A concise route to the core pentasaccharide of N-linked glycoproteins

Article abstract of DOI:10.1016/S0040-4039(03)00101-1

A concise preparation of the common pentasaccharide core of the N-linked glycoproteins is described. The reducing end glycal is functionalized at the level of chitobiose, which is then β-mannosylated using Crich's direct coupling protocol. Deprotection of the branching mannose residue, and di-α-mannosylation complete the synthesis.

Full text of DOI:10.1016/S0040-4039(03)00101-1

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 1791–1793
A concise route to the core pentasaccharide of N-linked
glycoproteins
Vadim Y. Dudkin,a,* Justin S. Millera and Samuel J. Danishefskya,b,
*
aLaboratory for Bioorganic Chemistry, The Sloan-Kettering Institute for Cancer Research, 1275 York Avenue, New York, NY
10021, USA
bDepartment of Chemistry, Columbia University, Havemeyer Hall, New York, NY 10027, USA
Received 12 December 2002; revised 9 January 2003; accepted 10 January 2003
Abstract—A concise preparation of the common pentasaccharide core of the N-linked glycoproteins is described. The reducing
end glycal is functionalized at the level of chitobiose, which is then b-mannosylated using Crich’s direct coupling protocol.
Deprotection of the branching mannose residue, and di-a-mannosylation complete the synthesis. © 2003 Elsevier Science Ltd. All
rights reserved.
Our laboratory is currently developing a major pro-
gram in the total synthesis of glycopolypeptides. Such
targets can potentially be used in the development of
new diagnostic assays and synthetic vaccines.1,2 Thus,
the preparation of glycopolypeptides is no longer
viewed from the primary perspective of a purely aca-
demic challenge. While the challenges are indeed daunt-
ing, possibilities for building constructs which carry
medically valuable information abound with the
increasing maturity of the dynamic field of glycobiol-
ogy.3 At the present time, chemical synthesis is
arguably the only viable way to prepare significant
quantities of homogeneous samples of complex gly-
copeptide targets where the structure of the oligosac-
charide can be varied at will.
the directness required to service our growing program.
Fortunately, recent developments in carbohydrate syn-
thesis now allow for major simplifications in the elabo-
ration of the core region saccharides. In particular,
triflate-mediated direct b-mannosylation methodology,
developed by Crich and associates,10–12 was envisioned
for the formation of the key b-mannosyl–chitobiose
linkage.13,14 The Kochetkov amination reaction15 fol-
lowed by the Lansbury peptide conjugation procedure16
enables access to glycopeptides without reliance on
intermediate anomeric azides. Moreover, we have
recently discovered that such glycans with all of the
hydroxyl groups liberated are readily available from the
global deprotection of reducing polybenzylated precur-
sors using sodium in liquid ammonia.17 Figure 1
expresses globally our new synthetic approach taking
the advantage of the above findings.
The abundance of N-linked glycoproteins in nature and
the important role they play in cellular interactions
prompt major interest in their synthesis by chemical
means.4 It has also been one of the focal research goals
of this laboratory.5 The glycal assembly method6 has
proven itself to be applicable in the preparation of the
‘symmetrical’ N-linked glycans.7 Indeed two years ago
we reported a route to the signature core region of
N-linked glycans.8,9 In that synthesis a b-linked glucose
was introduced at C4 of the chitobiose. Subsequently,
epimerization at C2 of the glucose was accomplished by
oxidation to a 2-ketoderivative followed by reduction
to produce the interior mannose. While this method did
provide the core pentasaccharide, the synthesis lacked
We started with the known disaccharide 1,7 which is
available from 3,6-di-O-benzylglycal.18 In order to sim-
plify the final steps of the synthesis, it was preferable to
* Corresponding authors.
Figure 1.
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(03)00101-1

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V. Y. Dudkin et al. / Tetrahedron Letters 44 (2003) 1791–1793
functionalize the reducing end double bond at an early
stage. Towards this end, glycal 1 was subjected to
iodosulfonamidation (Scheme 1).19 Following exposure
of the addition products to hydrolysis in THF, both
isomeric iodosulfonamides were cleanly converted into
the reducing disaccharide 2. Interestingly, the anomeric
hydroxyl group in 2 adopts exclusively the a configura-
tion, as evidenced by 1H NMR measurements. This
finding can be explained by the intramolecular hydro-
gen bonding stabilization of the a anomer. Further-
more, this anomeric configuration can be preserved
during silylation with excess TBSOTf which gives 3 as a
clean a anomer. Deacetylation of 3 following the Zem-
ple´n protocol afforded the disaccharide acceptor 4 in
53–60% for the four steps starting from 1.
Regioselective cleavage of the 4,6-benzylidene ring was
achieved by borane reduction in the presence of
dibutylboron triflate.23 Addition of 3 equiv. of Lewis
acid was required to complete the conversion of 7 into
the desired 3,6-diol 8. Simultaneous glycosylation of the
primary and secondary hydroxyl groups in 8 was then
necessary to form the desired pentasaccharide. Several
donors containing ester protection at mannose O2 were
screened using various coupling conditions. Glycosyla-
tion of 8 with an excess of thiomannoside 9 in acetoni-
trile using Sinay¨ radical cation activation24,25 was the
most convenient, and provided di-a-mannoside 10 in
good yield and complete anomeric selectivity (Scheme
3).
No indication of orthoester formation was found as
pentasaccharide 10 was readily saponified producing
‘symmetrical’ diol 11. The C2 hydroxyls of the terminal
mannose residues in 11 serve as attachments points for
lactosamine spacer units, enabling it to be used in the
preparation of complex-type glycopeptides. On the
other hand, anomeric silyl protection can easily be
removed using tetrabutylammonium fluoride to give
reducing pentasaccharide 12.26
Happily, triflate-mediated mannosylation of 4 (Scheme
2) was highly efficient.20 The coupling reaction, per-
formed at −78°C with 1.5 equiv. of
4
in
dichloromethane following the activation of the sulfox-
ide 521 with triflic anhydride, afforded the mixture of
anomers in 85–91% yield and 8/1 b/a ratio.22 Although
an excess of glycosyl acceptor is normally employed in
this type of coupling in order to achieve better
diastereomeric ratios, the unreacted acceptor is recov-
ered and can be recycled. The isomeric trisaccharides
were separated following the removal of the p-methoxy-
benzyl group from mannose 3-OH using ceric ammo-
nium nitrate in wet acetonitrile, providing alcohol 7 as
a single anomer in 74% yield.
Compound 12, now reachable in a robust way, serves
as a flexible building block in a variety of constructions
of highly complex and potentially valuable N-linked
glycans. Such extensions and applications of the chem-
istry described above will be reported in due course.
Scheme 1. Reagents and conditions: (a) I(coll)2ClO4, PhSO2NH2; (b) Et3N, H2O/THF; (c) TBSOTf, 2,6-lutidine, CH2Cl2; (d)
NaOMe/MeOH; 53–60% for four steps.
Scheme 2. Reagents and conditions: (a) i. Tf2O, DTBMP, CH2Cl2, −78°C, ii. 4, 85–91% (b/a=8/1); (b) CAN, MeCN/H2O, 74%;
(c) Bu2BOTf, BH3·THF, THF, 72%.

V. Y. Dudkin et al. / Tetrahedron Letters 44 (2003) 1791–1793
1793
Scheme 3. Reagents and conditions: (a) (BrC6H4)3NSbCl6, MeCN, 74%; (b) NaOMe/MeOH, 89%; (c) TBAF/AcOH, THF, 81%.
Acknowledgements
12. Crich, D.; Sun, S. X. J. Am. Chem. Soc. 1998, 120,
435–436.
13. Crich, D.; Dudkin, V. J. Am. Chem. Soc. 2001, 123,
6819–6825.
14. See preceding article.
15. Likhosherstov, L. M.; Novikova, O. S.; Derevitskaja, V.
A.; Kochetkov, N. K. Carbohydr. Res. 1986, 146, C1–C5.
16. Cohen-Anisfeld, S. T.; Lansbury, P. T. J. Am. Chem. Soc.
1993, 115, 10531–10537.
This work was supported by the NIH (AI16943). We
thank Drs. Andrzej Zatorski and Ulrich Iserloh for the
preparation of starting materials and for helpful discus-
sions, and Dr. George Sukenick and Ms. Sylvi Rusli
(NMR Core Facility, CA-02848) for mass spectral
analyses.
17. Iserloh, U.; Dudkin, V.; Wang, Z. G.; Danishefsky, S. J.
Tetrahedron Lett. 2002, 43, 7027–7030.
18. Seeberger, P. H.; Cirillo, P. F.; Hu, S. H.; Beebe, X.;
Bilodeau, M. T.; Danishefsky, S. J. Enantiomer 1996, 1,
311–323.
19. Griffith, D. A.; Danishefsky, S. J. J. Am. Chem. Soc.
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20. Dudkin, V. Y.; Crich, D. Tetrahedron Lett. 2003, 44,
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21. Crich, D.; Li, H. M.; Yao, Q. J.; Wink, D. J.; Sommer,
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22. For the use of sulfoxides in glycosylations, see: (a)
Kahne, D.; Walker, S.; Cheng, Y.; Engen, D. V. J. Am.
Chem. Soc. 1989, 111, 6881–6882; Gildersleeve, J.; Pascal,
R. A.; Kahne, D. J. Am. Chem. Soc. 1998, 120, 5961–
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24. Zhang, Y. M.; Mallet, J. M.; Sinay, P. Carbohydr. Res.
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