Cyclic glycopeptidomimetics through a versatile sugar-based scaffold

Article abstract of DOI:10.1016/j.bmcl.2009.04.008

Cyclic peptidomimetics are attracting structures to obtain a distinct, bioactive conformation. Even more attractive are sugar-containing cyclic peptidomimetics which present turn structures induced by the pyranose ring when incorporated in cyclic peptides

Full text of DOI:10.1016/j.bmcl.2009.04.008

Bioorganic & Medicinal Chemistry Letters 19 (2009) 3841–3844
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Cyclic glycopeptidomimetics through a versatile sugar-based scaffold
Maria Altamura a, Elisa Dragoni b,e, Angela Simona Infantino b, Laura Legnani c, Steve B. Ludbrook d,
Gloria Menchi b, Lucio Toma c, Cristina Nativi b,e,
*
a Menarini Ricerche S.p.A., via dei Sette Santi 1, I-50131 Firenze, Italy
b Dipartimento di Chimica Organica, Universita’ di Firenze, via della Lastruccia, 13-Polo Scientifico e Tecnologico, Sesto F.no, I-50019 Firenze, Italy
c Dipartimento di Chimica Organica, Universita’ di Pavia, via Taramelli 10, I-27100 Pavia, Italy
d Dept of Screening and Compound Profiling, GlaxoSmithKline R&D, New Frontiers Science Park, Harlow, Essex CM19 5AW, UK
e CERM, Universita’ di Firenze, via Sacconi, 6-Polo Scientifico e Tecnologico, Sesto F.no, I-50019 Firenze, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Cyclic peptidomimetics are attracting structures to obtain a distinct, bioactive conformation. Even more
attractive are sugar-containing cyclic peptidomimetics which present turn structures induced by the
pyranose ring when incorporated in cyclic peptides. The use of a new and versatile saccharidic scaffold
to achieve sugar-based peptidomimetics is here reported together with the successful synthesis of diaste-
reomerically pure cyclic SAA peptidomimetics 15 and 16.
Received 16 February 2009
Revised 31 March 2009
Accepted 3 April 2009
Available online 9 April 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Glycopeptides
Peptidomimetics
Sugar aminoacids
Tachykinins
Integrins
Low bioavailability, poor stability towards peptidases and rapid
excretion displayed by a large number of peptides of therapeutic
and biological interest, prompted an intensive search for pepti-
domimetics.1–3 As a matter of fact, peptidomimetics molecules,
are expected to have the same therapeutic effects as their peptide
counterparts, with the added advantages of metabolic stability and
bioavailability.
Cyclization of peptides affords structure of special interest to ob-
tain a distinct, bioactive conformation, and several SAAs have been
used as building blocks for peptide scaffolds and conformational
restrained peptido-mimetics.2,7 Indeed, this approach has been suc-
cessfully evaluated for biologically active, cyclic RGD peptides3 or
cyclic peptides with the b-turn motif of somatostatin, containing
tetrapeptide Phe-D
-Trp-Lys-Thr.2,8
A possible approach of such peptidomimetics is to replace the
peptide, or part of the peptide, by a scaffold that distributes in
the space the epitopes. Ideally, an attractive scaffold should easily
be available as chiral, enantiomerically pure, building block. It
should be highly functionalized to introduce various side chains
and its flexibility or rigidity could be modulated. Therefore, the
use of sugars as scaffolds in mimetic synthesis is a strategy that
has been widely studied and which still draws attention.4–6
Sugar derivatives can be incorporated in the peptide backbone
or in the side chains; modification of peptide skeleton by introduc-
ing carbohydrate derivatives is documented leading to a real
improvement of the properties of the peptides. Moreover, because
of the presence of pyranose ring, sugar aminoacids (SAAs) have
been found to introduce turn structures when incorporated in lin-
ear or cyclic peptides.5
Consequently, efficient syntheses and availability of suitable
saccharidic scaffolds allowing the systematic modification of the
amino acids and the carbohydrate residues, as well as the size of
the macrocycle, remain a challenging and attracting target to fully
exploit the potential of such hybride structure, in the search of new
active compounds.
We report herein the use of the a-O-linked glycohomoglutamate
1, (Scheme 1), easily obtained as distereomerically pure compound
by chemo-, regio- and stereoselective cycloaddition between glucal
2 and aspartic acid derivative 3,9 as versatile, multifunctionalized
scaffold in the synthesis of cyclic SAA-peptidomimetics 15 and
16 (Scheme 2).
Scaffold 1, characterized by two orthogonally protected carbox-
yls, a protected a-amino function and a selectively functionalized
monosaccharidic unit, was successfully employed to obtain the
monosilyl derivative 4 which, in turn, was oxidized with TEMPO/
BAIB to give the glucuronic derivative 5 (see Scheme 1) in 54%
yield. No sulfoxide derivative 6 was observed under these reaction
conditions (see Supplementary data).10 Condensation reactions
* Corresponding author.
E-mail address: cristina.nativi@unifit.it (C. Nativi).
0960-894X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2009.04.008

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M. Altamura et al. / Bioorg. Med. Chem. Lett. 19 (2009) 3841–3844
TIPSO
S
TIPSO
HO
O
H3CO
O
HO
O
+
Ref. 9
TIPSO
TIPSO
NHBoc
COOBn
O
S
O
H3CO
NHBoc
2
3
O
HOOC
1
HO
COOBn
HO
O
TEMPO-BAIB
TBAF, CH2Cl2
rt, 30 min
HO
O
TIPSO
CH2Cl2, rt, 22 h
TIPSO
H3CO
1
S
O
H3CO
O
S
85%
54%
NHBoc
O
NHBoc
HO
O
HO
O
COOBn
4
5
COOBn
TIPSO
S
O
H3CO O
O
NHBoc
6
COOBn
Scheme 1. Synthesis of scaffold 5 and structure of cyclo SAA peptidomimetics 15 and 16.
between 5 and 7 as well as between 5 and 8 were performed using
standard conditions (see Supplementary data) affording respec-
tively compounds 9 and 10 (Scheme 2). Peptidic chains 7 and 8
as well as SAA derivatives 9 and 10 have been obtained relying
on standard solid-phase strategy, using 2-chlorotrityl resin. Re-
moval of the resin, gave linear glycopeptides 11 (62%, over 10
steps) and 12 (25%, over 11 steps) which were treated with triflu-
oroacetic acid to take off the Boc protecting group quantitatively
affording derivatives 13 and 14, as trifluoroacetic salts. Intramolec-
ular condensation of the latter, and suitable deprotection, gave the
cyclo glycopeptides: cyclo(SAA5-Ala-Phe-Phe-Phe-Ala) 15, and cy-
clo(SAA5-Glu-Ile-Leu-Asp-Val) 16, (see Supplementary data) which
were prepared to be tested as tachykinin NK2 receptor ligand and
space regions. We thus synthesized compound 15 which showed
some, although limited, activity in the binding assay at the human
tachykinin NK2 receptor. Affinity of compound 15 at the human
NK2 receptor was determined in binding experiments by using
membranes of Chinese Hamster Ovary (CHO) cells stably transfec-
ted with the human NK2 receptor.14 The compound was tested for
its ability to displace the radioligand [125I]-neurokinin A (0.15 nM)
after 30 min incubation at room temperature, resulting in 25%
inhibition of the radioligand binding at 10 lM concentration.
Capitalizing on this encouraging result, further work is ongoing
in order to increase the biological activity through the modulation
of the peptide sequence assisted by molecular dynamics
calculations.
integrin (
a
4b1) inhibitor respectively (Scheme 2).
It is known that a requisite for a good recognition by NK2 recep-
tors is the presence of two close aromatic rings in the axial position
about the backbone ring.15 We performed a preliminary modeling
study of compound 15 to locate its allowed conformations;16 the
molecule showed a high degree of conformational flexibility and
several populated geometries. Among them, two significant con-
formations show two of the three phenyl rings oriented in the
same region of the space, though they do not belong to adjacent
aminoacidic residues as in 17. These conformations, reported in
Figure 2, are stabilized by a b-turn motif and support the potential
interest in such cyclic glycopeptidic structures.
In a previous paper11 we have shown how insertion of aromatic
groups on a suitable sugar derived scaffold can mimic the structure
of highly active tachykinin NK2 antagonists such as the small
monocyclic pseudopeptide 1712 (Fig. 1).
Indeed, some of the compounds we obtained showed binding
affinities at the human tachykinin NK2 receptor at submicromolar
concentrations, while molecular dynamics calculations indicated
that the structures of 17 and that of one of the best sugar derived
compounds were quite well superimposable. As tachykinin NK2
antagonists continue to raise interest as potential new drugs for
the therapy of gastrointestinal and CNS diseases,13 we turned our
attention to the synthesis of compounds in which small peptide se-
quences, containing aromatic aminoacids (Phe) and alanine resi-
dues as chiral spacers, are linked to the sugar derived rigid
scaffold 5 in order to distribute the aromatic moieties in suitable
Integrins are a family of membrane-spanning adhesion receptors
composed of non- covalently linked
aand b subunits which combine
to give a wide amountof heterodimers. In recentyears antagonists of
very late antigen-4 (VLA4, also known as integrin a4b1) have shown
great promise in treating inflammatory disorders.17–19 VLA4 recog-
Trp
Phe
HN
H
N
HN
O
O
O
O
HN
H
N
17
Figure 1. Structure of compound 17.
Figure 2. 3D plot of representative conformations for compound 15.

M. Altamura et al. / Bioorg. Med. Chem. Lett. 19 (2009) 3841–3844
3843
COOBn
O
O
O
O
O
O
H
N
H
N
H
N
H
R
R
H2N
H2N
N
O
N
H
N
H
O
N
H
N
H
O
O
O
O
7
COOBn
8
O
O
O
O
O
O
H
H
H
NH
NH
O
NH
O
O
N
O
N
N
NH
NH
OH
N
N
R
H
H
HO
O
O
HO
O
O
TIPSO
TIPSO
S
O
S
O
TFA (1% in CH2Cl2)
62% (over 10 steps)
NHR'
OBn
NHBoc
OBn
O
O
OCH3
OCH3
9
O
O
TFA
11
R' = Boc;
R' = H2+CF3COO- ; 13
CH2Cl2
> 90%
COOBn
COOBn
O
O
O
O
O
O
H
N
H
H
H
N
H
H
O
N
N
O
N
N
N
N
H
OH
R
N
N
H
O
H
H
O
O
HO
O
HO
O
O
TIPSO
TIPSO
S
COOBn
NHR'
S
O
COOBn
O
TFA (1% in CH2Cl2)
NHBoc
O
O
OCH3
OCH3
OBn
OBn
10
O
12
O
R' = Boc;
TFA
CH2Cl2
R' = H2+CF3COO-; 14
R
25% (over 11 steps)
=
O
Cl
R'OOC
O
O
O
O
O
N
H
N
HN
H
HN
NH
O
NH
O
O
O
O
HN
HN
HO
RO
HO
RO
COOR'
S
S
O
O
HN
O
HN
O
H
O
O
HN
N
OMe
OMe
O
O
COOBn
COOR'
R = TIPS; 18
R = H; 15 (50%)
R = TIPS; R' = Bn; 19
R = H; R' = Bn; 20
CsF, DMF
rt
TBAF, THF
rt
H2/ Pd(OH)2
MeOH
R = H; R' = OH; 16 (40%, over two steps)
Scheme 2. Synthesis of open-chain SAA peptides 13 and 14.
nizes polypeptide domains containing the Glu-Ile-Leu-Asp-Val ami-
noacid sequence whose specificity and efficacy is determined by the
contest (flanking residues, conformational presentation, etc.). A
number of peptidomimetics have been proposed as anti-VLA4
agents and in this contest the cyclo SAA peptidomimetic 16 was pre-
molecular calculations, a pentapeptidic sequence should well fit
to form macrocyclic structures, therefore biologically engaging
compounds 15 and 16 were prepared as single diastereoisomers.
Cyclo(SAA5-Ala-Phe-Phe-Phe-Ala) 15 was tested as tachykinin
NK2 ligand, showing some, although limited, activity which can
likely be rationalized on the basis of a correct orientation of its
phenyl groups. Cyclo(SAA5-Glu-Ile-Leu-Asp-Val) 16, however, did
pared and tested for inhibitory activity against
a4b1 integrin by
assessing ability to inhibit the adhesion of U937 and Jurkat cells to
a VCAM-1 coated surface20 demonstrating a lack of inhibition of
not exhibit activity against a4b1 integrin. Relying on the versatility
these processes up to a concentration of 100
l
M.
and potentials of the scaffold 5 we propose herein, assisted by
molecular modeling studies, further work is ongoing to prepare
an array of cyclic SAA peptidomimetics containing proper peptidic
chains to interact with specific biological targets.21
In summary, the monosaccharidic scaffold 5, efficiently ob-
tained as diastereomerically pure compound, was successfully
used to synthesize cyclic SAA peptidomimetics. As suggested by

3844
M. Altamura et al. / Bioorg. Med. Chem. Lett. 19 (2009) 3841–3844
6. Chakraborty, T. K.; Srinivasu, P.; Tapadar, S.; Mohan, B. K. Glycoconjugate J.
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for helpful discussions on MS and NMR spectra and Ente Cassa di
Risparmio di Firenze for Ph.D. grant of E.D.
Supplementary data
Supplementary data associated with this article can be found, in
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approach at the B3LYP/6-31G(d) level. Then, the energy was recalculated using
a continuum solvent model (PCM) to take into account the effect of water.
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cyclo(SAA5-Arg-Gly-Asp), that contains the RGD motif usually known to confer
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affinity for the
avb3 integrin receptor when the tripeptide moiety assumes a
kinked conformation. Calculations showed that the desired conformation is
largely populated though this analog exhibits
conformational freedom.
a certain degree of