Demixing libraries of saccharides using a multi-linker approach in combination with a soluble polymeric support

Article abstract of DOI:10.1016/S0040-4039(02)00851-1

A new method for demixing libraries of compounds that are attached to a soluble polymeric support by tagging starting materials with selective cleavable linkers is described.

Full text of DOI:10.1016/S0040-4039(02)00851-1

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 4691–4694
Demixing libraries of saccharides using a multi-linker approach
in combination with a soluble polymeric support
Galal A. Elsayed, Tong Zhu and Geert-Jan Boons*
Complex Carbohydrate Research Center, 220 Riverbend Road, Athens, GA 30602, USA
Received 22 March 2002; revised 29 April 2002; accepted 30 April 2002
Abstract—A new method for demixing libraries of compounds that are attached to a soluble polymeric support by tagging starting
materials with selective cleavable linkers is described. © 2002 Elsevier Science Ltd. All rights reserved.
Liquid polymer-, dendrimer- and fluorous supported
syntheses are emerging as attractive alternatives for
solid-phase organic chemistry.1 These synthetic
approaches have the favorable properties of traditional
solution phase chemistry, while the macromolecular or
fluorous properties of the liquid supporting system
facilitate product purification. For example, com-
pounds that are linked to polyethylene glycol
monomethyl ether (MPEG)2 are soluble in many
organic solvents; however, due to the helical structure
of MPEG, it has a high propensity to crystallize in
diethyl ether and ethanol and, thus, product purifica-
tion by crystallization can be accomplished at each
stage of the synthesis. Dendrimer supported synthesis
exploits that compounds can be easily separated from
excess reagents by size exclusion chromatography,3
whereas fluorous supported synthesis relies on selective
extraction in a fluorocarbon solvent.4 In general, these
liquid polymer supported methods have similar reaction
kinetics compared to traditional solution-phase chem-
istry; they do not require the use of a large excess of
reagents, and they allow supported intermediates and
final products to be easily characterized. Despite these
attractive features, liquid polymer supported methods
have rarely been used in combinatorial chemistry5 and
one of their main shortcomings is that there are no
general methods for synthesizing mixtures of com-
pounds that at the end of a sequence of reactions can
easily be separated to give individual compounds.6,7
A
notable exception is a method developed by Curran and
co-workers8 whereby four different compounds can be
linked to four different fluorous tags of increasing
fluorine content. The tagged compounds can then be
mixed and after multiple reactions, demixed by fluorous
chromatography to provide individual compounds.
Herein we report a novel and general method for
demixing liquid phase supported compounds by an
orchestrated release from the supporting system by
selective cleavage of different linkers. Thus, in the
proposed strategy, a series of substrates will be tagged
Figure 1. Building blocks for library synthesis.
* Corresponding author. Tel.: +1-706-542-9161; fax: +1-706-542-4412;
e-mail: gjboons@ccrc.uga.edu
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)00851-1

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G. A. Elsayed et al. / Tetrahedron Letters 43 (2002) 4691–4694
with a series of selective cleavable linkers. These linker-
tagged compounds can then be mixed, attached to a
soluble support and used in a series of combinatorial
reactions. At the end of the reaction sequence, individ-
ual compounds can be obtained by selective cleavage of
the linkers. The new method was applied to the prepa-
ration of a library of 18 disaccharides9–13 using glycosyl
acceptors 1–3 that are tagged by unique linkers, glyco-
syl donors 4–6 and MPEG as the liquid supporting
system.14
Compound 1 has a free C-3 hydroxyl and is functional-
ized with a phenolic ester linker (Fig. 1), which is stable
towards Lewis acid conditions used in glycosylations
but can be cleaved within minutes by treatment with
hydrogen peroxide/Et3N.15,16 After detachment, a p-
hydroxyl benzyl ether will be obtained which can easily
be removed by oxidation with DDQ.17 Compound 2
has a free C-2 hydroxyl and is derivatized with a
succinic ester linker.18 This ester is substantially more
stable than the phenolic ester linkage of 1 and requires
treatment with sodium methoxide for cleavage. Glyco-
syl acceptor 3 has a free C-4 hydroxyl and is derivatized
with an acid sensitive p-alkoxybenzyl linker,19 which is
orthogonal with the linkers of 1 and 2. The linkers of
1–3 contain a carboxylic acid moiety, which will be
utilized for the attachment to amino-functionalized
MPEG.
Scheme 1. Reagents and conditions: (i) PMBCl, NaH, DMF
(95%); (ii) aq. AcOH then Bu2Sn(OMe)2, p-allyloxybenzyl
chloride, Bu4NI (62%); (iii) BnBr, NaH, DMF (82%); (iv)
Pd(PPh3)4, EtOH (63%); (v) glutaric anhydride, Py, DMAP
(79%); (vi) TFA, DCM (30%); (vii) BH3·Me3N, AlCl3 (68%);
(viii) (Bu3Sn)2O, succinic anhydride (60%); (ix) Bu2Sn(OMe)2,
p-allyloxybenzyl chloride, Bu4NI (80%); (x) Br(CH2)5CO2Et,
Cs2CO3, DMF; then MeOH, NaOH (86%).
Spacer modified 1 was synthesized from known galac-
toside 720 by first installation of a p-hydroxybenzyl
group at C-6 (11), which was acylated with glutaric
anhydride (Scheme 1). Thus, the C-3 hydroxyl of 7 was
protected as a temporary p-methoxybenzyl ether (PMB)
by reaction with PMBCl and NaH in DMF to give
fully protected 8 in a yield of 95%. The benzylidene
acetal of 8 was removed using aqueous acetic acid and
the C-6 hydroxyl of the resulting diol was functional-
ized as a p-allyloxybenzyl ether by first formation of an
intermediate 4,6-O-stannylene acetal,21 which was
regioselectively reacted with p-allyloxybenzyl chloride
in the presence of t-butylammonium bromide to give 9.
Next, the remaining C-4 hydroxyl of 9 was benzylated
under standard conditions to give 10, which was treated
with Pd(PPh3)4 in refluxing ethanol to remove the phe-
nolic allyl ether to afford 11. Reaction of 11 with
glutaric anhydride in the presence of DMAP in pyridine
gave linker modified 12 in a yield of 61% and finally,
the target compound 1 was obtained by removal of the
PMB ether by treatment with TFA in dichloromethane.
hydroxyl of diol of 1123 was selectively functionalized
as a p-allyloxybenzyl ether to give 16 by reaction of an
intermediate stannylene acetal21 with p-allyloxybenzyl
chloride in the presence of n-Bu4NI in toluene.
Removal of the allyl ether of 16 by treatment with
Pd(PPh3)4 in refluxing ethanol afforded 17 in a high
yield. The target compound 3 was obtained by alkyla-
tion of the p-hydroxybenzyl ether of 17 with ethyl
6-bromohexanoate in the presence of Cs2CO3 followed
by hydrolysis of the ethyl ester using aqueous sodium
hydroxide in methanol.
With ample quantities of linker-modified acceptors in
hand, attention was focused on the combinatorial syn-
thesis of a library of eighteen disaccharides (Scheme 2).
Mixing of compounds 1–3 followed by attachment to
amino-modified MPEG (Mw 5000) by standard amide
bond formation afforded library 18. The mixture of
immobilized acceptors (18) could be easily purified by
precipitation with diethyl ether and inspection of the
NMR spectrum of the resulting material indicated that
the three monosaccharides were present in approxi-
mately equal molar quantities. The monosaccharide
library 18 was split into three pools and the first pool
was glycosylated with thiogalactosyl donor 424 using
NIS/TMSOTf25 as the activator and dichloromethane/
diethyl ether as the solvent mixture. The resulting
Target compound 2 was easily obtained by regioselec-
tive opening of the benzylidene acetal of 1322 with
BH3·Me3N complex in the presence of AlCl3 followed
by selective acylation of the primary alcohol of the
resulting 14 with succinic anhydride. The latter reaction
only proceeded with high regioselectivity when first an
intermediate tri-n-butylstannyl ether was formed by
reaction with (Bu3Sn)2O.
Spacer modified 3 was also prepared from an interme-
diate p-hydroxybenzyl ether (17), which in this case,
was alkylated with 6-bromohexanoate. Thus, the C-6

G. A. Elsayed et al. / Tetrahedron Letters 43 (2002) 4691–4694
4693
or TMSOTf,28 respectively, as the activators. The
resulting libraries of disaccharides were purified by
precipitation followed by filtration and washing and
then demixed by selective cleavage of the linkers. All
disaccharides were deprotected using standard proce-
dures to give the following disaccharides: a/b-
2)-b- -Galp-OMe (a/b=4:1, 61%); a/b- -Galp(1-3)-
b- -Galp-OMe (a/b=9:1, 65%); a/b- -Galp(1-4)-b-
Galp-OMe (a/b=3/1, 45%); a/b- -Glcp(1-2)-b- -Galp-
OMe (a/b=3/1, 55%); a/b- -Glcp(1-3)-b- -Galp-OMe
(a/b=1/1, 62% ), a/b- -Glcp(1-4)-b- -Galp-OMe (a/
D-Galp(1-
D
D
D
D
D-
D
D
D
D
D
D
b=3/2, 50%); a/b-
D
-GlcNH2p(1-2)-b-
D
-Galp-OMe (a/
b=3/1, 55%); a/b-
(a/b=5/1, 60%); a/b-
(a/b=9/1, 50%).29
D
-GlcNH2p(1-3)-b-
D
D
-Galp-OMe
-Galp-OMe
D
-GlcNH2p(1-4)-b-
In conclusion, we have developed a new method for
demixing libraries of compounds that are attached to a
soluble polymeric support by tagging starting materials
with selective cleavable linkers. Major attractions of the
methodology are that libraries of linker-tagged
monosaccharides can repeatedly be used in glycosyla-
tions with different glycosyl donors to give a large
number of oligosaccharide libraries. Each of these
libraries can then be demixed by simple chemical
manipulations to give well-defined products. Unlike
deconvolution procedures based on tagging of beads,
the method described here provides preparative quanti-
ties of material that can be characterized by conven-
tional methods. This is important because
oligosaccharide synthesis is prone to side-product for-
mation and there are also no reliable strategies for
deblocking oligosaccharides attached to the polymeric
support. It is to be expected that the new methodology
can be applied to other types of liquid supported syn-
thesis and in particular the combination of selective
cleavable linkers with fluorous tags will be attractive to
demix a relatively large number of compounds. Cur-
rently, we are expanding the new methodology by
developing several other linkers that will be compatible
with the existing linkers and by employing temporary
protecting groups to prepare larger oligosaccharides.
Scheme 2. Combinatorial synthesis and demixing disaccha-
ride library.
MPEG-bound disaccharides (19) were easily purified by
selective precipitation, filtration and washing.
The next stage of the synthesis entailed demixing of
library 19 by selective cleavage of the linkers followed
by precipitation of the MPEG-bound compounds
(Scheme 2) to give the individual disaccharides 20, 22
and 23. Thus, library 19 was first treated with H2O2 and
Et3N in dichloromethane for 10 min after which the
MPEG-bound compounds were precipitated by addi-
tion of diethyl ether and collected by filtration. The
filtrate, which contained the released disaccharide 20,
was concentrated and the crude reaction product ana-
lyzed by NMR and Maldi-Tof MS. The analysis
showed only the presence of disaccharide 20 and the
absence of any of the other disaccharides and intact
acceptor indicating that the cleavage was indeed selec-
tive and that the glycosylation had proceeded to com-
pletion. The crude reaction mixture was purified by
silica gel column chromatography to give 20 in an
overall yield of 65% as a mainly the a anomers (a/b=9/
1). The precipitated MPEG, which contained the two
remaining disaccharides, was dissolved in methanol and
treated with NaOMe to cleave the succinic ester linkage
to release 22. This compound was isolated in a 61%
yield as a mixture of anomers (a/b=4/1) by the same
procedure as described for 20. Finally, disaccharide 23
was obtained in an overall yield of 45% (a/b=3/1) by
treatment of the polymer with TFA in dichloromethane
followed by a standard isolation procedure.
Acknowledgements
We thank the Egyptian Cultural and Educational
Bureau and the Office of the Vice President for
Research, The University of Georgia, Athens for finan-
cial support.
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