [C42H42NO5S]+ 672.2784; found: 672.2797.
General glycosylation procedures
Method A: Glycosylation in the presence of Ag2CO3 and TMSOTf. A mixture of a glycosyl
donor (0.041-0.044 mmol), a glycosyl acceptor (0.037-0.040 mmol), and molecular sieves (3 Å,
80 mg) in 1,2-DCE or DCM (1.0 mL) was stirred under argon for 1 h at rt. The mixture was then
cooled to 0 or -70 °C (see Tables), Ag2CO3 (0.041-0.044 mmol) and TMSOTf (0.165-0.178
mmol) were added, and the resulting mixture was stirred under argon for 1 h. After that, the
external cooling was removed, the reaction mixture was allowed to warm to rt, and stirring was
continued for the time indicated in Tables. The solid was filtered off through a pad of Celite and
rinsed successively with dichloromethane. The combined filtrate (~30 mL) was washed with
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H2O (2 x 10 mL). The organic phase was separated, dried over sodium sulfate, filtered, and
concentrated under reduced pressure. The residue was purified by column chromatography on
silica gel (ethyl acetate-hexane gradient elution) to afford the corresponding disaccharide. If
necessary, further purification was accomplished by size-exclusion column chromatography on
Sephadex LH-20.
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Method B: Glycosylation in the presence of NIS and TfOH or TMSOTf. A mixture of a glycosyl
donor (0.041-0.044 mmol), a glycosyl acceptor (0.037-0.040 mmol), and molecular sieves (3 Å,
80 mg) in 1,2-DCE or DCM (1.0 mL) was stirred under argon for 1 h at rt. After that, NIS
(0.165-0.178 mmol) and TfOH (0.041-0.044mmol) or TMSOTf (0.041 mmol) were added, and
the resulting mixture was stirred for the time and temperature indicated in Tables. The solid was
filtered off through a pad of Celite and rinsed successively with dichloromethane. The combined
filtrate (~30 mL) was washed with 10% aq. Na2S2O3 (2 x 10 mL) and H2O (2 x 10 mL). The
organic phase was separated, dried over sodium sulfate, filtered, and concentrated under reduced
pressure. The residue was purified by column chromatography on silica gel (ethyl acetate-hexane
gradient elution). If necessary, further purification was accomplished by size-exclusion column
chromatography on Sephadex LH-20.
Methyl
6-O-(2,3,4,6-tetra-O-benzoyl-β-D-glucopyranosyl)-2,3,4-tri-O-benzyl-α-D-
glucopyranoside (11). The title compound was obtained from donor 2 and acceptor 719 by
general glycosylation procedure in 87% yield as a white amorphous solid. Analytical data for 11
was in accordance with that previously reported.20
Methyl
4-O-(2,3,4,6-tetra-O-benzoyl-β-D-glucopyranosyl)-2,3,6-tri-O-benzyl-α-D-
glucopyranoside (12). The title compound was obtained from donor 2 and acceptor 819 by the
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general glycosylation procedure in 69% yield as a clear syrup. Analytical data for 12 was in
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accordance with that previously reported.20
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Methyl
3-O-(2,3,4,6-tetra-O-benzoyl-β-D-glucopyranosyl)-2,4,6-tri-O-benzyl-α-D-
glucopyranoside (13). The title compound was obtained from donor 2 and acceptor 919 by the
general glycosylation procedure in 78% yield as a clear syrup. Analytical data for 13 was in
accordance with that previously reported.19
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Methyl
2-O-(2,3,4,6-tetra-O-benzoyl-β-D-glucopyranosyl)-3,4,6-tri-O-benzyl-α-D-
glucopyranoside (14). The title compound was obtained from donor 2 and acceptor 1019 by the
general glycosylation procedure in 79% yield as a clear syrup. Analytical data for 14 was in
accordance with that previously reported.25
Methyl
6-O-(2,3,4,6-tetra-O-benzyl-D-glucopyranosyl)-2,3,4-tri-O-benzyl-α-D-
glucopyranoside (15). The title compound was obtained from donor 4 and acceptor 7 by the
general glycosylation procedures in 91% yield (α/β = 1.1/1) as a colorless foam. Analytical data
for 15 was in accordance with that previously reported.27
Methyl
4-O-(2,3,4,6-tetra-O-benzyl-D-glucopyranosyl)-2,3,6-tri-O-benzyl-α-D-
glucopyranoside (16). The title compound was obtained from donor 4 and acceptor 8 by the
general glycosylation procedure in 68% yield of 16 (α/β = 1/1.1) as a clear syrup. Analytical data
for 16 was in accordance with that previously reported.28
Methyl
3-O-(2,3,4,6-tetra-O-benzyl-D-glucopyranosyl)-2,4,6-tri-O-benzyl-α-D-
glucopyranoside (17). The title compound was obtained from donor 4 and acceptor 9 by the
general glycosylation procedure in 70% yield (α/β = 1.4/1) as a clear syrup. Analytical data for
17 was in accordance with that previously reported.29
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Methyl
2-O-(2,3,4,6-tetra-O-benzyl-D-glucopyranosyl)-3,4,6-tri-O-benzyl-α-D-
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8
glucopyranoside (18). The title compound was obtained from donor 4 and acceptor 10 by the
general glycosylation procedure in 81% yield (α/β = 1.2/1) as a colorless foam. Analytical data
for 18 was in accordance with that previously reported.30
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Methyl
6-O-(2,3,4,6-tetra-O-benzyl-D-galactopyranosyl)-2,3,4-tri-O-benzyl-α-D-
glucopyranoside (19). The title compound was obtained from donor 6 and acceptor 7 by the
general glycosylation procedure in 70% yield (α/β = 1/1.3) as a clear syrup. Analytical data for
19 was in accordance with that previously reported.31
Methyl
4-O-(2,3,4,6-tetra-O-benzyl-D-galactopyranosyl)-2,3,6-tri-O-benzyl-α-D-
glucopyranoside (20). The title compound was obtained from donor 6 and acceptor 8 by the
general glycosylation procedure in 54% yield (α/β = 6.0/1) as a clear syrup. Analytical data for
20 was in accordance with that previously reported.32
Methyl
3-O-(2,3,4,6-tetra-O-benzyl-D-galactopyranosyl)-2,4,6-tri-O-benzyl-α-D-
glucopyranoside (21). The title compound was obtained from donor 6 and acceptor 9 by the
general glycosylation procedure in 62% yield (α/β = 1.7/1) as a clear syrup. Analytical data for
21 was in accordance with that previously reported.33
Methyl
2-O-(2,3,4,6-tetra-O-benzyl-D-galactopyranosyl)-3,4,6-tri-O-benzyl-α-D-
glucopyranoside (22). The title compound was obtained from donor 6 and acceptor 10 by the
general glycosylation procedure in 75% yield (α/β = 2.0/1) as a clear syrup. Analytical data for
22 was in accordance with that previously reported.34
1H NMR monitoring experiments
SIn glycoside 2 in the presence of NIS. A solution of donor 2 (0.04 mmol) and NIS (0.04-0.16
mmol) in CDCl3 (1.0 mL) was stirred in a round bottom flask under argon for 5 min at rt. The
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resulting solution was quickly transferred into a standard 5 mm NMR tube and H NMR
spectrum was recorded at 15, 25, and 60 min time points. The recorded spectra are presented in
Scheme 2 and the SI.
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SIn glycoside 2 in the presence of TMSOTf. A solution of donor 2 (0.0206 mmol) and TMSOTf
(0.0206-0.0824 mmol) in CDCl3 (1.0 mL) was stirred in a round bottom flask under argon for 5
min at rt. The resulting solution was quickly transferred into a standard 5 mm NMR tube and 1H
NMR spectrum was recorded at 15, 25, and 45 min time points. The recorded spectra are
presented in Scheme 4 and the SI.
SIn glycoside 2 in the presence of TMSOTf and Ag2CO3. A solution of donor 2 (0.0206 mmol)
and TMSOTf (0.0206-0.0824 mmol) in CDCl3 (1.0 mL) was stirred in a round bottom flask
under argon for 5 min at rt. After that, Ag2CO3 (0.0206 mmol) was added and stirring was
continued for 5 min at rt. The solid was filtered off, the filtrate was transferred into a standard 5
1
mm NMR tube, and H NMR spectrum was recorded at 0.16, 0.25, 0.41, 0.75, and 1 h
timepoints. The recorded spectra are presented in Scheme 4 and the SI.
Competition experiments
Experiment A. A mixture of ethylthio glucoside 26 (26.1 mg, 0.044 mmol), SIn glucoside 4 (30
mg, 0.044 mmol), acceptor 7 (41.5 mg, 0.089 mmol), and molecular sieves (3 Å, 80 mg) in 1,2-
DCE (1.0 mL) was stirred under argon for 1 h at rt. Triflic acid (8.0 μL, 0.00894 mmol) and NIS
(12.1 mg, 0.0536 mmol) were added, and the resulting mixture was stirred under argon for 30
min at rt. The solid was then filtered off through a pad of Celite and rinsed successively with
dichloromethane. The combined filtrate (~30 mL) was washed with 10% aq. Na2S2O3 (2 x 10
mL). The organic phase was separated, dried over sodium sulfate, filtered, and concentrated
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under reduced pressure. The residue was purified by column chromatography on silica gel (ethyl
acetate - hexane gradient elution) to produce 15 in 98% yield. Also recovered were compounds
28 (HRMS [M+Na]+: calcd for [C42H40INO5SNa]+ 820.1570, found: 820.1596) and 29 (HRMS
[M+Na]+: calcd for. [C44H45NO5S2Na]+ 754.2637, found: 754.2645).
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Experiment B. A mixture of ethylthio glucoside 26 (26.1 mg, 0.044 mmol), SIn glucoside 4 (30
mg, 0.044 mmol), glucosyl acceptor 7 (41.5 mg, 0.089 mmol), and molecular sieves (3 Å, 80
mg) in 1,2-DCE (1.0 mL) was stirred under argon for 1 h at room temperature. Ag2CO3 (12.3
mg, 0.044 mmol) was added and the resulting mixture was stirred for 5 min at rt. After that,
TMSOTf (32.4 μL, 0.178 mmol) was added, and the resulting mixture was stirred under argon
for 30 min at rt. The reaction was then quenched with triethyl amine (one drop). The solid was
filtered off through a pad of Celite and rinsed successively with dichloromethane. The combined
filtrate (~ 30 mL) was washed with H2O (2 x 10 mL). The organic phase was separated, dried
over sodium sulfate, filtered, and concentrated under reduced pressure. The residue was purified
by column chromatography on silica gel (ethyl acetate - hexane gradient elution) to produce 15
in 96% yield. Also recovered was thioglycoside 26 in 97% yield.
Experiment C. A mixture of S-benzoxazolyl glucoside 2742,45 (30.1 mg, 0.045 mmol), SIn
glucoside 4 (30 mg, 0.045 mmol) glucosyl acceptor 7 (41.5 mg, 0.089 mmol), and molecular
sieves (3 Å, 80 mg) in 1,2-DCE (1.0 mL) was stirred under argon for 1 h at rt. After that, freshly
activated AgOTf (10.7 mg, 0.089 mmol) was added, and the resulting mixture was stirred under
argon for 20 min at rt. The solid was filtered off through a pad of Celite and rinsed successively
with dichloromethane. The combined filtrate (~ 30 mL) was washed with H2O (2 x 10 mL). The
organic phase was separated, dried over sodium sulfate, filtered, and concentrated under reduced
pressure. The residue was purified by column chromatography on silica gel (ethyl acetate/
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hexane gradient elution). to produce 15 in 96% yield. Also recovered was SIn derivative 4 in
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90% yield.
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X-ray crystal structure determination for compounds 2 and 6
The obtained crystals were mounted on a MiTeGen cryoloop in random orientations. For
compound 2, preliminary examination and data collection were performed using a Bruker X8
Kappa Apex II Charge Coupled Device (CCD) Detector system single crystal X-Ray
diffractometer equipped with an Oxford Cryostream LT device. All data were collected using
graphite monochromated Mo K radiation (= 0.71073 Å) from a fine focus sealed tube X-Ray
source. Preliminary unit cell constants were determined with a set of 36 narrow frame scans.
Typical data sets consist of combinations of  and  scan frames with typical scan width of 0.5
and counting time of 10 seconds/frame at a crystal to detector distance of 4.0 cm.
For compound 6, preliminary examination and data collection were performed using a Bruker
Venture Duo Photon-II single crystal X-Ray diffractometer equipped with an Oxford Cryostream
LT device. Data sets were collected using an Incoatec IµS micro-focus source (Cu) with multi-
layer mirror optics. Preliminary unit cell constants were determined from a set of 180o fast phi
scan frames (1 sec exposure, 1 scan). Typical data sets consist of combinations of  and  scan
frames with typical scan width of 1.0 and counting time of 1 to 5 seconds/frame at a crystal to
detector distance of 3.7 cm.
The collected frames were integrated using an orientation matrix determined from the narrow
frame scans. Apex II and SAINT software packages46 were used for data collection and data
integration. Analysis of the integrated data did not show any decay. Final cell constants were
determined by global refinement of reflections harvested from the complete data set. Collected
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data were corrected for systematic errors using SADABS46 based on the Laue symmetry using
equivalent reflections.
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Structure solution and refinement were carried out using the SHELXTL- PLUS software
package.47 The structures were solved by direct methods and refined successfully in the
monoclinic space group, P21. Full matrix least-squares refinements were carried out by
minimizing w(Fo2-Fc2)2. The non-hydrogen atoms were refined anisotropically to
convergence. All hydrogen atoms were treated using appropriate riding model (AFIX m3).
Absolute structure determination resulted in Flack-x parameters of 0.02(4) and 0.011(11) for
compounds 2 and 6, respectively. Complete listings of positional and isotropic displacement
coefficients for hydrogen atoms and anisotropic displacement coefficients for the non-hydrogen
atoms are listed in the SI.
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ASSOCIATED CONTENT
Supporting Information
Additional experimental details, characterization data, NMR spectra for all new and selected
known compounds, tables of calculated and observed X-ray structure factors. This material is
available free of charge via the Internet at
AUTHOR INFORMATION
Corresponding Author
* Department of Chemistry and Biochemistry, University of Missouri – St. Louis, One
University Boulevard, St. Louis, Missouri 63121, USA; demchenkoa@umsl.edu
Author Contributions
The manuscript was written through contributions of all authors. / All authors have given
approval to the final version of the manuscript.
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