Direct O-glycosidation of resin bound thioglycosides

Article abstract of DOI:10.1039/c2ob06883d

The application of the safety-catch linker concept to solid-phase glycoconjugate synthesis is described. The process allows for direct conjugation of resin bound glycans to complex aglycones during cleavage. Large excesses of either coupling partner are not required, and even very hindered alcohols serve as acceptors in the reaction. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2ob06883d

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Direct O-glycosidation of resin bound thioglycosides†
Son Hong Nguyen, Adam H. Trotta, John Cao, Timothy J. Straub and Clay S. Bennett*
Received 8th November 2011, Accepted 19th December 2011
DOI: 10.1039/c2ob06883d
The application of the safety-catch linker concept to solid-
phase glycoconjugate synthesis is described. The process
allows for direct conjugation of resin bound glycans to
complex aglycones during cleavage. Large excesses of either
coupling partner are not required, and even very hindered
alcohols serve as acceptors in the reaction.
The introduction or alteration of carbohydrate chains on a small
molecule can have a profound effect on its biological activity.1
Such effects can include altering the molecule’s target,2 reducing
toxicity,3 or increasing potency.4 As a consequence, glycodiver-
sification holds enormous promise for the discovery of novel
therapeutics. This approach is currently underutilized, however,
due to limitations that preclude the routine, parallel, and auto-
mated synthesis of glycoconjugate libraries.
A central focus of our group’s research is developing methods
for the automated synthesis of small molecule glycoconjugate
Fig. 1 A. Standard approaches to glycoconjugate construction using
libraries. Over the past decade, solid-phase5 and fluorous-phase6
oligosaccharide synthesis have emerged as methods of choice
for automated carbohydrate synthesis. Solid-phase synthesis in
particular has received a significant amount of attention, includ-
ing the development of combinatorial approaches to oligosac-
charide library synthesis.7 While this approach has proven its
utility for carbohydrate library construction, its application to the
synthesis of small molecule glycoconjugate libraries has been
comparatively underexplored. This is due to a scarcity of
methods for attaching aglycones to the reducing end of an
immobilized glycan, either during solid-phase synthesis,8 or
upon cleavage from the resin. As a result, glycoconjugate syn-
thesis requires multiple solution-phase manipulations, a process
that is not readily amenable to automation or large-scale library
synthesis (Fig. 1A).
solid-phase oligosaccharide synthesis require multiple solution-phase
transformations. B. The use of a safety-catch linker for direct glycocon-
jugate synthesis upon cleavage described in this work.
synthesis.9 In other words, the linker must be orthogonal to all
solid-phase manipulations, and selectively activated at the end of
the synthesis. In this context a thioglycoside linker appeared to
be an ideal choice.
Thioglycosides are orthogonal to the Lewis acidic conditions
used to activate many donors in oligosaccharide synthesis, and
can themselves be selectively activated with a variety of thiophi-
lic reagents.10 Surprisingly, although thioglycoside-based linkers
have been known for quite some time,11 reports of activating
them for transfer to an aglycone have been limited to examples
that used a large excess (>20 equivalents) of simple acceptors.12
While such approaches are acceptable for solvolysis, the use of a
large excess of acceptor is impractical when valuable molecules,
such as complex natural products, are to serve as aglycones. To
address this issue we decided to examine thiophilic promoters
for the ability to activate carbohydrates immobilized through a
thioglycoside linker for transfer to a small excess (<2 equiva-
lents) of a complex aglycone.
We envisioned that a method for directly transferring an
immobilized oligosaccharide to an aglycone during cleavage
would significantly reduce the number of solution-phase manip-
ulations necessary to obtain the desired target (Fig. 1B). For
such an approach to be effective, it requires a linker analogous to
the “safety-catch” linkers developed for solid-phase peptide
Department of Chemistry, Tufts University, Medford, MA 02155, USA.
E-mail: clay.bennett@tufts.edu; Fax: +1 617-627-0621; Tel: +1-617-
627-0621
†Electronic supplementary information (ESI) available: Synthesis of 1
and 14, experimental procedures and spectroscopic data. See DOI:
10.1039/c2ob06883d
In our initial screening we examined mannose-bearing resin 1
(Table 1). We chose JandaJel as a solid support because this
resin had been reported to work particularly well in solid-phase
oligosaccharide synthesis.7c To determine the optimal reagent
combination, we examined different promoters for the ability to
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Table 1 Preliminary reagent screening
Entry
Promotera
Donor : acceptor ratio
Solvent
T
Yield (%)
1
2
3
4
5
N-(phenylthio)-ε-caprolactam/Tf2O
Ipy2BF4
N-fluoro-2,6-dichloropyridinium triflate
NIS/TfOH
1 : 1.5
1 : 1.5
1 : 1.3
1 : 1
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
−70 °C to rt
−35 °C
20
13
NR
NR
40
−35 °C
rt
benzenesulfinyl piperidine/Tf2O
1 : 2
−40 °C to rt
a Ipy2BF4 = bis(pyridine)iodonium(I)tetrafluoroborate, NIS = N-iodosuccinimide, AW300MS = acid washed 3 Å molecular sieves.
Table 2 BSP/TF2O optimization
Entry
Donor : acceptor ratio
Solvent
Resin
T
Yield (%)
1
2
3
4
5
6
7
8
1 : 2
2 : 1
1 : 2
2 : 1
2 : 1
2 : 1
1 : 1.5
1 : 1.5
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
DCE
toluene
toluene
toluene
JandaJel
JandaJel
JandaJel
JandaJel
JandaJel
JandaJel
JandaJel
Merrifield resin
−40 °C to rt
−40 °C to rt
−60 °C to rt
−60 °C to rt
−40 °C to rt
−40 °C to rt
−60 °C to rt
−60 °C to rt
40
42
30
21
16
50
71%
31%
aTTBP = 2,4,6-tri-tert-butylpyrimidine, DCE = 1,2-dichloroethane, AW300MS = acid washed 3 Å molecular sieves.
transfer the immobilized sugar to cholesterol (2), a model accep-
tor. A number of different promoter systems were screened for
the ability to catalyze the glycosylation reaction.13–16 From this
study we identified the benzenesulfinyl piperidine/triflic anhy-
dride (BSP/Tf2O) promoter as the best candidate for further
optimization (Table 1, entry 5).17
Having identified BSP/Tf2O as the optimal promoter for
glycan transfer, we next turned our attention to optimizing the
reaction conditions. Using cholesterol as an acceptor we found
that toluene was the optimal solvent for the reaction (Table 2,
entry 6). Further optimization revealed that initiating the reaction
at lower temperature (−60 °C) and slowly warming to room
temperature provided the product in 71% yield (Table 2, entry
7). Finally, attempts to use the less expensive Merrifield resin as
a replacement for JandaJel led to a dramatic decrease in yield
(Table 2, entry 8), presumably due to its poorer swelling proper-
ties. Accordingly, all subsequent studies were carried out using
JandaJel as the solid support.
reactivity in CH2Cl2. Under these conditions we found we were
able to isolate the desired glycoconjugate in 46% yield (Table 3,
entry 1). The lower yield was presumably a result of hindrance
about the neopentyl center of the acceptor. Indeed, less hindered
alcohols provided the product in much higher yields (Table 3,
entries 2–4). Pleasingly, both Fmoc and Boc amino acids 5 and
6 were effective acceptors, implying that this approach could
have utility in generating libraries of glycopeptides. Finally, the
very hindered tertiary alcohol acceptor cedrol 8 provided the
product in low yield (Table 3, entry 5), in accordance with our
observation that more hindered acceptors are less efficient in the
reaction. In all cases, the products were all α-anomers due to par-
ticipation of the C-2 acetate.
We next examined disaccharide 14 as a model of a more hin-
dered oligosaccharide donor (Table 4). Pleasingly, the increased
steric encumbrance at C-2 in 14 did not have a deleterious effect
on the reaction, and yields were similar to those observed with
the monosaccharide acceptors (Table 4). Additionally, in all
cases the reaction afforded the product exclusively as the
α-anomer.
We examined the scope of the reaction with several acceptors,
representing model aglycones. When stearyl glycyrrhetinate (4)
was used as an acceptor in toluene, the reaction failed to provide
any product. Reasoning that this may be due to poor solubility
of 4 in toluene at low temperature, we next examined its
In conclusion we have demonstrated that it is possible to trans-
fer carbohydrates immobilized to a solid phase through a
through a thiol linker directly to complex molecule aglycones.
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Table 3 Scope of reaction with monosaccharide resin 1
Table 4 Scope of the reaction with disaccharide resin 14
Entry
Acceptor
Product
Yield (%)
1
2
3
2
5
7
15
16
17
66
60
75
Entry
Acceptor
Product
Yield (%)
TTBP = 2,4,6-tri-tert-butylpyrimidine, AW300MS = acid washed 3 Å
molecular sieves.
1
2
3
4
5
4
5
6
7
8
9
46
69
65
75
20
10
11
12
13
Notes and references
TTBP = 2,4,6-tri-tert-butylpyrimidine, AW300MS = acid washed 3 Å
molecular sieves.
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alcohols serve as acceptors. To our knowledge this is the first
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Acknowledgements
Support for this research was provided in part by a Tufts Univer-
sity Faculty Research Fund Award.
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