STICS: Surface-tethered iterative carbohydrate synthesis

Article abstract of DOI:10.1039/b817684a

A new surface-tethered iterative carbohydrate synthesis (STICS) technology is presented in which a surface functionalized 'stick' made of chemically stable high surface area porous gold allows one to perform cost efficient and simple synthesis of oligosaccharide chains; at the end of the synthesis, the oligosaccharide can be cleaved off and the stick reused for subsequent syntheses.

Full text of DOI:10.1039/b817684a

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STICS: surface-tethered iterative carbohydrate synthesisw
Papapida Pornsuriyasak,a Sneha C. Ranade,a Aixiao Li,a M. Cristina Parlato,a
Charles R. Sims,a Olga V. Shulga,ab Keith J. Stine*ab and Alexei V. Demchenko*a
Received (in College Park, MD, USA) 8th October 2008, Accepted 14th January 2009
First published as an Advance Article on the web 18th February 2009
DOI: 10.1039/b817684a
A new surface-tethered iterative carbohydrate synthesis (STICS)
technology is presented in which a surface functionalized ‘stick’
made of chemically stable high surface area porous gold allows one
to perform cost efficient and simple synthesis of oligosaccharide
chains; at the end of the synthesis, the oligosaccharide can be
cleaved off and the stick reused for subsequent syntheses.
Carbohydrates are the most abundant molecules on Earth, yet
our ability to chemically synthesize these complex natural
compounds to keep pace with rapidly evolving areas of
glycomics is still far from being satisfactory.1–4 In spite of
significant progress in the area of synthetic carbohydrate
chemistry, there is a constant need for improved methods.5–14
Recent years have witnessed a new loop of discovery of high
throughput technologies for expeditious oligosaccharide
assembly; their syntheses in a one-pot fashion,15–19 by using
an automated synthesizer,20,21 or in a microreactor22 are only
a few of the recent notable breakthroughs.
Herein we describe a new complementary technology
Scheme 1 Outline of the STICS concept.
that we call STICS (surface-tethered iterative carbohydrate
synthesis). This technique is aimed at overcoming some of the
limitations associated with traditional solid-phase synthesis of
oligosaccharides,23,24 such as limited use of molecular sieves,
large volume of waste solvent, resin poisoning, reagent
trapping, and difficulties in the construction of compound
libraries. As illustrated in Scheme 1, at the basis of the STICS
concept is a surface functionalized ‘stick’ made of chemically
stable high surface area material that would simplify the
transformation of the solid support-bound molecules between
the reaction vessels. The glycosyl acceptor-anchored stick (A)
is placed in the reaction vessel (B), containing sufficient
quantities of the glycosyl donor, promoter and molecular
sieves. Upon completion of the reaction, the stick, func-
tionalized by a protected terminal sugar moiety, is trans-
ferred into another vessel (C) containing an appropriate
solvent, wherein excess reagent is being rinsed off.
Subsequently, the stick is dipped into vessel D containing a
certain reagent to remove a strategically placed temporary
substituent P. This transformation results in the formation of
the second-generation glycosyl acceptor. To conclude the cycle,
the stick is then briefly placed in a thick-wall container
connected to the vacuum (E). Upon completion of this
sequence, again, a hydroxyl-modified stick is available for
consequent glycosylation. To repeat the cycle, coupling–
washing–deprotection–drying steps are performed once again.
At the end of the synthesis, the oligosaccharide can be cleaved
off from the stick. Depending on the type of the attachment
used, the oligosaccharide can be deprotected directly on the
stick to be used for immunoassay or molecular recognition
studies.25
The ‘‘stick’’ material has been prepared in the form of free-
standing plates of very high surface area that is then covered
with the organic anchor bound to a carbohydrate acceptor.
For the high surface area stable material we chose nanoporous
gold (NPG) that excellently suits our purpose as it provides the
benefits of high surface area with the chemical stability of
gold and access to the well-established chemistry of surface
modification using thiol or disulfide derivatives that can form
self-assembled monolayers (SAMs).26 The NPG plates have
been prepared by dealloying commercially available 10 carat
white gold (Hoover and Strong, 41.8 at% gold, 5 at% Ag,
30–35 at% Cu, 8–9 at% Zn, and 15–20 at% Ni) in nitric acid
and characterizing the resulting NPG using field-emission
SEM and tapping mode AFM.27 The thickness of the sheet
used was 250 mm for these efforts at supported synthesis and
pieces were cut of dimensions of 8.0 ꢀ 8.0 mm.
a Department of Chemistry and Biochemistry, University of
Missouri – St. Louis, One University Boulevard, St. Louis,
Missouri 63121, USA. E-mail: demchenkoa@umsl.edu
b Center for Nanoscience, University of Missouri – St. Louis,
One University Boulevard, St. Louis, Missouri 63121, USA.
E-mail: kstine@umsl.edu
w Electronic supplementary information (ESI) available: Experimental
procedures for the synthesis of all new compounds and their 1H
and 13C NMR spectra. See DOI: 10.1039/b817684a
The synthesis of the carbohydrates on NPG makes use of a
series of common glycosyl donors (1–4,28–31 Fig. 1). The NPG
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Table 1 STICS of disaccharides 8 and 9
Entry
Donor
Acceptor
Promoter
Product
Yield (%)
1
2
3
4
5
6
7
8
9
1
2
3
4
1
2
3
4
1
2
3
4
5
5
5
5
6
6
6
6
7
7
7
7
MeOTf
TMSOTf
MeOTf
MeOTf
MeOTf
TMSOTf
MeOTf
MeOTf
MeOTf
TMSOTf
MeOTf
TMSOTf
8
8
8
8
9
9
9
9
9
9
9
9
31
39
32
42
35
33
44
65a
48
50
62
65
10
11
12
Fig. 1 Glycosyl donors 1–4 and tethered glycosyl acceptors 5–7.
a
When this reaction was reproduced on the 10-plate assembly, an
improved yield of 72% was achieved.
surface of the plates is then modified with self-assembled
monolayers of anchor–linker–monosaccharide conjugates
bearing the first partially protected sugar unit (glycosyl acceptor).
The anchor compounds are di- or tri-thioderivatives designed to
bond strongly to the surface and do not desorb in the organic
solvents used in the subsequent preparative steps.
The free-standing plates can be also slotted into Teflon
assemblies (sticks) of ten plates (or greater). For example,
the reaction between 4 and 6 performed on the 10-plate stick
assembly yielded disaccharide 9 in 72% (7.8 mg), which was a
marginal improvement in comparison to the single plate
experiment (65%, entry 8, Table 1). The estimated cost of
material required to prepare the 10-plate assembly is $18.
To demonstrate further versatility of the developed
technique, we performed the synthesis of trisaccharide 11.
For this purpose 6-O-TBDPS-protected glycosyl donor 10
was reacted with the anchored glycosyl acceptor 6 in the
presence of MeOTf (Scheme 2). The tethered disaccharide
intermediate was then treated with Bu4NF in THF and the
resulting 60-OH acceptor was glycosylated with glycosyl donor
4. After that, trisaccharide 11 was cleaved off with NaOMe in
MeOH, and characterized as per-benzoate 12 obtained in 52%
overall yield.
The trithiol derivative 5 and the lipoic acid derivative 7 are
expected to be covalently bound to the gold surface, while the
derivative 6 with its three sulfide groups should be adsorbed
through multiple attractive Au–S interactions, as found for
dialkylsulfides.32,33 The formation of self-assembled mono-
layers of trithiols,34–36 lipoic acid37 including derivatives linked
to carbohydrates,38 resorcin[4]arene tetrasulfides39 and a
trithiaadamantyl derivative35 have been reported. Depending
on the structure, the anchor is covalently bonded to the
monosaccharide acceptor either via a succinoyl linker segment
(5 and 6) or directly (7) designed so that after the desired series
of glycosylation reactions is completed, it can be easily
cleaved.
The surface coverage of these anchor-linker species was
assessed gravimetrically and was determined to be B0.9–1.1 mmol
per 8 ꢀ 8 mm plate. Typical results of single-step glycosylation
experiments on a single plate are summarized in Table 1. The
main motivation for these preliminary experiments was to
identify the best donor-tethered acceptor–promoter combina-
tion. This was achieved by direct comparison of donors 1–4
and acceptors 5–7. The glycosyl donor was used in excess
(5 equiv.) and the reaction was only performed once.
Expectedly, repetitive glycosylations could positively affect
the yield,40 yet this was not our primary intention at this stage
of the method development. As evident from the results
presented in Table 1, S-benzoxazolyl glycoside 4 provided
marginally higher yields than other glycosyl donors tested
(1–3), and was particularly effective with the primary glycosyl
acceptors (6 and 7, entries 8 and 12, respectively). Since the
disaccharides 8 and 9 in the single-plate experiments were
obtained in amounts less than 1 mg, the most reliable tech-
nique to determine the yields was HPLC using a standardized
calibration plot. Detailed information on this approach can be
found in the ESIw.
Scheme 2 STICS of trisaccharide 11.
It is important to note that the use of a stack of NPG plates
also allows a split-and-mix combinatorial approach, single
plate analysis, conduct of biological assays, etc. The stick
can be disassembled and shuffled as needed for the creation
of combinatorial libraries of carbohydrates. To illustrate this,
we performed the synthesis of an alternative trisaccharide
derivative (see ESIw). It has been also demonstrated that the
remaining stick can be reused with high efficiency (see ESIw).
In conclusion, we have developed a new technology for
expeditious oligosaccharide synthesis that elaborates on the
advantageous features of traditional solid-phase synthesis. This
new approach offers a useful alternative for high throughput
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directed and combinatorial synthesis of carbohydrates or other
bioorganic molecules as well as biological screening thereof.
Considering the manipulative character of STICS strategy,
further automation can be envisaged, and is under pursuit in
our laboratories.
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