Formation of phostonic acids during the reduction of azidonucleosidephosphonic acids

Article abstract of DOI:10.1016/S0040-4039(01)01919-0

Reduction of 5′-azido-3′,5′-dideoxy-3′-(phosphorylmethyl) nucleosides, either with H₂/Pd/C or with dithiothreitol, leads to cyclic phosphonate esters (phostonic acids) in addition to the desired 5′-amino compounds. The phostonic acids are obtained as the main products when a 20-fold excess of dithiothreitol is used.

Full text of DOI:10.1016/S0040-4039(01)01919-0

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 8841–8843
Formation of phostonic acids during the reduction of
azidonucleosidephosphonic acids†
Michael Morr,a,* Christel Kakoschkea and Ludger Ernstb
aGBF-Gesellschaft fu¨r Biotechnologische Forschung, Mascheroder Weg 1, D-38124 Braunschweig, Germany
bDepartment of Chemistry, Technical University of Braunschweig, D-38092 Braunschweig, Germany
Received 12 September 2001; revised 10 October 2001; accepted 14 October 2001
Abstract—Reduction of 5%-azido-3%,5%-dideoxy-3%-(phosphorylmethyl)nucleosides, either with H2/Pd/C or with dithiothreitol, leads
to cyclic phosphonate esters (phostonic acids) in addition to the desired 5%-amino compounds. The phostonic acids are obtained
as the main products when a 20-fold excess of dithiothreitol is used. © 2001 Elsevier Science Ltd. All rights reserved.
Phosphonic acid analogs of nucleotides are interesting
objects of biological studies because the stability of
their PꢀC bonds makes them resistant towards the
action of phosphatases.1 Phostonic acids, cyclic esters
of phosphonic acids, are also known from various
classes of biomolecules.2 Some time ago, we reported
the synthesis of 2%,3%-dideoxy-3%-(phosphonomethyl)-
nucleosides which are phosphonate analogs of 2%-
deoxy-3%-nucleotides.3 In connection with this work
and with the studies by Moffatt et al.1 we became
interested in the synthesis and properties of 5%-amino-
3%,5%-dideoxy-3%-(phosphonomethyl)-nucleosides such as
1 and of the corresponding 2%,3%,5%-trideoxy nucleosides
such as 4 (Scheme 1).
sideproducts. In fact, these phostonic acids can be
made the main products when a large excess of the
reducing agent dithiothreitol is used. Cyclization of 1
or 4 with water-soluble carbodiimide yields the novel
cAMP analog 3 and the cdTMP analog 6, respec-
tively, which contain the 3%-CH2 and 5%-NH groups in
the newly formed six-membered ring. The present
communication describes the synthetic pathways to 1–
3 and to 4–6.
Moffatt’s compound 71 was treated with p-toluenesul-
fonyl chloride in dry pyridine (0°C, 5 h) to give tosy-
late 8, which was reacted with lithium azide in
dimethylformamide to furnish good yields (>85%) of
azide 9 (Scheme 2). Acetolysis of the 1,2-O-isopropyli-
dene group of 9 gave crystalline 1,2-di(O-acetyl)-5-
The preparation of 1 and 4 from their azido precur-
sors was accompanied by the formation of title com-
azido-3,5-dideoxy-3-(diethoxyphosphorylmethyl)-b-
D-
pounds
2
and 5a/5b, respectively, as major
ribofuranose (10). Compound 10 was converted to 11
Scheme 1.
Keywords: nucleosides; phosphonic acids and derivatives; cyclonucleotide analogs.
* Corresponding author. Tel.: +49-531-6181-722; fax: +49-531-6181-795; e-mail: mmo@gbf.de
† In memoriam Dr. Ludger Witte (1948–2001).
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)01919-0

8842
M. Morr et al. / Tetrahedron Letters 42 (2001) 8841–8843
Scheme 2.
(60% yield) with N6-benzoyladenine using the one-pot
procedure of Vorbru¨ggen with potassium perfluorobu-
tanesulfonate and chlorotrimethylsilane in acetoni-
trile.4,5 Ester 11 was hydrolyzed with 1N NaOH to give
the sodium salt of phosphonic acid 12, which was
purified by ion-exchange chromatography on DEAE-
In order to test whether the formation of a phostonic
acid during catalytic reduction is specific to adenosine
derivatives, hydrogenation of the thymine analog 17
was also investigated (Scheme 3). Tosylation of com-
pound 14, previously described by us,3 gave 15, which
was converted to azide 16. Alkaline hydrolysis of the
ester groups of 16 was not successful due to the lack of
neighboring group assistance by the 2%-OH group.
Hence, 16 was transesterified8 with bromotrimethylsi-
lane in the presence of 2,4,6-trimethylpyridine and
hydrolyzed with water to give azido acid 17. However,
this reaction was accompanied by some epimerization
of C-1% and the product contained ca. 30% of the
a-anomer. After ion-exchange chromatography the
anomeric mixture of 17 was obtained in 65% yield. The
subsequent hydrogenation in methanol was monitored
by thin-layer chromatography which showed that
aminoacid 4 was the main product, accompanied by ca.
38% of phostonic acids 5a/5b. After ion-exchange chro-
matography the latter mixture was separated into the
anomers by HPLC on a C8 column. Cyclization of 4
−
Sephadex (HCO3 form) and eluted with a gradient of
water/triethylammonium bicarbonate buffer. Subse-
quent hydrogenation of 12 with H2 in methanol in the
presence of Pd/charcoal gave the expected aminophos-
phonic acid 1 and, surprisingly, phostonic acid 2, a
cAMP analog that had been prepared in a different
way by Moffatt in 10% yield.6 Aminoacid 1 was
cyclized with 1-(3-dimethylaminopropyl)-3-ethylcar-
bodiimide (‘WSC’, 20-fold excess) to give the cAMP
analog 3, in which the 3%- and 5%-oxygens are replaced
by carbon and nitrogen, respectively. The 31P NMR
spectrum of the crude product, however, also contained
a foreign signal at l=+43 ppm characteristic of a
five-membered ring, which indicated7 that some cycliza-
tion involving the 2%-OH group had also occurred to
give phostonic acid 13.
Scheme 3.

M. Morr et al. / Tetrahedron Letters 42 (2001) 8841–8843
8843
with WSC gave a 70% yield of cyclic 3%,5%-aminophos-
phonate 6.
2: 1H NMR [400 MHz, D2O, l(HOD)=4.80)]: l=8.20,
8.19 (both s; H-2, H-8), 6.14 (d; H-1%), 4.58 (br. d;
H-2%), 4.47–4.36 (m; H-5%), 4.21–4.12 (m; H-4% and
A recent paper by Reardon et al.9 on the reduction of
3%-azido-3%-deoxythymidine (AZT) and of AZT-5%-
monophosphate with dithiothreitol (DTT), which gave,
H-5% ), 2.61 (br. m; H-3%), a2.01 (ddd; PCHa), 1.82 (dt;
b
PCHb); J(1%,2%)=1.3, J(3%,PCHa)=3.2, J(3%,PCHb)=
13.5, J(PCHa,PCHb)=(−)13.9, J(PCHa,P)=(−)17.0,
J(PCHb,P)=(−)15.9. 13C NMR [101 MHz, D2O, l(int.
Dioxan)=67.4]: 156.3 (Cq; C-6), 153.5 (CH; C-2), 148.9
(Cq; C-4), 140.1 (CH; C-8), 119.5 (Cq; C-5), 93.3 (CH;
C-1%), 78.2 (CH; d, JPC=4.4 Hz; C-4%), 76.9 (CH; d,
among other products, 9% of
D-threo-thymidine 3%,5%-
cyclic monophosphate, led us to apply DTT for the
reduction of our above azidophosphonates. Both in the
reduction of 12 and in that of 17 at neutral pH with a
20-fold excess of DTT we obtained the phostonic acids
2 and 5, respectively, as the main products (over 50%
yield). The reduction of 5%-azido-5%-deoxyadenosine 3%-
phosphate did not give cAMP, neither catalytically nor
with DTT. The formation of phostonic acids 2 and 5 as
opposed to cAMP could be favored by the conforma-
tion of the PꢀC moiety at C-3% (gauche–anti ), which
differs from that of a 3%-phosphate group.3 The leaving
group is possibly the partially hydrogenated azido
group (ꢀNHꢀNꢁNH) as suggested by Reardon et al.9
J
PC=14.7 Hz; C-2%), 67.8 (CH2; d, JPC=5.8 Hz; C-5%),
45.0 (CH; d, JPC=5.5 Hz; C-3%), 23.3 (CH2; d, JPC
=
122.5 Hz; PCH2). 31P NMR [162 MHz, D2O, ext.
H3PO4]: l=21.9. HRMS (ESI): m/z=326.0689 (M−
H)−, calcd for C11H13N5O5P 326.0655.
3: 1H NMR [300 MHz, D2O, l(HOD)=4.80)]: l=8.26,
8.25 (both s; H-2, H-8), 6.14 (d; H-1%), 4.57 (d; H-2%),
4.02 (td; H-4%), 3.47 (ddd; H-5%), 3.16 (td; H-5% ), 2.55
a
(br. m, SJ=37.8 Hz; H-3%), 1.96 (ddd; PCHab), 1.77
(‘q’; PCHb); J(1%,2%)=1.5, J(2%,3%)=4.4, J(3%,4%)=11.2,
J(3%, PCHa)=3.1, J(3%,PCHb)=13.4, J(PCHa,PCHb)=
(−)13.7, J(PCHa,P)=(−)18.3, J(PCHb,P)=(−)13.4,
J(4%,5%)=4.4, J(4%,5% )=11.0, J(5%,5% )=(−)12.5, J(5%,P)=
The structures of all new compounds were verified by
1
the usual spectroscopic techniques, in particular by H,
13C and 31P NMR spectroscopy. Characteristic features
of the cyclized compounds 2 and 3 are the spin–spin
coupling between 31P and the protons at C-5% in 3 (not
visible in 2 because of the complexity of the spectrum
a
b
a
23.0, J(5% ,P)=1.6 Hz. 13C NMR [b75 MHz, D2O, al(int.
b
Dioxan)=67.4]: 155.9 (Cq; C-6), 153.1 (CH; C-2), 148.6
(Cq; C-4), 139.7 (CH; C-8), 119.2 (Cq; C-5), 92.1 (CH;
C-1%), 80.2 (CH; d, JPC=5.2 Hz; C-4%), 77.0 (CH; d,
due to the coincidence of the H-4% and H-5% chemical
b
shifts) and the coupling between 31P and C-5% in 2 and
3. In the comparison of the 13C NMR spectra of 2 and
3 (phostonic acid versus phostamic acid) the replace-
ment of O by NH is indicated by the shielding of C-5%
(l=44.5 in 3 versus 67.8 in 2), by the shielding of the
protons at C-5% (l=4.5–4.1 in 3 versus 3.47 and 3.16 in
J
PC=13.5 Hz; C-2%), 44.9 (CH; d, JPC=4.5 Hz; C-3%),
44.4 (CH2; d, JPC=2.0 Hz; C-5%), 25.6 (CH2; d, JPC
=
109.4 Hz; PCH2). 31P NMR [121 MHz, D2O, ext.
H3PO4]: l=23.7. HRMS (ESI): m/z=325.0821 (M−
H)−, calcd for C11H14N6O4P 325.0814.
1
2), and by the large decrease of JPC (109.4 Hz in 3
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1: 1H NMR [400 MHz, D2O, pD=8.6, l(HOD)=
4.80)]: l=8.37, 8.29 (both s; H-2, H-8), 6.18 (d; H-1%),
4.95 (dd; H-2%), 4.42 (ddd; H-4%), 3.59 (dd; H-5%), 3.44
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PCHb); J(1%,2%)=1.9, J(2%,3%)=5.9, J(3%,P)=8.7, J(3%,
4%)=9.8, J(3%,PCHa)=8.8, J(3%,PCHb)=5.8, J(PCHa,
PCHb)=(−)14.9, J(PCHa,P)=(−)16.7, J(PCHb,P)=
(−)17.4, J(4%,5%)=3.3, J(4%,5% )=6.4, J(5%,5% )=(−)13.8Hz.
a
b
13C NMR [75 MHz, D2O, pD=8.6, al(bint. Dioxan)=
67.4]: 156.0 (Cq; C-6), 153.1 (CH; C-2), 148.9 (Cq; C-4),
140.5 (CH; C-8), 119.4 (Cq; C-5), 91.0 (CH; C-1%), 82.5
(CH; d, JPC=14.3 Hz; C-4%), 77.5 (CH; d, JPC=5.2 Hz;
C-2%), 41.9 (CH2; C-5%), 41.8 (CH; d, JPC=3.1 Hz; C-3%),
25.1 (CH2; d, JPC=127.9 Hz; PCH2). 31P NMR [162
MHz, D2O, ext. H3PO4]: l=19.1. HRMS (ESI): m/z=
343.0935 (M−H)−, calcd for C11H16N6O5P 343.0920.