Synthesis and biological evaluation of inosine phosphonates

Article abstract of DOI:10.1039/b9nj00574a

The 4'-phosphonomethoxy analogs of inosine and 2',3'-dideoxyinosine ere synthesized and tested for their activity against HCV and HIV, but found to be nactive. During the course of this synthetic investigation, an unexpected oxidative leavage of the 2',3'

Full text of DOI:10.1039/b9nj00574a

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www.rsc.org/njc | New Journal of Chemistry
Synthesis and biological evaluation of inosine phosphonateswz
Mikhail Abramov and Piet Herdewijn*
Received (in Montpellier, France) 16th October 2009, Accepted 13th November 2009
First published as an Advance Article on the web 25th January 2010
DOI: 10.1039/b9nj00574a
The 40-phosphonomethoxy analogs of inosine and 20,30-dideoxy-
inosine were synthesized and tested for their activity against
HCV and HIV, but found to be inactive. During the course of
this synthetic investigation, an unexpected oxidative cleavage of
the 20,30-bond of 20-deoxyinosine was observed.
the oxidation of 20-deoxyadenosine1,6 and 20,30-O-isopropyl-
ideneinosine.7 The product formed under these conditions was
the result of oxidative cleavage of the 20–30 carbon–carbon
bond of the deoxyribose sugar, with the formation of
the dicarboxylic acid (Scheme 2). This reaction is rather
unexpected for 20-deoxyribonucleosides and might be
explained by the oxidative cleavage of an enol intermediate.
When we tried to oxidize unprotected 20-deoxyadenosine with
potassium permanganate under basic conditions, mass
spectrometry showed the formation of a compound with a
molecular mass that may correspond to 2-(carboxy(adenin-9-
yl)methoxy)-3-hydroxypropanoic acid. The oxidation of
ribo- or 2-hexenepyranosyl nucleosides, with cleavage of the
20–30 carbon–carbon bond of the sugar moiety, resulting in
dialdehyde formation, is known.8
The chemistry used for the synthesis of 50-modified (20-deoxy)-
adenosine nucleosides1,2 does not always work for the synthesis
of inosine congeners, and in some cases the hypoxantine base
needs to be protected. We present here the synthesis of two
inosine derivatives with a phosphonomethoxy substituent at
the 40-position. This kind of isostere/isoelectronic replacement
has proven in the past to lead to bioactive compounds.3,4
A particular problem encountered was the oxidation of the
50-hydroxyl function of the 20-deoxyinosine to a carboxylic
acid group. This conversion could be carried out using the
N-oxoammonium salt of 2,2,6,6-tetramethylpiperidine-1-oxyl.
Given the interest in phosphonate nucleosides in the antiviral
field, the obtained compounds were tested against HIV, RSV
and in a HCV replicon assay.
The platinum-catalyzed oxidation by air, following the
procedure of Moss et al.,9 also failed. Finally, the oxidation
of the N1-protected 20-deoxyinosine, 5, utilizing the in situ-
generation of the N-oxoammonium salt derived from
TEMPO10 and following deprotection with methanolic
ammonia, gave key acid 6. Furthermore, the reaction
generated only acetic acid and iodobenzene as by-products,
which were easily removed from the precipitated product
to yield acid 6 in analytical purity. Decarboxylative dehydration
of acid 6 into furanoid glycal 7 was carried out with
N,N-dimethylformamide dineopentyl acetal in good yield.1
The haloetherification reaction of glycal 7 with dibenzyl
(hydroxymethyl)phosphonate11 mediated by iodine mono-
bromide, followed by the base (DBU) promoted elimi-
nation of hydrogen bromide1 or oxyselenenation with the
aid of phenylselenyl chloride, and then oxidative (hydrogen
peroxide) elimination of the phenylselenyl group1,12 gave rise
to olefin 8.
Two possible procedures for the synthesis of phosphonate
analogs of nucleotides are known. A Lewis acid-mediated
glycosidation of a phosphonate alcohol onto an anomeric
acetate could be used to introduce the desired phosphonate,
as reported previously.3,5 Phosphonate isoesters of nucleoside
monophosphates could also be prepared using a highly stereo-
selective asymmetric oxyselenenylation of a furanoid glycal or
a subsequent electrophilic addition of iodine monobromide to
glycal onto the least hindered a-face of the ribose, followed by
the introduction of the protected hydroxymethylphosphonate
ester onto the b-face.2
The phosphonomethoxy derivatives of inosine, 1, and 20,30-
dideoxyinosine, 2, were prepared starting from 20-deoxy-
inosine (3) in 11 steps, as illustrated in Scheme 1. There were
two key transformations in the sequence: firstly, the regiospecific
oxidation of the 50-hydroxyl group into a 40-carboxylic
acid, and secondly, the regiospecific and stereoselective intro-
duction of the phosphonate group into the b-configuration.
Direct oxidation of 3 with chromium trioxide in pyridine or
potassium permanganate in basic aqueous solution did
not result in the desired 40-carboxylic acid, as described for
Saturated phosphonate 2 was prepared from 8 by hydro-
genation using catalytic Pd/C with simultaneous removal of
the phosphonate benzyl ester and the 2-N-benzyloxymethyl
protecting groups. Bis-hydroxylation of the double bond in 8
was accomplished using catalytic osmium tetraoxide and N-
methylmorpholine oxide13 as the oxidizing reagent. The one
step deprotection of the generated single diol isomer1 resulted
in phosphonate isostere 1 of inosine monophosphate in
high yield.
In conclusion, a synthetic scheme has been developed
leading to the 40-phosphonomethoxy derivatives of inosine
and of 20,30-dideoxyinosine. Both compounds showed
activity when tested again HIV-1, RSV and in a HCV replicon
assay.14
Laboratory of Medicinal Chemistry, Rega Institute for Medical
Research, K.U. Leuven, Minderbroedersstraat 10, 3000, Leuven,
Belgium. E-mail: piet.herdewijn@rega.kuleuven.be;
Fax: +32 16 33.73.40; Tel: +32 16 33.73.41
w Electronic supplementary information (ESI) available: Experimental
procedures and characterization of compounds 1, 2 and 4–8. See DOI:
10.1039/b9nj00574a
The authors thank Dr Richard Mackman (Gilead Sciences)
for testing the compounds against HCV, HIV and RSV.
z This article is part of a themed issue on Biophosphates.
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This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2010
New J. Chem., 2010, 34, 875–876 | 875

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Scheme 1 Synthesis of the 40-phosphonomethoxy derivatives 1 and 2 of inosine and 20,30-dideoxyinosine. Reagents and conditions: (i) MMTrCl,
Py; (ii) Ac2O, Py, 90%; (iii) BnOCH2Cl, DIPEA, DCM, 70%; (iv) TsOH, MeOH–DCM, TEA, 0 1C, 83%; (v) TEMPO, iodobenzene diacetate,
MeCN–H2O, 97%; (vi) NH3, MeOH, 92%; (vii) N,N-dimethylformamide dineopentyl acetal, DMF, 130 1C, 76%; (viii) PhSeCl, dibenzyl
hydroxymethylphosphonate, LiClO4, DCM, ꢁ70 1C, 42%; (ix) 30% H2O2, dioxane, 0 1C, 45%; (x) IBr, dibenzyl hydroxymethylphosphonate,
DCM, ꢁ25 1C, 37%; (xi) DBU, dioxane, 65 1C, 80%; (xii) H2, Pd/C, MeOH, 95%; (xiii) OsO4, N-methylmorpholine N-oxide, 70%; (xiv) H2,
Pd/C, MeOH, 95%.
4 R. L. Mackman, C. G. Boojamra, V. Prasad, L. Zhang, K.-Y. Lin,
O. Petrakovsky, D. Babusis, J. Chen, J. Douglas, D. Grant,
H. C. Hui, C. U. Kim, D. Y. Markevitch, J. Vela, A. Ray and
T. Cihlar, Bioorg. Med. Chem. Lett., 2007, 17, 6785–6789.
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This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2010
876 | New J. Chem., 2010, 34, 875–876