(R)-9-[2-(Hydroxyphosphinylmethoxy)propyl]adenine as the precursor molecule for antivirals

Article abstract of DOI:10.1016/j.tetlet.2014.05.061

The biological importance and practical significance of phosphonates have been a major driving force for antiviral research in the past two decades. We describe in this Letter the potential of the H-phosphinate derivative (R)-9-[2-(hydroxyphosphinylmethoxy)propyl]adenine as the versatile synthetic intermediate in the preparation of N-alkylphosphonamidate and alkylphosphonate series.

Full text of DOI:10.1016/j.tetlet.2014.05.061

Tetrahedron Letters xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
(R)-9-[2-(Hydroxyphosphinylmethoxy)propyl]adenine
as the precursor molecule for antivirals
⇑
Loic Roux, Bruno Canard, Karine Alvarez
Laboratoire d’Architecture et Fonction des Macromolécules Biologiques, Aix-Marseille Université, CNRS UMR 7257, Equipe ‘Chimie Médicinale Antivirale’, Parc scientifique de
Luminy, 163 av. de Luminy, 13288 Marseille Cedex 9, France
a r t i c l e i n f o
a b s t r a c t
Article history:
The biological importance and practical significance of phosphonates have been a major driving force for
antiviral research in the past two decades. We describe in this Letter the potential of the H-phosphinate
derivative (R)-9-[2-(hydroxyphosphinylmethoxy)propyl]adenine as the versatile synthetic intermediate
in the preparation of N-alkylphosphonamidate and alkylphosphonate series.
Received 14 April 2014
Revised 13 May 2014
Accepted 14 May 2014
Available online xxxx
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
ANPs
Antivirals
Phosphinate chemistry
N-Alkylphosphonamidate
Alkylphosphonate
Phosphorus-containing compounds became key in various
medicinal techniques such as antisense strategy, antigene or gene
silencing technique, and in drug therapy making use of modified
nucleotides in antiviral or antitumoral treatments.1 During the past
decades, a significant interest has emerged in the preparation and
study of phosphonic acids and their derivatives. As it was
presumed that the carbonAphosphorus bond was unable to be
hydrolyzed by enzymes involved in natural phosphate bond cleav-
age, they were considered as more stable analogues than natural
phosphates. Amongst phosphonate derivatives, acyclic nucleoside
phosphonates (ANPs)1 are outstanding molecules in the fight
against DNA virus and retrovirus infections. Three of such com-
pounds have been licenced for the treatment of HCMV infections
in AIDS patients (cidofovir, VistideÒ), HIV, and chronic HBV
infections (Adefovir dipivoxil, HepseraÒ, and Tenofovir disoproxil
fumarate, VireadÒ). In addition to these indications, there are
various other clinical conditions in which ANPs were found to be
of therapeutic interest.2
desired therapeutic effects, as it is apparent from recent studies
on the inhibition of HIV and HBV infections.5,8
In this Letter, we explore new series of analogues. (R)-9-
[2-(Hydroxyphosphinylmethoxy)propyl]adenine
1 (Scheme 1),
H-phosphinate precursor in PMPA series offers a unique opportu-
nity to introduce various modifications of the phosphorus center
by changing the oxidation conditions. We do not intend to give
an exhaustive coverage of all possible modifications leading to
new potential therapeutics, but explore the flexibility of the
H-phosphinate chemistry whose exploration lags somehow behind
that of H-phosphonate chemistry.9
We developed an original method to synthesize H-phosphinate
intermediate 13,6 that can serve as precursor to generate modi-
fied acyclic nucleoside phosphonates (Scheme 1). This key
precursor was then used to synthesize
a
-boranophosphonate 33
and
a
-thiophosphonates 45,8 derived from PMPA.
The phosphorus atom in H-phosphinate 1 is an electrophilic
center, however, it can be transformed into a nucleophilic center
under certain conditions. Indeed, H-phosphinate exists as an equi-
librium mixture of two tautomeric forms:10 a tetra-coordinated
phosphinate form and a tri-coordinated phosphonite form. The
tervalent form, due to the presence of lone electron pair on the
phosphorus center, reacts readily with various electrophiles. On
the contrary, in a tetra-coordinated form, the lack of the lone of
electron pair on the phosphorus center reduces the reactivity to
electrophiles. Nevertheless, by using Atherton–Todd conditions11
(silylating agent, carbon tetrachloride and pyridine), the phosphi-
nate form can be transformed into phosphonamidate P(V) species
In previous articles, we focused our researches on the impact of
the modification of the phosphorus environment in (9-[2-(phos-
phonomethoxy)ethyl]adenine [PMEA] and (R)-9-[2-(phosphono-
methoxy)propyl]adenine [PMPA] derivatives to improve their
efficacy as antivirals.3–7 Some of these modifications as thiophosph-
onate derivatives [S-PMPA] and [S-PMEA] turned out to produce
⇑
Corresponding author. Tel.: +33 4 91 82 55 70; fax: +33 4 91 26 67 20.
E-mail address: karine.alvarez@afmb.univ-mrs.fr (K. Alvarez).
http://dx.doi.org/10.1016/j.tetlet.2014.05.061
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Roux, L.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.05.061

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L. Roux et al. / Tetrahedron Letters xxx (2014) xxx–xxx
Reactions with nucleophiles. Synthesis of compounds 8 and 9
Nucleos(t)ides, phosphoramidates, and phosphonamidates
have been extensively studied,17 due to their prodrug activity
against viruses and in cancer therapy. In the course of our studies
on the chemistry of nucleoside H-phosphinates, we have investi-
gated the synthesis of N-alkyl phosphonamidates. The bis-silyl
phosphonite of 1 was treated under mild Atherton–Todd reaction
conditions11 (pyridine and carbon tetrachloride) to produce the
pyridinium phosphonamidate intermediate, which was subse-
quently substituted by n-butylamine or benzylamine, leading to
N-alkyl phosphonamidates 8 and 918 (Scheme 2). Using Ather-
ton–Todd conditions, an important amount of the side product
identified as the trichloromethylphosphinate was obtained. To
minimize its formation, the quantity of carbon tetrachloride has
been reduced from 50 to 12 equiv. Inspection of the 31P NMR
spectra during the monitoring of the reaction revealed resonance
signals at 20 ppm characteristics of the formation of compounds 8
and 9.19 Unfortunately isolation of n-butyl phosphonamidate 8
and benzyl phosphonamidate 9 was impossible due to their par-
tial instability during workup and chromatography under aque-
ous conditions. As outlined in Figure 1, chromatogram of the
purification under aqueous conditions (Fig. 1–Panel A) of com-
pound 8 revealed the presence of three compounds. HPLC-MS
coupling analysis (Fig. 1–Panel B and C–S30 gradient) allowed us
to identify the compound with retention time tR = 0. 35 min,
ESI-MS [M+H] m/z = 287.94 as phosphonate 2 ; the compound
with retention time tR = 1.52 min, ESI-MS [M+H] m/z = 342.0 as
n-butyl phosphonamidate 8 ; the compounds with retention
Scheme 1. General structure of H-phosphinate 1 as the versatile precursor, and
some oxidative transformations.
(pyrimidium adduct) which can be efficiently attacked by
nucleophiles. In order to obtain an effective activation, N,O-bis
(trimethylsilyl)acetamide (BSA) was chosen as the silylating
agent12 and H-phosphinate 1 was treated with BSA in pyridine,
or DCM with Et3N, and the reaction mixture was stirred at room
temperature for 16 h (Scheme 2).
Reactions with electrophiles. Synthesis of alkylphosphonates 5,
6, and 7
t
times; R = 2.01 and 2.09 min, ESI-MS [M+H] m/z = 611.7 as com-
pound 10 as a diastereoisomeric mixture. The same results were
observed with benzyl phosphonamidate 9 (tR = 6.85 min, ESI-MS
[M+H] m/z = 376.8) which decomposed rapidly in phosphonate
2 and compound 11 (tR = 9.36 and 9.71 min, ESI-MS [M+H] m/
z = 646.0).
It has to be noted that the lability of PAN bonds in phospho-
namidates has been explored.20 The PAN bond in a phosphonami-
date monoester, is labile and hydrolyzed faster than that in a
phosphoramidate diester,20 so the observed decomposition should
be predictable. However the formation of compounds 10 and 11 is
more surprising. The SPARC pKa calculator allowed us to determine
pKa values of compounds 8 and 9, respectively 3.40 and 3.30.21 We
may hypothesize that under aqueous conditions, the N-alkyl
For the formation of PAC bonds, a variety of metal catalyzed
cross-coupling reactions have been developed.13 To perform oxida-
tive addition of H-phosphinate substrates, palladium-catalyzed
additions to alkenes14 and base-promoted addition to alkenes, car-
bonyls, and alkyl halides as Arbuzov-like reaction have been
described.15,16
Compounds 5, 6, and 7 were prepared by reaction of the
bis-silyl phosphonite of 1 with corresponding alkyl halides/benzyl
bromide, ethyl 2-bromoacetate, or methyl iodide, during 2 h. Reac-
tions led to expected compounds 5, 6, and 7 in 53%, 39%, and 57%
yield, respectively. 31P NMR spectra of compounds 5, 6, and 7
revealed resonance signals between 29 and 37 ppm.
Scheme 2. Synthesis of compounds 5, 6, 7, 8, 9, 10, and 11. Reagents and conditions: (a) BSA, TEA in DCM, rt, 16 h; (b) R1X (X = Br for 5 and 6 or X = I for 7), 2 h then H2O, then
dowex Na+ exchange after purification; (c) BSA in pyridine, rt, 16 h; (d) CCl4 and NH2R2 (R2 = n-butyl for 8 and R2 = Bn for 9), 2 h then H2O.
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L. Roux et al. / Tetrahedron Letters xxx (2014) xxx–xxx
3
Figure 1. Analysis of N-alkylphosphonamidate 8. Panel A: chromatogram of compound 8 before purification (MPLC analysis). Panel B: chromatogram of compound 8 after
purification (HPLC–MS analysis, S30 gradient). Panel C: Mass fingerprints (ESI+) of pics observed in panel B.
moiety is a good living group and the nucleophilicity of the oxygen
negatively charged is enough to attack phosphorus atom and leads
to the formation of the dimeric species.
J = 5 Hz), characteristic of predominant thiono form for thi-
ophosphonate 4.
H-phosphinate chemistry offers a versatile method for synthe-
sis of various phosphonate compounds, combining advantages of
tricoordinated phosphorus(III) and tetracoordinated phospho-
rus(V). Although the focus of our research has been nucleotide
chemistry, the H-phosphinate chemistry is not, by any means, con-
fined to this class of compounds. It is likely that the demand will
grow regarding other bioactive phosphorus compounds to be used
as potential therapeutic agents. In this context, H-phosphinate
chemistry might provide necessary tools for modification of
phosphorus compounds by means of introducing multiple modifi-
cations and even controlling the stereochemistry.
The 31P NMR chemical shifts of phosphinate 1 and modified-
phosphonates 2, 3, 4, 5, 6, and 7 are reported in Figure 2. These data
should be useful for synthesis monitoring and qualitative identifi-
cation of these specific compounds and, to some extent, related
compounds by extrapolation. For example, 31P NMR signal of 1
around 25 ppm with a huge PAH coupling constant (JPH = 528 Hz)
are replaced by characteristic downfield resonance signal around
115 ppm, due to the displacement of electron density from the
phosphorus center to the negatively charged coordinated borane
for boranophosphonate 3 or by resonance signal at 60 ppm (t,
Figure 2. 31P RMN chemical shifts of compounds 1, 2, 3, 4, 5, 6, and 7.
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L. Roux et al. / Tetrahedron Letters xxx (2014) xxx–xxx
5. Barral, K.; Weck, C.; Payrot, N.; Roux, L.; Durafour, C.; Zoulim, F.; Neyts, J.;
Acknowledgments
Balzarini, J.; Canard, B.; Priet, S.; Alvarez, K. Eur. J. Med. Chem. 2011, 46, 4281–
4288.
This work was supported by Grant from the French ‘Agence
Nationale de Recherche contre le Sida et les Hépatites Virales’
(ANRS), by SIDACTION. Loic Roux is particularly grateful to SIDAC-
TION for a his Ph.D. fellowship. We wish to thank, A. Lebrun and Dr.
G. Valette (Université Montpellier II, Montpellier, France) for their
expertise in conducting NMR experiments and mass spectrometry
analysis.
6. Canard, B.; Alvarez, K.; Barral, K.; Romette, J. L.; Neyts, J.; Balzarini, J. WO/2008/
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Supplementary data
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Supplementary data (detailed experimental procedures and
spectroscopic data for compounds 5, 6, 7, 8, 9, 10 and 11) associ-
ated with this article can be found, in the online version, at
http://dx.doi.org/10.1016/j.tetlet.2014.05.061.
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