New and efficient approach to aryl phosphoramidate derivatives of AZT/d4T as anti-HIV prodrugs

Article abstract of DOI:10.1055/s-2004-834800

Ester exchange of diaryl phosphite with AZT or d4T produced aryl AZT/d4T H-phosphonate, and the following reaction with amino acid esters in the presence of hexachloroethane and triethylamine yielded membrane-soluble anti-HIV prodrugs aryl phosphoramide d

Full text of DOI:10.1055/s-2004-834800

2600
LETTER
New and Efficient Approach to Aryl Phosphoramidate Derivatives of
AZT/d4T as Anti-HIV Prodrugs
Approach to Aryl
P
hosphorami
e
date
D
eriva
s
tives of
A
h
ZT/d4T u Li,a Hua Fu,*a Yuyang Jiang,a,b Yufen Zhao,a Jinyong Liua
a
The Key Laboratory of Bioorganic Phosphorus Chemistry and Chemical Biology, Ministry of Education, Department of Chemistry,
Tsinghua University, Beijing 100084, P. R. China
b
The Key Lab of Chemical Biology, Guangdong Province, College of Shenzhen, Tsinghua University, Shenzhen 518057, P. R. China
Fax +86(10)62781695; E-mail: fuhua@mail.tsinghua.edu.cn
Received 18 August 2004
We realize that Atherton–Todd reaction is an efficient
method for the construction of P-N bond from phosphite-
containing P-H bond, however, to our knowledge, it was
only used for synthesis of dialkoxylphosphoramidates
from H-dialkoxylphosphonates, there are no reports about
preparation of aryl phosphoramidates from aryl H-phos-
phonates. We attempted the following procedure: ester-
change reaction of diphenyl phosphite with AZT pro-
duced phenyl AZT 5¢-H-phosphonate diester, and the
following Atherton–Todd reaction with phenylalanine
methyl ester in the presence of tetrachloromethane and
triethylamine only yielded trace amount of phenyl meth-
oxyphenylalaninyl phosphate of AZT. When hexachloro-
ethane replaced tetrachloromethane in the Atherton–Todd
reaction above, a target product was obtained in good
yield. The result encouraged us to develop a general meth-
od for synthesis of aryl phosphoramide derivatives of
AZT and d4T.
Abstract: Ester exchange of diaryl phosphite with AZT or d4T pro-
duced aryl AZT/d4T H-phosphonate, and the following reaction
with amino acid esters in the presence of hexachloroethane and tri-
ethylamine yielded membrane-soluble anti-HIV prodrugs aryl
phosphoramide derivatives of AZT and d4T in good yields.
Key words: aryl phosphoramidate, anti-HIV, AZT, d4T
Some dideoxynucleosides (ddNs) have emerged as effi-
cient drugs against human immunodeficiency virus
(HIV),1–8 and the anti-retroviral effects of these com-
pounds involve their conversion, through cellular en-
zymes, to the corresponding 5¢-triphosphates (ddNTPs)
which interact with HIV-associated reverse transcriptase.
The ddNTPs may act as competitive inhibitors, preventing
the incorporation of the natural substrates (dNTPs), or as
alternate substrates incorporated in the growing DNA
chain, leading to termination of the newly synthesized vi-
ral nucleic acid.9 However, in many cases the unnatural
ddNs have poor affinity for nucleoside kinases,10 one pos-
sibility to improve the efficiency of ddNs could be to by-
pass the phosphorylation steps. Unfortunately, these polar
nucleotides are not able to cross the cell membrane effi-
ciently,11 and they are readily dephosphorylated in extra-
cellular fluids and on cell surfaces by nonspecific
phosphohydrolases.12–15 Hence, the strategies of tempo-
rarily masking or reducing the phosphate negative charges
of nucleoside 5¢-monophosphates (NMPs) with neutral
substituents to prodrugs are used, these prodrugs would be
freed to the corresponding NMPs once inside the cell.
McGuigan et al. have designed and synthesized aryl phos-
phoramidate derivatives of AZT and d4T which show
good efficacy against HIV.16–18 The typical synthesis of
the phosphoramidate derivatives is from sequential reac-
tions of phosphoryl chloride with phenol or p-substituted
phenol, amino acid methyl ester and nucleoside,16 howev-
er, the method usually needs a long reaction time and ab-
solute water-free condition, and sometimes yields are very
low. So it is necessary to develop a simple, rapid and gen-
eral approach to aryl phosphoramidate derivatives.
Et3N
PCl3
+
X
OH
3
P(OAr)3
THF
2
1
3
X = H (a), Cl (b)
O
O
ArO
3
P OAr
HO
P
OH
+
2
P(OAr)3
H
3
H
5
4
Scheme 1 Synthetic route of diaryl phosphites
The synthetic route of diaryl phosphite is shown in
Scheme 1 according to known procedures.19,20 Reaction
of 3.0 equivalents of phenol or p-chlorophenol with
phosphonyl chloride in THF at room temperature almost
quantitatively yielded triphenyl or tri-p-chlorophenyl
phosphite in the presence of triethylamine. After two
hours, triethylamine hydrochloride was filtered, the solu-
tion was concentrated by rotary evaporation, and the cor-
responding triphenyl phosphite was obtained. The amount
of 0.5 equivalents of phosphoric acid was added to triaryl
phosphite, the mixture was heated at 70 °C for one hour
and at 150 °C for three hours. The diaryl phosphite was
quantitatively transferred, and it could be used for the fol-
lowing reaction without any purification. AZT or d4T in
dry THF was added dropwise to 1.5 equivalents of diaryl
SYNLETT 2004, No. 14, pp 2600–2602
2
2
.1
1
.2
0
0
4
Advanced online publication: 20.10.2004
DOI: 10.1055/s-2004-834800; Art ID: U23404ST
© Georg Thieme Verlag Stuttgart · New York

LETTER
Approach to Aryl Phosphoramidate Derivatives of AZT/d4T
2601
O
O
N
NH
O
P
O
N
NH
O
P
NH
N
O
O
O
ArO
R
HO
O
O
O
ArO
ArO
O
NH
CH
O
H
N3
C2Cl6, Et3N
CHCOOCH3
O
P
8
N3
COOCH3
6
O
N
N3
ArO
OAr
H2N
O
N
or
O
or
O
O
N
NH
R
H
NH
NH
or
Yield: 70–78%
O
5
O
ArO
R
P
O
P
O
O
O
HO
O
NH
CH
O
H
9
7
Yield: 92–98%
COOCH3
R = H (c), CH(CH3)2 (d),
CH2Ph (e)
Scheme 2 Synthetic route of aryl phosphoramidate derivatives of AZT and d4T
phosphite and a catalytic amount of triethylamine (about Todd reaction using tetrachloromethane as chlorination
0.2 equiv) in dry THF at –5 °C. The ester-change reaction reagent.22 Aryl nucleoside 5¢-H-phosphonate diester
of diaryl phosphite with nucleoside completed within 20 could transferred into tricoordinated phosphite (10) in the
minutes. Amino acid methyl ester, triethylamine and presence of triethylamine, reaction of 10 with hexachloro-
CH2Cl2 solution of hexachloroethane were sequentially ethane formed pentacoordinated phosphane intermediate
added to the reaction solution above, and the Atherton– 11, and then 11 changed into the corresponding chloro-
Todd reaction produced the target products, aryl phos- phosphate 12 with releasing tetrachloroethylene and tri-
phoramides derivatives of AZT and d4T, in good yields ethylamine hydrochloride. The following reaction of 12
as shown in Table 1. The synthetic route to the target with amino acid methyl ester yielded aryl phosphorami-
products is shown in Scheme 2, and their structure was date derivatives of AZT/d4T.
identified by 31P NMR, 1H NMR, 13C NMR and ESI-MS21
after isolation by silica gel column chromatography using
EtOAc–MeOH (40:1) as eluent.
+
–
OHNEt3
+
–
O
ONu
Et3NHO
C2Cl6
ONu
P
Et3N
ArO
11
P
H
ArO
ONu
P
CCl2
Table 1 31P NMR and Yields of the Synthesized Compounds
ArO
Cl
10
CCl2
6 or 7
Com- Ar
pound
NuOH
R
31P NMR
(d/ppm)
Yield
(%)a
Cl
.
-
Et3NHCl
O
O
8ac
8ad
8ae
9ac
9ad
9ae
8bc
8bd
8be
9bc
9bd
9be
Ph
AZT
AZT
AZT
d4T
H
4.11, 4.34
4.29, 4.56
3.99, 4.13
4.37, 4.98
4.23, 4.77
3.30, 3.70
4.63, 4.99
4.49, 4.75
3.45, 3.89
4.31, 4.72
4.42, 4.90
3.36, 3.91
64
72
67
65
71
67
63
68
60
69
67
72
NH2R
-HCl
-
C2Cl4
ArO
ONu
P
P
ArO
ONu
Ph
iso-Propyl
Benzyl
H
HN
R
Cl
12
8 or 9
Ph
Scheme 3 Proposed formation mechanism of aryl phosphoramidate
Ph
derivatives
Ph
d4T
iso-Propyl
Benzyl
H
In summary, ester-change reaction of diaryl phosphite
with AZT or d4T in THF–CH2Cl2 almost quantitatively
yielded asymmetric nucleoside H-phosphonate diesters,
the following Atherton–Todd reaction using hexachloro-
ethane as chlorination agent produced aryl phosphorami-
date derivatives of AZT/d4T in good yields under mild
conditions. The method is rapid, convenient and efficient,
and it could be used for preparation of other phosphorami-
dates.
Ph
d4T
p-Cl-Ph
p-Cl-Ph
p-Cl-Ph
p-Cl-Ph
p-Cl-Ph
p-Cl-Ph
AZT
AZT
AZT
d4T
iso-Propyl
Benzyl
H
d4T
iso-Propyl
Benzyl
d4T
a Isolated yield of the last two steps.
General Procedure for Preparation of Aryl Phosphoramidate
Derivatives.
Diaryl phosphite (1.5 mmol; about 85%, 0.41 g) and a catalytic
amount of Et3N (about 3 drops) were dissolved in 5 mL of dry THF
and cooled to –5 °C. AZT (1.0 mmol, 0.27 g) in 5 mL of dry THF
was added dropwise to the solution at this temperature under the
A possible formation mechanism of aryl phosphoramidate
derivatives of AZT/d4T was proposed in Scheme 3, and it
could be similar to the pathway of the classical Atherton–
Synlett 2004, No. 14, 2600–2602 © Thieme Stuttgart · New York

2602
X. Li et al.
LETTER
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warmed from –5 °C to r.t. during this process. At –5 °C, 1.1 mmol
of amino acid methyl ester hydrochloride, Et3N (TEA, 0.31 g, 3.0
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CHCl3, washed with sat. NaHCO3 solution (10 mL), 1 M HCl (10
mL) and distilled H2O (2 × 15 mL). The organic phase was dried
over anhyd Na2SO4, and the solvent was removed under reduced
pressure. The residue was purified by column chromatography on
silica gel with EtOAc–MeOH (40:1), the solvent was evaporated by
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The authors would like to thank the financial supports from the
Excellent Dissertation Foundations by the Chinese Ministry of
Education (No. 200222) and the Excellent Young Teacher Program
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Synlett 2004, No. 14, 2600–2602 © Thieme Stuttgart · New York