Novel functionalized acylphosphonates as phosphonoformate analogs

Full text of DOI:10.1021/jo9611834

7212
J . Org. Chem. 1996, 61, 7212-7216
Novel F u n ction a lized Acylp h osp h on a tes a s
P h osp h on ofor m a te An a logs
Alan R. Glabe, Katherine L. Sturgeon,
Sally B. Ghizzoni, W. Kenneth Musker,* and
J oyce N. Takahashi*,1
Department of Chemistry, University of California,
Davis, California 95616
Received J une 24, 1996
F igu r e 1.
In tr od u ction
mation we used a modified Michaelis-Arbuzov reaction,7
the scope and utility of which we necessarily explored.
In this modified version of the Michaelis-Arbuzov reac-
tion, acylphosphonates result from the combination of a
phosphorus-containing nucleophile and an acyl halide.
R-Substituted acyl chlorides with bromo-, tributylsily-
loxy-, or trans-epoxide substituents were successfully
transformed, while those with cis-epoxides or mono
substituted epoxide substituents were not. While PFA
is stable as the trisodium salt, most acylphosphonic acids
are most stable at about pH 6 and are isolated as the
amine salt8 and then, if necessary, converted to the
sodium or potassium salt by ion exchange.
In 1978, B. Oberg reported that phosphonoformic acid
(O2CPO33-, PFA) specifically inhibited cell-free DNA
polymerase from herpes simplex virus (HSV). Since that
time PFA and acylphosphonates have been shown to be
polymerase inhibitors in avian myeloblastosis (AMS) and
human immunodeficiency virus type 1 (HIV-1). PFA, 1,
is a potent inhibitor of the reverse transcriptase of HIV-1
and is the active ingredient in Foscarnet.2 However,
PFA’s clinical value is limited by its toxic side effects
which may arise from its high ionic character at physi-
ologic pH.3 Most modifications to PFA, such as the
esterification of either the carboxylate or the phosphonate
groups, have been designed to decrease PFA’s highly ionic
nature, but these changes did not produce a more potent
antiviral agent.4 We have used two other approaches
which utilize hydroxamic acid derivatives of PFA5 and
acylphosphonic acids which resemble PFA homologs in
that they bear an oxygen-containing group on the R-car-
bon atom. In this work we describe the synthesis of four
novel R-substituted acylphosphonates (R-hydroxyacetyl)-
phosphonate, 2, (trans-2,3-epoxybutanoyl)phosphonate,
3, (2,3-epoxypropanoyl)phosphonate, 4, and (cis-2,3-ep-
oxybutanoyl)phosphonate, 5, designed to be less ionic
than PFA while retaining the acylphosphonate moiety
(Figure 1). Additionally, the three (epoxyacyl)phospho-
nates (3-5) were designed to be affinity labels for reverse
transcriptase, the enzyme responsible for replicating
HIV-1.6 Since our preliminary report on N-hydroxy-
phosphonoformamide, 6, the synthesis of this compound
has been improved.5
Resu lts a n d Discu ssion
The structure of (R-hydroxyacetyl)phosphonate, 2, is
a combination of glycolic acid and phosphonic acid. Since
this transformation could not be accomplished directly
by condensation, the hydroxy groups were protected by
treating glycolic acid with a solution of 2 equiv of tert-
butyldimethylsilyl chloride (TBDMSCl), imidazole (4
equiv), and DMF (Scheme 1).9 After workup and removal
of the volatiles under reduced pressure, tert-butyldim-
ethylsilyl (tert-butyldimethylsilyloxy)acetate was ob-
tained in 91% yield and converted to the acyl chloride
by treatment with oxalyl chloride. The acyl chloride was
then combined with tris(trimethylsilyl) phosphite to yield
bis(trimethylsilyl) [(tert-butyldimethylsilyloxy)acetyl]-
phosphonate. Deesterification of the silyl ester was
achieved by dissolution in a methanol-pyridine solution.
The tert-butyldimethylsilyl protecting group was removed
by passing the pyridinium salt through H+ Dowex and
purified by passing pyridinium (R-hydroxyacetyl)phos-
phonate through Sephadex (Na+) (67%).10
Preparation of the functionalized acylphosphonates
required three different synthetic schemes with the key
and yield-limiting step in each scheme being the forma-
tion of the carbon-phosphorus bond. For this transfor-
Since Breuer had already developed a protocol for the
preparation of numerous acylphosphonates, including
R,â-unsaturated acylphosphonates, the most facile route
to 3 seemed to involve the epoxidation of dimethyl (E)-
but-2-enoylphosphonate followed by deesterification.8b
However, attempts to epoxidize the unsaturated acylphos-
(1) aka J oyce Takahashi Doi.
(2) (a) Sundquist, B.; Oberg, B. J . Gen. Virol. 1979, 45, 273-281.
(b) Richman, D. D. In Design of Anti-AIDS Drugs; De Clerq, E., Ed.;
Elsevier Science: Netherlands, 1990; p 351.
(3) Seidel, E. A.; Koenig, S.; Polis, M. A. AIDS 1993, 7, 941-945.
(4) (a) Iyer, R. P.; Phillips, L. R.; Biddle, J . A.; Thakker, D. R.; Egan,
W. Tetrahedron Lett. 1989, 30, 7141-7144. (b) Neto, C. C.; Stein, J .
M.; Sarin, P. S.; Sun, D. K.; Bhongle, N. N.; Piralta, R. K.; Turcotte, J .
G. Biochem. Biophys. Res. Commun. 1990, 1, 458-463.
(5) Doi, J . T.; Ma, Q.-F.; Rowley, G. L.; Kenyon, G. L. Med. Chem.
Res. 1991, 1, 226-229. Correction in the RT assay solution reported
in this paper: The oligo(dT)12-18 primer-template concentration should
be nM instead of mM.
(6) (a) Larder, B. A.; Purifoy, D. J . M.; Powell, K. L.; Darby, G.
Nature 1987, 327, 716-717. (b) Fersht, A. Enzyme Structure and
Mechanism, 2nd ed.; W. H. Freeman and Co.: New York, 1985; pp
252-253. (c) Fersht, A. Enzyme Structure and Mechanism, 2nd ed.;
W. H. Freeman and Co.: New York, 1985; pp 422-423.
(7) (a) Emsley, J .; Hall, D. Chemistry of Phosphorus; Harper and
Row: London, 1976; pp 117-119. (b) Gerrard, W.; Green, W. J . J .
Chem. Soc. 1951, 2250-2253. (c) Corbridge, D. E. C. Phosphorus: An
Outline of its Chemistry, Biochemistry and Technology, 4th ed.; Elsevier
Science Publishers Co., Inc.: New York, 1990. (d.) Arbuzov, A. E.;
Kamay, G. X.; Belorossova, O. N. J . Gen. Chem. 1945, 15, 766. (e)
Britteli, D. R. J . Org. Chem. 1985, 50, 1845.
(8) (a) Dixon, H.; Sparkes, M. Biochem. J . 1991, 272, 767. (b)
Karaman, R.; Goldblum, A.; Breuer, E. J . Chem. Soc., Perkin Trans. 1
1989, 765-774.
(9) Wissner, A.; Grudzinskas, C. V. J . Org. Chem. 1978, 43, 3972.
(10) Corey, E. J .; Ponder, J . W.; Ulrich, P. Tetrahedron Lett. 1980,
21, 137.
S0022-3263(96)01183-8 CCC: $12.00 © 1996 American Chemical Society

Notes
J . Org. Chem., Vol. 61, No. 20, 1996 7213
Sch em e 1
Sch em e 2
phonate with m-CPBA resulted in the formation of
dimethyl (E)-but-2-enoyl phosphate presumably via a
Baeyer-Villiger type rearrangement.11 Since the use of
other oxidants such as dimethyldioxirane12 also proved
to be ineffectual, an alternate scheme was devised.
phosphites resulted in cleaner looking 31P NMR spectra,
but we were unable to further purify the corresponding
acylphosphonates. Dimethyl acylphosphonates are readily
deesterified by treatment with 2 equiv of trimethylsilyl
bromide (TMSB).14 However, the presence of an epoxide
required the use of an additional 1 equiv of TMSB. The
silicon of TMSB behaves as a Lewis acid which coordi-
nates to the oxirane oxygen and further polarizes the
C-O bonds to create a partial positive charge on the ring
carbons to facilitate ring opening by bromide ion. Treat-
ment of the demethylation product, bis(trimethylsilyl)
[3-bromo-2-(trimethylsilyloxy)butanoyl]phosphonate, with
a methanol-triethylamine solution produced triethylam-
monium (3-bromo-2-hydroxybutanoyl)phosphonate which
was then converted without purification to 3 by stirring
in a 40 °C aqueous solution of NaHCO3 and NaOH (pH
9) for 40 min.15 Because both the epoxide opening and
reformation steps occur with inversion of configuration,
the net result is retention of configuration. Since the
epoxidation of methyl crotonate produces a racemic
mixture, the final product, 3, is racemic.
The preparation of 3 began with the epoxidation of
methyl crotonate (Scheme 2). Methyl trans-2,3-epoxybu-
tanoate was obtained in 89% yield by refluxing methyl
crotonate and m-CPBA in CH2Cl2 overnight. The ester
was saponified with alcoholic KOH to produce the potas-
sium salt in 49% yield.13 The low yield probably resulted
from the opening of the oxirane by hydroxide to either
initiate polymerization or diol formation. After the salt
was converted to the acyl chloride (72%) by treatment
with oxalyl chloride, formation of dimethyl (trans-ep-
oxybutanoyl)phosphonate was accomplished by adding 1
equiv of trimethyl phosphite to a solution of the acyl
chloride in benzene. Purified phosphonate ester was
obtained in low yield by Kugelrohr distillation (13%). A
significant competing reaction to acylphosphonate forma-
tion was nucleophilic attack by trimethyl phosphite on
the oxirane. This resulted in the production of dimethyl
(E)-but-2-enoylphosphonate and trimethyl phosphate as
Initially, the protocol used for 3 was used for the
preparation of 4 and 5. However, when the epoxy acyl
chlorides required for the syntheses of 4 and 5 (2,3-
epoxypropanoyl chloride and cis-2,3-epoxybutanoyl chlo-
ride, respectively) were treated with either trimethyl
phosphite or tris(trimethylsilyl) phosphite, no epoxy
acylphosphonates were detected. Since the less stable
cis and monosubstituted epoxides apparently do not
withstand the Arbuzov reaction intact, the syntheses of
4 and 5 required a different approach (Scheme 3).
(2S,3R)-2-Bromo-3-hydroxybutanoic acid was prepared in
72% yield by the nitrous acid deamination of L-threonine
1
suggested by the H and 31P NMR spectra of the crude
product. The enoylphosphonate and trimethyl phosphate
presumably arise from a mechanism similar to that of
olefin and phosphine oxide formation in Wittig reactions.
We attempted to reduce the amount of side product due
to epoxide opening by using phosphites with increased
bulk and found that triisopropyl and tris(trimethylsilyl)
(11) (a) Sprecher, M.; Nativ, E. Tetrahedron Lett. 1968, 42, 4405-
4408. (b) Quin, L. D.; Kislaus, J . C.; Mesch, K. A. J . Org. Chem. 1983,
48, 4466-4472. (c) Gordon, N. J .; Evans, S. A. J . Org. Chem. 1993,
58, 4516-4519.
(12) (a) Adam, W.; Curci, R.; Edwards, E. Acc. Chem. Res. 1989,
22, 205-211. (b) Adam, W.; Hadjiarapoglou, L.; Nestler, B. Tetrahedron
Lett. 1990, 31, 331-334. (c) Murray, R. W.; J eyaraman, R. J . Org.
Chem. 1985, 50, 2847-2853.
(14) McKenna, C. E.; Higa, M. T.; Cheung, N. H.; McKenna, M.-C.
Tetrahedron Lett. 1977, 2, 155-8.
(15) (a) Guss, C. O. J . Org. Chem. 1952, 17, 678-682. (b) Guss, C.
O.; Rosenthal, R. J . Am. Chem. Soc. 1955, 2549.
(13) Singh, S. P.; Kagan, J . J . Org. Chem. 1970, 35, 3839.

7214 J . Org. Chem., Vol. 61, No. 20, 1996
Notes
Sch em e 3
using a solution of KBr, NaNO2, and H2SO4.16 While the
nitrous acid deamination may also be accomplished using
NaNO2 and concd HCl to give (2S,3R)-2-chloro-3-hydrox-
ybutanoic acid, higher yields of the bromo compound were
obtained. Before preparing the acyl chloride, the hy-
droxyl group was protected as the acetate ester by
treating the acid with a slight excess of acetyl chloride
at ambient temperature. Removal of the volatiles under
reduced pressure yielded (2S,3R)-2-bromo-3-(ethanoy-
loxy)butanoic acid in quantitative yield. Formation of the
acyl chloride was affected by treating the acid with oxalyl
chloride and removing the volatiles in vacuo. For the
formation of the acylphosphonates, phosphites with vary-
ing steric bulk and nucleophilicity were tried before
relying upon tris(trimethylsilyl) phosphite. Tris(tri-
methylsilyl) phosphite was added to the acyl chloride to
form bis(trimethylsilyl) (2S,3R)-[2-bromo-3-(ethanoylox-
y)butanoyl]phosphonate which was deesterified without
isolation by treatment with KHCO3 (aq) to give 40%
potassium (2S,3R)-[2-bromo-3-(ethanoyloxy)butanoyl]-
phosphonate. The final transformations, removal of the
protecting group and epoxide formation, were completed
in one step by use of a solution of water, methanol, and
1.5 equiv of K2CO3 at ambient temperature for 1 h to
yield 5.
The preparation of 4 was accomplished starting with
L-serine and using the same methodology. The yields for
the corresponding nitrous acid deamination (52%) of
L-serine and the formation of potassium (S)-[2-bromo-3-
(ethanoyloxy)propanoyl]phosphonate (21%) were lower
than those obtained for similar steps in the preparation
of 5.
Compounds 3-5 all contain one or two chiral centers.
While the method used to prepare 3 results in a racemic
mixture, the method used to prepare 4 and 5 is stereo-
selective. By using Scheme 3 and starting with enan-
tiomerically pure amino acids, it is possible to prepare
any one of the stereoisomers of 3-5.
accomplished by adding phenyl chlorothioformate to a
cooled solution of bis(trimethylsilyl) phosphite, triethy-
lamine, and anhydrous ethyl ether. Because the product,
bis(trimethylsilyl) [(phenylthio)carbonyl]phosphonate,
proved to be a potent allergen and vessicant, it was used
without further purification. This led to decreased yields
of the ensuing products. However, the use of methyl
chlorothioformate in this synthesis resulted in bis-
(trimethylsilyl) [(methylthio)carbonyl]phosphonate, an
easily handled, vacuum distillable oil which was obtained
in high purity in 41% yield.17 The hydroxamic acid was
formed by adding a solution of O-(trimethylsilyl)hydroxy-
lamine (1 equiv) and ethyl ether dropwise to bis(tri-
methylsilyl) [(methylthio)carbonyl]phosphonate dissolved
in ethyl ether. Removal of the volatiles under reduced
pressure yielded an oil which was then treated with an
ice cold methanol-pyridine solution to remove the tri-
methylsilyl groups to give the pyridinium salt of 6 which
was purified by passing the pyridinium salt through
(Na+) Sephadex.
All of the compounds are more effective inhibitors of
recombinant HIV-1 RT P-66 from the yeast, Saccharo-
myces cerevisiae, than pyrophosphate, but less effective
than PFA.1 The IC50 values are 1:4:3:2:5:fosfomycin )
0.3:170:200:240:700:>5000 µM. The antibiotic fosfomy-
cin, (-)-(1R,2S)-(1,2-epoxypropyl)phosphonic acid, is in-
cluded to show that the acyl group is a necessary part of
more effective structures. The IC50 values were deter-
mined by use of solutions of varying concentrations of
inhibitors in the enzyme assay.5 Although these pyro-
phosphate analogs appear to be less effective than
phosphonoformate as an inhibitor of HIV-1 RT P66, they
may be effective inhibitors of other viruses.
Exp er im en ta l Section
Negative ion fast atom bombardment (-FAB) high resolution
mass spectra (HRMS) were obtained from Dr. Dan J ones at the
Facility for Advanced Instrumentation, University of California,
Davis. Acetonitrile and methylene chloride were distilled from
CaH2 and stored under N2. Diethyl ether, benzene, and toluene
were distilled from Na and stored under N2. Bis(trimethylsilyl)
phosphite was prepared by the method of Sekine, Okimoto,
We improved upon the method previously used by Doi
to prepare 6.5 As in Doi’s synthesis, we started by
forming the carbon-phosphorus bond via a Michaelis-
Arbuzov reaction. In the previous synthesis, this was
(16) Larcheveque, M.; Petit, Y. Tetrahedron Lett. 1987, 28, 1993-
1996.
(17) Chrzanowski, R. L.; Han, J . C.-Y.; McIntosh, C. L. J . Agric. Food
Chem. 1979, 27, 550-4.

Notes
J . Org. Chem., Vol. 61, No. 20, 1996 7215
Yamada, and Hata.18 All other reagents were available from
commercial sources and used without further purification.
Cation exchange column chromatography was performed using
SP SEPHADEX C25 (washed with NaCl and deionized water)
and DOWEX 50× 8-100 (washed with ethanol and deionized
water).
of potassium trans-2,3-epoxybutanoate (3.0 g, 21 mmol) and
benzene (50 mL) at room temperature. The mixture was stirred
at room temperature for an additional 3.5 h after which the
potassium chloride was removed by suction filtration and
washed with cold benzene. Concentration of the filtrate in vacuo
gave trans-2,3-epoxybutanoyl chloride (1.8 g, 72%) as a pale
yellow liquid. IR (neat) υ 1816 cm-1 (CdO str); 1H NMR (CDCl3)
δ 1.45 (d, J ) 4.8 Hz, 3 H, CH3), 3.34 (m, 2 H, CH); 13C NMR
(CDCl3) δ 17.70, 54.34, 56.43, 165.41.
Trimethyl phosphite (4.6 mL, 40 mmol) was added dropwise
under nitrogen atmosphere to a room temperature solution of
trans-2,3-epoxybutanoyl chloride (6.1 g, 40 mmol) and benzene
over a 30 min period. The solution was stirred at ambient
temperature overnight. After the volatiles were removed under
reduced pressure, dimethyl (trans-2,3-epoxybutanoyl)phospho-
nate was purified by vacuum distillation (Kugelrohr). Yield 13%,
bp 50-100 °C/0.05 Torr; IR (neat) υ 1750 cm-1; 1H NMR (CDCl3)
δ 1.39 (d, J ) 6.6 Hz, 3 H, CH3), 3.17 (m, 1 H, CH), 3.21 (m, 1
H, CH), 3.78 (d, J ) 11.1 Hz, 6 H, POCH3); 13C NMR (CDCl3) δ
17.86, 54.48, 55.07, 55.23, 171.86; 31P NMR (CDCl3) δ 3.62 (m),
10.8 Hz).
Trimethylsilyl bromide (930 mg, 6.2 mmol), 2-methyl-2-butene
(2 drops), and dry acetonitrile (5 mL) were combined and swirled
in an ice bath for 20 min before being slowly added (15 min) to
a mixture of dimethyl (trans-2,3-epoxybutanoyl)phosphonate
(374 mg, 1.9 mmol) and dry acetonitrile (3 mL). The resulting
mixture was stirred for 3 h at room temperature while under
nitrogen. After concentrating the reaction mixture in vacuo, a
cold solution of methanol (10 mL) and triethylamine (2 mL) was
added and swirled for 15 min. Concentration under reduced
pressure yielded a light yellow residue, triethylammonium (3-
bromo-2-hydroxybutanoyl)phosphonate (1.105 g) which was used
without further purification. 31P NMR (D2O) δ 0.90, singlet.
Crude triethylammonium (3-bromo-2-hydroxybutanoyl)phos-
phonate (1.105 g) was dissolved in saturated sodium bicarbonate
(2 mL). Enough 6 N sodium hydroxide was added to increase
the pH to 9.0. The mixture was stirred in a 40 °C water bath
for 40 min, acidified with 6 N HCl to pH 6, reduced in vacuo to
ca. 30% of its original volume, and chilled in an ice bath. A few
drops of cold acetone were added to the cold solution, and the
large amount of white crystalline solid that precipitated out of
solution was removed by suction filtration. The aqueous filtrate
was lyophilized to give 3 (310 mg, 97%) as a white solid. IR
(KBr) υ 1625 cm-1 (CdO str); 1H NMR (D2O) δ 1.36 (d, J ) 5.1
Hz, 3H, CH3), 3.12 (qd, J ) 5.1, 2.1 Hz, 1H, epoxy), 3.17 (d, J )
2.1 Hz, 1H, epoxy); 13C NMR (D2O) δ 16.68, 54.85, 56.86, 160.90;
31P NMR (D2O) δ 3.14 s; HRMS (-FAB) for C4H6PO5, calcd
164.9953, obs 164.9980.
P ot a ssiu m (cis-2,3-E p oxyb u t a n oyl)p h osp h on a t e (5).
(2S,3R)-2-Bromo-3-hydroxybutanoic acid was synthesized by a
modification of the method of Izumiya.21 Sodium nitrite (9.2 g,
133 mmol) was added in portions to a 0 °C solution of L-threonine
(10.0 g, 84 mmol), KBr (35.0 g, 294 mmol), and 2.5 N H2SO4
(180 mL) at a rate such that the reaction temperature did not
exceed 5 °C (1 h). The solution was stirred for an additional
2.5 h at 0 °C and extracted with diethyl ether (5 × 50 mL). The
ether layer was dried over Na2SO4(s), concentrated in vacuo, and
further dried under high vacuum for 24 h to give (2S,3R)-2-
bromo-3-hydroxybutanoic acid as a yellow liquid (11.1 g, 72%).
1H NMR (DMSO-d6) δ 1.14 (d, J ) 6.3 Hz, 3H, CH3), 3.90
(quintet, J ) 6.3 Hz, 1H, CHOH), 4.22 (d, J ) 6.3 Hz, 1H, CHBr);
13C NMR (DMSO-d6) δ 19.57, 52.03, 67.01, 172.29.
Sod iu m r-Hyd r oxya cetylp h osp h on a te (2). tert-Butyldi-
methylsilyl (tert-butyldimethylsilyloxy)acetate was prepared by
the method of Wissner and Grudzinskas.9 Glycolic acid (4.19 g,
0.055 mol) and tert-butyldimethylchlorosilane (17.72 g, 0.0117
mol) were stirred in 40 mL of dry DMF. Imidazole (15.62 g,
0.2295 mol) was added to the mixture and stirred under N2 for
18 h. The mixture was poured into deionized water (ap-
proximately 250 mL) and extracted with petroleum ether (3 ×
100 mL). The organic fractions were combined, washed with
saturated NaHCO3, dried over MgSO4, and concentrated in
vacuo to give 15.28 g (91%) of 2 as a white solid. This solid
product was further dried over P2O5 under reduced pressure for
2 days prior to use. 1H NMR (CDCl3) δ 4.169 (s, 2H,OCH2C),
0.898 (s, 18H, 2 C(CH3)3), 0.261 (s, 6H, C(O)OSi(CH3)2), 0.082
(s, 6H, Si(CH3)2O); (lit.9 1H NMR (CDCl3) δ 4.14 (s, 2), 0.87 (s,
18), 0.22 (s, 6), 0.04 (s, 6)). tert-Butyldimethylsilyl (tert-bu-
tyldimethylsilyloxy)acetate (2.01 g, 0.0066 mol) was dissolved
in 10 mL of dry CH2Cl2 containing 4 drops of DMF. A solution
of 4.09 mL 2 M oxalyl chloride/methylene chloride and 5.0 mL
of dry methylene chloride was added dropwise under N2 for a
period of 40 min.9 After stirring the solution at ambient
temperature for 1 h, the volatiles (unreacted oxalyl chloride)
were removed in vacuo to yield a yellow oily residue. The residue
was mixed with 8.0 mL of dry benzene and cooled <0 °C before
the dropwise addition of tris(trimethylsilyl)phosphite (2.2 mL,
0.0066 mol). After 15 min, the ice bath was removed and the
mixture was stirred for a total of 1 h. The mixture was
concentrated under vacuum, and then a solution of (0.53 mL,
0.0066 mol) pyridine and (1.34 mL, 0.033 mol) methanol was
added dropwise to form a gooey white precipitate. This material
was passed through a column of DOWEX (H+) using cold
deionized water as the eluent.10 The resultant (hydroxyacetyl)-
phosphonic acid was neutralized with pyridine before chromato-
graphing on a Sephadex Na+ column with deionized water. The
eluent was lyophilized to give 0.24 g of 2 (67%). 1H NMR (D2O)
31
δ 3.880 (t, J ) 11 Hz), OCH2); P NMR (D2O) δ 17.057 (t, J )
11 Hz); HRMS (-FAB) for C2H4O5P, calcd 138.9784, obs
138.9784.
Sod iu m (tr a n s-2,3-E p oxyb u t a n oyl)p h osp h on a t e (3).
Methyl trans-2,3-epoxybutanoate was prepared by the method
of Danishefsky.19 A solution of methyl crotonate (9.4 g, 94
mmol), 80% m-chloroperoxybenzoic acid (22.4 g, 104 mmol), and
CH2Cl2 (70 mL) was refluxed overnight. The reaction mixture
was suction filtered, and the solid (m-chlorobenzoic acid) was
washed with cold CH2Cl2. The mother liquor was washed
successively with 5% Na2CO3, water, and saturated NaCl, dried
(Na2SO4), and concentrated in vacuo to give methyl trans-2,3-
epoxybutanoate (9.7 g, 89%) as a liquid. 1H NMR (CDCl3) δ 1.40
(d, J ) 5.1 Hz, 3 H, CH3), 3.21 (d, J ) 1.9 Hz, 1 H, CH), 3.25
(qd, J ) 5.1, 1.9 Hz, 1 H, CH), 3.78 (s, 3 H, OCH3); 13C NMR
(CDCl3) δ 17.42, 52.65, 54.12, 54.83, 169.97.
Potassium trans-2,3-epoxybutanoate was prepared by a modi-
fication of the method of Kagan.13 A cold solution of 85% KOH
(4.7 g, 72 mmol) and ethanol (75 mL) was added in two parts to
a 0 °C solution of methyl trans-2,3-epoxybutanoate (8.3 g, 72
mmol) and ethanol (25 mL), and the solution was stirred at room
temperature overnight. The potassium salt was suction filtered,
washed with cold ethanol, and recrystallized from ethanol to give
potassium trans-2,3-epoxybutanoate (4.9 g, 49%) as a white solid.
IR (KBr pellet) υ 1603 cm-1 (CdO str); 1H NMR (D2O) δ 1.35 (d,
J ) 5.1 Hz, 3 H, CH3), 3.11 (qd, J ) 5.1 Hz, 2.4 Hz, 1 H, CH),
3.17 (d, J ) 2.4 Hz, 1 H, CH); 13C NMR (D2O) δ 16.63, 54.77,
56.77, 176.57.
Acetyl chloride (4.4 g, 55 mmol) was added in one portion to
a cold solution of (2S,3R)-2-bromo-3-hydroxybutanoic acid (6.3
g, 45 mmol) and CH2Cl2 (40 mL). The ice bath was removed,
and the solution was stirred at ambient temperature overnight
while under nitrogen. The mixture was concentrated in vacuo
for 24 h to give pure (2S,3R)-2-bromo-3-(ethanoyloxy)butanoic
1
acid. Yield 100%; H NMR (CDCl3) δ 1.43 (d, J ) 6.6 Hz, 3H,
trans-2,3-Epoxybutanoyl chloride was prepared by a modifica-
tion of the method of Miyano.20 A 2 M solution of oxalyl chloride
(11.6 mL, 23 mmol) in CH2Cl2 was slowly added to a suspension
CH3), 2.13 (s, 3H, C(O)CH3), 4.39 (d, J ) 6.3 Hz, 1H, CHBr),
5.35 (quintet, J ) 6.3 Hz, 1H, C(O)CH), 11.01 (s, 1H, C(O)OH);
13C NMR (CDCl3) δ 17.48, 20.59, 47.70, 69.74, 170.47, 170.97.
(18) Sekine, M.; Okimoto, K.; Yamada, K.; Hata, T. J . Org. Chem.
1981, 46, 2097.
(19) Danishefsky, S.; Zamboni, R.; Kahn, M.; Etheredge, S. J . J . Am.
Chem. Soc. 1981, 103, 3460-3467.
(20) Miyano, M. J . Am. Chem. Soc. 1965, 87, 3958-3962.
(21) (a) Izumiya, N. Bull. Chem. Soc. J pn. 1953, 26, 53-56. (b)
Shimohigashi, Y.; Waki, M.; Izumiya, N. Bull. Chem. Soc. J pn. 1977,
52, 949-950.
(22) Plattner, J .; Gless, R.; Rapoport, H. J . Am. Chem. Soc. 1972,
94, 8613-8615.

7216 J . Org. Chem., Vol. 61, No. 20, 1996
Notes
Gen er a l P r oced u r e for th e P r ep a r a tion of P ota ssiu m
2-Br om o-3-(eth a n oyloxy)a cyl]p h osp h on a tes. A 2 M solu-
tion of oxalyl chloride in CH2Cl2 (27 mL, 54 mmol) was added
dropwise over 1.5 h to a stirred solution of 2-bromo-3-ethanoy-
loxy butanoic acid (45 mmol) and CH2Cl2 (50 mL) at ambient
temperature under nitrogen. The mixture was refluxed over-
night. Removal of the volatiles in vacuo (ca. 100 mmHg) yielded
the acyl chloride as a yellow liquid. Toluene (100 mL) was added
to the crude acyl chloride and the solution cooled to -78 °C. Tris-
(trimethylsilyl) phosphite (12.0 mL 36 mmol) was added to the
cold solution via dropping funnel over 2 h. The cold bath was
removed, and a solution of KHCO3 (11.3 g, 113 mmol) and water
(40 mL) was slowly added with stirring. The layers were
separated, and the organic layer was extracted with 20 mL of
5% KHCO3. The aqueous fractions were combined, and if
needed, adjusted to pH 6 with 6 N HCl, and lyophilized to give
the crude product as a yellow solid. Purification was effected
by triturating the crude product in methanol (0.2 g crude/mL
methanol) for 30 min, removing the solid by gravity filtration,
and concentrating the filtrate in vacuo.
Potassium (2S,3R)-[2-bromo-3-(ethanoyloxy)butanoyl]phos-
phonate was produced in 40% yield from the reaction of (2S,3R)-
2-bromo-3-(ethanoyloxy)butanoic acid with oxalyl chloride fol-
lowed by reaction of the acid chloride with tris(trimethylsilyl)
phosphite. 1H NMR (D2O) δ 1.34 (d, J ) 6.3 Hz, 3H, CH3), 2.11
(s, 3H, C(O)CH3), 4.35 (d, J ) 5.7 Hz, 1H, CHBr), 5.27 (quintet,
J ) 6.1 Hz, 1H, C(O)OCH); 13C NMR (D2O) δ 16.93, 20.85, 48.76,
70.43, 171.05, 172.14; 31P NMR (D2O) δ 0.90, (s).
Potassium (2,3-epoxypropanoyl)phosphonate, 4, was prepared
from potassium (S)-[2-bromo-3-(ethanoyloxy)propanoyl]phos-
phonate in the same manner as 5 to give 4 as a yellow solid
1
(28%). IR (KBr) υ 1632 cm-1 (CdO str); H NMR (D2O) δ 2.81
(dd, J ) 2.9, 5.7 Hz, 1H, epoxy), 2.97 (dd, J ) 5.1, 5.7 Hz, 1H,
epoxy), 3.40 (dd, J ) 2.9, 5.7 Hz, 1H, epoxy); 13C NMR (D2O) δ
46.38, 49.80, 161.65; 31P NMR (D2O) δ 3.60, s. HRMS (-FAB)
for C3H4PO5, calcd 150.9796; obs 150.9794.
N-Hyd r oxyp h osp h on ofor m a m id e (6) was prepared by a
variation of method of Doi.5 Bis(trimethylsilyl) [(methylthio)-
carbonyl]phosphonate was prepared by the method of Chrza-
nowski, Han, and McIntosh.17 Bis(trimethylsilyl)phosphonate
(4.42 g, 0.0196 mol) was dissolved in 20 mL of anhydrous diethyl
ether and cooled to <20 °C in an ice bath. Triethylamine (2.8
mL, 0.0201 mol) was added dropwise to the solution with
stirring. Methyl chlorothioformate (1.52 mL, 0.0177 mol) was
then added dropwise to the solution at <10 °C. After complete
addition, the solution was allowed to come to room temperature
and stirred for 16.5 h. Triethylamine hydrochloride was re-
moved by filtration under N2 and the filtrate concentrated under
reduced pressure. The product was distilled under reduced
pressure to afford 2.17 g (41%), bp 102-124 °C (2.25 mmHg)
(lit.17 bp 100-103 °C (0.25 mmHg)). 1H NMR (C6D6) δ 2.217 (s,
3H, SCH3), 0.392 (s, 18H, 2Si(CH3)3); 31P NMR (CDCl3) δ
-20.959 (s, 1). The [(methylthio)carbonyl]phosphonate (0.20 g,
6.7 × 10-4 mol) was dissolved in 5.0 mL of dry ethyl ether.
O-(Trimethylsilyl)hydroxylamine (0.050 mL, 6.7 × 10-4 mol)
was mixed with 2 mL of dry diethyl ether and added dropwise
to the [(methylthio)carbonyl]phosphonate under N2 <20 °C and
then stirred for 6 h. After concentrating the solution under
reduced pressure to 1/3 original volume, the precipitate/liquid
was centrifuged and separated. Further concentration of the
liquid in vacuo afforded an oil to which a cold solution of pyridine
(0.0247 mL, 3.08 × 10-4 mol) in 5 mL of methanol was added
dropwise. The resulting solid/liquid was separated and the solid
washed with 2 × 5 mL of methanol. Concentration of the
methanol solution afforded an oily product which produced a
solid upon trituration with diethyl ether (20 mL). The solid was
chromatographed on Sephadex (Na+) in a cold room (1.7 °C) with
deionized water as the eluent. The first six fractions (ap-
proximately 5.5 mL/fraction) which contained product were
combined and lyophilized to give 0.07 g (23%) of product. 31P
NMR (D2O) δ -1.165 (s, 1) (lit.5 31P (D2O) δ -3.7). The 31P
chemical shift of the acyl phosphonates are dependent on the
pH of the solution, e.g. the chemical shift of 6 was -0.926 ppm
at pH 6.5 and -1.439 ppm at pH 5.
Potassium (2S,3R)-[2-bromo-3-(ethanoyloxy)butanoyl]phos-
phonate (3.0 g, 10.3 mmol) was dissolved in methanol (15 mL),
and water (10 mL). Potassium carbonate (2.1 g, 15.5 mmol) was
added, and the solution was stirred for 1 h at room temperature.
The solution was acidified to pH 6 with 6 N HCl and concen-
trated under reduced pressure to give crude 5 as a yellow solid.
The crude solid was triturated with methanol for 30 min and
suction filtered to remove the solid. The filtrate was concen-
trated in vacuo to yield 5 as a pale yellow solid (692 mg, 41%):
1
IR (KBr) υ 1629 cm-1 (CdO str); H NMR (D2O) δ 1.26 (d, J )
5.4 Hz, 3H, CH3), 3.31 (quintet, J ) 5.3 Hz, 1H, epoxy), 3.51 (d,
J ) 5.1 Hz, 1H, epoxy); 13C NMR (D2O) δ 16.92, 55.46, 57.15,
161.20; 31P NMR (D2O) δ 3.70, (s) ; HRMS (-FAB) for C4H6-
PO5, calcd 164.9953, obs 164.9943.
P ota ssiu m (2,3-Ep oxyp r op a n oyl)p h osp h on a te (4). (S)-
2-Bromo-3-hydroxypropanoic acid was prepared by the same
method as (2S,3R)-2-bromo-3-hydroxybutanoic acid using L-
serine as starting material in a modification of the method of
Izumiya.21 Yield, 52%; 1H NMR (CDCl3) δ 3.92 (dd, J ) 5.7,
11.3 Hz, 1H, CHOH), 4.02 (dd, J ) 7.8, 11.3 Hz, 1H, CHOH),
4.33 (dd, J ) 5.7, 7.8 Hz, 1H, CHBr); 13C NMR (CDCl3) δ 44.91,
63.04, 170.30.
(S)-2-Bromo-3-(ethanoyloxy)propanoic acid was prepared by
the same method as (2S,3R)-2-bromo-3-(ethanoyloxy)butanoic
acid by use of acetyl chloride on (S)-2-bromo-3-hydroxypropanoic
acid. Yield 100%; 1H NMR (CDCl3) δ 2.15 (s, 3H, CH3), 4.52
(m, 3H), 11.62 (s, 1H, COOH); 13C NMR (CDCl3) δ 20.26, 40.18,
64.18, 170.62, 171.45.
Potassium (S)-[2-bromo-3-(ethanoyloxy)propanoyl]phospho-
nate was prepared in 21% yield from (S)-2-bromo-3-(ethanoy-
loxy)propanoic acid, oxalyl chloride, and tris(trimethylsilyl)
phosphite, using the general procedure. 1H NMR (D2O) δ 2.149
(s, 3H, C(O)CH3), 4.521 (m, 3H); 13C NMR (D2O) δ 20.09, 44.75,
65.89, 172.90, 173.06; 31P NMR (D2O) δ 1.45, (s).
Ack n ow led gm en t. The authors thank the Univer-
sity of California Universitywide AIDS Research Pro-
gram for their award (R91DO85) to J oyce N. Takahashi,
the University of California, Davis, and for their 1990
Professional Development Leave Award to J oyce N.
Takahashi, and Professor George L. Kenyon, Professor
of Chemistry and Pharmaceutical Chemistry, for his
help during the initial period of this work.
J O9611834