Solid-phase capture-release strategy applied to oligosaccharide synthesis on a soluble polymer support

Article abstract of DOI:10.1002/1521-3773(20011217)40:24<4725::AID-ANIE4725>3.0.CO;2-9

A highly simplified synthesis of a tetrasaccharide was possible by a repeated coupling/capture-release cycle. In the first cycle, a glycosyl acceptor (1) bound to a soluble polymer reacted with a glycosyl donor (2) having a chloroacetyl group as a tempora

Full text of DOI:10.1002/1521-3773(20011217)40:24<4725::AID-ANIE4725>3.0.CO;2-9

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R. C. Johnson, R. E. Dickerson, Science 1994, 263, 348 355; b) A.
Draganescu, J. R. Levin, T. D. Tullius, J. Mol. Biol. 1995, 250, 595
608; c) G. Meinke, P. B. Sigler, Nat. Struct. Biol. 1999, 6, 471 477.
[8] A major minor groove DNA-binding ligand made of a hairpin
polyamide and a 11-mer oligonucleotide has been previously descri-
bed: J. W. Szewczyk, E. E. Baird, P. B. Dervan, Angew. Chem. 1996,
108, 1596 1598; Angew. Chem. Int. Ed. 1996, 35, 1487 1489.
[9] C. Zimmer, U. Wahnert, Prog. Biophys. Mol. Biol. 1986, 47, 31 112.
[10] T. E. Ellemberger, C. J. Brandl, K. Struhl, S. C. Harrison, Cell 1992, 71,
1223 1237.
[11] M. Coll, C. A. Frederick, A. H. J. Wang, A. Rich, Proc. Natl. Acad. Sci.
USA 1987, 84, 8385 8389.
[12] The model was constructed by superimposing appropriate base pairs,
by using the INSIGHTII software package on a SGI workstation.
[13] The synthesis of 2 and 1 have been previously communicated: M. E.
Figure 4. Autoradiograms showing the binding of hybrid 5 to 32P-labeled
DNAs. Lanes 1 6: T/CREhs, 5: 0, 7.7, 19, 38, 58, 77 nm respectively;
Lanes 7 11: T/CREhsm, 5: 7.7, 19, 38, 58, 77 nm respectively; lanes 12 14:
CREhs, 5: 38, 77, 154 nm respectively. Binding reactions were performed
over 10 min at 48C using <1 nm labeled DNAs in a binding mixture (20 mL)
containing 20 mm tris(hydroxymethyl)aminomethane (pH 7.5), 100 mm
KCl, 2 mm MgCl2, 2 mm ethylenediaminetetraacetate, 10% glycerol,
0.3 mgmLÀ1 N,O-bovine serum albumin (BSA), and 2% NP-40. The
products were resolved by polyacrylamide gel electrophoresis using a 10%
nondenaturing acrylamide gel and 0.5X TBE (25 mm tris borate and 0.5 mm
EDTA) buffer.
¬
ƒ
ƒ
Vazquez, A. M. Caamano, L. Castedo, D. Gramberg, J. L. Mascarenas,
Tetrahedron Lett. 1999, 40, 3625 3629.
[14] The target site of this sequence does not match exactly with that of the
model, but it was used on the basis of the reported preferred (1:1)
recognition sites for distamycin, see: D. Rentzeperis, L. A. Marky, T. J.
Dwyer, B. H. Geierstanger, J. G. Pelton, D. A. Wemmer, Biochemistry
1995, 34, 2937. See also reference [15].
CREhs: 5'-d(CGACGTCATCGGAGGTCCT)-3'
3'-d(GCTGCAGTAGCCTCCAGGA)-5'
[15] F.-M. Chen, F. Sha, Biochemistry 1998, 37, 11143 11151.
[16] We have calculated an approximate Kd value of 0.8 Æ 0.5mm by CD
titration of compound 2 withthis dsDNA.
makes non-specific electrostatic contacts to the phosphate
groups.[18]
In conclusion, appropriate linking of a b-ZIP basic region to
a minor groove binding tripyrrole allowed for specific binding
to its cognate DNA site. The hybrid compound 5 shows
considerably higher affinity for its designated target DNA
sequence than that of its isolated components for their
respective cognate subsites. Although further refinement of
the design is necessary to obtain compounds with higher
affinities and better specificities, the work described herein
confirms the viability of this new type of major minor groove
DNA-binding molecules.
[17] The synthesis of 5 was carried out by using similar procedures as for
the synthesis of 1, and is described in the Supporting Information.
‡
Electrospray mass spectrometry: m/z [M‡H ] found: 3271.4; calcd:
3270.8.
[18] A related mode of interaction has been previously observed with
¬
microgonotropens, see: D. Sengupta, A. Blasko, T. C. Bruice, Biorg.
Med. Chem. 1996, 4, 803 813, and references therein.
Received: July 23, 2001 [Z17576]
Solid-Phase Capture Release Strategy
Applied to Oligosaccharide Synthesis on a
Soluble Polymer Support**
[1] For reviews on DNA-binding proteins, see: a) C. O. Pabo, R. T. Sauer,
Annu. Rev. Biochem. 1992, 61, 1053 1095; b) S. K. Burley, Curr. Opin.
Struct. Biol. 1994, 4, 3 11; c) B. Alberts, D. Bray, J. Lewis, M. Raff, K.
Roberts, J. D. Watson, Molecular Biology of the Cell, Garland
Publishing, New York, 1994, chap. 9; d) C. Branden, J. Tooze,
Introduction to Protein Structure, Garland Publishing, New York,
1999, chap. 8 10; e) D. S. Latchman, Eukaryotic Transcription Fac-
tors, Academic Press, London, 1998, chap. 8; f) N. M. Luscombe, S. E.
Austin, H. M. Berman, J. M. Thornton, Genome Biology 2000, 1, 1
37.
Hiromune Ando, Shino Manabe, Yoshiaki Nakahara,
and Yukishige Ito*
Advances in the techniques for oligosaccharide synthesis
have lagged behind those for other classes of biological
oligomers. For the preparation of oligopeptides and oligo-
[2] H. C. Hurst, Protein Profile 1996, 3, 1 72.
[3] a) R. V. Talanian, C. J. McKnight, P. S. Kim, Science 1990, 249, 769
771; b) B. Cuenoud, A. Shepartz, Science 1993, 259, 510 513; c) T.
Morii, M. Shimomura, M. Morimoto, I. Saito, J. Am. Chem. Soc. 1993,
115, 1150 1151; d) T. Morii, Y. Saimei, M. Okagami, K. Makino Y.
Sugiura, J. Am. Chem. Soc. 1997, 119, 3649 3655; e) Y. Aizawa, Y.
Sugiura, T. Morii, Biochemistry 1999, 38, 1626 1632.
[*] Dr. Y. Ito, Dr. H. Ando, Dr. S. Manabe
RIKEN (The Institute of Physical and Chemical Research)
and CREST, Japan Science and Technology Corporation (JST)
2
1 Hirosawa, Wako-shi, Saitama 351-0198 (Japan)
Fax : (‡81)48-462-4680
E-mail: yukito@postman.riken.go.jp
[4] The use of an azobenzene as a dimerizing unit allows the DNA affinity
of the peptides to be photomodulated: A. M. Caamanƒo, M. E.
Prof. Dr. Y. Nakahara
Department of Applied Biochemistry, Tokai University
and CREST, JST
¬
ƒ
Vazquez, J. MartÌnez-Costas, L. Castedo, J. L. Mascarenas, Angew.
Chem. 2000, 112, 3234 3237; Angew. Chem. Int. Ed. 2000, 39, 3104
3107.
Kitakaname 1117, Hiratsuka-shi, Kanagawa 259-1292 (Japan)
[**] This work was supported in part by the Mizutani Foundation for
Glycoscience (grant to Y.I.), the RIKEN Presidential Fund (grant to
S.M.), and the Ministry of Education, Culture, Sports, Science and
Technology (Grant-in-Aid for Scientific Research to S.M.). We thank
Dr. Teiji Chihara and his staff for the elemental analyses, Ms. Yasuko
Tanaka for her contribution at the exploratory stage of this work, and
Ms. Akemi Takahashi for her technical assistance.
[5] Recently Zondlo and Shepparzt have developed a smart strategy for
achieving monovalent DNA recognition which is based on grafting the
DNA-binding residues into the helix of an aPP protein: a) N. J.
Zondlo, A. Schepartz, J. Am. Chem. Soc. 1999, 121, 6938 6939;
b) J. W. Chin, A. Schepartz, J. Am. Chem. Soc. 2001, 123, 2929 2930.
[6] D. Stanojevic, G. L. Verdine, Nat. Struct. Biol. 1995, 2, 450 457.
[7] This system would somewhat mimick the mode of recognition of a
number of transcription factors that bind DNA by interacting with
boththe major and minor grooves. See for example: a) J.-A. Feng,
Supporting information for this article is available on the WWW under
http://www.angewandte.com or from the author.
Angew. Chem. Int. Ed. 2001, 40, No. 24
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nucleotides, the automated synthesizer has become a routine
tool, in which polymer-support synthesis plays a key role.
Considering the structural diversity of glycoconjugate-
derived oligosaccharides,[1] the potential utility of oligosac-
charide synthesis on a polymer support is obvious.[2] This
technology would greatly facilitate glycoconjugate synthesis
and could possibly reap the benefits of automation.[3] How-
ever, several problems must be solved before this goal can be
reached: firstly, the difficulty in monitoring reaction progress;
secondly, the reduced reactivity of substrates bound to the
polymer support; and thirdly, the limitations in scaling up.
Addressing these issues, we recently developed a method-
ology for the monitorable synthesis of oligosaccharides on a
soluble polymer support based on the ™tag reporter∫ con-
cept.[4] It exploited low molecular weight poly(ethylene)glycol
(PEG),[5] which served both as a ™reporter∫ in the monitoring
of the reaction by matrix-assisted laser desorption/ionization
time-of-flight (MALDI-TOF) mass spectrometry and as a
™tag∫ in the separation of support-bound materials from the
reaction mixture due to its high polarity.[6, 7]
stage, excess donor and side product(s) can be removed. The
coupled product (C), potentially contaminated withunreact-
ed acceptor A1, is then captured onto solid phase by a
chemoselective reaction between CA and a resin-bound thiol
group (D). After removal of Fmoc, the exposed amine
function should cyclize spontaneously to release a disacchar-
ide (A2), which is now ready for the next coupling. Repetition
of this cycle should provide facile access to correctly
assembled oligomers An.
To examine the adequacy of this strategy, we selected a
lactosamine repetition sequence as the target. It is well
recognized that polylactosamine [(Galb1 !4GlcNAcb1 !3)n]
is an important structural motif of Asn-linked[11] and Ser/Thr-
linked[12] glycoproteins and of glycosphingolipids.[13]
PEG-supported monosaccharide 2, which carries the nitro-
modified Wang resin-type linker,[14] was prepared as depicted
in Scheme 2. Previously reported 1[4] was treated withPEG
O
OH
O
Me
n
Although syntheses on polymer supports can indeed be
monitored,[4, 8] the success of this method assumes all reac-
tions can be driven to near completion. This seems to be a
difficult requirement, considering that typical O-glycosyla-
tions proceed in 50 80% yield.[9] Now, we wishto report a
refined strategy for the synthesis of PEG-tagged oligosac-
charides that circumvents this obstacle.
This new approach is based on the concept of resin-aided
capture release (Scheme 1).[10] It employs a glycosyl acceptor
bound to a low molecular weight PEG support (A1) in
combination witha chloroacetyl (CA) carrying donor ( B), as
described before.[4] The PEG-bound component can be
recovered by filtration through a pad of silica gel. At this
R
NO2
OH
O
O
O
O
O
c
a
AcO
BnO
HO
OMP
HO
BnO
OMP
NPhth
OMP
NPhth
BnO
NPhth
4
1
2, R = NO2
b
3, R = NHAc
Scheme 2. Preparation of PEG-supported monosaccharide 2. a) PEG
monomethyl ether, DEAD, Ph3P, THF, RT, 18 h, 99%; b) acetylacetone,
Zn/Cu, THF, RT, 16 h, then Ac2O, Et3N; c) DDQ, ClCH2CH2Cl/H2O, RT,
4 h, 92% overall. DEAD ˆ diethyl azodicarboxylate, DDQ ˆ 2,3-dichloro-
5,6-dicyanobenzoquinone, MP ˆ p-MeOC6H4, Phth ˆ phthaloyl.
methyl ether (av Mw ˆ 550) under Mitsunobu con-
O
O
Cl
ditions. Nearly quantitative carbohydrate incorpora-
O
X
1
tion was supported by H NMR spectroscopy and
O
O
B
O
O
confirmed by the cleavage experiment performed
under the conditions reported by Kusumoto et al. for
the removal of the nitrobenzyl group.[15] Namely, the
nitro group of 2 was first converted to the acetamido
group in 3, which was then treated with DDQ[16] to
afford compound 4 in high overall yield.
Galactosyl donor 9, which has a chloroacetyl
group, was prepared from phenylsulfanylgalactoside
5
[17] via 6 8 (Scheme 3). The coupling of 2 and 9 was
performed withdimethyl(methylsulfanyl)sulfonium
triflate (DMTST)[18] in CH2Cl2 (Scheme 4). To eval-
uate the efficacy of the capture release process only
a substoichiometric amount (0.9 equiv) of 9 was used
for this reaction. The reaction progress was moni-
tored by MALDI-TOF mass spectrometry as de-
scribed previously.[4, 19] The mixture was passed
through a pad of silica gel that was first washed with
AcOEt, and the PEG-bound component was eluted
withAcOEt/MeOH 3:1. Analysis of this fraction by
1H NMR spectroscopy revealed the presence of
disaccharide 10 and unreacted acceptor 2 in a ratio of
about 10:3 (Figure 1A). This indicates that the
O
HO
R
O
Cl
O
O
R
A1
I
(coupling)
C
SH
O
NHFmoc
II
O
(capture)
O
O
D
O
O
R
HO
III
O
O
(release)
A2
O
O
O
S
O
R
Repeat
I, II, III
(n times)
O
NHFmoc
O
S
O
O
N
H
O
O
O
O
HO
O
R
n+1
An
Scheme 1. Capture release strategy for the synthesis of disaccharides A2 and
oligosaccharides An on a polymer support. Fmoc ˆ 9-fluorenylmethoxycarbonyl.
4726
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glycosylation proceeded in a highly efficient manner (ca. 85%
based on donor 9) and that the entropic disadvantage inherent
to reactions on a polymer support was kept to a minimum
through the use of low molecular weight PEG. Additionally,
potential contamination by moisture due to the hygroscopic
nature of PEG was practically inconsequential.
d, e
OH
O
OH
O
a, b, c
HO
PMBO
HO
HO
SPh
SPh
SPh
OPMB
OH
6
5
OBn
O
BnO
OBn
O
BnO
HO
f, g
SPh
The mixture of disaccharide 10 and unreacted 2 was then
subjected to the capture release cycle. According to our
expectations, thiol-containing resin 12 generated from com-
mercially available StBu-protected cysteine-loaded Wang
resin 11 efficiently captured the coupled product. This step
was monitored by chloroacetyl color test with p-nitrobenzyl-
O
OR
Cl
OH
O
7
8, R = H
g
9, R = Tol
Scheme 3. Preparation of galactosyl donor 9. a) (MeO)2CMe2, CSA,
MeCN, RT, 10 min; b) p-methoxybenzyl chloride (PMBCl), NaH, DMF,
RT, 15 h; c) 60% AcOH, RT, 13 h, 56% overall; d) benzyl bromide, NaH,
DMF, RT, 5 h; e) DDQ, CH2Cl2/H2O, RT, overnight, 79% from 6;
f) (ClCH2CO)2O, pyridine, CH2Cl2, À408C, 3 h, 33 %; g) p-MeC6H4COCl
(TolCl), pyridine, CH2Cl2, 08C !RT, 3 h, 85%. CSA ˆ 10-camphorsulfonic
acid, DMF ˆ dimethylformamide.
pyridine piperidine.[4, 20] Subsequent release was effected by
[21]
treatment with4-(aminometyhl)piperidine
to afford 14
withexcellent purity. As apparent from 1H NMR analysis,
unreacted acceptor 2 was completely removed (Figure 1B).
At this stage, the coupled product was fully characterized as
disaccharide 15, which was obtained from 14
after cleavage from PEG (Scheme 4).
Further chain elongation was performed
O
according to Scheme 5. Namely, the second
glucosamine residue was incorporated by using
trichloroacetimidate 16. Crude trisaccharide 17
was then subjected to capture release to give
18. After final glycosylation withthioglycoside
9 and capture release, tetrasaccharide 20 was
cleaved from PEG to provide 21. Complete
deprotection was performed in a standard
manner to give the free tetrasaccharide
Galb1!4GlcNAcb1!3Galb1!4GlcNAcb1 !
OMP.
We have developed a resin-aided capture
release strategy for oligosaccharide synthesis on
a polymer support. As an initial demonstration
of the strategy, we examined the construction of
an oligosaccharide consisting solely of syntheti-
cally straightforward 1,2-trans (b) glycosidic
linkages. More systematic studies should prove
the generality of this strategy to a wide range of
glycoconjugate-derived glycans.
NO2
O
O
HO
OMP
BnO
NPhth
OBn
O
BnO
O
2
SPh
a
OTol
Cl
O
9
O
NO2
O
O
O
Cl
OTol
O
O
OMP
NPhth
O
BnO
BnO
SR
OBn
11, R = StBu
12, R = H
b
10
c
O
NHFmoc
O
O
NO2
O
Received: June 1, 2001
Revised: August 13, 2001 [Z17211]
O
O
S
OTol
O
O
OMP
NPhth
O
O
BnO
NHFmoc
BnO
OBn
O
: Wang resin
[1] A. Kobata, Acc. Chem. Res. 1993, 26, 319 324.
[2] Recent reviews: Y. Ito, S. Manabe, Curr. Opin.
Chem. Biol. 1998, 2, 701 708; H. M. I. Osborn, T. H.
Khan, Tetrahedron 1999, 55, 1807 1850; P. M.
St. Hilaire, M. Meldal, Angew. Chem. 2000, 112,
1210 1228; Angew. Chem. Int. Ed. 2000, 39, 1162
1179; P. H. Seeberger, W.-C. Hasse, Chem. Rev. 2000,
100, 4349 4393.
13
d
: PEG monomethyl ether
O
NO2
O
O
OH
O
O
e
OTol
OTol
HO
BnO
AcO
O
OMP
NPhth
OMP
O
OBn
O
BnO
BnO
[3] O. J. Plante, E. R. Palmacci, P. H. Seeberger, Science
2001, 291, 1523 1527.
NPhth
BnO
OBn
[4] H. Ando, S. Manabe, Y. Nakahara, Y. Ito, J. Am.
Chem. Soc. 2001, 123, 3848 3849.
15
14
Scheme 4. Capture release of disaccharide 10. a) DMTST, MS 4ä/CH2Cl2, À20 !08C,
14 h, 94%; b) nBu3P, EtOH/CH2Cl2/H2O (4:6:1), RT, 3 h, 98%; c) 12 (3 equiv), iPr2EtN
(3 equiv), CH2Cl2/MeCN (1:1), RT, 14 h; d) 4-(aminomethyl)pyridine (15 equiv), THF, RT,
12 h, then Amberlyst 15E, 82% from 10; e) acetylacetone, Zn/Cu, THF, RT, 18 h, then Ac2O,
Et3N; f) DDQ, ClCH2CH2Cl/H2O, RT, 3 h, 86% from 14.
[5] L. Jiang, R. C. Hartley, T.-H. Chan, Chem. Commun.
1996, 2193 2194.
[6] Review: P. H. Toy, K. D. Janda, Acc. Chem. Res.
2000, 33, 546 554. For pioneering work on PEG-
supported oligosaccharide synthesis: S. P. Douglas,
Angew. Chem. Int. Ed. 2001, 40, No. 24
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COMMUNICATIONS
D. M. Whitfield, J. J. Krepinsky, J. Am. Chem.
Soc. 1995, 117, 2116 2117.
[7] Recent examples of tag-assisted product iso-
lation: V. Pozgay, Org. Lett. 1999, 1, 477 480;
H. Perrier, M. Labelle, J. Org. Chem. 1999, 64,
2110 2113; S.-Q. Zhang, K. Fukase, S. Kusu-
moto, Tetrahedron Lett. 1999, 40, 7479 7483;
K. Itami, K. Mitsudo, T. Kamei, T. Koike, T.
Nokami, J. Yoshida, J. Am. Chem. Soc. 2000,
122, 12013 12014; S. V. Ley, A. Massi, F.
RodrÌguez, D. C. Horwell, R. A. Lewthwaite,
M. C. Pritchard, A. M. Reid, Angew. Chem.
2001, 113, 1088 1090; Angew. Chem. Int. Ed.
2001, 40, 1053 1055.
[8] P. H. Seeberger, X. Beebe, G. D. Sukenick, S.
Pochapsky, S. Danishefsky, Angew. Chem. 1997,
109, 507 510; Angew. Chem. Int. Ed. Engl.
1997, 36, 491 493; T. Kanemitsu, O. Kanie, C.-
H. Wong, Angew. Chem. 1998, 110, 3574 3577;
Angew. Chem. Int. Ed. 1998, 37, 3415 3418.
[9] F. Baressi, O. Hindsgaul, J. Carbohydr. Chem.
1995, 14, 1043 1087.
[10] Review: A. Kirschning, H. Monenschein, R.
Wittenberg, Chem. Eur. J. 2000, 6, 4445 4450.
[11] M. Hummel, H. C. Hedrich, A. Hasilik, Eur. J.
Biochem. 1997, 245, 428 433.
[12] K. Maemura, M. Fukuda, J. Biol. Chem. 1992,
267, 24379 24386.
[13] K. Watanabe, S. Hakomori, R. A. Childs, T.
Feizi, J. Biol. Chem. 1979, 254, 3221 3228.
[14] Similar linkers were reported previously: S.
Manabe, Y. Nakahara, Y. Ito, Synlett 2000,
Figure 1. 1H NMR spectra of crude 10 before capture (A), released disaccharide 14 (B), PEG-
supported acceptor 2 (C), and disaccharide 15 after cleavage (D), and schematic reaction sequence.
The starred signals belong to unreacted 2.
O
1241 1244; X. Wu, M. Grathwohl, R. R. Schmidt, Org. Lett. 2001,
3, 747 750.
[15] K. Fukase, T. Yoshimura, M. Hashida, S. Kusumoto, Tetrahedron
Lett. 1991, 32, 4019 4022; K. Fukase, K. Egusa, Y. Nakai, S.
Kusumoto, Mol. Diversity 1996, 2, 182 188.
NO2
O
OBn
O
O
O
OTol
l
HO
O
O
O
CCl3
OMP
NPhth
O
BnO
BnO
NPhth
BnO
NH
OBn
[16] Y. Oikawa, T. Yoshioka, O. Yonemitsu, Tetrahedron Lett. 1982, 23,
885 888.
16
14
[17] K. C. Nicolaou, N. J. Bockovich, D. R. Carcanague, J. Am. Chem.
Soc. 1993, 115, 8843 8844.
[18] P. F¸gedi, P. J. Garegg, Carbohydr. Res. 1986, 149, c9-c12.
[19] Due to the statistical distribution of the length of the PEG chain,
which consists of 8 20 ethylene glycol units, the progress of the
reaction was clearly seen in the mass spectrum by the shift of a
group of signals with a characteristic mountainlike shape (m/z
980 1450, highest peak at 1212) to a higher molecular weight
region (m/z 1570 2020, highest peak at 1751).
O
a, b
NO2
17, R = CA
18, R = H
NPhth
O
O
BnO
RO
OTol
O
O
BnO
O
OMP
NPhth
O
OBn
OBn
BnO
ƒ
¬
[20] O. Kuisle, M. Lolo, E. Quinoa, R. Riguera, Tetrahedron 1999, 55,
14807 14812.
c, b
[21] L. Carpino, B. J. Cohen, K. E. Stephens, Jr., S. Y. Sadat-Aalaee, J.-
H. Tien, D. C. Langridge, J. Org. Chem. 1986, 51, 3732 3734.
O
NO2
OBn
O
BnO
RO
NPhth
O
O
BnO
O
OTol
O
19, R = CA
20, R = H
O
O
OMP
OTol
O
BnO
OBn
NPhth
BnO
OBn
d, e
OBn
O
BnO
AcO
NPhth
O
OH
O
BnO
OTol
O
O
O
OMP
OTol
O
BnO
OBn
NPhth
BnO
OBn
21
Scheme 5. Synthesis of tetrasaccharide 21. a) TMSOTf/CH2Cl2, À208C, 1 h,
95%; b) 12, iPr2EtN, MeCN/CH2Cl2, RT, then 4-(aminomethyl)pyridine, THF,
RT, 79 % (18), 80% (20); c) 9, DMTST, MS 4ä, CH2Cl2, 0 !108C, 5 h, 88 %;
d) acetylacetone, Zn/Cu, THF, RT, 18 h, then Ac2O, Et3N; e) DDQ,
ClCH2CH2Cl/H2O, RT, 3 h, 72% overall. TMSOTf ˆ trimethylsilyl triflate.
4728
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Angew. Chem. Int. Ed. 2001, 40, No. 24