Efficient polymer-assisted strategy for the deprotection of protected oligosaccharides

Article abstract of DOI:10.1002/anie.200601128

(Chemical Equation Presented) New strategies in sugar synthesis: A prelinker containing a dihydropyranyl moiety and an activated ester can be attached to a sugar, which is then loaded onto an amino-modified support by amidation. Standard protecting-group manipulations are possible. Final cleavage of the linker under mildly acidic conditions provides the fully deprotected oligosaccharides. TFA=trifluoroacetic acid.

Full text of DOI:10.1002/anie.200601128

Angewandte
Chemie
Supported Oligosaccharides
DOI: 10.1002/anie.200601128
Efficient Polymer-Assisted Strategy for the
Deprotection of Protected Oligosaccharides**
Hiroshi Tanaka, Tadasuke Ishida, Nobuatsu Matoba,
Hirokazu Tsukamoto, Haruo Yamada, and
Takashi Takahashi*
Oligosaccharides play important roles in cell-surface events
through carbohydrate–protein and carbohydrate–carbohy-
drate interactions.[1] The chemical synthesis of structurally
defined oligosaccharides would be highly desirable in struc-
ture–activity studies because oligosaccharides from natural
sources can be produced in only limited qualities. Recent
progress in oligosaccharide synthesis has resulted in a number
of new and efficient glycosidation methodologies,which are
amenable to the synthesis of protected oligosaccharides 1 by
standardized and routine protocols.[2] However,deprotection
of the protected oligosaccharides 1!2,including the cleavage
of various O-protecting groups and the replacement of N-
protecting groups with N-acetyl groups is difficult to achieve
by standardized protocols (Scheme 1). The complete depro-
tection of protected oligosaccharides frequently requires
careful selection of the reaction solvents to prevent the
partially deprotected intermediates from precipitating.
Herein,we describe an efficient method for the deprotection
of protected oligosaccharides based on a polymer-assisted
strategy and its application to the synthesis of dimeric and
trimeric Lewis X derivatives.
Scheme 1. Polymer-assisted strategy for deprotection of the protected
oligosaccharides 1. Bn=benzyl, PNP=para-nitrophenyl, Phth=phthalyl,
Troc=2,2,2-trichloroethoxycarbonyl.
would smoothly undergo deprotection because they are
aggregated to only a very limited extent. A Birch reduction
was adapted for removing the solid-supported benzyl ethers
and esters on 3.[3] The THP linker would survive deprotection
reactions and can be cleaved under mildly acidic conditions to
release the fully deprotected oligosaccharide 2 without
anomerization or cleavage of the glycosidic bonds.[4,5] The
ease of handling of solid-supported compounds would be
effective not only for the high-speed synthesis of a single
target oligosaccharide but also for the deprotection of a
protected oligosaccharide library.[6] The polymer-supported
protected oligosaccharide 3 can be prepared using the
following methodology: 1) acetal formation of the protected
saccharides 1 with prelinker 4 containing a dihydropyranyl
(DHP) moiety and an activated ester and 2) subsequent
Our polymer-assisted strategy for the deprotection of
protected oligosaccharides 1 is illustrated in Scheme 1. The
solid-supported protected oligosaccharide 3 linked through a
tetrahydropyranyl (THP) linker was designed as a key
intermediate. The solid-supported complex oligosaccharides
[*] Dr. H. Tanaka, T. Ishida, Dr. N. Matoba, Dr. H. Tsukamoto,[+]
Prof. Dr. H. Yamada,[#] Prof. Dr. T. Takahashi
Department of Applied Chemistry
Graduate School of Science and Engineering
Tokyo Institute of Technology
2-12-1 Ookayama, Meguro, Tokyo 152-8552 (Japan)
Fax: (+81)3-5734-2884
E-mail: ttak@apc.titech.ac.jp
Homepage: www.apc.titech.ac.jp/ꢀttakahas/index.html
[+] Current address:
Graduate School of Pharmaceutical Sciences, Tohoku University
Aobayama, Aoba-ku, Sendai 980-8578 (Japan)
[#] Current address:
Department of Chemistry, Faculty of Science
Okayama University of Science
Okayama, Ridai 1-1, Okayama 700-0005, Japan
[**] This work was supported by a Grant-in-Aid for Young Scientists (B)
(grant-in-aid no. 16710155) from the Ministry of Education, Culture,
Sports, Science, and Technology.
Scheme 2. Reagents and conditions: a) LiAlH4, THF; b) tert-butyl bro-
moacetate, NaH, DMF; c) 1n aq. NaOH, dioxane; d) 4-nitrophenol,
EDCI, DIEA, CH2Cl2, 42% from 6. DMF=N,N-dimethylformamide,
EDCI=1-ethyl-3-(3-dimethylaminopropyl)carbodiimide, DIEA=diiso-
propylethylamine.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Communications
Table 1: Deprotection of the O-benzyl-protected glucose derivative 8
Exposure of the solid-supported glucose derivatives to
mildly acidic conditions permitted the products to be
released. An HPLC analysis of the released products using
an evaporative light-scattering detector (ELSD) revealed that
only the benzyl ethers supported on the ArgoPore resin
underwent complete reductive cleavage under the Birch
reduction conditions (Table 1,entry 1). These results suggest
that the substrates on the surface of the polymer might
selectively undergo Birch reduction because the solution of
liquid ammonia in THF did not cause the hydrophobic
polymers to swell. Further purification of the product
based on a polymer-assisted strategy.[a]
Entry
10
Loading yield
Conv.
[%][c]
Purity
Overall yield
(8!11) [%]
(8!10) [%][b]
of 11 [%][c]
1
2
3
3
10a
10b
10c
10d
99
97
99
93
>99
59
trace
3
>99
58
trace
3
91
–
–
–
[a] CSA=(+)-camphorsulfonic acid, PS=polystrene, TFA=trifluoro-
acetic acid. [b] The yield was estimated by the release of 8 from 10.
[c] The conversion and purity were estimated by HPLC analysis of the
cleaved materials using ELSD.
amidation of the resulting activated ester with the
solid-supported amines 5 to give 3,irreversibly.
The irreversible loading reaction would enable
the complete immobilization of the protected
oligosaccharides 1.[7]
The preparation of prelinker 4 is outlined in
Scheme 2. The reduction of aldehyde 6 followed
by alkylation of the resulting alcohol with tert-
butyl a-bromoacetate provided the tert-butyl
ester 7. Hydrolysis of the tert-butyl ester 7
under basic conditions followed by esterification
with 4-nitrophenol afforded prelinker 4 in 42%
overall yield from 6.
We first examined the polymer-assisted
deprotection of methyl 2,3,4-O-tribenzylgluco-
side (8) using the solid supports 5a–d (Table 1).
Treatment of 8 with 1.2 equiv of prelinker 4 under
acidic conditions provided the protected gluco-
side 9,which contains an activated ester,as a
mixture of diastereomers. The subsequent ami-
dation of 9 with 3.0 equiv of the solid-supported
amines 5a–d afforded the solid-supported pro-
tected glucoses 10a–d. The yields of 10a–d based
on 8,estimated by cleavage of 8 from 10,were
excellent. The removal of the benzyl ethers was
achieved by treatment of the solid-supported
protected glucose derivatives 10a–d with lithium
in liquid ammonia and tetrahydrofuran at ꢁ308C
for 1 h.
Scheme 3. Reagents and conditions: a) 12 (1.8 equiv), 13 (1.5 equiv), AgOTf
(1.8 equiv), CH2Cl2, 4-Š M.S., ꢁ208C; b) 15 (1.0 equiv), NIS (1.5 equiv), ꢁ108C;
c) 14 (5.0 equiv), NIS (5.5 equiv), 77% based on 15; d) thiourea, 2,6-lutidine, DMF,
99% for 18, 92% for 20, 70% for 22; e) 12 (1.8 equiv), 13 (1.5 equiv), AgOTf
(1.5 equiv), CH2Cl2, 4-Š M.S., ꢁ208C, then 18 (1.0 equiv), NIS (1.5 equiv), ꢁ108C,
then, 14 (5.0 equiv), NIS (5.5 equiv), 56% based on 18; f) 12 (1.8 equiv), 13
(1.5 equiv), AgOTf (1.8 equiv), CH2Cl2, 4-Š M.S., ꢁ208C, then 20 (1.0 equiv), NIS
(1.5 equiv), ꢁ108C, then, 14 (10 equiv), NIS (10.5 equiv), 44% based on 20.
Bz=benzoyl, M.S.=molecular sieves, NIS=N-iodosuccinimide, Tf=trifluorometha-
nesulfonyl.
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Angewandte
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released from the ArgoPore resin provided a-
methyl glucoside 11 in 91% yield based on 8
(Table 1,entry 1). These results indicate that
the substrate on the ArgoPore resin was not
released during the Birch reduction. To the
best of our knowledge,this is the first report
on an application of a Birch reduction to
solid-phase synthesis.
We next synthesized the trimeric and
dimeric Lewis X derivatives 29 and 30 by a
one-pot glycosylation and the polymer-
assisted deprotection procedure (Scheme 3).
The hepta- and decasaccharides 29 and 30 are
important tumor-associated antigens[8] and
have served as effective synthetic targets for
demonstrating the feasibility of new method-
ologies owing to the complexity of their
branched structure,which contains both a-
and b-glycosidic linkages.[9–11]
A one-pot glycosylation was used to
prepare the protected hepta- and decasac-
charides
20
and
22,respectively
(Scheme 3).[12] The chemo-,stereo-,and
regioselective glycosylation of glucosamine
13 at the C4 hydroxy group with the galactosyl
bromide 12 provided the glycosylated thio-
glycoside 16. Subsequent activation of thio-
glycoside 16 to couple with alkyl galactoside
15 at the 3-position,followed by the a-
selective glycosylation of the remaining hy-
droxy group at the C3 position with thiofuco-
side 14 provided tetrasaccharide 17 in 77%
overall yield based on 15. Removal of the
chloroacetyl group at the C3 position on 17
provided alcohol 18 in 99% yield,which was
used as an acceptor in the next one-pot
glycosylation. A one-pot glycosylation using
these three building blocks—12, 13,and 14—
and acceptor 18 provided heptasaccharide 19
in 57% yield based on 18. Subsequent depro-
tection of the C3 protecting group gave the
heptasaccharide acceptor 20 in 92% yield. A
one-pot glycosylation using the three building
blocks 12, 13,and 14 and heptasaccharide 20
Scheme 4. Reagents and conditions: a) 4 (3 equiv), CSA, CH2Cl2, room temperature; b) amino-
methyl ArgoPore 5a (10 equiv), DIEA, DMF, room temperature, 12 h; c) NH2NH2·H2O/EtOH (1:5),
reflux, 6 h, then AcOH, DIC, DIEA, room temperature, 6 h; d) lithium, THF, liq. NH3, ꢁ788C, 1.5 h,
reflux, 1 h, then MeOH, 12 h; (l); e) TFA, MeOH/CH2Cl2 (1:19), 58% yield for 20!29, 58% yield
afforded the protected decasaccharide 21 in for 22!30. DIC=1,3-diisopropylcarbodiimide.
44% yield based on 20. Removal of the
chloroacetyl protecting group gave the deca-
saccharide 22,containing a hydroxy group,in
70% yield.
Deprotection of the protected trimeric and dimeric
Lewis X epitopes 20 and 22 based on the polymer-supported
strategy was examined (Scheme 4). The treatment of 20 and
22 with 3 equiv of prelinker 4 in the presence of CSA at room
temperature provided the protected oligosaccharides 23 and
24,which contain an activated ester. After removal of the
remaining prelinker 4 by flash column chromatography,the
activated esters 23 and 24 were reacted with 10 equiv of the
solid-supported amine 5a in DMF under basic conditions for
12 h to give the solid-supported protected oligosaccharides 25
and 26,respectively. The phthalyl groups of 25 and 26 were
removed by treatment with NH2NH2·H2O/EtOH (1:5) at
reflux for 6 h.[13] Acetylation of the resulting amines provided
the solid-supported acetamides 27 and 28,respectively.
Oligosaccharides 27 and 28 were then subjected to Birch
reduction conditions and then cleaved from the resin under
mildly acidic conditions. 1H NMR spectra of the released
products revealed that most of the solid-supported benzyl
esters on the complex oligosaccharides smoothly underwent
cleavage in the Birch reduction. Purification of the released
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Communications
[12] H. Tanaka,N. Matoba,H. Tsukamoto,H. Takimoto,H. Yamada,
T. Takahashi, Synlett 2005,824 – 828.
[13] Treatment of 10a with NH2NH2·H2O/EtOH (1:5) at reflux for
12 h did not reduce the loading amount of the solid-supported
10a. These results indicated that the THP linker could be stable
during cleavage of the phthalyl groups.
products afforded the fully deprotected trimeric and dimeric
Lewis X epitopes 29 and 30,each in 58% overall yield,based
on 20 and 22,respectively. Although the loading and release
of these complex protected oligosaccharides would be
difficult to achieve without loss of material,this method
should be of practical use for the synthesis of oligosaccharide
libraries.
In conclusion,an efficient polymer-assisted method for
the deprotection of protected oligosaccharides has been
demonstrated. The prelinker 4 composed of a DHP unit and
an activated ester was effective not only for the loading of the
protected oligosaccharides from the solution but also for the
release of the fully deprotected oligosaccharide from the
resin. ArgoPore was the best support for the cleavage of solid-
supported benzyl ethers,which was achieved using Birch
reduction conditions.
Received: March 22,2006
Revised: May 30,2006
Published online: August 17,2006
Keywords: Birch reduction · glycoconjugates · glycosylation ·
.
oligosaccharides · solid-phase synthesis
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