Microwave-assisted cleavage of phosphate, phosphonate and phosphoramide esters

Article abstract of DOI:10.1016/j.tetlet.2006.06.126

A mild and rapid protocol for cleavage of phosphate, phosphonate and phosphoramide esters. The scope and limitations of this microwave-assisted reaction is explored here.

Full text of DOI:10.1016/j.tetlet.2006.06.126

Tetrahedron Letters 47 (2006) 6281–6284
Microwave-assisted cleavage of phosphate, phosphonate and
phosphoramide esters
G. D. Kishore Kumar, David Saenz, G. L. Lokesh and Amarnath Natarajan*
Chemical Biology Program, Department of Pharmacology and Toxicology, University of Texas Medical Branch,
Galveston, TX 77555-0650, USA
Received 17 May 2006; accepted 23 June 2006
Available online 18 July 2006
Abstract—A mild and rapid protocol for cleavage of phosphate, phosphonate and phosphoramide esters. The scope and limitations
of this microwave-assisted reaction is explored here.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Phosphorylation of proteins among other things facili-
tates protein–protein interactions that are important
for a variety of cellular functions, including cell divi-
sion.1 Therefore, phosphate mimics viz., phosphonates
and phosphoramides are valuable chemical tools to
probe cellular functions.2 For example, sphingosine-1-
phosphate, is a suspect in a plethora of diseases states.3
Due to the metabolic lability of the phosphate group on
sphingosine-1-phosphate it was difficult to use sphingo-
sine-1-phosphate as a chemical probe.4 The recent avail-
ability of the metabolically resistant phosphonate and
phosphoramide mimics of sphingosine-1-phosphates
has made possible investigations into the role of sphin-
gosine-1-phosphate in the intracellular functions leading
to the disease states.5,6 Phosphate, phosphonate and
phosphoramide functionality are also used in enhancing
the solubility of the drugs and in some cases they
function as prodrugs.7–10 Phosphoramides and phos-
phonates have also been used in the synthesis of a-
aminophosphonic acids and aminoalkylphosphonic
acids, respectively, as surrogates for the corresponding
amino acids in biological systems.11,12 Therefore, a mild
method to introduce these functionalities into small
molecule/peptides will be advantageous, for example;
(1) in developing chemical tools to study phosphoryl-
ation dependent biological interactions, (2) to incorpo-
rate these as solubilizing groups in drug development
and (3) particularly using the phosphoramide groups
to generate prodrugs.
The phosphate, phosphonate and phosphoramide group
are usually carried through multi-step synthesis as their
corresponding esters. The established methodology to
remove the ester protecting groups of phosphonates in-
volves the use of trimethylsilyl bromide (TMSBr) over
long reaction times.13,14 Herein, we describe a micro-
wave-assisted version of this methodology that reduces
the reaction time to minutes and yields pure products
as phosphonic acids upon work-up. In addition, we have
evaluated the scope of this methodology by extending it
to phosphate and phosphoramide esters.
Microwave-assisted organic transformations reduce
chemical reaction times from hours to minutes, improve
reproducibility and result in high yielding reactions.15,16
During the cleavage of diethyl-(3-phenyl-allyl)-phospho-
nate ester by TMSBr, we found that the reaction times
were reduced from 8 h at room temperature to 10 min
at 100 ꢁC in the microwave (CEM-Discover mono-mode
microwave apparatus). This observation led us to inves-
tigate the use of microwave for cleavage of phosphonate
ethyl esters.
The readily available diethyl-(3-phenyl-allyl)-phospho-
nate ester was chosen as the substrate to optimize the
reaction conditions. Dielectric loss (e00) reflects the effi-
ciency of a solvent to convert microwave energy to ther-
mal energy.17 Three solvents with varying e00 were
chosen for the optimization. Table 1 summarizes the
optimization of solvent and temperatures for the cleav-
age reaction. Time course and ratio of reagents required
for completion of reaction in the microwave was moni-
tored by TLC.
*
Corresponding author. Tel.: +1 409 772 9748; fax: +1 409 772
9818; e-mail: amnatara@utmb.edu
0040-4039/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.06.126

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G. D. Kishore Kumar et al. / Tetrahedron Letters 47 (2006) 6281–6284
Table 1. Optimization of reaction conditions for microwave-assisted cleavage of phosphonate esters
O
P
O
OEt
P
OH
2 eq. TMSBr
Microwave
OEt
OH
1
Entry
Solvent
Dielectric loss (e00)
Microwave (W read back)
Temperature (ꢁC)
Time (min)
Yielda (%)
1
2
3
4
5
6
7
8
PhCH3
THF
0.096
0.348
2.325
2.325
2.325
2.325
2.325
2.325
175
250
250
220
100
35
100
100
100
80
60
40
10
10
10
10
10
10
480
400
65
55
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
>95
>95
>95
>95
92
No
No
rt
40
93
a Yield of isolated product.
Pd/C
H2
TMSBr (2 equiv) and a reaction time of 10 min were
required to drive the reaction to completion in the
microwave. The reactions were quenched with a 95:5
methanol–water mixture and concentrated to yield the
final product. The microwave was set to deliver 250 W
of power; however, the read back on the instrument
(see Supporting data) varied with solvents and tempera-
tures (Table columns 2, 4 and 5). Since the reaction pro-
ceeds well at 40 ꢁC and 35 W of power, all further
experiments were carried out at 40 ꢁC with a power set-
ting of 50 W. Based on the isolated yields, it was clear
that acetonitrile was the solvent of choice and the reac-
tion reaches completion at 40 ꢁC. By comparing entries
6–8 (Table 1), it is clear the reaction proceeds faster
under the microwave conditions without compromising
product yields.
O
O
P
P
EtO
OEt
OEt
EtO
( >98%)
2
HO2C
O
P
Pd/C
H2
EtO
OEt
O
O
CbzN
N
CbzN
HN
NH
DIC, HOBt
DMAP, DIPEA
(96%)
P
OEt
(98%)
OEt
3
O
O
O
BnCOCl
TEA 0oC
O
P
N
N
O
P
N
OEt
OEt
(49%)
OEt
5
OEt
4
Scheme 1. Synthesis of phosphonate esters 2 and 5.
To explore the scope of this reaction, a combination of
commercially available and easily accessible starting
phosphonates was used as their corresponding ethyl
esters. Compound 2 was synthesized by catalytic Pd/C
hydrogenation of the corresponding olefin 1.18 The reac-
tion proceeds smoothly and the product was isolated in
a quantitative yield. The starting material 5 was synthe-
sized in three steps starting from readily available Cbz-
protected piperizine. Cbz-piperizine was coupled to
(diethoxy-phosphoryl)-acetic acid using standard pep-
tide coupling procedures to yield 3.19 The Cbz group
on the resulting compound 3 was deprotected by hydro-
genation to yield 4.20 In the final step, the free amine on
the piperizine was reacted with phenylacetyl chloride to
yield the desired phosphonate ester 5.21 Scheme 1 illus-
trates the synthesis of 2 and 5. Compounds in Table 2,
entries 3 and 4 are commercially available and were used
without further purification. Table 2 summarizes the
yields of the cleavage reaction of the phosphonates.
The microwave-assisted reaction is mild as it tolerates
labile functional groups and is a high yielding reaction.
on the substrates. The reactions were monitored for
completion by TLC. Table 3 summarizes the yields of
the ester and the product obtained in these reactions.
Phosphoramides generated from the primary amines un-
dergo cleavage to give the corresponding phosphorami-
dic acids in high yields (Table 3, entries 1, 2, 4 and 5). In
a control experiment, benzyl phosphoramide (Table 3,
entry 4) when treated with TMSBr takes 6 h at 40 ꢁC
in the hood reach completion and the corresponding
acid was isolated in >90% yield. Although the yields in
the hood and the microwave are comparable, it is clear
that the reaction in the microwave is faster.
We then explore the scope and limitations of this micro-
wave-assisted cleavage of esters. Phosphoramide esters
generated from alkyl, propargyl, benzyl and aryl amines
were cleaved rapidly to their corresponding phosphor-
amide in high yields (Table 3; entries 1, 2, 4 and 7). A sub-
strate containing
a benzyl ester and a primary
phosphoramide ester (Table 3, entry 3), when subjected
to microwave-assisted cleavage by TMSBr, yields a mix-
ture of products. The desired product was isolated in
modest yield, however, the mixture includes products
in which the benzyl ester was also cleaved. This suggests
that the compounds containing benzyl esters might not
be stable to the reaction conditions. Also the phosphor-
We then turned our attention to phosphate and phos-
phoramide esters. The phosphate and phosphoramide
esters were prepared in good yields from the correspond-
ing alcohol or amine and diethylchlorophosphate.12 The
reaction proceeds smoothly at 0 ꢁC in 3–6 h depending

G. D. Kishore Kumar et al. / Tetrahedron Letters 47 (2006) 6281–6284
6283
Table 2. Microwave-assisted cleavage of phosphonate esters
ported by the smooth conversion of an aryl phosphate
ester to the corresponding aryl phosphate (Table 3, en-
try 9).
TMSBr (2 eq.)
O
O
P
R
R
CH3CN
P
OH
OEt
OH
OEt
MW, 10min
In conclusion, we have identified a rapid microwave-
assisted method to generate phosphates, phosphonates
and phosphoramides from their corresponding ethyl es-
ters. The cleavage of the esters proceeds faster under
microwave conditions. Although this methodology
works well for phosphonate esters, phosphoramide es-
ters from primary amines and aryl phosphate esters, it
is not suitable for benzyl phosphate ester and phosphor-
amide esters generated from secondary amine. To the
best of our knowledge, this is the first report that de-
scribes the cleavage of phosphoramide esters using
TMSBr.
Entry
R
Yielda (%)
1
2
3
4
2
5
96
94
92
96
–CH2CN
–CH2CH2Br
a Yield of isolated product.
Table 3. Synthesis of phosphate and phosphoramide esters followed
by microwave-assisted cleavage22
O
Cl
P OEt
O
P
O
P
TMSBr
MW
OEt
R
X
OH
R
X
R
X
OEt
Acknowledgements
TEA
OH
OEt
We would like to thank the Parallel Synthesis & Medic-
inal Chemistry Facility UTMB for the CEM-Discover
mono-mode microwave apparatus. A.N. thanks the
University of Texas Medical Branch and the American
Cancer Society (Grant IRG-96-152-07) for funding.
Entry
R
X
Ester
Product
yielda (%)
yielda (%)
1
N
47
94
2
3
N
N
63
51
89
Supplementary data
H
O
27b
Supplementary data associated with this article can
be found, in the online version, at doi:10.1016/j.tetlet.
2006.06.126.
O
4
6
7
N
N
N
68
73
21
93
References and notes
X
76c
96
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cleavage (Table 3, entry 6). In the case of phosphate
esters, cleavage of the benzyl phosphate ester yields
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an SN2 type displacement of the phosphate group by
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General procedure for microwave-assisted cleavage of
phosphate, phosphonate and phosphoramide esters: A sep-
tum-sealed microwave tube charged with phosphate or
phosphonate or phosphoramide esters (1 equiv) and tri-
methylsilylbromide (2 equiv) in CH3CN was irradiated in
a CEM-Discover mono-mode microwave cavity (50 W,
40 ꢁC, 10 min). After completion of the reaction indicated
by TLC, the reaction mixture was quenched with
CH3OH–H2O (95:5) and concentrated to give the corre-
sponding phosphates or phosphonates or phosphoramidic
acids in good yields. Table 2, entry 2. 1H NMR (300 MHz,
DMSO-d6) d: 2.90–2.98 (m, 2H), 3.20–3.40 (m, 8H), 3.70–
3.72 (m, 2H), 7.18–7.26 (m, 5H), 8.73 (br s, 2 · OH). 13C
NMR (75 MHz, DMSO-d6) d: 169.66, 165.02, 136.33,
129.57, 128.94, 126.99, 49.34, 47.10, 42.18, 36.87. 15P
NMR (121 MHz, DMSO-d6) d: 17.35. MS (ESI)
C14H19N2O5P (M+H)+ 327.28. Table 3, entry 1. 1H
NMR (300 MHz, DMSO-d6) d: 1.54–1.75 (m, 6H), 1.73–
1.75 (m, 6H), 2.48–2.49 (m, 3H), 7.74 (br s 2 · OH). 13C
NMR (75 MHz, DMSO-d6) d: 29.16, 35.91, 40.90. 15P
NMR (121 MHz, DMSO-d6) d: 0.90. MS (ESI)
C10H18NO3P (M+H)+ 232.06. Table 3, entry 2. 1H
NMR (300 MHz, CD3OD) d: 3.09 (s, 1H), 3.21 (br s,
1H, NH), 3.62 (m, 2H). 13C NMR (75 MHz, CD3OD) d:
76.52, 75.03, 29.02. 15P NMR (121 MHz, CD3OD) d:
0.904. MS (ESI) C3H6NO3P (M+H)+ 136.62. Table 3,
22. General procedure for synthesis of phosphate and phosphor-
amide esters: Diethyl chlorophosphate (1 equiv) was added
drop wise to a cooled (0 ꢁC) solution of phenol or amine
(1 equiv) and triethylamine (1.2 equiv) in dry CH2Cl2
under nitrogen. The reaction was warm to room temper-
ature and stirring was continued at until completion of
reaction (3–6 h). The reaction mixture was extracted with
1 M HCl and a saturated aqueous solution of NaHCO3.
The organic phase was dried (Na2SO4) and concentrated
in vacuo. The residue was purified by column chromatog-
raphy. Table 3, entry 1 ester. 1H NMR (300 MHz, CDCl3)
d: 1.33 (t, J = 7.2 Hz, 2 · CH3, 6H), 1.59–1.63 ( m, 6H),
1.80–1.81 (m, 6H), 2.05–2.10 (m, 3H), 4.07 (q, J = 7.2 Hz,
2 · CH2, 4H). 13C NMR (75 MHz, CDCl3) d: 62.38, 44.72,
36.43, 30.06, 16.68. 15P NMR (121 MHz, CDCl3) d: 8.01.
MS (ESI) C14H26NO3P (M+H)+ 288.56. Table 3, entry 9
1
entry 4. H NMR (300 MHz, DMSO-d6) d: 4.03 (s, 2H),
7.36–7.45 (m, 5H), 8.17 (br s, 2 · OH). 13C NMR
(75 MHz, DMSO-d6) d: 138.12, 129.50, 129.29, 128.01,
40.35. 15P NMR (121 MHz, DMSO-d6) d: 1.086. MS (ESI)
1
ester. H NMR (300 MHz, CDCl3) d: 1.39 (t, J = 7.2 Hz,
2 · CH3, 6H), 3.95 (s, 3H), 4.28 (q, J = 7.2 Hz, 2 · CH2,
4H), 7.47–7.48 (m, 3H), 9.92 (br s, CHO, 1H). 13C NMR
(75 MHz, CDCl3) d: 190.99, 150.4, 145.3, 132.1, 125.31,
121.60, 111.03, 65.24, 56.42. 15P NMR (121 MHz, CDCl3)
d: ꢀ5.56. MS (ESI) C12H17O6P (M+H)+ 289.63.
1
C7H10NO3P (M+H)+ 188.28. Table 3, entry 7. H NMR
(300 MHz, DMSO-d6) d: 7.26 (d, J = 8.4 Hz, 2H), 7.64 (d,
J = 8.7 Hz, 2H), 9.88 (br s, 2 · OH). 13C NMR (75 MHz,
DMSO-d6) d: 133.38, 133.21, 125.24, 120.22. 15P NMR
(121 MHz, DMSO-d6) d: 0.88. MS (ESI) C6H7BrNO3P
(M+H)+ 253.52. Table 3, entry 9. 1H NMR (300 MHz,
DMSO-d6) d: 3.82 (s, 3H), 7.41–7.48 (m, 3H), 9.86 (s, 1H),
10.2 (br s, 2 · OH). 13C NMR (75 MHz, DMSO-d6) d:
192.29, 151.42, 146.39, 133.26, 124.51, 121.07, 112.37,
56.55. 15P NMR (121 MHz, DMSO-d6) d: ꢀ5.49. MS
(ESI) C8H9O6P (M+H)+ 234.01.
Table 1, entry 5 ester. 1H NMR (300 MHz, CDCl3) d: 1.29
(t, J = 6.9 Hz, 2 · CH3, 6H), 2.99–3.06 (m, 2H), 3.29–3.68
(m, 8H), 3.69–3.72 (m, 2H), 4.09 (q, J = 6.9 Hz, 2 · CH2,
4H), 7.18–7.28 (m, 5H). 13C NMR (75 MHz, CDCl3) d:
169.77, 163.61, 134.71, 129.05, 128.60, 127.18, 63.08,
46.94, 42.10, 41.74, 34.50, 32.94, 16.72. MS (ESI)
C18H27N2O5P (M+H)+ 383.48.