Solution and solid-phase chemoselective synthesis of (1-6)-amino(methoxy) di- and trisaccharide analogues.

Article abstract of DOI:10.1039/b203605c

Disaccharide and trisaccharide mimics containing the amino(methoxy) interglycosidic linkage were obtained by chemoselective condensation of unprotected aldoses in an aqueous environment both in solution and in solid phase.

Full text of DOI:10.1039/b203605c

Solution and solid-phase chemoselective synthesis of
(1-6)-amino(methoxy) di- and trisaccharide analogues
Francesco Peri,* Alexander Deutman, Barbara La Ferla and Francesco Nicotra*
Department of Biotechnology and Biosciences, University of Milano-Bicocca, P.zza della Scienza 2, 20126
Milano, Italy. E-mail: francesco.peri@unimib.it; Fax: +39 0264483565; Tel: +39 0264483453
Received (in Cambridge, UK) 15th April 2002, Accepted 15th May 2002
First published as an Advance Article on the web 12th June 2002
Disaccharide and trisaccharide mimics containing the ami-
no(methoxy) interglycosidic linkage were obtained by chem-
oselective condensation of unprotected aldoses in an aque-
ous environment both in solution and in solid phase.
The observation that a large number of biological processes are
based on oligosaccharide recognition by protein receptors,
inspired the synthesis of oligosaccharide analogues in which the
O-glycosidic linkage, generally not directly involved in the
interaction with the receptor, is replaced by other bonds. C–C
linked oligosaccharides are an interesting class of mimetics
presenting resistance to chemical degradation and glycosidase
digestion, while having biological activity and conformational
properties suitable to substitute natural oligosaccharides in
protein–sugar interactions.1 Analogues containing a sulfur2
Scheme 1
atom at the interglycosidic linkage as well as carbopeptoids3 or
carbonucleotoids4 in which the interglycosidic bond has been
mixtures. After complete removal of the solvents, crude
replaced by a more rigid carboxyamide or phosphoramide bond,
reaction products were treated with Ac2O, pyridine and DMAP,
have been prepared. From a methodological point of view, the
and fully acetylated disaccharides 9, 10, 11 and 12 were isolated
and characterised by MALDI-TOF mass spectrometry and
synthesis of oligosaccharides and their analogues is still far
from routine, both in solution and in solid phase,5 requiring
NMR spectroscopy.‡ Disaccharides 9 and 12 were obtained in
extensive use of orthogonal protecting groups and strictly
anhydrous conditions. Synthetic strategies based on peptide-
1
the b form with total stereoselectivity, whereas the H-NMR
analysis of 10 revealed the presence of a small amount of the a
form (b+a = 7+1). In the case of 11 a mixture of a and b
anomers was formed with a being the most abundant (b+a =
1+5).§ These results are coherent with the stereochemical
like bond formation (carbopeptoids)3 or on the principle of
chemoselective ligation, provide a powerful tool for the
convergent preparation of oligosaccharide mimics. In partic-
ular, sugars bearing an aminoxy group at the anomeric position
have been reacted with keto functions present in a second
sugar.6,7 Preliminary studies also indicate the possibility to link
chemoselectively a molecule with an aminoxy function to the
anomeric centre of a free aldose. Exploiting an R-ONH2, an
open chain sugar-oxime is formed, whereas when a R-ONHRA is
used, the cycle of the sugar is restored.8 Following this last
approach, if RA is a sugar unit, oligosaccharide mimics could be
obtained in aqueous media, without protections and in a
iterative way.
In this communication, we describe our preliminary results
on the development of this promising approach. Compounds
5–8, a new class of disaccharide mimics, isosteric to natural
disaccharides, have been synthesised from unprotected aldoses
such as
D-glucose,
D-mannose,
D-galactose and
D-N-acet-
ylglucosamine, and 4, an unprotected sugar bearing a methox-
yamino group at C-6.†
Methyl 6-deoxy-6-methoxyamino- -glucopyranoside 4 was
D
prepared according to Scheme 1. Compound 1 was detritylated
(97% yield) and oxidized at C-6 (Dess–Martin periodinane,
95% yield) to afford aldehyde 2 which was converted into the
O-methyloxime by treatment with O-methylhydroxylamine
hydrochloride in pyridine. Zemplén de-O-acetylation gave
compound 3 (85% yield over the two steps) the reduction of
which with NaCNBH3 in glacial acetic acid afforded compound
4 in 86% yield. Compound 4 was reacted with
galactose, -mannose and -N-acetylglucosamine affording,
D
-glucose, -
D
D
D
respectively, disaccharide analogues 5, 6, 7 and 8 (Scheme 2).
The coupling reactions proceeded in good yields (except for 7)
at room temperature in 4–6 hours, and were carried out either in
aqueous acetic acid–sodium acetate buffer (pH = 4.5) or in 1+1
(v/v) water–acetic acid or in 1+2 acetic acid–DMF as solvent
Scheme 2
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CHEM. COMMUN., 2002, 1504–1505
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behaviour of the same reducing sugars in the reaction with N,O-
dialkyl hydroxylamines and N-methylhydroxylamino peptides
described in a precedent paper.8
the interglycosidic amino(methoxy) bond of 9 proved to be
resistant to prolonged TFA treatment.
In conclusion, this chemoselective and iterative approach for
the synthesis of oligosaccharide mimics is promising either in
solution and on solid phase. Work is in progress to apply the
method to the synthesis of other oligosaccharide mimics and
study their conformational properties.
In order to investigate the possibility of using this approach
iteratively for the preparation of oligosaccharide mimetics,
sugar 14 (Scheme 3), obtained from 13 following the procedure
of Scheme 1, was reacted with 4 to afford disaccharide 15 (75%
yield). Upon reduction of the oxime group of 15 with
NaCNBH3, and subsequent reaction of the obtained amino(me-
thoxy) disaccharide with 14, trisaccharide mimic 16 was
obtained (65% yield), which can be reduced for further
chemoselective elongation. Compound 16 was fully O-acety-
lated and converted into derivative 17 which was characterised.
Monomer 14 is an excellent scaffold for the synthesis of
oligosaccharide analogues, presenting the two complementary
functionalities required for chemoselective ligation, one of
which, the amino(methoxy) group, masked as O-methylox-
ime.
Notes and references
† The synthesis of (1 ? 4)amino(methoxy) disaccharides containing
glucose and galactose was recently achieved in a non-chemoselective way
by reacting protected 4-deoxy-4-methoxyaminoglycosides acceptors with
glycosyl bromide donors in Koenigs–Knorr conditions.7
‡ Selected spectral data (1H-NMR, 400 MHz, CDCl3); protons of the
monosaccharide unit derived from 4 are numbered 1-6, 1A-6A and 1B-
6Bnumbers refer to the other units, methyl signals of acetates have been
omitted. Compounds 10-b and 11-a are described as selected peaks from
spectra of anomeric mixtures. Compound 9: d (ppm) 5.43 (t, 1H, J = 9.3
Hz, H-3), 5.20 (m, 2H, H-2A and H-3A), 5.04 (t, 1H, J = 9.6 Hz, H-4A), 4.87
(d, 1H, J = 3.7 Hz, H-1), 4.81 (t, 1H, J = 9.4 Hz, H-4), 4.80 (dd, 1H, J =
3.7, 9.3 Hz, H-2), 4.28 (d, 1H, J = 9.0 Hz, H-1A), 4.24 (dd, 1H, J = 5.3, 12.2
Hz, H-6Aa), 4.08 (dd, 1H, J = 2.0, 12.2 Hz, H-6Ab), 3.95 (m, 1H, H-5), 3.62
(ddd, 1H, J = 2.0, 5.3, 9.5 Hz, H-5A), 3.45 (s, 3H, OCH3), 3.40 (s, 3H,
OCH3), 3.16 (dd, 1H, J = 8.1, 14.6 Hz, H-6a), 3.03 (dd, 1H, J = 2.2, 14.6
Hz, H-6b). Compound 10-b (major isomer): d (ppm) 5.45 (t, 1H, J = 9.3
Hz, H-3), 5.36 (m, 2H, H-2A and H-3A), 5.01 (dd, 1H, J = 3.5, 9.9 Hz, H-4A),
4.88 (d, 1H, J = 3.7 Hz, H-1), 4.83 (t, 1H, J = 9.3 Hz, H-4), 4.81 (dd, 1H,
J = 3.7, 9.3 Hz, H-2), 4.26 (d, 1H, J = 9.3 Hz, H-1A), 4.14 (m, 2H, H-6Aa,
H-6Ab), 3.98 (m, 1H, H-5), 3.86 (m, 1H, H-5A), 3.49 (s, 3H, OCH3), 3.41 (s,
3H, OCH3), 3.16 (dd, 1H, J = 8.2, 14.6 Hz, H-6a), 3.03 (dd, 1H, J = 2.0,
14.6 Hz, H-6b). Compound 11-a (major isomer): d (ppm) 5.57 (dd, 1H, J =
1.8, 3.4 Hz, H-2A), 5.41 (t, 1H, J = 9.4 Hz, H-3), 5.32 (dd, 1H, J = 3.4, 9.7
Hz, H-3A), 5.19 (t, 1H, J = 9.7 Hz, H-4A), 4.85 (d, 1H, J = 3.6 Hz, H-1), 4.79
(t, 1H, J = 9.4 Hz, H-4), 4.75 (dd, 1H, J = 3.6, 9.4 Hz, H-2), 4.2–4.0 (m,
5H, H-1A, H-6Aa, H-6Ab, H-5, H-5A), 3.45 (s, 3H, OCH3), 3.40 (s, 3H, OCH3),
3.11 (dd, 1H, J = 7.7, 14.6 Hz, H-6a), 2.80 (br d, 1H, J = 14.6 Hz, H-6b).
Compound 12 d (ppm) 5.85 (d, 1H, J = 8.8 Hz, H-1A), 5.47 (t, 1H, J = 10.2
Hz, H-3), 5.10 (t, 1H, J = 9.4 Hz, H-3’), 5.05 (m, 2H, H-4A, H-4), 4.91 (d,
1H, J = 3.6 Hz, H-1), 4.81 (dd, 1H, J = 3.6, 10.2 Hz, H-2), 4.31 (t, 1H, J
= 9.3 Hz, H-2A), 4.25 (dd, 1H, J = 5.3, 12.2 Hz, H-6’a), 4.11 (dd, 1H, J =
2.3, 12.2 Hz, H-6Ab), 3.94 (m, 1H, H-5), 3.58 (m, 1H, H-5A), 3.50 (s, 3H,
OCH3), 3.40 (s, 3H, OCH3), 3.26 (dd, 1H, J = 2.1, 15.3 Hz, H-6a), 2.93 (dd,
1H, J = 5.9, 15.3 Hz, H-6b). Compound 17 d (ppm) 7.16 (d, 1H, J = 7.1
Hz, H-6B), 5.09 (m, 1H, H-5), 5.01 (t, 1H, J = 9.3 Hz, H-2A), 5.40–4.80 (m,
9H, H-2B, H-3B, H-4B, H-3A, H-4A, H-1, H-2, H-3, H-4), 4.22 (d, 1H, J = 9.3
Hz, H-1B), 4.20 (d, 1H, J = 9.3 Hz, H-1A), 3.90 (m, 2H, H-5A, H-5B), 3.76
(s, 3H, OCH3), 3.41 (s, 3H, OCH3), 3.38 (s, 3H, OCH3), 3.35 (s, 3H, OCH3),
3.20 (dd, 1H, J = 2.5, 14.6 Hz, H-6a), 2.92 (dd, 1H, J = 8.6, 14.6 Hz, H-
6b), 2.82 (m, 1H, H-6Aa), 2.72 (dd, 1H, J = 7.3, 14.1 Hz, H-6Ab).
Scheme 3
Finally, we aimed to extend the chemoselective ligation to
polymer-bound sugars in order to evaluate the possibility of
automation of the process on solid phase. In sharp contrast with
the majority of solid phase glycosylation methods, this
condensation can be effected in the presence of water, does not
require any glycosylation promoter and, in the case of -glucose
D
§ It was impossible to isolate anomers from mixtures by chromatography
for both compounds 10 and 11.
or N-acetylglucosamine, is stereoselective affording only the b-
disaccharide mimic.
D-glucose was reacted with 4-methoxytrityl chloride resin
1 A. Paveda, J. L. Asensio, T. Palat, R. J. Linhardt and J. Jimenez-Barbero,
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(Novabiochem) in the presence of TBAI and sym-collidine in
DMF–pyridine (4+1) for 12 h, affording sugar linked to polymer
through C-6 position (loading: 0.8 mmol g21). The polymer-
bound sugar was reacted with a 4-fold excess of 4 in DMF–
AcOH (5:1) for 48 h at rt (Scheme 4). The disaccharide 5 was
then cleaved from the polymer (5% TFA in CH2Cl2–DMF 1+1).
After solvent evaporation, the crude product was acetylated
giving pure 9 (95% yield without purification). Interestingly,
4 K. C. Nicolaou, H. Florke, M. G. Egan and V. A. Estevez, Tetrahedron
Lett., 1995, 36, 1775.
5 P. H. Seeberger and W.-C. Haase, Chem. Rev., 2000, 100, 4349.
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Chem. Soc., 1997, 119, 9905; L. A. Marcaurelle and C. R. Bertozzi, J.
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Scheme 4
CHEM. COMMUN., 2002, 1504–1505
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