New mannose derivatives: The tetrazole analogue of mannose-6-phosphate as angiogenesis inhibitor

Article abstract of DOI:10.1016/j.bmcl.2015.11.059

Two novel compounds with mannose-derived structure, bearing a tetrazole (compound 3) and a sulfone group (compound 4) in terminal position, have been prepared from methyl α-d-mannopyranoside in reduced number of steps. The angiogenic activity of 3 and 4 has been screened using the chick chorioallantoic membrane (CAM) method. Tetrazole 3 has been identified to possess a promising bioactivity, being identified as angiogenesis inhibitor, with 68% of neovascular vessels when compared to control (PBS).

Full text of DOI:10.1016/j.bmcl.2015.11.059

Bioorganic & Medicinal Chemistry Letters 26 (2016) 636–639
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
New mannose derivatives: The tetrazole analogue
of mannose-6-phosphate as angiogenesis inhibitor
a
b
a
Ca˘ta˘lina Ionescu a, , Simona Sippelli , Loïc Toupet , Véronique Barragan-Montero
⇑
a Université de Montpellier, Place Eugène Bataillon, 34095 Montpellier, France
b GMCM UMR 6626, Campus de Beaulieu, 35042 Rennes Cedex, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Two novel compounds with mannose-derived structure, bearing a tetrazole (compound 3) and a sulfone
group (compound 4) in terminal position, have been prepared from methyl -mannopyranoside in
reduced number of steps. The angiogenic activity of 3 and 4 has been screened using the chick chorioal-
lantoic membrane (CAM) method. Tetrazole 3 has been identified to possess a promising bioactivity,
being identified as angiogenesis inhibitor, with 68% of neovascular vessels when compared to control
(PBS).
Received 15 October 2015
Revised 13 November 2015
Accepted 17 November 2015
Available online 18 November 2015
a
-D
Keywords:
Mannose-6-phosphate analogues
Iodination
Ó 2015 Elsevier Ltd. All rights reserved.
Carboxylic acid
Tetrazole
Sulfone
Angiogenesis is the process of generating new capillary blood
vessels from pre-existing ones.1 In healthy adults, angiogenesis is
normally absent, excepting two specific phenomena: the cuta-
neous wound healing2 and the intervention in female reproductive
functions.3,4 An abnormal vascularisation (either insufficient,
either excessive) can cause or contribute to the development of
various diseases.5 Therefore, angiogenesis activators have wide
applications in medicine, in the treatment of diseases caused by
insufficient angiogenesis, linked to the ischemia of a part of the
vascular system. At the same time, a considerable amount of atten-
tion has been dedicated to angiogenesis inhibitors, due to their
intensive employment, among other therapeutic applications, in
cancer treatment. It has been demonstrated that tumors cannot
develop to a volume bigger than 1–2 mm3 without the participa-
tion of blood vessels.6 Therefore, angiogenesis inhibitor therapy7
became an important actor in cancer therapy. It is supposed hat
angiogenesis inhibitors act by inhibiting the development of blood
vessels inside tumors and by normalizing the blood vessel
network, facilitating the access of medication at tumor site.
mannose-6-phosphate/insulin like growth factor II receptor
(M6P/IGFIIR), is a 275 kDa, P-type glycoprotein,11 whose main
function is represented by the transport of newly synthesized
enzymes from the cell membrane or from Golgi apparatus to
lysosomes.12 The binding specificity of the receptor has been inten-
sively studied.13 In our ongoing research on mannose-6-phosphate
(M6P) and its derivatives, different M6P analogues have been syn-
thesized and their affinity for the M6P/IGFIIR has been tested.14–17
In a recent study, we have reported the synthesis of different M6P
analogues, bearing various functional groups (azido, aminomethyl,
carboxyl, malonate, sulfonate, carboxymethyl, phosphonate) in the
C6 position of the carbohydrate and a methyl group in anomeric
position, using a method involving a key cyclic sulfate intermedi-
ate. The angiogenic activity of these compounds has been evalu-
ated and the tested analogues proved to be angiogenesis
regulators.18 The most potent angiogenesis inhibitor in this series
was the methyl mannopyranoside of M6P itself (MeM6P) (com-
pound 1, Fig. 1), but its therapeutic applications are limited by
its instability in biological environment, caused by the presence
of hydrolytic enzymes. Besides MeM6P, another compound with
promising antiangiogenic properties was the carboxylic acid 2.
Because tetrazoles are known as nonclassical bioisosteres of car-
boxylic acids,19,20 the evaluation of the angiogenic properties of
compound 3 became an interesting target, presented in this Letter.
At the same time, we prepared a second mannose-6-functionalized
derivative: the sulfone 4. Sulfones have previously been
investigated as phosphate mimics in carbohydrate series, both in
the anomeric21 and terminal position of carbohydrates, when a
Angiogenesis is
a complex process, involving numerous
biological mediators.8 Recent studies indicate that the cation-
independent mannose-6-phosphate receptor (CI-M6PR) is also
involved in angiogenesis.9,10 CI-M6PR, also known as the
⇑
Corresponding author at present address: Faculty of Sciences (Chemistry
Department), University of Craiova, 107 I Calea Bucuresßti, 200144 Craiova, Romania.
Tel./fax: +40 757 571 247.
E-mail address: catalinagurgui@yahoo.co.uk (C. Ionescu).
http://dx.doi.org/10.1016/j.bmcl.2015.11.059
0960-894X/Ó 2015 Elsevier Ltd. All rights reserved.

C. Ionescu et al. / Bioorg. Med. Chem. Lett. 26 (2016) 636–639
637
N N
N N
N
N
Compounds previously tested
N
HN
HN
N
HN
N
i
OBn
OH
OH
O
O
O
+
BnO
BnO
HO
HO
HO
HO
3
10
OH
9
OCH3
OCH3
O
HO
OH
O
HO2C
P
ii
OH
OH
O
O
HO
HO
HO
HO
Scheme 2. Two-steps synthesis of 3, by deprotection of hydroxyl groups of 9
followed by methylation of the obtained reaction mixture. Reactions, conditions
and yields: (i) BCl3, Cl2H2; 3 min at À78 °C; (ii) SOCl2/MeOH, 3 h at rt; 36% over the
two steps (i) and (ii).
OCH3
OCH3
MeM6P (1)
2
N N
HN
N
O S O
OBn
O S O
OH
OH
O
O
O
HO
HO
BnO
HO
HO
BnO
5
OCH3
OCH3
OCH3
3
4
Novel compounds tested in this work
Figure 1. Mannose derivatives functionalized in terminal position.
dimethylene sulfone linker has been used as a replacer of the phos-
phodiester group in oligonucleotides.22 Compound 4 is not a bioi-
sostere of M6P. It has heteroatoms in the C6 position, but, unlike
other M6P analogues tested by now, it does not possess negatively
charged groups in the C6 position at physiologic pH. Moreover,
besides the biological applications of 4, intermediate 5, which is
used for the preparation of the final compound 4, can open
important perspectives in the synthetic carbohydrate chemistry.
Carbanions of methylene groups in
a
position of sulfones can react
Figure 2. ORTEP drawing of 5.
with variety of electrophiles, like, for example, carbonyl
a
compounds, followed by subsequent elimination of the sulfone
using samarium diiodide or sodium/mercury amalgam23 (Julia
olefination method).24
trifluoride diethyletherate (0.3 equiv/hydroxyl group) have given
the same result.
In order to prepare compounds 3 and 4, we have chosen an
efficient pathway that involves the iodinated compound 7 as
common intermediate in the synthesis of the desired carbohy-
drates (Scheme 1). This intermediate was obtained from methyl
The tetrazole derivative 3 was prepared using the nitrile inter-
mediate 8, obtained by reacting 7 with sodium cyanide in DMF.
The conversion of nitriles to tetrazoles supposes, in a classical
manner, the usage of hydrazoic acid, prepared from sodium azide
and acids30 or of tributyltin azide, prepared from sodium azide
and tri-n-butyltin chloride.31 Other methods include the usage of
Al(N3)3, prepared in situ from trimethylaluminium and
trimethylsilyl azide32 or salts of hydrazoic acid.33 We decided to
use the last method, already applied with success in glucose series,
using sodium azide and ammonium chloride, that allowed us to
obtain the tetrazole 9 in 70% yield. The last step was represented
by carbohydrate hydroxyl groups’ deprotection. Based on our
knowledge, no literature data has reported by now the synthesis
of an unprotected glycoside bearing a tetrazole moiety in terminal
a-D
-mannopyranoside, using an Appel reaction25 and among vari-
ous protocols available26–28 we have chosen the procedure of
Skaanderup et al.,26 followed by introduction of benzyl protective
groups on carbohydrate hydroxyls. Because benzylation under
standard basic conditions (benzyl bromide and sodium hydride
or potassium hydroxide) is not compatible with our iodinated
compound, we have realized this step in acid catalysis,26 with
benzyl trichloroacetimidate prepared from benzyl alcohol and
1,1,1-trichloroacetonitrile.29 The benzylation reaction is usually
realized with triflic acid as catalyst, but our tests with boron
OH
I
I
O S O
O S O
OH
O
OBn
O
OH
O
OBn
O
OH
i
vi
ii
vii
O
HO
HO
BnO
BnO
HO
HO
BnO
BnO
HO
HO
7
OCH3
6
OCH3
OCH3
5
OCH3
OCH3
4
iii
N N
N N
HN
N
HN
N
CN
OBn
O
OBn
iv
v
OH
O
O
BnO
BnO
BnO
BnO
HO
HO
8
9
OCH3
OCH3
OCH3
3
Scheme 1. Synthesis of 3 and 4. Reactions, conditions and yields: (i) I2, PPh3, Imidazole, THF, 2 h at refluxing temperature, 81%; (ii) benzyl trichloroacetimidate, triflic acid,
dioxane, 10 min at 0 °C, 67%; (iii) NaCN, DMF, 2 h at 70 °C, 87%; (iv) NaN3, NH4Cl, DMF, 144 h at 95 °C, 70%; (v.a) BCl3, CH2Cl2, 3 min at À78 °C; (v.b) SOCl2, methanol, 3 h at rt;
36% over the two steps v.a and v.b; (vi) sodium phenylsulfinate, DMF, 12 h at 60 °C, 67%; (vii) H2, Pd/C, ethyl acetate/methanol, HCl, 2 h at rt, 80%.

638
C. Ionescu et al. / Bioorg. Med. Chem. Lett. 26 (2016) 636–639
Figure 3. Chorioallantoic membrane (CAM) assays realized with MeM6P (1) as inhibitor, its phosphonate analogue 10 as angiogenesis activator, PBS as control, carboxylic
acid 2 and its non-classical bioisostere, tetrazole 3. The given values represent the extent of angiogenesis, 100% being the value quoted for PBS (control).
position. In our case, the deprotection of mannose hydroxyl groups
proved to be very delicate. Initial attempts of classical hydrogenol-
ysis at different hydrogen pressures, in neutral or acidic solvents,
were inefficient. Another method known for benzyl groups depro-
tection is represented by the action of boron trichloride.34 The
usage of this compound in methyl glycosides remains limited, as
BCl3 is known to break glycosidic bonds.35 Indeed, by reacting 9
with BCl3 in CH2Cl2, at À78 °C, the tetrazole mannopyranoside 3
has been obtained, but together with its corresponding mannopy-
ranose, bearing a hydroxyle group in anomeric position (Scheme 2).
The crude reaction mixture has been methylated, using the
SOCl2/MeOH couple, in order to enrich the mixture in the desired
derivative 3.
The sulfone 4 has been prepared in two steps starting from 7:
displacement of iodine with p-toluenesulfinate36 and benzyl
groups’ elimination (Scheme 1). Because initial attempts of depro-
tection by hydrogenolysis at neutral pH were inefficient, this step
was realized in the presence of HCl in order to obtain compound
4. Single-crystal X-ray analysis of 5 confirmed its structure. Crys-
tallographic data (excluding structural factors) for the structure
of compound 5 have been deposited at the Cambridge Crystallo-
graphic Data Centre with the deposition number CCDC 679336.
An ORTEP diagram of 5 is shown in Figure 2.
methyl 6-deoxy-6-iodo-a-D-mannopyranose as common interme-
diate, in six steps for tetrazole 3 (12% yield) and in four steps for
sulfone 4 (29% yield). Both products’ angiogenic activity has been
evaluated using the CAM assay. Tetrazole 3 showed good inhibi-
tory properties (70% compared to PBS), thus being an interesting
new carbohydrate derivative with possible applications in anti-
cancer therapy. Moreover, this discovery is important because
the number of monosaccharides with angiogenesis inhibitor prop-
erties reported by now is extremely limited.18
Acknowledgment
This work was supported by the Ministère de la Recherche et de
la Technologie (MRT).
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.bmcl.2015.11.
059.
References and notes
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during the 7th embryonic day and were grown in ovo at 38 °C for
4 days. Angiogenic response quantification was realized by mea-
suring the area of neovascularization on the surface of each mem-
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being defined as 100%) (Fig. 3).
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