Intramolecular monodealkylation during the attempted synthesis of diethylphosphonoacetohydroxamic acid

Article abstract of DOI:10.1002/hc.10082

The reaction of diethyl methyl phosphonoacetate (1) with hydroxylamine in NaOH solution resulted in the loss of one of the phosphorus ethyl groups, and yielded monoethylphosphonoaceto-hydroxamic acid (2) as the major product (79%) and diethylphosphonoacetic acid (3) as the minor product (21%). A series of control experiments were carried out to elucidate the sequence of the reactions leading to 2. When the reaction of 1 with NH₂OH was carried out in NaHCO₃ solution, a transient product 4 was also observed, which slowly transformed to 2. Compound 4 was assigned the structure diethylphos-phonoacetohydroxamic acid. There was no dealkylation observed at the phosphorus when 1 was reacted with methoxylamine or when O-methyl diethylphosphonoacetohydroxamate (7) was placed in alkaline solution. The dealkylation at phosphorus was interpreted in terms of intramolecular nucleophilic catalysis by the hydroxamic OH group attacking the phosphorus in 4, involving cyclic 1, 2,5-oxazaphospholidine intermediates.

Full text of DOI:10.1002/hc.10082

Heteroatom Chemistry
Volume 14, Number 1, 2003
Intramolecular Monodealkylation
During the Attempted Synthesis of
Diethylphosphonoacetohydroxamic Acid
Ravit Chen and Eli Breuer
Department of Medicinal Chemistry, The School of Pharmacy, The Hebrew University of Jerusalem,
P.O. Box 12065, 91120 Jerusalem, Israel
Received 2 February 2002; revised 15 April 2002
INTRODUCTION
ABSTRACT: The reaction of diethyl methyl phos-
phonoacetate (1) with hydroxylamine in NaOH so-
lution resulted in the loss of one of the phosphorus
ethyl groups, and yielded monoethylphosphonoaceto-
hydroxamic acid (2) as the major product (79%) and
diethylphosphonoacetic acid (3) as the minor product
(21%). A series of control experiments were carried
out to elucidate the sequence of the reactions lead-
ing to 2. When the reaction of 1 with NH2OH was
carried out in NaHCO3 solution, a transient product
4 was also observed, which slowly transformed to 2.
Compound 4 was assigned the structure diethylphos-
phonoacetohydroxamic acid. There was no dealkyla-
tion observed at the phosphorus when 1 was reacted
with methoxylamine or when O-methyl diethylphos-
phonoacetohydroxamate (7) was placed in alkaline so-
lution. The dealkylation at phosphorus was interpreted
in terms of intramolecular nucleophilic catalysis by the
hydroxamic OH group attacking the phosphorus in 4,
involving cyclic 1,2,5-oxazaphospholidine intermedi-
Hydroxamic derivatives have been attracting consid-
erable interest in recent years due to the inhibitory
effect they possess on several enzymes of consider-
able medical significance such as aminopeptidases
[1], lipoxygenases [2], angiotensin converting en-
zyme [3], endothelin converting enzyme [4], and
metalloproteinases [5]. In addition, there exist re-
ports on hydroxamic acids having antimalarial [6]
and anticancer [7] activity as well as synergism with
some anti-AIDS drugs [8]. In contrast to the multi-
tude of existing hydroxamic acids of various struc-
tures, there are only a few reports on molecules con-
taining hydroxamic groups along with phosphorus
containing functions [9].
In the course of our explorations of the chemical
and biological properties of phosphorus containing
hydroxylamine derivatives, such as oxyiminophos-
phonates [10] and phosphonohydroxamates, [11],
which may serve as biological chelating agents, we
turned our attention to phosphonoacetohydroxamic
acid derivatives. Since phosphonoacetate esters are
easily available via Arbuzov reactions of the corre-
sponding ꢀ-bromoacetates with trialkyl phosphites,
we considered it reasonable that reactions of the
phosphonoacetates with hydroxylamine, followed by
TMSBr-mediated dealkylation would constitute a
convenient approach to these compounds.
ꢀ
C
ates.
2003 Wiley Periodicals, Inc. Heteroatom Chem
14:67–71, 2003; Published online in Wiley InterScience
(www.interscience.wiley.com). DOI 10.1002/hc.10082
This work was presented at the 13th International Conference
on Phosphorus Chemistry in 1995 in Jerusalem; also published in
Phosphorus, Sulfur and Silicon, 1996, 111, 797.
Correspondence to: Eli Breuer; e-mail: breuer@cc.huji.ac.il.
Contract grant sponsor: German–Israeli Foundation for Scien-
tific Research.
RESULTS AND DISCUSSION
Present address of Ravit Chen: Department of Organic Chem-
istry, Israel Institute for Biological Research, P.O. Box 19, 74100
As a model experiment, we reacted diethyl methyl
phosphonoacetate (1) with hydroxylamine in
Ness Ziona, Israel.
c
ꢀ
2003 Wiley Periodicals, Inc.
67

68 Chen and Breuer
TABLE 1 Composition of the Reaction Mixture of 1 +
NH2OH/NaHCO3/H2O as Function of Time
Composition/Time
Compound
18 h
48 h
72 h
1 (δP = 23.1)
4 (δP = 26.2)
2 (δP = 16.9)
3 (δP = 27.3)
7%
22%
37%
34%
0%
7%
59%
33%
0%
0%
65%
35%
FIGURE 1 Structures of compounds 4, 5 and 6.
sodium hydroxide solution. Examination of this
reaction mixture by 31P NMR spectroscopy revealed
two products (Scheme 1). The major product (δP =
16.9 (triplet of triplets, 79%)) was identified as mo-
noethylphosphonoacetohydroxamic acid (2) and the
minor product (δP = 27.3 (triplet of quintets, 21%))
was identified as diethylphosphonoacetic acid (3).
The isolation of 2 indicated that in addition to the
expected carboxylate ester to hydroxamic acid trans-
formation, unexpected monodealkylation had taken
place at the phosphonic site, while carboxylic acid
3 clearly resulted from the hydrolysis of the carboxy-
late ester. Since, in 3, both P-ethoxy groups remained
intact, it seems that the loss of one P-OEt group in
the course of the formation of 2 cannot be explained
by a simple base-catalyzed phosphonate ester hy-
drolysis. This article describes a series of simple
experiments that were carried out to elucidate the
sequence of events leading to product 2.
First, we repeated the reaction of 1 with hydrox-
ylamine in a weaker base, namely sodium bicarbon-
ate solution (control experiment no. 1 in the Ex-
perimental Section). Monitoring of this reaction by
means of 31P NMR spectroscopy revealed a transient
product (δP = 26.2, Table 1), in addition to 2 and 3
described previously. The structure of this product
was assigned as diethylphosphonoacetohydroxamic
acid (4, Fig. 1) on the basis of its chemical shift in
the 31P NMR spectrum resembling that of the corre-
sponding O-methylhydroxamate diester (7). This as-
signment is also supported by the subsequent mono
de-ethylation of 4 to 2, as it is apparent from the in-
crease in the percentage of the latter at the expense
of the former at 48 and 72 h (Table 1).
To clarify the role of the base in the reactions of
1, we compared the effects of two alkaline condi-
tions on 1, without NH2OH (control experiment no. 2
in the Experimental Section). The results, listed in
Table 2, show that under weak basic conditions (5%
NaHCO3), the hydrolysis occurs only at the carboxy-
late ester with the formation of 3. We have not ob-
served monoethylphosphonoacetic acid (5, Fig. 1).
The latter was formed only at higher pH (1N NaOH).
Compound 5 was identified by comparison with
an authentic sample that we synthesized. We have
not observed at any point the third possible prod-
uct, namely methyl monoethylphosphonoacetate (6,
Fig. 1), which therefore, cannot be considered as
an intermediate in the formation of 2. These results
show that the phosphorus monodealkylation is not ob-
served, under comparable conditions, without the si-
multaneous formation of the hydroxamic function.
We examined the effect of added hydroxylamine
in basic solutions of diethylphosphonoacetic acid
(3) by monitoring the solutions by 31P NMR spec-
troscopy, as described in the Experimental Section in
control experiments no. 3. There was a slow forma-
tion (∼70% in 24 h, as in the previous experiment) of
monoethylphosphonoacetic acid (5) as the sole prod-
uct in NaOH solution, whether accompanied or not
by hydroxylamine. In NaHCO3 solution, there was no
formation of 5 observed after 240 h, whether in the
presence or in the absence of hydroxylamine.
To test the assumption that the hydroxy group
of the hydroxamic function plays a role in the
We carried out a series of further experiments
to determine the roles of the base and of hydroxy-
lamine in the phosphonate de-ethylation reaction.
TABLE 2 The Decomposition of Diethyl Methyl Phospho-
noacetate (1) in the Presence of Base as Function of Time
Time
(h)
Compound
Compound
Base
3a (%)
5b (%)
5% NaHCO3
18
48
72
1
24
72
30
50
70
100
26
0
0
0
0
0
74
100
1N NaOH
aδP = 27.3.
bδP = 19.3.
SCHEME 1 The reaction of diethyl methyl phosphono-
acetate with hydroxylamine.

Intramolecular Monodealkylation During the Attempted Synthesis of Diethylphosphonoacetohydroxamic Acid 69
monodealkylation, we synthesized O-methyl diethyl-
As mentioned earlier, triester 1 can be converted
to monoester 5 by NaOH but not by NaHCO3. As
the latter is sufficiently basic to ionize the COOH
group, lack of dealkylation by it clearly means that
the NaOH-catalyzed de-ethylation at the phospho-
rus does not result from intramolecular catalysis
by the carboxylate anion. This is not surprising,
considering that such a mechanism would require
the formation of a four-membered cyclic, strained
intermediate.
In summary, the de-ethylation leading to the for-
mation of phosphonomonoester hydroxamic acid
(2), can be accounted for by a mechanism, shown
in Scheme 3, which consists of (a) the initial forma-
tion of diethylphosphonoacetohydroxamic acid (4),
(b) followed by intramolecular attack of the anion
of the hydroxamic OH on the phosphorus to yield
an oxazaphospholidine phosphorane intermediate 8,
(c) departure of an ethoxy group to give the five-
membered cyclic phosphonate 9, and (d), which then
undergoes ring-opening through the second hetero-
cyclic phosphorane 10 to the final product.
phosphonoacetohydroxamate (7) by reacting di-
ethylphosphonoacetyl chloride with methoxylamine
(Scheme 2). This compound was obtained as a sta-
ble oil, showing two signals δP = 22.03 and 20.2 (∼70
and ∼30%, respectively), presumably because of the
presence of two rotamers, observable as a result of
the high barrier of rotation around the amide C N
bond. O-methyl hydroxamate (7) underwent no visible
dealkylation at the phosphorus. In addition to the syn-
thesis of 7, we also carried out an experiment to react
1 with methoxylamine (control experiment no. 4 in
the Experimental Section). In this experiment (data
not shown), there was initial formation of 3 and 7, in
the approximate ratio of 3:1. After about 2 months at
ambient temperature, all of O-methyl hydroxamate
(7) was eventually converted to 3 as the sole product.
The results hitherto presented indicate that the
hydroxamic function has a catalytic role in the de-
ethylation at phosphorus. To observe whether this
can occur intermolecularly, an experiment was car-
ried out in which O-methyl hydroxamate (7) was
allowed to react with benzohydroxamic acid in
NaHCO3 solution. After 72 h, only the hydrolysis of
the O-methyl hydroxamate to carboxylate 3 was ob-
served to the extent of about 45%. This experiment
indicates that the hydroxamic group does not catalyze
an intermolecular dealkylation reaction at a noticeable
rate under comparable conditions.
The results presented here are consistent with an
intramolecular nucleophilic catalysis mechanism, in
which the anion of the hydroxamic OH group at-
tacks the phosphorus intramolecularly. This mecha-
nism is not refuted by the negative result of the con-
trol experiment employing benzohydroxamic acid,
in which catalysis can only be intermolecular and
the molecules need to be brought together by ran-
dom collisions in order to react. Intramolecular re-
actions, in contrast, are normally faster than their
intermolecular analogs because the reacting groups
are closer together from the start, provided that their
interaction is not prevented by steric or strain fac-
tors. The operation of such intramolecular cataly-
sis by aliphatic and phenolic OH groups has been
demonstrated in phosphate diesters and triesters in
recent years [12].
EXPERIMENTAL
NMR spectra were recorded on a Varian VXR-300S
instrument. The spectra were referenced to SiMe4 as
an internal standard (1H) and to H3PO4 in D2O as
an external standard (31P). Positive chemical shifts
are downfield from that of the reference. All spectra
were taken using a repetition time sufficiently long
1
for complete relaxation. Broad band H decoupling
did not affect 31P signal integration, as confirmed by
SCHEME 2 Synthesis of O-methyl diethylphosphonoaceto-
hydroxamate.
SCHEME 3 Mechanism proposed for the formation of
compound 2.

70 Chen and Breuer
comparing the integration of decoupled to that of
coupled signals.
order to precipitate Et3N·HCl, which was filtered off.
The evaporation of the solvent from the filtrate gave
the product as an oil, 1.1 g, 90%.
NMR (D2O): 1H: δ = 1.2 (t, 6H, 3 JHH = 6.9 Hz), 2.8
(d, 2H, 2 JPH = 21.3 Hz), 3.62 and 3.65 (2 singlets, ∼80
and ∼20%, respectively, 3H), 4.0 (dq, 2H, 3 JHH = 7.1
Disodium Ethylphosphonoacetohydroxamate(2)
To a mixture of a solution of methyl diethyl phos-
phonoacetate (1, 1.84 ml, 10 mmol) in doubly dis-
tilled water (10 ml) and a solution of hydroxy-
lamine hydrochloride (0.85 g, 12 mmol) in doubly
distilled water (10 ml) was added dropwise a 4 N
NaOH solution until the pH remained constant at
12 (about 5 ml). The reaction mixture was stirred
at ambient temperature for 24 h. 31P NMR spec-
troscopy of the solution showed two signals: δP =
16.9 (triplet of triplets, 79%) and δP = 27.3 (triplet
of quintets, 21%). The solution was concentrated
to about half of its volume, and the residue was
treated with 2-PrOH (50 ml) causing precipitation
of the main product, which was filtered off and
3
Hz, JPH = 8.1 Hz). 31P: (δ): 24.1 (m, ∼25%); 23.3
(m, ∼75%). IR (Neat): ν = 3470w, 3183m, 2984m,
1674s, 1516m, 1467m, 1328w, 1244s, 1162m, 1040s,
974m cm−1. MS-FAB Found: [M]+ = 225.3, calcd.
MW = 225.18.
Monoethylphosphonoacetic Acid (5)
To a solution of methyl diethyl phosphonoacetate
(1.0 ml, 5.4 mmol) in distilled water (10 ml) was
added dropwise a 4 N NaOH solution until the pH
remained constant at 12 (about 5 ml). The reac-
tion mixture was stirred at ambient temperature
for 24 h. Monitoring the reaction mixture by 31P
NMR spectroscopy indicated the decay of the sig-
nal at δ 23.1 ppm, belonging to the starting mate-
rial, and the development of a transient signal: δ
27.3 ppm belonging to diethylphosphonoacetic acid.
The solution was concentrated, and the residue was
treated with MeOH (20 ml) causing precipitation of
the product, which was filtered off and dried. NMR
(D2O): 1H: δ = 1.15 (t, 3H, 3 JHH = 7.2 Hz), 2.55 (d, 2H,
2 JPH = 20.7 Hz), 3.8 (dq, 2H, 3 JHH = 7.2 Hz, 3 JPH = 7.3
Hz). 31P: (δ): 19.3 (tt, 2 JPH = 20.6 Hz, 3 JPH = 7.3 Hz).
1
dried, yielding 1.5 g, 66%. NMR (D2O): H: δ = 1.1
3
2
(t, 3H, JHH = 6.9 Hz), 2.5 (d, 2H, JPH = 19.8 Hz),
3.7 (dq, 2H, JHH = 7.2 Hz, JPH = 7.8 Hz). 31P: (δ):
3
3
2
3
16.5 (tt, JPH = 19.5 Hz, JPH = 7.8 Hz). IR (Nujol):
ν = 3170m, 2900s, 1660m, 1630m, 1560w, 1340w,
1190s, 1040s cm−1. FAB-MS Found: [M + H]+ =
228.2 [M − Na + H]+ = 206.4, calcd MW = 227.06.
Anal calcd for C4H8Na2NO5P: C, 21.14; H, 3.55; N,
6.16. Found: C, 22.16; H, 4.13; N, 6.13.
Diethylphosphonoacetic Acid (3) (Synthesized
According to Malevannaya et al. [13])
Phosphonoacetic Acid
1
3
NMR (D2O): H: δ = 1.19 (t, 6H, JHH = 6.9 Hz), 3.0
Methyl diethyl phosphonoacetate (5 ml, 23.8 mmol)
was refluxed with 21% HCl (100 ml) for 24 h. After
evaporation of the solvent to dryness the residue was
recrystallized from diethyl ether, yielding 2 g, 60%
yield of product, mp 140–143◦C. NMR (D2O): 1H: δ =
(d, 2H, 2 JPH = 21.6 Hz), 4.06 (dq, 4H, 3 JHH = 6.9 Hz,
3 JPH = 7.5 Hz). 31P: (δ): 27.3 (tqu, JPH = 20.5 Hz,
2
3 JPH = 7.8 Hz). IR (Neat): ν = 2988m, 2936m, 2633w,
2361w, 1729s, 1395w, 1241s, 1165w, 1120w, 1026s,
976s cm−1. Anal calcd for C3H13O5P: C, 36.74; H, 6.68.
Found: C, 35.42; H, 6.69.
2
2
2.6 (d, 2H, JPH = 19.5 Hz). 31P: (δ): 15.5 (t, JPH
=
21.7 Hz).
O-Methyl Diethylphosphonoacetohydroxamate
Control Experiment No. 1
To a solution of methoxylamine hydrochloride (0.8 g,
9.5 mmol, freshly dried over P2O5) suspended in dry
CH2Cl2 (45 ml) was added dropwise triethylamine
(1.7 ml, 12 mmol) followed by slow addition of di-
ethylphosphonoacetyl chloride (1.2 g, 5.6 mmol),
causing the immediate appearance of precipitate.
Examination of the reaction mixture by 31P NMR
spectroscopy, after it had been stirred at ambient
temperature for 24 h, revealed the absence of the
starting material’s signal at δ14.75, and two new
signals at δP 22.03 and 20.2 (∼70 and ∼30%, respec-
tively), indicating the formation of the desired prod-
uct as two rotamers. After removal of the solvent,
the residue was treated with ethyl acetate (50 ml) in
To a solution of methyl diethyl phosphonoacetate
(0.2 ml, 1 mmol) in NaHCO3 solution (5%, 5 ml) was
added NH2OH·HCl (69.5 mg, 1 mmol). The solution
was kept at ambient temperature for 72 h and mon-
itored by 31P NMR spectroscopy.
Control Experiments No. 2
Two samples of methyl diethyl phosphonoacetate
(0.2 ml, 1 mmol) were taken. One was dissolved in
NaHCO3 solution (5%, 5 ml) and the other in NaOH
solution (1 N, 5 ml). The solutions were kept at am-
bient temperature by for 72 h and monitored by 31
P
NMR spectroscopy. The results are listed in Table 2.

Intramolecular Monodealkylation During the Attempted Synthesis of Diethylphosphonoacetohydroxamic Acid 71
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(a) Two portions of diethylphosphonoacetic acid
(0.2 g, 1 mmol) were dissolved in NaOH solution
(1 N, 5 ml). To one of the solutions was added
NH2OH·HCl (69.5 mg, 1 mmol). Both solutions
were kept at ambient temperature for 24 h and
monitored by 31P NMR spectroscopy.
(b) Two portions of diethylphosphonoacetic acid
(0.2 g, 1 mmol) were dissolved in NaHCO3 so-
lution (5%, 5 ml). To one of the solutions was
added NH2OH·HCl (69.5 mg, 1 mmol). Both so-
lutions were kept at ambient temperature by for
240 h and monitored by 31P NMR spectroscopy.
Control Experiment No. 4
To a solution of methyl diethyl phosphonoacetate (1,
0.2 ml, 1 mmol) in NaHCO3 solution (5%, 5 ml) was
added NH2OMe·HCl (83.5 mg, 1 mmol). The solution
was kept at ambient temperature by for 60 days and
monitored by 31P NMR spectroscopy.
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Control Experiment No. 5
O-methyl diethylphosphonoacetohydroxamate (7,
23 mg, 0.1 mmol) and benzohydroxamic acid (17 mg,
0.1 mmol) were dissolved in NaHCO3 solution (5%,
2 ml). The solution was kept at ambient tempera-
ture by for 7 days and monitored by 31P NMR spec-
troscopy.
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