Ethyl 4-(diethoxyphosphinyl)-3-oxobutanoate: Selective synthesis of β-keto phosphonates

Article abstract of DOI:10.1081/SCC-120002016

The selective synthesis of ethyl 4-(diethoxyphosphinyl)-3-oxobutanoate (7) and the utilization of its trimethylsilyl enolether (14) in a tandem desilylation Horner-Wittig condensation is illustrated.

Full text of DOI:10.1081/SCC-120002016

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ETHYL 4-(DIETHOXYPHOSPHINYL)-3-OXOBUTANOATE:
SELECTIVE SYNTHESIS OF β-KETO PHOSPHONATES
a
b
a
Charl du Pisani , David F. Schneider & Pieter C. R. Venter
a
Chemistry Department , University of Stellenbosch , Stellenbosch, 7602, South Africa
b
Chemistry Department , University of Stellenbosch , Stellenbosch, 7602, South Africa
Published online: 16 Aug 2006.
To cite this article: Charl du Pisani , David F. Schneider & Pieter C. R. Venter (2002) ETHYL 4-(DIETHOXYPHOSPHINYL)-3-
OXOBUTANOATE: SELECTIVE SYNTHESIS OF β-KETO PHOSPHONATES, Synthetic Communications: An International Journal
for Rapid Communication of Synthetic Organic Chemistry, 32:2, 305-314, DOI: 10.1081/SCC-120002016
To link to this article: http://dx.doi.org/10.1081/SCC-120002016
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SYNTHETIC COMMUNICATIONS, 32(2), 305–314 (2002)
ETHYL 4-(DIETHOXYPHOSPHINYL)-
3
-OXOBUTANOATE: SELECTIVE
SYNTHESIS OF b-KETO
PHOSPHONATES
Charl du Pisani, David F. Schneider,*
and Pieter C. R. Venter
Chemistry Department, University of Stellenbosch,
Stellenbosch 7602, South Africa
ABSTRACT
The selective synthesis of ethyl 4-(diethoxyphosphinyl)-3-
oxobutanoate (7) and the utilization of its trimethylsilyl
enolether (14) in a tandem desilylation Horner-Wittig conden-
sation is illustrated.
1
–7
b-Keto phosphonates (1) in general are utilized as versatile starting
materials for the synthesis of a wide array of a,b-unsaturated carbonyl
compounds, b-hydroxy derivatives and for liquid–liquid extraction of
metal ions. Consequently, the development of reliable methods for the selec-
tive synthesis of b-keto phosphonates (1), i.e. devoid of the concomitant
5
formation of undesired enol phosphates (2), were actively pursued by
4
various laboratories during recent years.
–7
*
Corresponding author.
3
05
Copyright & 2002 by Marcel Dekker, Inc.
www.dekker.com

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3
06
DU PISANI, SCHNEIDER, AND VENTER
Our laboratory is especially interested in the synthetic potential of
phosphonates (3) derived from 3-oxobutanoates, since the corresponding
phosphinyl stabilized carbanions (4) are ambident nucleophiles that may
react as a-or g-anions with appropriate electrophiles, e.g. a,b-unsaturated
1
,2
carbonyl compounds. Furthermore, successive treatment of the phospho-
nates (3) with sodium hydride and butyllithium easily generates the corres-
ponding dianions (5) which are nucleophiles with substantially enhanced
reactivity, and may be regarded as promising candidates for the annulation
1
–3
of a,b-unsaturated carbonyl compounds.
Direct condensation between triethyl phosphite and ethyl 4-bromo-
-oxobutanoate (6) produces a mixture of the phosphonate (7) (Arbusov
reaction product) and the enol phosphate (8) (Perkow reaction product).
In our hands this condensation yielded a ca 1:1 mixture of the phosphonate
3
8
(
was performed at 110 C.
7) and enol phosphate (8) in a combined yield of 90% when the reaction
ꢀ
1,2

ORDER
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SYNTHESIS OF b-KETO PHOSPHONATES
307
Various methods for the selective synthesis of phosphonates of type (3)
were consequently developed during recent years, but these methods gener-
9
,10
ally necessitate the use of excessive amounts of strong bases
1
implementation of a rather cumbersome deprotection step.
and the
Although
1–13
the masked b-ketophosphonate (9) could be utilized in Horner-Wittig con-
densations, deprotection of the reaction products was still an inevitable
manipulation.
1
1
Although condensation of the 4-halo-3-alkoxy-2-butenoates (10) with
trialkyl phosphates yielded the corresponding phosphonates (11), acid-
catalysed hydrolysis, either before or after Horner-Wittig condensation, is
1
4,15
an essential and critical step in this procedure.
In our endeavour to develop a selective synthesis for the phosphonate
(
7), the dianion of ethyl acetoacetate was treated with diethyl phospho-
1
6
chloridite followed by air oxidation. The phosphonate (7) was indeed
produced selectively under these conditions but a yield of only 35% could
be realized.
In order to achieve a higher yield of (7), eliminate the use of strong
bases but still maintain the desired selectivity, we decided to protect the
carbonyl group of the bromoester (6) as an enolether which must be
stable under Arbusov conditions, but on the other hand be labile
enough to allow selective removal of the protecting group under mild con-
ditions without affecting the ester group. With these prerequisites in
mind we elected to apply the trimethylsilyl group as the protecting
1
7
substituent.

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3
08
DU PISANI, SCHNEIDER, AND VENTER
Silylation of the bromoester (6) with hexamethyldisilazane (HMDS)
1
7
and a catalytic amount of imidazole yielded an 82 : 18 mixture of the
desired (E )-and ( Z )-silyl enolethers (12) and (13) in high yield. The mixture
of enolethers (12) and (13) could be distilled under vacuum but was gener-
ally utilized without the need for further purification.
However, an even higher yield of the silyl enolethers (12) and (13) was
achieved when the catalytic amount of imidazole was omitted from the
reaction mixture. The intermediate formation of 1-(trimethylsilyl)imidazole
1
7,18
as the active silylating agent is, contrary to general belief,
not an essen-
tial element for the successful outcome of these silylation processes. It was
furthermore found that the above mixture of silyl enolethers (12) and (13)
was effectively desilylated at room temperature upon treatment with a sus-
pension of protonated imidazole, or the precipetate that was formed during
the silylation of the bromoester (6) with HMDS/imidazole.
When the mixture of silyl enolethers (12) and (13) was treated with
triethylphosphite, the Arbusov reaction proceeded selectively to yield a
product mixture which consisted of the silylated phosphonate (14, 82%),
the phosphonate (7, 8%) and the enol phosphate (8, 10%) according to
NMR analysis. The formation of small amounts of enol phosphate (8)
and phosphonate (7) indicated that some desilylation of the enol ethers
(
12) and (13) took place under the reaction conditions, while limited desi-
lylation of the phosphonate (14) could also have contributed to the forma-
tion of the phosphonate (7). We eventually found that the silylated
phosphonate (14) can conveniently be synthesized as the sole reaction prod-
uct in a one-pot procedure by successive treatment of the bromoester (6)
with HMDS and triethyl phosphite. The silylated phosphonate (14) could
not be distilled due to extensive desilylation but was sufficiently pure for
further utilization. Removal of HMDS prior to the addition of triethyl
phosphite to the reaction mixture was essential for the successful outcome
of the one-pot procedure, since the Arbusov reaction appeared to be

ORDER
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SYNTHESIS OF b-KETO PHOSPHONATES
309
inhibited by the presence of HMDS. Desilylation of the phosphonate (14)
was almost instantly and cleanly effected by treatment with silica gel.
In order to establish whether the silylated phosphonate (14) could as
such be utilized in a Horner-Wittig condensation, it was consecutively trea-
ted with one molar equivalent each of sodium bis(trimethylsilyl)amide
(
NaHMDS) and benzaldehyde. No Horner-Wittig condensation took
place but complete desilylation of the phosphonate (14) was effected instead.
Horner-Wittig condensation of the silylated phosphonate (14), which was
pretreated with two molar equivalents of NaHMDS, proceeded smoothly to
1
9
yield of 3 : 1 mixture of the keto-and enolesters ( 15) and (16) in good yield.
The desilylation that accompanied the Horner-Wittig condensation most
probably resulted from interaction of the silylated phosphonate (14) with
the first molar equivalent of base to generate the intermediate phosphono-
2
0
enolate (17) and tris(trimethylsilyl)amine (18). This was followed by

ORDER
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3
10
DU PISANI, SCHNEIDER, AND VENTER
deprotonation and Horner-Wittig condensation of the resultant phospho-
nate dianion (19) with the carbonyl compound. The total failure of
the Horner-Wittig condensation when the silylated phosphonate (14) was
treated with one molar equivalent of base is, therefore, not surprising.
To summarise we conclude that our results illustrate that the silylated
phosphonate (14) can now be synthesized in high yield according to a very
convenient one-pot procedure and utilized directly in tandem desilylation/
Horner-Wittig condensations. Desilylation of the phosphonate (14) can be
achieved in a few minutes at room temperature in the presence of silica gel to
produce the phosphonate (7) in quantitative yield.
EXPERIMENTAL
All operations were carried out in an argon atmosphere. Merck silica
gel 60 (particle size 0.063–0.200 mm) was used for column chromatography.
NMR spectra were recorded for solution in deuteriochloroform on a Varian
VXR 300 instrument, while mass spectra were taken on a Varian 311A
spectrometer. Multiplicities were confirmed by APT and SFORD experi-
ments and HETCOR experiments were performed to interpret the more
complex NMR spectra.
1. Condensation of the Dianion of Ethyl Acetoacetate with Diethyl
Phosphochloridite Followed by Air Oxidation: Ethyl acetoacetate (1.81 g,
3
3.9 mmol) in THF (20 cm ) was added dropwise to an oil-free suspension
1
of sodium hydride (0.37 g, 15.4 mmol) in THF (20 cm ) at 0 C and the
3
ꢀ

ORDER
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SYNTHESIS OF b-KETO PHOSPHONATES
311
resulting mixture stirred for an additional 5 min. Butyllithium (16.9 mmol)
ꢀ
in hexane was then added dropwise to the reaction mixture at ꢁ15 C and
stirring was maintained for 30 min before diethyl phosphochloridite (13 g,
20 mmol) was slowly added and the temperature of the reaction mixture was
allowed to reach room temperature during the course of 30 min. Air was
bubbled through the reaction mixture overnight, saturated aqueous ammo-
3
nium chloride (20 cm ) was added followed by extraction with ether. The
residue from the dried (MgSO ) ether extract was chromatographed on silica
4
gel and the ether eluate yielded ethyl 4-(diethoxyphosphinyl )-3-oxobutanoate
ꢀ
1
,2
(
7) (1.13 g, 35%), b.p. 130 C (air-bath temp.)/0.05 mm Hg; dH 1.27 (t, J
7.0 Hz, 3H), 1.31 (t, J 7.0 Hz, 6H), 3.25 (d, JPH 23.0 Hz, 2H), 3.65 (s, 2H),
4.12 (dq, JHH 7.0 and JPH 7.0 Hz, 4H).
2. (E )- and (Z )-Ethyl-4-Bromo-3-trimethylsiloxy-2-butenoate (12) and
13): Ethyl 4-bromo-3-oxobutanoate (6) (10.79 g, 51.6 mmol) was added
(
dropwise to hexamethyldisilazane (HMDS) (28.19 g, 174.0 mmol) at room
temperature and the resulting mixture was stirred for 6 h at room tempera-
3
ture before ether (20 cm ) was added. The mixture was then filtered through
a 0.45 mm Teflon filter via a syringe and the ether and HMDS removed
under reduced pressure at 30 C. The residue consisted of an 82 : 18 mixture
ꢀ
of pure (E )-ethyl 4-bromo-3-trimethylsiloxy-2-butenoate (12) and (Z )-ethyl
4-bromo-3-trimethylsiloxy-2-butenoate (13) in quantitative yield (14.4 g,
99%) which was directly utilized without the need for further purification;
dH (E-12) 0.31 (s, 9H), 1.28 (t, J 7.2 Hz, 3H), 4.15 (s, 2H), 4.16 (q, J 7.0 Hz,
H), 5.16 (s, 1H); d (E-12) 0.70 (SiMe ), 14.30 (CH ), 28.33 (CH Br), 60.0
2
C
3
3
2
(
0
CH ), 101.25 (¼CH), 165.20 and 166.53 (¼C< and >C¼O); d (Z-13)
2
H
.31 (s, 9H), 1.26 (t, J 7.2 Hz, 3H), 3.78 (s, 2H), 4.14 (q, J 7.0 Hz, 2H), 5.39
(
s, 1H); dC (Z-13) 0.70 (SiMe ), 14.30 (CH ), 28.33 (CH Br), 60.0 (CH ),
3
3
2
2
1
01.25 (¼CH), 165.20 and 166.53 (¼C< and >C¼O). Partial desilylation
ꢀ
followed by dimerization took place during distillation at 60 C(air-bath
ꢁ4
temp.)/7.5 ꢂ 10 mm Hg.
3. Condensation of Triethyl Phosphite with a Mixture of (E )- and
Z )-Ethyl 4-Bromo-3-trimethylsiloxy-2-butenoate (12) and (13): Triethyl
(
phosphite (6.21 g, 37.4 mmol) was added dropwise to an 82 : 18 mixture of
E )-and ( Z )-ethyl 4-bromo-3-trimethylsiloxy-2-butenoate (12) and (13) (9.52 g,
(
3
3.9 mmol) at room temperature, the resulting yellow reaction mixture stir-
ꢀ
red for 15 min at room temperature and for an additional 17 h at 50 C,
before the excess of triethyl phosphite was removed and reduced pressure
1
at 30 C. H NMR analysis of the residue established that it consisted of an
ꢀ
1
,2
8
: 10 : 82 mixture of ethyl 4-(diethoxyphosphinyl )-3-oxobutanoate (7)
1
,2
ꢀ
(
(
8%), ethyl 3-(diethoxyphosphinyloxy)-3-butenoate (8) (10%), b.p. 100 C
air-bath temp.)/0.08 mm Hg; dH 1.25 (t, J 7.0 Hz, 3H), 1.33 (dt, JPH 7.0
and JHH 7.0 Hz, 6H), 3.22 (s, 2H), 4.17 (dq, JPH 6.5 and JHH 7.0 Hz, 4H),

ORDER
REPRINTS
3
12
DU PISANI, SCHNEIDER, AND VENTER
4
2
.17 (q, J 7.0 Hz, 2H), 4.69 (dd, JPH 2.3 and JHH 1.8 Hz, 1H), 5.01 (dd, JPH
.4 and JHH 1.8 Hz, 1H) and (E )-ethyl 4-(diethoxyphosphinyl )-3-trimethyl-
siloxy-2-butenoate (14) (82%).
. One-pot Synthesis of (E )-Ethyl 4-(Diethoxyphosphinyl)-3-tri-
methylsiloxy-2-butenoate (14): Ethyl 4-bromo-3-oxobutanoate (6) (1.34 g,
.4 mmol) and HMDS (3.53 g, 21.9 mmol) were stirred for 6 h at room
4
6
3
temperature, the reaction mixture diluted with ether (20 cm ), filtered
through a pad of celite on a sintered glass filter under argon pressure and
the excess of HMDS and ether evaporated under reduced pressure at 30 C.
Triethyl phosphite (1.22 g, 7.3 mmol) was added to the clear residue at room
temperature and the reaction mixture stirred for 15 min at room tempera-
ꢀ
ꢀ
ture and 7.5 h at 55 C. The unreacted triethyl phosphite was removed under
ꢀ
reduced pressure at 30 C and NMR analysis of the residue established that
(
E )-ethyl 4-(diethoxyphosphinyl )-3-trimethylsiloxy-2-butenoate (14) (1.80 g,
8
7
4
1
3%); dH 0.31 (s, 9H), 1.27 (t, J 7.1 Hz, 3H), 1.35 (dt, JPH 1.6 and JHH
.0 Hz, 6H), 3.55 (d, JPH 22.9 Hz, 2H), 4.12 (dq, JPH 7.3 and JHH 7.3 Hz,
H), 4.13 (q, J 7.0 Hz, 2H), 5.17 (d, JPH 3.3 Hz, 1H); dC ꢁ 0.01 (SiMe ),
3
4.36 (CH ), 16.34 (J 6.7 Hz, CH ), 31.92 (J 134.3 Hz, CH ), 59.61
3
PC
3
PC
2
(
1
CH ), 61.96 (J 6.1 Hz, CH ), 100.67 (J
2
9.3 Hz, ¼CH), 163.24 (J
PC
2
PC
PC
2.1 Hz, ¼C<), 167.24 (J 2.7 Hz, > C¼O), was formed exclusively in
PC
this one-pot procedure. The phosphonate (14) could not be distilled due
to extensive desilylation but was sufficiently pure for further utilization.
5. Condensation of (E )-Ethyl 4-(Diethoxyphosphinyl)-3-trimethylsiloxy-
-butenoate (14) with Benzaldehyde in the Presence of Two Molar Equivalents
2
of NaHMDS: HMDS (0.615 g, 4.0 mmol) was added to sodium amide
ꢀ
(
0.48 g, 1.2 mmol) and the mixture stirred for 30 min at 50 C followed by
ꢀ
3
h at 80 C. THF (9 cm ) was added to the reaction mixture, the resulting
2
solution of NaHMDS cooled (0 C) and the silylated phosphonate (14)
ꢀ
3
0.162 g, 0.5 mmol) in THF (11 cm ) added dropwise at 0 C. Stirring was
ꢀ
(
maintained for 15 min at 0 C before benzaldehyde (0.104 g, 1.0 mmol) was
ꢀ
ꢀ
ꢀ
added at 0 C. The reaction mixture was stirred for an additional 2 h at 0 C
ꢀ
before the solvent was evaporated under reduced pressure at 30 C, the
3
residue treated with saturated aqueous sodium chloride (80 cm ), and the
aqueous mixture extracted with ether. The residue from the dried (MgSO4)
ether extract was chromatographed on silica gel with ether-petroleum ether
1
9 :1) as eluent. H NMR analysis of the solid eluate established that it
19
(
consisted of a 3 : 1 mixture of (E )-ethyl 5-phenyl-3-oxo-4-pentenoate (15)
1
9
1
9
and (2E, 4E )-ethyl 3-hydroxy-5-phenyl-2,4-pentadienoate (16) (0.774 g,
ꢀ
19
ꢀ
7
1%), m.p. (from hexane) 46 C (lit., m.p. 46 C).
. Desilylation of (E )-Ethyl 4-(Diethoxyphosphinyl)-3-trimethylsiloxy-
-butenoate (14): A suspension of silica gel (20 g) in a solution of the silylated
6
2
3
phosphonate (14) (3.32 g, 10 mmol) in ether (40 cm ) was stirred for 5 min at

ORDER
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SYNTHESIS OF b-KETO PHOSPHONATES
313
room temperature, the suspension filtered and the silica gel washed on the
filter with ethanol. The combined filtrates were evaporated under reduced
ꢀ
pressure at 30 C to give an almost quantitative yield of pure 4-(diethoxy-
1
phosphinyl )-3-oxobutanoate (7) (2.63 g, 99%).
,2
ACKNOWLEDGMENTS
We thank mr HSC Spies for the NMR spectra and many valuable
discussions and ms V Truter for the mass spectra. Financial support by the
University of Stellenbosch and technical support by mr MC de Jongh and
his staff is gratefully appreciated.
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Received in the UK December 7, 2000

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