2-Pyridylphosphonates: A new type of modification for nucleotide analogues

Article abstract of DOI:10.1016/S0040-4039(01)00115-0

Suitably protected dithymidine H-phosphonates afforded the corresponding dinucleoside 2-pyridylphosphonates upon treatment with N-methoxypyridinium tosylate in acetonitrile in the presence of 1,8-diazabicylo[5.4.0]undec-7-ene (DBU). The reaction was rapid (ca. 5 min), practically quantitative and proceeded stereospecifically, most likely with retention of configuration at the phosphorus centre.

Full text of DOI:10.1016/S0040-4039(01)00115-0

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 2217–2220
2
-Pyridylphosphonates: a new type of modification for nucleotide
analogues
a
a
a,b,
Tommy Johansson, Annika Kers and Jacek Stawinski
*
a
Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University, S-106 91 Stockholm, Sweden
Institute of Bioorganic Chemistry, Polish Academy of Sciences, Noskowskiego 12/14, 61-704 Poznan, Poland
b
Received 28 November 2000; revised 12 January 2001; accepted 17 January 2001
Abstract—Suitably protected dithymidine H-phosphonates afforded the corresponding dinucleoside 2-pyridylphosphonates upon
treatment with N-methoxypyridinium tosylate in acetonitrile in the presence of 1,8-diazabicylo[5.4.0]undec-7-ene (DBU). The
reaction was rapid (ca. 5 min), practically quantitative and proceeded stereospecifically, most likely with retention of configuration
at the phosphorus centre. © 2001 Elsevier Science Ltd. All rights reserved.
9
Simple dialkyl phosphonate diesters bearing a 2-pyridyl
moiety (e.g. 6, Fig. 1), apart from being widely used as
corrosion inhibitors, dispersing and emulsifying agents,
antistatics, and lubricant additives in various techno-
common approach used is that of Redmore and con-
sists of the reaction of N-methoxypyridinium salts with
sodium dialkyl phosphites in the corresponding dialkyl
H-phosphonates as solvents. Unfortunately, the reac-
tion is confined to simple dialkyl H-phosphonates, is
highly sensitive to experimental conditions and usually
affords the desired products in variable yields (low to
moderate), partly due to decomposition of the reactants
during the extended reaction time (overnight). Other
methods, e.g. nucleophilic substitution of halides in
deactivated pyridine rings with sodium dialkyl
1
logical processes, are also known as potent insecti-
2
3
4
cides, fungicides and herbicides. Some of them also
5
possess a pronounced cytokinin activity. Recently,
anti-proliferating and anti-platelet activating factor
6
(
anti-PAF) activities of 2-pyridylphosphonates have
been reported, and quinolylphosphonic acid derivatives
were found to be potent antagonists of AMPA/kainate
and thus of potential use as therapeutic agents in the
10
11
phosphite or with triethyl phosphite, are even less
efficient and require harsh reaction conditions. The
most recent approach to the preparation of 2-
pyridylphosphonates using N-trifluoromethanesulpho-
nylpyridinum triflate and trialkyl phosphites suffers
7
acute treatment of stroke and head trauma.
This diverse array of biological activity of 2-
pyridylphosphonates prompted us to develop a syn-
thetic method for incorporation this functionality into
nucleotides, as a potentially useful modification in
1
2
from a rather lengthy protocol and low overall yields.
8
designing new antisense/antigene agents.
As part of our programme in developing new synthetic
methods for biologically important phosphate deriva-
There are only a few synthetic methods for the prepara-
tion of dialkyl esters of 2-pyridylphosphonates and
practically none of them can be applied to the synthesis
of natural products bearing this functionality. The most
1
3
tives based on H-phosphonate chemistry, we have
recently reported an efficient conversion of H-phospho-
nate diesters into the corresponding 4-pyridylphospho-
O
O
OEt
P O
OEt
OEt
H
OEt
EtO
P
H
OEt
EtO
P
EtO
P
OEt
OX
P
O
pTs
N
OSiMe3
N
N
OEt
OMe
OMe
1
2
3
4
5
6
Figure 1.
*
Corresponding author. E-mail: js@organ.su.se
0
040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00115-0

2
218
T. Johansson et al. / Tetrahedron Letters 42 (2001) 2217–2220
nate derivatives using a pyridine–trityl chloride–DBU
reagent system. Unfortunately, 2-pyridylphosphonate
derivatives cannot be prepared by this method.
methoxypyridinium salt 2, the formaldehyde produced
can easily add to silylphosphite 3 present in the reaction
mixture to afford silylated hydroxymethylphosphonate
1
4
4
(X=SiMe ). Apparently, silyl phosphite 3 itself was
3
Since activation of the pyridine ring to nucleophilic
basic enough to trigger the decomposition of pyri-
attack by the formation of an N-alkoxypyridinium
dinium salt 2, since formation of hydroxymethylphos-
1
5
salt seemed attractive both in terms of the availability9
of suitable substrates and the desired regioselectivity,
we decided to explore this avenue for the preparation of
phonate 4 (X=SiMe ) was also observed (albeit in
3
lower yield, ca. 55%) in the absence of an external base.
2
-pyridylphosphonate analogues of nucleotides.
We found, however, that diethyl H-phosphonate 1
reacted fast with pyridinium salt 2 (2 equiv.) in acetoni-
trile in the presence of 1,8-diazabicylo[5.4.0]undec-7-ene
To this end diethyl H-phosphonate 1 (l =7.6 ppm) in
P
methylene chloride was silylated with trimethylsilyl
chloride (TMS-Cl, 3 equiv.) in the presence of triethy-
lamine (TEA, 2 equiv.) to produce a nucleophilic phos-
(
DBU) (4 equiv.) affording, within 5 min, the desired
18
diethyl 2-pyridylphosphonate 6 (l =10.9 ppm, 80%)
P
19
and hydroxymethylphosphonate 4 (X=H, l =24.6
P
phorus species, diethyl trimethylsilyl phosphite
3
20
ppm, 20%). This indicated that apparently under the
(
l =127.1 ppm), and this was allowed to react with
P
reaction conditions nucleophilic attack of diethyl phos-
3
1
pyridinium salt 2 (3 equiv.). After ca. 3 h, P NMR
spectroscopy revealed in the reaction mixture the pres-
ence of several phosphorus-containing species (includ-
ing 30% of the starting material), but none of them
resonated in the range of chemical shifts expected for
phite anion (generated from 1 and DBU) on the pyri-
dine ring of
2
was significantly faster than
decomposition of 2 due the presence of DBU. In agree-
ment with this, no formation of pyridylphosphonate 6
31
was observed ( P NMR spectroscopy) when pyri-
dinium salt 2 was mixed with DBU prior to the addi-
tion of H-phosphonate 1.
2
-pyridylphosphonate 6 (the expected l ca. 10 ppm).
P
The replacement of TMS-Cl by bis(trimethylsi-
lyl)acetamide (BSA, 3 equiv.) as a silylating agent
caused significant changes in product distribution, and
after 3 h the formation of a major product (>90%)
The efficacy of this approach in the synthesis of a new
type of nucleotide analogue with a 2-pyridylphospho-
nate internucleotide linkage (Scheme 1) was assessed by
reacting, in acetonitrile, a suitably protected dinu-
cleoside H-phosphonate 7 (diastereomeric mixture,
resonating at l =22.1 ppm was observed. The chemi-
P
cal shift value suggested a dialkoxyphosphinoyl group
bound to an aliphatic carbon, but an indistinct splitting
pattern in the H– P NMR coupled spectrum pre-
vented further spectral identification. Fortunately, the
compound resonating at l =22.1 ppm was rather sta-
ble which permitted its isolation by silica gel chro-
1
31
lP=8.44 and 9.35 ppm) with N-methoxypyridinium
salt 2 (2 equiv.) and DBU (4 equiv.). The reaction was
rapid and went to completion within 5 min producing
as a sole nucleotidic material a 1:1 mixture of
P
1
13
31
matography. Analysis of the H, C and P NMR
spectra showed that the compound in question was not
a possible intermediate in this reaction, 1,2-dihydropy-
ridylphosphonate 5, but silylated hydroxymethylphos-
diastereomeric 2-pyridylphosphonates derivatives
9
(
lP=10.6 and 11.0 ppm). In contrast to 4-
14
pyridylphosphonates, no intermediacy of the corre-
1
6
sponding dihydropyridine 8 could be detected in this
phonate 4 (X=SiMe , comparison with authentic
3
31
sample). Formation of hydroxymethylphosphonate 4
derivative in this reaction can probably be traced back
to known instability of N-alkoxypyridinium salts,
which under basic conditions collapse to pyridine and
reaction using P NMR spectroscopy. After work-up
(see below), 2-pyridylphosphonates 9 were isolated by
silica gel column chromatography in 86% yield and
their chemical identity was confirmed by MS and NMR
data.
17
the corresponding aldehydes. In the instance of N-
RO
T
RO
T
RO
T
O
O
O
pTs
N
O
H
O
H
O
H
OMe
H
P O
O
OMe
P O
O
H P O
O
DBU
Acetonitrile
N
N
T
T
T
O
O
O
RO
H
RO
H
RO
H
8
9
7
7
a (R ) ("fast") (δ = 8.4 ppm);
9a (RP) (δP = 10.6 ppm);
R = 4,4'-dimethoxytrityl
9b (SP) (δP = 11.0 ppm);
R = 4,4'-dimethoxytrityl
P
P
R = 4,4'-dimethoxytrityl
7b (S ) ("slow") (δ = 9.4 ppm);
P
P
R = 4,4'-dimethoxytrityl
Scheme 1.

T. Johansson et al. / Tetrahedron Letters 42 (2001) 2217–2220
2219
To assess a stereochemical course of the formation of
-pyridylphosphonates 9 (Scheme 1), the reaction was
carried out on the separate diastereomers of
References
2
1. Redmore, D. US Pat. 3,673,196, 1972 (Petrolite Corp.).
2. Protopopova, G. V.; Reidalova, L. I.; Pavlenko, A. F.;
Sologub, L. S.; Kukhar, V. P.; Petrenko, V. S. Fiziol.
Akt. Veshchestva 1980, 12, 16–18.
. (a) Kukhar, V. P.; Cherepenko, T. I.; Pavlenko, A. F.
Dokl. Akad. Nauk Ukr. RSR, Ser. B: Geol. Khim. Biol.
Nauki 1982, 60–63; (b) Akiba, K.; Tsuzuki, K. Jpn.
Kokai Tokkyo Koho 61221102, 1986 (Toyo Soda Mfg.
Co., Ltd, Japan).
21
31
dithymidine H-phosphonate 7. It was found ( P
NMR spectroscopy) that H-phosphonate diester 7a
afforded 2-pyridylphosphonate 9a, while the dia-
stereomer 7b, the isomeric 2-pyridylphosphonate 9b,
exclusively. This established the transformation as
stereospecific. Assuming the reaction pathway as in
Scheme 1, it seems most likely that it proceeds with
overall retention of configuration.
3
4
. Malhotra, S. K.; Evoy, I. L. US Pat. 4,606,757, 1986
A typical procedure for the preparation of 2-pyridylphos-
phonates 9
(
Dow Chemical Co., USA).
5. Pavlenko, A. F.; Konstantinova, A. M.; Sologub, L. S.;
Moshchitskii, S. D. Fiziol. Akt. Veshchestva 1976, 8,
The separate diastereomers of H-phosphonate diester 7
21–22.
(
0.88 mmol) in acetonitrile (4 mL) were treated with
6
. Murray, K. J.; Porter, R. A.; Prain, H. D.; Warrington,
B. H. PCT Int. Appl. 9,117,987, 1991 (Smith Kline and
French Laboratories Ltd, UK).
. Desos, P.; Lepagnol, J. M.; Morain, P.; Lestage, P.;
Cordi, A. A. J. Med. Chem. 1996, 39, 197–206.
. (a) Wagner, R. W. Nature 1994, 372, 333–335; (b)
Vasquez, K. M.; Wilson, J. H. TIBS 1998, 23, 4–9.
. Redmore, D. J. Org. Chem. 1970, 35, 4114–4117.
N-methoxypyridinium tosylate (2 equiv.) and DBU (4
equiv.). CAUTION: DBU has to be added as the last
reagent to the reaction mixture to ensure the efficient
formation of 2-pyridylphosphonate 9 (see also in the
7
8
9
3
1
text). After 5 min ( P NMR) the reaction mixture was
concentrated, partitioned between 10% aq. NaHCO3
(
20 mL) and CH Cl , (20 mL), and the aqueous layer
2 2
was extracted with CH Cl , (2×20 mL). The organic
2
2
1
0. (a) Moshchitskii, S. D.; Sologub, L. S.; Pavlenko, A. F.;
layer was dried over anh. Na SO , concentrated and the
2
4
Kukhar, V. P. Zh. Obshch. Khim. 1977, 47, 1263–1267;
residue was purified by silica gel column chromatogra-
phy using toluene–ethyl acetate–methanol (49:49:2, v/v/
v). Yields 9a, 86% (white, microcrystalline solid); 9b
(
b) Ivashchenko, Y. N.; Sologub, L. S.; Moshchitskii, S.
D.; Kirsanov, A. V. Zh. Obshch. Khim. 1969, 39, 1695–
1697.
22
85% (white, microcrystalline solid).
1
1. (a) Achremowicz, L. Synthesis 1975, 653–654; (b)
Boenigk, W.; Haegele, G. Chem. Ber. 1983, 116, 2418–
2425.
12. Haase, M.; Goerls, H.; Anders, E. Synthesis 1998, 195–
200.
13. Kers, A.; Kers, I.; Kraszewski, A.; Sobkowski, M.;
Szab o´ , T.; Thelin, M.; Zain, R.; Stawinski, J. Nucleosides
Nucleotides 1996, 15, 361–378.
In conclusion, we have developed an efficient protocol
for the preparation of a new type of nucleotide ana-
logues bearing 2-pyridylphosphonate internucleotide
linkage. The method makes use of easily available
starting materials and affords the target compounds in
high yields under mild reaction conditions. The distinc-
tive feature of 2-pyridylphosphonate modification is
that it permits further functionalisation (e.g. via quater-
nisation of the pyridine nitrogen) and thus can be of
importance in designing new antisense/antigene agents
with improved biomembrane penetration and tuneable
chemical, biological and pharmacokinetic properties.
Apart from this, the ability of 2-pyridylphosphonates to
form bidentate complexes with metal cations can be
exploited in the development of new artificial nucleases
or specific probes for investigation of electron transfer
1
1
1
4. Kers, A.; Stawinski, J. Tetrahedron Lett. 1999, 40, 4263–
266.
5. Katritzky, A. R.; Lunt, E. Tetrahedron 1969, 25, 4291–
305.
4
4
6. Although undesired in the context of these investigations,
this reaction may constitute a convenient way for the
preparation of hydroxymethylphosphonates. Further
studies are in progress.
7. (a) Katritzky, A. J. Chem. Soc. 1956, 2404–2408; (b)
Feely, W.; Lehn, W. L.; Boekelheide, V. J. Org. Chem.
1
(
ET) phenomena in nucleic acids.
1
957, 22, 1135; (c) Coats, N. A.; Katritzky, A. J. Org.
Studies on elaboration other heteroaromatic phospho-
nate systems for the purpose of modification of nucleic
acids are in progress in this Laboratory.
Chem. 1959, 24, 1836–1837.
1
8. Comparison with the authentic sample. Isolated in 74%
yield from the reaction mixture by silica gel
chromatography.
1
2
9. Comparison with the authentic sample.
0. At this stage of the investigation no attempt was made to
optimise the procedure in order to eliminate the forma-
tion of hydroxymethylphosphonate by-products. How-
ever, it seems likely that by using other
N-alkoxypyridinium salts and varying the amount of
DBU, the formation of hydroxymethylphosphonates can
be eliminated or significantly suppressed. Further studies
are in progress in this Laboratory.
Acknowledgements
We are indebted to Professor P. J Garegg for his
interest in this work. Financial support from the
Swedish Natural Science Research Council and the
Swedish Foundation for Strategic Research is gratefully
acknowledged.

2
220
T. Johansson et al. / Tetrahedron Letters 42 (2001) 2217–2220
+
2
1. Stawinski, J.; Str o¨ mberg, R.; Zain, R. Tetrahedron Lett.
992, 33, 3185–3188.
2. Compounds 9a and b were >98% pure and their chemical
HRMS [M+H]
found: 1212.4369; C H N O P
67 67 5 15
1
requires: 1212.4371. Some diagnostic spectral data [com-
pound, l ; lH(H-6 py); lH1%; lC(C-2 py) (JCP)]: 9a: 10.63 ppm;
8.42 ppm (1H); 6.39 ppm (2H); 150.36 ppm (227 Hz). 9b:
11.02 ppm; 8.58 ppm (1H); 6.38 ppm (2H); 150.94 ppm
(228).
2
P
1
13
31
1
1
identity was confirmed by H, C, P, and H– H
+
correlated NMR spectroscopy. 9a: HRMS [M+H]
found: 1212.4366; C H N O P requires: 1212.4371. 9b:
67
67
5
15
.