Cross metathesis assisted solid-phase synthesis of glycopeptoids

Article abstract of DOI:10.1021/ol300808c

A solid-phase synthesis of glycopeptoids was explored through olefin cross metathesis (CM). Peptoids and sugar derivatives with appropriate olefin moieties were coupled in the presence of an olefin metathesis catalyst to afford glycopeptoids in good yield

Full text of DOI:10.1021/ol300808c

ORGANIC
LETTERS
XXXX
Vol. XX, No. XX
000–000
Cross Metathesis Assisted Solid-Phase
Synthesis of Glycopeptoids
Sharaf Nawaz Khan,† Arim Kim,† Robert H. Grubbs,†,‡ and Yong-Uk Kwon*,†
Department of Chemistry and Nano Science, Ewha Womans University, Seoul 120-750,
South Korea, and The Arnold and Mabel Beckman Laboratory of Chemical Synthesis,
Division of Chemistry and Chemical Engineering, California Institute of Technology,
Pasadena, California 91125, United States
yukwon@ewha.ac.kr
Received April 2, 2012
ABSTRACT
A solid-phase synthesis of glycopeptoids was explored through olefin cross metathesis (CM). Peptoids and sugar derivatives with appropriate
olefin moieties were coupled in the presence of an olefin metathesis catalyst to afford glycopeptoids in good yields. This systematic solid-phase
CM study can provide facile access to the molecular sources of glycopeptidomimetics and postchemical modifications on various molecular
scaffolds.
Glycosylation, a complex post-translational modifica-
tion, involves the attachment of glycans to proteins and
lipids through a series of enzymatic reactions. It plays vital
roles in cellÀcell recognition, protein folding, stabilization,
cell growth regulation, cell differentiation, immunological
response, metastasis, and bacterial and viral infections.1
Unlike transcription or translation, it is a nontemplate
driven process and abberant glycosylation can lead to
numerous genetic disorders.2 Glycoconjugates have been
separated from natural sources to explore glycosylation,
but they are very difficult to obtain in adequate purity and
quantity.
Therefore, a main goal is to manipulate the syntheses of
glycoconjugates and glycopeptides as biological probes
and lead compounds for glycobiology and drug discovery.
Peptides are promising units for the synthesis of glycocon-
jugates and glycopeptides, though their disadvantages
include sensitivity to proteases, limited cell permeability,
and poor bioavailability. Much work has been focused
toward modifying peptides to generate new peptidomi-
metics with improved pharmacokinetic characteristics,
including peptoids, N-substituted glycine oligomers with
side chains directly attached to the nitrogen atoms of
amide bonds.3 They possess advantages such as broad
chemical diversity, proteolytic stability, and improved cell
permeability over peptides.4
The solid-phase synthesis of peptoids is straightforward
and can be supported by a large number of amine
monomers.5 Moreover, peptoids can act as inhibitors of
proteinÀprotein interactions, molecular carriers, and po-
tent antimicrobial agents.6 Their advantages and charac-
teristics make them suitable for the development of
glycopeptoids as glycopeptide mimetics which can be
useful chemical tools in glycobiology and chemical glyco-
mics. So far, a few syntheses of N-, O-, C-, and S-linked
(3) (a) Adessi, C.; Soto, C. Curr. Med. Chem. 2002, 9, 963. (b) Patch,
J. A.; Barron, A. E. Curr. Opin. Chem. Biol. 2002, 6, 872.
(4) (a) Zuckermann, R. N.; Kerr, J. M.; Kent, S. B. H.; Moos, W. H.
J. Am. Chem. Soc. 1992, 114, 10646. (b) Olivos, H. J.; Alluri, P. G.;
Reddy, M. M.; Salony, D.; Kodadek, T. Org. Lett. 2002, 4, 4057.
(c) Udugamasooriya, D. G.; Dineen, S. P.; Brekken, R. A.; Kodadek,
T. J. Am. Chem. Soc. 2008, 130, 5744.
† Ewha Womans University.
‡ California Institute of Technology.
(1) (a) Varki, A. Essentials of Glycobiology; Cold Spring Harbor
Laboratory Press: New York, 1999. (b) Taylor, M. E.; Drickamer, K.
Introduction to Glycobiology; Oxford University Press: Oxford and New
York, 2003.
(5) Culf, A. S.; Ouellette, R. J. Molecules 2010, 15, 5282.
(6) (a) Patch, J. A.; Barron, A. E. J. Am. Chem. Soc. 2003, 125, 12092.
(b) Chongsiriwatana, N. P.; Patch, J. A.; Czyzewski, A. M.; Dohm,
M. T.; Ivankin, A.; Gidalevitz, D.; Zuckermann, R. N.; Barron, A. E.
Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 2794.
(2) Freeze, H. H. Nat. Rev. Genet. 2006, 7, 537.
r
10.1021/ol300808c
XXXX American Chemical Society

glycopeptoids have been reported.7 Usually modification
of glycoconjugates could be achieved by olefin cross
metathesis (CM),8 bioorthogonal ligation,9 the use of
transistion metal complexes,10 cross-coupling reactions,11
and azideÀalkyne cycloadditions.12
been extended to the aqueous phase where site-selective
modification of proteins bearing allyl sulfides has been
accomplished.8
The synthesis of glycopeptoids reported here involved
attaching sugar derivatives to alkenyl moiety-containing
peptoids through solid-phase CM (Figure 1). Peptoids
with various side chains were prepared on beads with
strategically positioned alkenyl components in different
chain lengths. O-Linked sugar-alkenyl derivatives (16À30)
were prepared from mannose, galactose, and glucose pre-
cursors (Scheme 1). Free sugars were peracetylated or
perbenzoylated and underwent selective anomeric depro-
tection by methylamine in THF.15 The obtained lactols
(8À11) were treated with trichloroacetonitrile in DBU to
produce glycosyl trichloroacetimidates (12À15). Glycosyl
imidate donors were activated by trimethylsilyltriflate,
followed by nucleophilic attack of primary alcohols such
as allyl alcohol, 3-buten-1-ol, 4-penten-1-ol, and 3-methyl-
3-buten-1-ol to furnish four sugar derivatives from each
monosaccharide in high yields.
CM has emerged as one of the most powerful tools in the
preparation of carbonÀcarbon bonds over the past decade
and has provided a convenient synthetic route to simple
alkenes and substituted precursors. The well-defined olefin
metathesis catalysts prepared by Grubbs et al. may tolerate
a variety of functional groups and have facilitated metath-
esis chemistry.13 Since the reactants and products of olefin
metathesis are alkenes, great care must be taken to design
reactions so as to avoid unnecessary side products. CM has
been intensively studied in solution, with few examples
reportedinthesolid phase.14 Theversatility ofCMhasalso
Scheme 1. Synthesis of Alkene-Containing Sugar Derivatives
Figure 1. General schematic representation for the solid-phase
synthesis of glycopeptoids via CM.
(7) (a) Saha, U. K.; Roy, R. Tetrahedron Lett. 1995, 36, 3635.
(b) Roy, R.; Saha, U. K. Chem. Commun. 1996, 2, 201. (c) Kim, J. M.; Roy,
R. Tetrahedron Lett. 1997, 38, 3487. (d) Marcaurelle, L. A.; Bertozzi,
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Y. A.; Boutureira, O.; Davis, B. G. Chem. Commun. 2009, 3714. (c) Lin,
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Bioconjugate Chem. 2005, 16, 1572.
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253. (b) Tilley, S. D.; Francis, M. B. J. Am. Chem. Soc. 2006, 128, 1080.
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K. B.; Finn, M. G. J. Am. Chem. Soc. 2003, 125, 3192. (b) Lin, P.-C.;
Ueng, S.-H.; Tseng, M.-C.; Ko, J.-L.; Huang, K.-T.; Yu, S.-C.; Adak,
A. K.; Chen, Y.-J.; Lin, C.-C. Angew. Chem., Int. Ed. 2006, 45, 4286.
(c) Van Kasteren, S. I.; Kramer, H. B.; Jensen, H. H.; Campbell, S. J.;
Kirkpatrick, J.; Oldham, N. J.; Anthony, D. C.; Davis, B. G. Nature
2007, 446, 1105. (d) Norgren, A. S.; Budke, C.; Majer, Z.; Heggemann,
C.; Koop, T.; Sewald, N. Synthesis 2009, 488.
To study solid-phase CM, 4-mer peptoids (31À32) were
first prepared with terminal units of allylamine or buteny-
lamine (3-buten-1-amine) on Rink Amide LL resin
(100À200 mesh, 0.4 mmol/g) (Scheme 2). The terminal
amine of the peptoids was capped by di-tert-butyl dicar-
bonate. The three most commonly used olefin metathesis
catalysts G1, G2, and HG2 were tested (Figure 1).16
Peptoids (31À32) with terminal allyl or butenyl groups
were reacted with mannosides (20À21) with allyl or bute-
nyl groups inthe presenceofthe catalystsunder microwave
or reflux conditions (for details, see Supporting
Information). The catalyst was used at 2À5 mol %, and
5 mol % was suitable for the catalysis of solid-phase CM.
Increasing the catalyst loading to 10 mol % increased the
homodimerization of the sugars in solution. G1 and G2
provided much lower yields; HG2 gave a higher yield.
In addition, allylÀallyl (with allyl units each from the
peptoids and the sugar derivatives) combinations were
(13) Grubbs, R. H. Handbook of Metathesis; Wiley-VCH: Weinheim,
Germany, 2003, and references therein.
(14) (a) Andrade, R. B.; Plante, O. J.; Melean, L. G.; Seeberger, P. H.
Org. Lett. 1999, 1, 1811. (b) Blackwell, H. E.; Clemons, P. A.; Schreiber,
S. L. Org. Lett. 2001, 3, 1185. (c) Kanemitsu, T.; Seeberger, P. H. Org.
Lett. 2003, 5, 4541. (d) Testero, S. A.; Mata, E. G. Org. Lett. 2006, 8,
4783. (e) Poeylaut-Palena, A. A.; Testero, S. A.; Mata, E. G. J. Org.
Chem. 2007, 73, 2024.
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Fujita, S.; Matsumura, F.; Wada, T. J. Org. Chem. 2010, 75, 2147.
(16) Khan, S. N.; Kim, A.; Grubbs, R. H.; Kwon, Y. U. Org. Lett.
2011, 13, 1582.
B
Org. Lett., Vol. XX, No. XX, XXXX

Table 2. Solid-Phase CM of Peptoids Containing Arbitrary
Sequences on Different Resinsa
Scheme 2. Optimization of Solid-Phase CM
a Reagents and conditions: (i) sugar derivative (20 equiv, 25 mM),
catalyst (5 mol %), DCM, 40 °C, 8 h; (ii) 92% TFA. *CM conversion
efficiency was determined by HPLC analysis. For details, see Supporting
Information.
sugar
conv
(%)c
entryb
peptoid
derivative
glycopeptoid
1
49
49
49
49
49
49
49
49
49
49
49
49
49
49
49
59
59
59
59
59
59
20
21
22
24
25
26
28
29
30
20
21
22
21
21
21
21
22
25
26
29
30
50
51
52
53
54
55
56
57
58
50
51
52
51
51
51
60
61
62
63
64
65
21
46
44
27
47
49
28
49
32
16
37
40
50
49
51
52
63
35
51
46
53
2
3
Table 1. Effects of Various Alkene-Containing Sugar Deriva-
tives on Solid-Phase CMa
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
entryb
sugar derivative
glycopeptoid
conv (%)c
1
16
17
18
19
20
21
22
23
24
25
26
27
39
40
41
42
35
36
43
44
45
46
47
48
12
15
30
<5
27
47
39
<5
29
40
46
<5
2
3
4
a Reagents and conditions: (i) sugar derivative (20 equiv, 25 mM),
HG2 (5 mol %), DCM, 40 °C, 8 h; (ii) 92% TFA. b Entries 1À9: Rink
Amide AM resin (0.4 mmol/g), sugar derivative (20 equiv, 25 mM);
entries 10À12: Rink Amide AM resin (0.2 mmol/g), sugar derivative
(20 equiv, 25 mM); entries 13À15: Rink Amide AM resin (0.2 mmol/g),
sugar derivative (50, 75, and 100 mM, respectively); entries 16À21:
TentaGel MB RAM resin (0.4 mmol/g), sugar derivative (20 equiv,
25 mM). c Conversion efficiency was determined by HPLC analysis.
5
6
7
8
9
10
11
12
a Reagents and conditions: (i) sugar derivative (20 equiv, 25 mM),
HG2 (5 mol %), DCM, 40 °C, 8 h; (ii) 92% TFA. b Rink Amide AM resin
(0.4 mmol/g). c Conversion efficiency was determined by HPLC analysis.
were reacted together with HG2. The CM yield of 38
gradually increased to 76% from 2 to 8 h before slowly
decreasing to 70% at 12 h (see Supporting Information).
This behavior might be due to the product of the first CM
reaction also participating in CM reactions with either the
peptoid or the sugar; this was confirmed by MALDI-TOF.
Therefore, 8 h was considered optimal for the CM
reactions.
The versatility of solid-phase CM methodology was
demonstrated by conducting reactions between peptoid
32 and sugar derivatives (16À27) (Table 1). The CM of
butenylÀbutenyl/pentenyl fragments generally proceeded
not suitable for the synthesis of glycopeptoids; allylÀ
butenyl combinations resulted in improved yields to some
extent. Surprisingly, butenylÀbutenyl combinations gave
much higher yields (up to 47%).
With this information in hand, butenylÀbutenyl combi-
nations were then used to study the relationship between
reaction time and yield. Peptoid 37 and sugar derivative 21
Org. Lett., Vol. XX, No. XX, XXXX
C

well and gave higher yields than allylÀallyl combinations.
CM reactions with sugar derivatives protected by acetyl
groups gave lower yields than those protected by benzoyl
groups. These results corroborate previous observations
that acetyl groups could chelate with metal alkylidene
complexes, hampering the reaction pathway and giving
nonmetathetic products.17 Sugar derivatives (19, 23, and 27)
with disubstituted alkenyl moieties were almost spectators
to CM even though this type of alkene participated in
solution-phase CM to give trisubstituted products.18 In most
cases, homodimerized peptoids were produced as byproducts
in moderate yields.
Furthermore, alkenyl moieties placed in the middle of
the peptoid sequences were tested to assess the steric effects
on the CM reaction under various reaction conditions
(Table 2). The best results were also achieved using bute-
nylÀbutenyl/pentenyl combinations, showing that there
was no strong steric hindrance. However, the homodimer-
izaton of peptoids on the resin was inevitable.
long polyethylene glycol linkers. Especially, it is very
desirable for protected amino or acid moieties such as
N-Boc-1,4-butanediamine (Nlys) and glycine tert-butyl
ester (Nasp) to be compatible with solid-phase CM, so as
to provide highly diverse glycopeptoids. Finally, for the
preparaton of debenzoylated glycopeptoids, the benzoyl
protecting groups of the sugar moiety were easily depro-
tected by using sodium methoxide in CH3OH/THF at
reflux to give almost quantitatively a free sugar-containing
glycopeptoid (see Supporting Information).
In conclusion, the solid-phase synthesis of glycopeptoids
by cross-metathesis resulted in good yields. The CM of
butenylÀbutenyl/pentenyl combinations was superior to
that of other allyl pairings. The stereochemistry of the
sugar did not affect the CM, and phosphine-free HG2 was
found to be the better catalyst in our study. The type of
resin was shown tohave no significant effectonsolid-phase
CM, and any side chains, including protected charged
moieties, could be employed. Such solid-phase CM reac-
tions can provide facile access to useful molecular sources
of glycopeptidomimetics and postchemical modification
on various molecular scaffolds.
Thus, we tried to change reaction conditions such as the
loadingcapacity oftheresin and theconcentration of sugar
derivatives to reduce the homodimerization of peptoids.
However, there was no dramatic improvement for the
production of CM products with a reduced loading capa-
city of the resin and an increased concentration of sugar
derivatives (Table 2, entries 10À15).
Acknowledgment. This research was supported by the
World Class University (WCU) program (R33-10169) and
the Basic Science Research Program (2009-0068960)
through the National Research Foundation of Korea
funded by the Ministry of Education, Science and Tech-
nology. This work was also supported by the Ewha Global
Top5 Grant 2011 of Ewha Womans University.
The effects of the resin on solid-phase CM were next
tested. Peptoid 59 was prepared on TentaGel MB RAM
resin (0.4 mmol/g), a commonly used resin in on-bead
assays. The CM reactions were carried out with sugar
derivatives containing butenyl or pentenyl fragments
(Table 2, entries 16À21) and were successful on both
hydrophobic Rink Amide resin and hydrophilic TentaGel
resin. Actually, TentaGel resin gave slightly better results
than Rink Amide resin because TentaGel resin contains
Supporting Information Available. Experimental de-
tails for the syntheses of sugar derivatives and peptoids
and solid-phase CM, 1H NMR, 13C NMR, and MALDI-
TOF spectral data, and RP-HPLC chromatograms. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(17) Morrill, C.; Funk, T. W.; Grubbs, R. H. Tetrahedron Lett. 2004,
45, 7733.
(18) (a) Chatterjee, A. K.; Grubbs, R. H. Org. Chem. 1999, 1, 1751.
(b) Chatterjee, A. K.; Sanders, D. P.; Grubbs, R. H. Org. Chem. 2002, 4,
1939.
The authors declare no competing financial interest.
D
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