Synthesis of novel S-neoglycopeptides from glycosylthiomethyl derivatives

Article abstract of DOI:10.1021/ol036186z

(Equation presented) Reaction of glycosylthiomethyl azides with amino acid and peptide derivatives containing aspartate and glutamate thio acids gave the corresponding glycosylthiomethyl amides in excellent yields. Another type of neoglycopeptides was obt

Full text of DOI:10.1021/ol036186z

ORGANIC
LETTERS
2004
Vol. 6, No. 7
1083-1085
Synthesis of Novel S-Neoglycopeptides
from Glycosylthiomethyl Derivatives
Xiangming Zhu, Kandasamy Pachamuthu, and Richard R. Schmidt*
UniVersita¨t Konstanz, Fachbereich Chemie, Fach M 725, D-78457 Konstanz, Germany
richard.schmidt@uni-konstanz.de
Received November 7, 2003 (Revised Manuscript Received February 16, 2004)
ABSTRACT
Reaction of glycosylthiomethyl azides with amino acid and peptide derivatives containing aspartate and glutamate thio acids gave the
corresponding glycosylthiomethyl amides in excellent yields. Another type of neoglycopeptides was obtained via reaction of glycosylthiomethyl
bromide with cysteine and homocysteine containing peptide derivatives, thus affording the corresponding S-(glycosylthiomethyl) peptides.
Glycoproteins are widely distributed in nature and play a
variety of biological roles.1 Their biosynthesis is based on
post-translational site-selective glycosylation of the peptide
backbone. Glycopeptides,2 which retain the carbohydrate-
peptide linkage region of a glycoprotein but lack its size and
complexity, are much more amenable to study and are
important models for glycoproteins. As a consequence,
significant attention has been paid to synthesize glycopeptides
during the past two decades.3 However, site-selective at-
tachment of glycosyl residues to serine and threonine or
asparagine residues in order to obtain natural O- and
N-glycopeptides is generally not possible. This has led to
considerable efforts toward the synthesis of modified glyco-
peptides, often termed neoglycopeptides or glycopeptide
mimetics.4 They possess non-native sugar-peptide linkages,5
and importantly, the resulting neoglycopeptides have found
use in various biological studies and demonstrated bio-
activities comparable to the native glycopeptides.5a,6,7
For the desirable site-selective attachment of glycosyl
residues to prepared peptides chemoselective reactions of
sugars with hydroxylamine and hydrazine derivatives have
been successfully employed (I-III in Figure 1). Addition-
ally, several groups have exploited the unique reactivity of
cysteine (Cys) or homocysteine (Hcy) residues. For example,
N-glycosyliodoacetamide has been designed7 and coupled
(4) Reviewed in: (a) Hang, H. C.; Bertozzi, C. R. Acc. Chem. Res. 2001,
34, 727-736. (b) Marcaurelle, L. A.; Bertozzi, C. R. Chem. Eur. J. 1999,
5, 1384-1390.
(5) There are a large number of reports on the synthesis of neoglyco-
peptides; for some recent examples, see: (a) Marcaurelle, L. A.; Rodriguez,
E. C.; Bertozzi, C. R. Tetrahedron Lett. 1998, 39, 8417-8420. (b) Macindoe,
W. M.; van Oijen, A. H.; Boons, G. J. Chem. Commun. 1998, 847-848.
(c) Peri, F.; Cipolla, L.; Ferla, B. L.; Dumy, P.; Nicotra, F. Glycoconjugate
J. 1999, 16, 399-404. (d) Wittmann, V.; Seeberger, S. Angew. Chem. 2000,
112, 4508-4512; Angew. Chem., Int. Ed. 2000, 39, 4348-4352. (e) George,
S. K.; Schwientek, T.; Holm, B.; Reis, C. A.; Clausen, H.; Kihlberg, J. J.
Am. Chem. Soc. 2001, 123, 11117-11125. (f) Peluso, S.; Imperiali, B.
Tetrahedron Lett. 2001, 42, 2085-2087. (g) Ramos, D.; Rollin, P.; Klaffke,
W. J. Org. Chem. 2001, 66, 2948-2956. (h) Macmillan: D.; Daines, A.
M.; Bayrhuber, M.; Flitsch, S. L. Org. Lett. 2002, 4, 1467-1470 and
references therein. (i) Carrasco, M. R.; Nguyen, M. J.; Burneil, D. R.;
MacLaren, M. D.; Hengel, S. M. Tetrahedron Lett. 2002, 43, 5727-5729.
(j) McGarvey, G. J.; Benedum, T. E.; Schmidtmann, F. W. Org. Lett. 2002,
4, 3591-3594. (k) Filira, F.; Biondi, B.; Biondi, L.; Giannini, E.; Gobbo,
M.; Negri, L.; Rocchi, R. Org. Biomol. Chem. 2003, 1, 3059-3063. (l)
Carrasco, M. R.; Brown, R. T.; Serafimova, I. M.; Silva, O. J. Org. Chem.
2003, 68, 195-197.
(1) (a) Varki, A. Glycobiology 1993, 3, 97-130. (b) Lis, H.; Sharon, N.
Eur. J. Biochem. 1993, 218, 1-27. (c) Dwek, R. A. Chem. ReV. 1996, 96,
683-720.
(2) Glycopeptides and related compounds: Synthesis, Analysis and
Applications; Large, D. G., Warren, C. D., Eds.; Marcel Dekker, Inc.: New
York, Basel, Hong Kong, 1997.
(3) For some recent reviews on glycopeptide synthesis, see: (a) Jansson,
A. M.; Hilaire, P. M. S.; Meldal, M. In Synthesis of peptides and
peptidomemetics; Goodman, M., Felix, A., Moroder, L., Toniolo, C., Eds.;
Houben-Weyl: Stuttgart, 2003; pp 235-332. (b) Davis, B. G. Chem. ReV.
2002, 102, 579-601. (c) Wittmann, V. In Glycoscience: Glycoproteins,
Fraser-Reid, B. O., Tatsuta, K., Thiem, J., Eds.; Springer-Verlag: Berlin,
2001; pp 2253-2352. (d) Herzner, H.; Reipen, T.; Schultz, M.; Kunz, H.
Chem. ReV. 2000, 100, 4495-4537. (e) Arsequell, G.; Valencia, G.
Tetrahedron: Asymmetry 1999, 10, 3045-3094. (f) Taylor, C. M.
Tetrahedron 1998, 54, 11317-11362.
(6) Liu, H.; Wang, L.; Brock, A.; Wong, C.-H.; Schultz, P. G. J. Am.
Chem. Soc. 2003, 125, 1702-1703.
(7) Davis, N. I.; Flitsch, S. L. Tetrahedron Lett. 1991, 32, 6793-6796.
10.1021/ol036186z CCC: $27.50 © 2004 American Chemical Society
Published on Web 03/04/2004

indicating decomposition of the resultant glycosylthiomethyl-
amines during the reduction. Another strategy based on direct
conjugation of the azides with thio acids11a was therefore
investigated.
To test the feasibility of this strategy, we reacted GTM-
N3 112 and Boc-Ala-SH 213 in the presence of 2,6-lutidine
to give amide 3 as shown in Scheme 1.14 The yields are good
Scheme 1. Synthesis of S-Glycosides 3 and 5 and
S-Neoglycopeptides 8, 11, and 13a
Figure 1. Some examples of non-native sugar-peptide linkages.
with cysteine sulfhydryl groups (IV in Figure 1) to synthe-
size, for instance, erythropoietin (EPO)-derived neoglyco-
peptides.5h Bromoethyl glycosides have been utilized to ligate
with cysteine-containing peptides to build up neoglyco-
peptides derived from mucins and T-cell stimulating glyco-
peptides V.5e Only very few papers have been directed at
the synthesis of S-linked neoglycopeptides containing sulfur
atoms in their glycosidic linkage. Such molecules are not
readily degraded by glycosidases, and they are also more
stable under a number of chemical conditions. Recently, a
new approach to the preparation of disulfide-linked neo-
glycopeptides VI was described wherein a 5-nitropyridine-
2-sulfenyl-activated thioglycoside was conjugated with a
preassembled peptide or protein sequence.5b,8 As part of our
ongoing interest in S-glycopeptides,9 we report here the
synthesis of two new types of neoglycopeptides, structures
VII and VIII.
a Reagents and conditions: (a) 2,6-lutidine, CHCl3, reflux, 65-
70 °C, 10-12 h.
The azido group has gained significant attention recently
due to its exceptional stability toward many synthetic
conditions and its ready transformation into various nitrogen-
containing functionalities.10-12 Hence, the most convenient
route to target structure VII would be through the conversion
of the glycosylthiomethyl azide12 (GTM-N3) into the corre-
sponding free amine followed by amide formation with Asp-
or Glu-containing peptides in their side chains. Unfortunately,
the reduction of GTM-N3 failed under various conditions,
giving rise to a mixture of byproducts after acetylation, thus
for all five examples, which include three different GTM-
N3 partners and five different thio acids which were prepared
in two steps from the corresponding acid according to known
procedures.15
Recently, we reported a new protocol for the synthesis of
S-glycopeptides in aqueous solution by direct S-glycosylation
of Cys or Hcy containing peptides with glycosyl bromides,
wherein chemical ligation techniques were utilized to prepare
the required peptides.9a Based on this work, it occurred to
us that ligation of GTM-Cl12 with peptides containing Cys
or Hcy residue would lead to the desired structure VIII.
Hence, as a test, peracetyl glucosylthiomethyl chloride12 was
treated with Boc-Cys-OBn16 under the same PTC conditions
as used before,9a i.e., Na2CO3, TBAHS, and EtOAc-H2O.
(8) (a) Davis, B. G.; Lloyd, R. C.; Jones, J. B. J. Org. Chem. 1998, 63,
9614-9615. (b) Gamblin, D. P.; Garnier, P.; Ward, S. J.; Oldham, N. J.;
Fairbanks, A. J.; Davis, B. G. Org. Biomol. Chem. 2003, 1, 3642-3644.
(9) (a) Zhu, X.; Pachamuthu, K.; Schmidt, R. R. J. Org. Chem. 2003,
68, 5641-5651. (b) Zhu, X.; Schmidt, R. R. Tetrahedron Lett. 2003, 44,
6063-6067. (c) Zhu, X.; Schmidt, R. R. Chem. Eur. J. 2004, 10, 875-
887.
(13) Lehmann, J.; Linden, A.; Heimgartner, H. HelV. Chim. Acta 1999,
82, 888-908.
(10) (a) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem., Int.
Ed. 2001, 40, 2004-2021. (b) Fazio, F.; Bryan, M. C.; Blixt, O.; Paulson,
J. C.; Wong, C. H. J. Am. Chem. Soc. 2002, 124, 14397-14402.
(11) (a) Shangguan, N.; Katukojvala, S.; Greenberg, R.; William, L. J.
J. Am. Chem. Soc. 2003, 125, 7754-7755. (b) Fazio, F.; Wong, C. H.
Tetrahedron Lett. 2003, 44, 9083-9085.
(14) The yields in Scheme 1 were based on recovered starting materials;
for details, see the Experimental Section (Supporting Information).
(15) (a) Mitin, Y. V.; Zapevalova, N. P. Int. J. Peptide Protein Res. 1990,
35, 352-356. (b) Hoeg-Jensen, T.; Holm, A.; Sorensen, H. Synthesis 1996,
383-387.
(12) Zhu, X.; Schmidt, R. R. J. Org. Chem. 2004, 69, 1081-1085.
(16) Trost, B. M.; Braslau, R. J. Org. Chem. 1988, 53, 532-537.
1084
Org. Lett., Vol. 6, No. 7, 2004

However, unexpectedly, no reaction took place even after
extended reaction times.
Scheme 3. Synthesis of S-Glycosides 24 and 26 and
S-Neoglycopeptides 28, 31, and 33a
To synthesize the desired S-neoglycopeptide VIII under
these aqueous conditions, the reactivity of the sugar moiety
had to be enhanced. For this purpose, the more reactive
glycosylthiomethyl bromides (GTM-Br) were prepared from
the corresponding glycosylthiols17 by treatment with di-
bromomethane in the presence of K2CO3, as shown in
Scheme 2. Glycosylthiomethyl bromides 14-19 were ob-
Scheme 2. Synthesis of Glycosylthiomethyl Bromides
(GTM-Br) from the Corresponding Glycosylthiol (Yield, %)
tained in good to excellent yields, although under the same
conditions, GlcNAc-derived bromide 20 was produced from
the corresponding thiol only in 25% yield due to 2-acetamido
group induced side reactions.18 Compared to 20, bromides
21 and 22 were obtained in much better yields, 49% and
84%, respectively.
With these more reactive GTM-Br species in hand,
reaction with amino acid and peptide derivatives containing
Cys and Hcy to give neoglycopeptides of general structure
type VIII was investigated. Bromide 14 was treated with
Boc-Cys-OBn 2316 under the above-mentioned PTC condi-
tions to give the desired S-glycoside 24 in very high yield
(Scheme 3).
a Reagents and conditions: (a) Na2CO3, EtOAc-H2O, TBAHS,
rt, 8 h; (b) NaHCO3, DMF-H2O, rt, 7 h; (c) Zn, Ac2O, Et3N,
ultrasonic bath, rt, 3 h.
The yields with other investigated thiol-containing starting
materials (25,19 27,9a 29,9a and 3220) were invariably good,
and the simplicity of the reaction conditions makes this an
attractive way of preparing neoglycopeptides. For larger
peptides (e.g., 27 and 32) that are not soluble in ethyl acetate,
a mixture of water and DMF proved to be a more effective
solvent. The N-Troc group could be converted to the
acetamide21 to produce the GlcNAc glycoside 31 as shown.
The formation of these tripeptide derivatives signals the
potential utility of this method for the formation of more
complex neoglycopeptides.22
species GTM-N3 and GTM-Br that are useful synthons for
anchoring sugars onto the side chain of Asp or Glu and Cys
or Hcy, respectively. The reaction of GTM-N3 with peptide-
derived thiol acids gave rise to one type of S-neoglycopeptide
VII. The ligation of GTM-Br with Cys- or Hcy-containing
peptides led to another type of S-neoglycopeptide VIII. Both
VII and VIII could be interesting mimetics of native
N-glycopeptides with enhanced enzymatic resistance and
chemical stability, thus providing good opportunities to probe
biological properties of glycopeptides at the molecular level.
In summary, we have presented efficient syntheses of two
types of novel S-neoglycopeptides based on the new sugar
Acknowledgment. This work was supported by the
Deutsche Forschungsgemeinschaft and the Fonds der
Chemischen Industrie. K.P. is grateful for an Alexander von
Humboldt Fellowship.
(17) All the glycosylthiols used here were prepared according to a known
procedure: Cˇerny´, M.; Vrkocˇ, J.; Staneˇk, J. Collect. Czech. Chem. Commun.
1959, 24, 64-69.
(18) Presumably, polymerization took place in this reaction, giving rise
to several byproducts as indicated by TLC.
(19) Zhu, X.; Schmidt, R. R. Eur. J. Org. Chem. 2003, 4069-4072.
(20) Pachamuthu, K.; Schmidt, R. R. Synlett 2003, 659-662.
(21) Zhu, X.; Schmidt, R. R. Synthesis 2003, 1262-1266.
(22) (a) In view of the distance between the sugar and the peptide
backbone, compounds 8 and 28 could be good mimetics of native
N-glycopeptides.
Supporting Information Available: Experimental pro-
1
cedures and H and 13C data for the new compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OL036186Z
Org. Lett., Vol. 6, No. 7, 2004
1085