Synthesis of 5,6-Spiroethers and Evaluation of their Affinities for the Bacterial A Site

Full text of DOI:10.1002/cbic.201100076

DOI: 10.1002/cbic.201100076
Synthesis of 5,6-Spiroethers and Evaluation of their Affinities for the
Bacterial A Site
Ioannis A. Katsoulis, Georgia Kythreoti, Athanasios Papakyriakou, Konstantina Koltsida,
Panoula Anastasopoulou, Christos I. Stathakis, Ioannis Mavridis, Thomas Cottin, Emmanuel Saridakis, and
Dionisios Vourloumis*[a]
Natural aminoglycoside antibiotics and their corresponding
chemical analogues are superb molecular tools for the explora-
tion of the subtle interactions that underlie the biological reg-
ulation of RNA sequences.[1] One prominent example is to be
found in molecules that target the bacterial ribosomal RNA,
cause the disruption of protein synthesis and by this validated
mechanism exert their pharmacological activity.[2] Understand-
ing of the structural elements that lead to molecular recogni-
tion of RNA is constantly improving with the identification of
aromatic interactions[3]—alongside the known electrostatic
ones[4]—that enable high-affinity binding. Moreover, it has
been theorized that the conformational versatility of the impli-
cated chemical entities plays an important role for specific
binding to the desired RNA site.[5] Despite the challenges that
still confront the discovery and pharmacological application of
new aminoglycoside conjugates, the potential of such chemi-
cal entities to interfere with diverse biological processes re-
mains highly attractive.[6]
Our past endeavours in this field have focused on the syn-
thesis and biological evaluation of rigid spirocyclic ethers[7,8]
that appear to capture the “bioactive” conformation within the
Scheme 1. Secondary structure of the bacterial decoding-site internal loop
(A-site) in 16S rRNA. The four base changes in the eukaryotic sequence are
A site of the ribosomal RNA (Scheme 1). In order to explore the
chemical space that can be accessed by these means further,
cyclic 5,6-spiroethers bearing triazole moieties were readily
synthesized and assessed for their binding potentials. The
choice of the triazole unit was predominantly governed by its
ease of preparation, either by Cu-catalysed “click chemistry”[9]
or by thermal 1,3-dipolar cycloaddition.[10] Moreover, triazoles
have unique biological potential, possibly arising from p stack-
ing with other aromatic[11] or carbohydrate moieties[3] of the
RNA target, and therefore comprise an important pharmaco-
phore for potential drug discovery.[12] A noteworthy difference
from past synthetic analogues of the same chemical family[7,8]
is the substitution pattern of the spiroether. The presented
compounds are extended in their “northwestern” flanks with a
set of functionalities that can satisfy and complement the con-
dition of “structural electrostatic complementarity”[4] within the
active site.
indicated by arrows. The recognition site for the 2-DOS moiety of aminogly-
cosides is boxed. The structures of neomycin B, paromomycin, neamine (ne-
omycin A) and kanamycin B all contain the 2-DOS core. Below right: general
structure of the analogues described in this work.
The preparation of the spiroether moiety began with the
common intermediate 1 (Scheme 2, 7:1 mixture of diastereo-
mers at C4), previously utilized in our synthetic 6,7-membered
series.[7] On iodoetherification with N-iodosuccinimide[13] this
furnished the primary iodide 2. This reaction appears to pro-
ceed stereoselectively, with regard to C9 (numbering present-
ed in Scheme 1), as revealed in the ensuing steps (vide infra),
with a possible transition state as depicted in Scheme 2. The
observed selectivity could result from the minimization of
steric interactions between the N3-Cbz and the electrophilic
iodine.[14] The iodide 2 was next converted into its correspond-
ing azide 3.[15] Upon removal of the masking ketal, we were
able to separate the two diastereoisomeric (at C4) 1,2-diols 4
and 5 (7:1 d.r.) by column chromatography. In order to verify
the absolute configuration of the major isomer, 4 was convert-
ed to the monobenzoate ester 6 and X-ray crystallographic
analysis provided decisive evidence for all its stereocenters
(Scheme 2).
[a] Dr. I. A. Katsoulis, Dr. G. Kythreoti, Dr. A. Papakyriakou, K. Koltsida,
P. Anastasopoulou, Dr. C. I. Stathakis, I. Mavridis, Dr. T. Cottin, E. Saridakis,
Dr. D. Vourloumis
Laboratory of Chemical Biology of Natural Products and
Designed Molecules, Institute of Physical Chemistry
N.C.S.R “Demokritos”, Agia Paraskevi–Athens, 15310 (Greece)
Fax: (+30)210-651-1776
Our synthetic efforts continued with both isomers, produc-
ing a set of 12 new triazoles (7–18, Table 1), either by the stan-
dard “click-chemistry” procedure[9] or through thermal 1,3-di-
E-mail: vourloumis@chem.demokritos.gr
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cbic.201100076.
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ChemBioChem 2011, 12, 1188 – 1192

matic interactions. Of the mono-
substituted analogues, the alco-
hol 13 represents the strongest
binder, with affinity in the low-
nm concentrations. This signifi-
cant improvement relative to
previously reported rigid spi-
roether analogues[7,8] motivated
a computational modelling at-
tempt to interpret their atomic-
level interactions with the A-site.
Docking of compounds 7–18
into a model of the rRNA A-site
was performed as described pre-
viously.[7,8] The calculated confor-
mations of the most potent
compound 13 exhibit
a high
degree of invariability with re-
spect to its orientation within
the A-site. This was also evident
for all spiroether triazole com-
pounds, because their docking
to the decoding centre predict-
ed at least four major clusters of
putative orientations. Lack of
any cocrystallographic data on
these analogues bound to RNA
makes our choice quite specula-
Scheme 2. Reagents and conditions: a) NIS (1.0 equiv), NaHCO3 (2.0 equiv), MeCN, 3 h, 0!258C, 78% yield;
b) NaN3 (3.5 equiv), DMF, 28 h, 508C, 89% yield; c) AcOH/THF/H2O (3:3:1, v/v/v), 488C, 12 h, 79% for 4 and 11%
for 5; d) 4-bromobenzoyl chloride (3.0 equiv), TEA (4.5 equiv), DMAP (0.2 equiv), CH2Cl2 (0.05m), 87%. NIS=N-io-
dosuccinimide; DMF=N,N-dimethylformamide; AcOH=acetic acid; THF=tetrahydrofuran; TEA=triethylamine;
DMAP=4-(dimethylamino)pyridine.
polar cycloaddition.[10] The selection of the reactant alkynes
was based on their sizes and on the nature and number of
their functional groups, with the goal of identifying major in-
teraction with the pertinent RNA sites. Specifically, the distance
between the H-bond donor and the triazole (9 vs 13), the im-
portance of dipolar as opposed to aromatic interactions (11 vs
13) and the natures (amine, amide, ether or hydroxy) and num-
bers of polar groups present (10, 13, 15–17), as well as the
overall sizes of the small molecules (13, 15, 16, 18), could thus
be evaluated.
tive. However, two of the highest-affinity conformations calcu-
lated for compound 13 were found to overlap with the posi-
tion of neamine in the corresponding cocrystal structure (see
the Supporting Information). In the first one (Figure 1A), the
aminocyclitol of 13 is proximal to ring II (2-DOS) of neamine,
such that the triazole ring protrudes towards the two extruded
A1492 and A1493 bases, stabilizing their conformation through
a direct hydrogen bond between the hydroxy constituent and
N7 of A1493. The aminocyclitol group provides the conserved
hydrogen-bonding functionality of 2-DOS with the C1407–
G1494 and U1406–U1495 base pairs. In the second “inverted”
orientation, the aminocyclitol moiety is predicted to superim-
pose with ring I of neamine (Scheme 1) such that the triazole
extends up to U1495 where it interacts with the phosphate
backbone of G1494 (Figure 1B). The observed affinities of the
spiroether triazole analogues could therefore potentially be ex-
plained by either binding mode.
The affinities of all the spiroether triazoles towards the RNA
A site were evaluated by RNA fluorescence assay (Table 1).[16]
A
model RNA oligonucleotide in which the adenine at position
1492 had been replaced by a fluorescent probe (2-aminopur-
ine, 2AP) was employed.[16] The binding of the minor isomers 7
and 8 to the RNA construct proved to be relatively weak (low-
mm concentrations, Table 1) in relation to that of the com-
pounds resulting from functionalization of the isomeric azide
4. Analogously to the reported six- and seven-membered ana-
logues,[7] the presence of the S configuration around the C4
quaternary centre is accompanied by improved affinity in most
of the cases studied. Generally, the sizes of the triazole sub-
stituents appear to be the determining factor for potency, ir-
respective of the hydrogen-bonding potential present in the
monosubstituted series (13 vs 9, 9 vs 15 and 17, Table 1). Bulk-
ier substituents (compounds 16 and 18) resulted in complete
inactivity. The analogue 11, however, represents an exception
to that observation, presumably because the expected unfa-
vourable steric load could be compensated by favourable aro-
In conclusion, we have successfully synthesized and evaluat-
ed small rigid molecular scaffolds that produce very good EC50
values for the ribosomal A-site construct utilized in our fluores-
cent studies. These adducts avoid the excessive electrostatic
charge of the natural products, which is a cause of adverse
side effects for this class of compounds.[18] The exact binding
mode (orientation and site) of the spiroether triazole ana-
logues is uncertain, and further biophysical biological[19] and
crystallographic efforts for this purpose are currently underway.
Nevertheless, we expect them to contribute significantly to un-
derstanding of the principles governing RNA recognition and
more specifically the dynamic interplay of the small-molecule
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ChemBioChem 2011, 12, 1188 – 1192

1H), 2.37 (brs, 1H), 2.34 (t, J=6.3 Hz, 1H), 2.06–1.92 (m, 2H), 1.94
(m, 1H), 1.83 (m, 1H), 1.72–1.51 (m, 11H), 1.35 ppm (m, 1H);
13C NMR (63 MHz, CDCl3): d=156.0, 155.6, 136.3, 136.2, 128.6,
128.5, 128.3, 128.2, 128.1, 111.8, 84.9, 82.0, 81.0, 67.1, 66.8, 55.5,
54.4, 52.1, 51.8, 49.6, 36.4, 30.8, 29.2, 25.0, 23.7 ppm; IR (neat): n˜ =
3309, 3032, 2931, 2096, 1699, 1523, 1230, 1042 cmꢁ1; HRMS-ESI:
m/z calcd for C28H32N2NaO7: 531.2107 [M+Na]+; found: 531.2103.
Indicative click-chemistry protocol for the preparation of
(2S,5S,6R,7S,8R,10S)-8,10-diamino-2-[(4-(hydroxymethyl)-1H-
1,2,3-triazol-1-yl)methyl]-1-oxaspiro[4.5]decane-6,7-diol (13): The
azide
4 (13 mg, 0.025 mmol) and propargyl alcohol (10 mL,
0.17 mmol) were dissolved in EtOH/H2O (2:1, 2 mL) at room tem-
perature. CuSO4·5H2O (2.0 mg, 8.0ꢁ10ꢁ3 mmol) was added to this
solution, followed by sodium ascorbate (4.0 mg, 0.031 mmol), and
the suspension was vigorously stirred for 18 h. The solvents were
removed in vacuo and the crude product was directly purified by
flash column chromatography on silica gel, with a CH2Cl2 to
CH2Cl2/MeOH (2:1) gradient elution system to afford the corre-
sponding triazole. This was directly dissolved in water, KOH (0.18 g,
3.2 mmol) was added, and the mixture was warmed at 1308C in a
sealed tube. The solvent was removed in vacuo and the crude
product was directly purified by flash column chromatography on
silica gel, with a methanol to methanol/NH4OH (aq.) (8:2) gradient
elution system to afford the triazole 13 (4.7 mg, 60% yield over
1
two steps). Rf =0.2 (MeOH/NH4OH (aq) 9:1, v/v); H NMR (500 MHz,
D2O): d=8.08 (s, 1H), 4.60 (m, 2H), 4.50 (m, 1H), 3.47 (t, J=9.8 Hz,
1H), 3.42 (m, 1H), 3.00 (m, 1H), 2.89 (m, 1H), 2.23–2.08 (m, 3H),
1.97 (m, 1H), 1.86 (m, 1H), 1.53 ppm (q, J=12.4 Hz, 1H); 13C NMR
(63 MHz, D2O): d=148.2, 126.6, 91.1, 81.8, 77.7, 77.5, 56.2, 55.6,
53.7, 52.2, 37.8, 31.8, 30.6 ppm; HRMS-ESI: m/z calcd for
C13H24N5O4: 314.1828 [M+H]+; found: 314.1822.
Indicative fluorescence binding assay: Desalted and gel-purified
RNA complementary strand oligonucleotides (Dharmagon, Inc.)
were annealed in sodium cacodylate buffer (30 mm, pH 6.8) at
658C for 3 min, followed by snap cooling on ice. Hybridization was
confirmed by analytical gel electrophoresis. The double-stranded
construct contains the bacterial decoding site sequence in which
the adenine at position 1492 has been substituted with the 2AP
fluorescent analogue.
Figure 1. Predicted binding modes of compound 13 in the ribosomal A-site.
A) Binding orientation of 13 with 2-DOS occupying the usual site of the ami-
noglycoside ring II, and B) alternative orientation with 2-DOS in the position
corresponding to the aminoglycoside ring I (stacked over G1491). Intermo-
lecular hydrogen bonding interactions are shown with red lines, carbon
atoms in RNA are coloured orange, those in compound 13 in cyan, whereas
all oxygen atoms are red, nitrogen blue and phosphorus yellow. Images
were prepared with VMD 1.8.6[17]
attributes with the adaptable structural and energetic land-
scape of the ribosomal A-site.[20]
Titrations of the tested compounds with the 2AP-labelled RNA bi-
partite construct were performed, with concentrations ranging
from 10 pm to 1 mm. Emission spectra were recorded at RNA con-
centrations of 20 nm, 100 nm, 500 nm and 1 mm (30 mm sodium ca-
codylate, pH 6.8) in 1 cm pathlength quartz cells. Fluorescence was
measured with a QuantaMaster 40 Steady State Spectrofluorimeter
at 258C. The excitation wavelength used was 310 nm, whereas
emission was monitored between 320 and 450 nm and normalized
at maximum emission wavelength of 370 nm. Half-maximal re-
sponse concentration (EC50) values were calculated by fitting dose–
response curves to the fluorescence intensities plotted against the
logs of ligand concentrations. Three replicate experiments per
compound were run. As control experiments, all ligands tested
were also added to the 2AP-labelled single-stranded B oligonucleo-
tide. Additionally, spermidine, a known nonspecific RNA binder,
was titrated in parallel, as a negative dose–response control.
Experimental Section
Iodoetherification and azidation: The homoallylic alcohol
1
(0.26 g, 0.47 mmol) was dissolved in dry acetonitrile (4.0 mL) at
08C, and NaHCO3 (78 mg, 0.93 mmol) and N-iodosuccinimide
(0.11 g, 0.47 mmol) were added successively. The suspension was
allowed to stir in the dark for 3 h and brine (5 mL) was added. The
mixture was extracted with AcOEt (3ꢁ15 mL), dried (MgSO4) and
concentrated in vacuo. The solid crude product was purified by
flash column chromatography on silica gel (prewashed with Et3N)
with a hexanes to hexanes/AcOEt (1:2) gradient elution system in
order to afford the iodide 2 in the form of a white amorphous
solid. (0.26 g, 78% yield). Sodium azide (71 mg, 1.087 mmol) was
added at 508C to a solution of the iodide 2 (250 mg, 0.362 mmol)
in dry DMF (4 mL). The reaction mixture was allowed to stir for
36 h, the solvent was then removed in vacuo, and the crude prod-
uct was purified by flash column chromatography on silica gel,
with a hexanes to hexanes/AcOEt (1:1) gradient elution system, to
afford azide 3 as a mixture of diastereomers (7:1 219 mg, 90%
Acknowledgements
This research is supported by Marie Curie Actions, Excellence
Teams (EXT) Grants, contract number MEXT-CT-2006–039149. We
would also like to thank Dr. I. Mavridis for crystallographic sup-
port, Prof. E. Theodorakis for assistance with HR-mass spectrosco-
1
yield); Rf =0.5 (hexanes/AcOEt 3:2, v/v); H NMR (500 MHz, CDCl3):
d=7.39–7.30 (m, 10H), 5.15–5.03 (m, 4H), 4.96 (brs, 1H, NH), 4.89
(m, 1H, NH), 4.20 (m, 1H), 3.86–3.72 (m, 2H), 3.66 (t, J=8.9 Hz,
1H), 3.50–3.40 (m, 1H), 3.34–3.26 (m, 1H), 3.07 (dd, J=12.4, 3.0 Hz,
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Keywords: aminoglycosides · click chemistry · ribosomal A
site · rigid scaffolds · RNA recognition
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Received: February 1, 2011
Published online on May 6, 2011
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