Selective methylphosphonylation of an echinocandin B analog derived from LY303366

Article abstract of DOI:10.1016/S0040-4020(03)00310-7

Echinocandin B (ECB) analog 1c was methylphosphonylated with the new reagent dimethyldiphosphonate 7. Selective functionalization of the phenol group was achieved in the presence of 11 other reactive alcohol and amide groups. The phosphonylation was best conducted in a mixture of THF and DMF using lithium t-butoxide as base. Methylphosphonate diester 1d was deprotected by hydrogenolysis to afford methylphosphonate monoester 1e, a potential prodrug for ECB analog 1c.

Full text of DOI:10.1016/S0040-4020(03)00310-7

Tetrahedron 59 (2003) 2667–2672
Selective methylphosphonylation of an echinocandin B analog
derived from LY303366
*
*
Uko E. Udodong, Marvin M. Hansen, Daniel E. Verral, Allen R. Harkness, John L. Grutsch,
William D. Miller and Bret Astleford
Chemical Process Research and Development, Lilly Research Laboratories, A Division of Eli Lilly and Company, Indianapolis,
IN 46285-4813, USA
Received 30 December 2002; revised 24 February 2003; accepted 24 February 2003
Abstract—Echinocandin B (ECB) analog 1c was methylphosphonylated with the new reagent dimethyldiphosphonate 7. Selective
functionalization of the phenol group was achieved in the presence of 11 other reactive alcohol and amide groups. The phosphonylation was
best conducted in a mixture of THF and DMF using lithium t-butoxide as base. Methylphosphonate diester 1d was deprotected by
hydrogenolysis to afford methylphosphonate monoester 1e, a potential prodrug for ECB analog 1c. q 2003 Published by Elsevier Science
Ltd.
1. Introduction
diester 1d. Selective functionalization of the phenol group
in high yield has been achieved despite the presence of
eleven other reactive alcohol and amide groups.
Fungal infections are a leading cause of nosocomal blood
stream infection. With the increasing resistance of these
infections to conventional therapies, the need for new, safe,
and effective antifungal agents has never been greater.
LY303366 (1a) is a cidal antifungal showing activity
against Candida and Aspergillus infections.1 It is a
semisynthetic variant of the cyclic peptide echinocandin B
(ECB, 2), a fermentation product of Aspergillus Nidulans.2
Other structural analogs of ECB such as the pneumocandins
(L-671,329), mulundocandin, and aculeacins have been
widely studied.3 The low aqueous solubility of LY303366
(,1 mg/mL) has hindered the development of an intra-
venous formulation. An increase in aqueous solubility could
conceivably be engineered through formulation technology
or, more attractively, through the advent of a prodrug. Early
studies suggested that phosphate and phosphonate deriva-
tives at the homotyrosine phenol residue afforded prodrugs
with the best combination of solubility and activity proper-
ties. Issues with the stability of the ECB nucleus uncovered
in the phosphorylation of 1a to produce phosphate 1b,4 led
us to consider dideoxy ECB methylphosphonate monoester
1e for prodrug development. Methylphosphonate monoester
1e has been reported to enhance the water solubility of 1c
200-fold while maintaining comparable antifungal activity.5
This paper presents the results of our studies on the
methylphosphonylation of 1c to produce methylphospho-
nate monoester 1e via dideoxy ECB methylphosphonate
An important consideration in any chemical manipulation of
1c is the acid and base lability of the polypeptide. Initial
studies employing the classical phosphonylating agent
methylphosphonic dichloride6 gave poor selectivity for the
homotyrosine phenol. Moreover, the HCl liberated during
hydrolytic work-up caused substantial decomposition. The
overall yield of 1e obtained in one step by treating 1c with
MeP(O)Cl2 in the presence of LiOTMS was 5–10%.
Although a number of other reagents have been reported7
in the literature for the phosphonylation of alcohols,
previous success in the selective phosphorylation of 1a4
with tetrabenzyl diphosphate led us to consider a prototype
dimethyldiphosphonate 7 (Fig. 1).
2. Results and discussions
The synthesis of dimethyldiphosphonate 7 involved self-
coupling of methylphosphonate monoester 6. Two options
(path a and path b) were available for the synthesis of
monoester 6 (Scheme 1). 1H-Tetrazole-catalyzed8a reaction
of 4-bromobenzyl alcohol with methylphosphonic
dichloride 3 led to diester 4 in 89% yield. Treatment of 4
with sodium iodide10 gave monoester 5 in 89% yield. This
reaction also produced benzyl iodide, a strong lachrymator,
as the by-product. Acidification of 5 led to methyl-
phosphonate monoester 6 in 99% yield. In the alternative
and more desirable procedure (path b), 4-bromobenzyl
alcohol was added to a slight excess of the dichloride 3 to
Keywords: methylphosphonylation; echinocandin B; LY303366.
*
Corresponding authors. Tel.: þ1-3172765821; fax: þ1-3172764507;
e-mail: udodong_uko@lilly.com
0040–4020/03/$ - see front matter q 2003 Published by Elsevier Science Ltd.
doi:10.1016/S0040-4020(03)00310-7

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U. E. Udodong et al. / Tetrahedron 59 (2003) 2667–2672
Figure 1.
provide a 1:4 mixture of diester 4 and monoester 8.8
Addition of aqueous sodium hydroxide to the reaction
mixture initially provided desired monoester 6, along with
dimethyldiphosphonate 7 as the major product. Formation
of dimethyldiphosphonate 7 can be explained by rapid
reaction of the initially formed anion 5 with unquenched
monochlorophosphonate monoester 8.9 Diphosphonate 7
could not be obtained cleanly by this procedure; so the
quenched reaction mixture, consisting of 4, 5, and 7, was
stirred for 12–24 h with aqueous base to hydrolyze 7 to
anion 5. The diester 4 was removed by extraction, and the
monoester 6 was isolated in good yield after acidification
and extraction. This procedure provides a practical one-step
synthesis of methylphosphonate monoester 6 in high yield
and purity.10
have been the preferred solvents due to good solubility of
the diphosphonate. However, at the end of the reaction the
dicyclohexylurea (DCU) was removed by filtration, and
these solvents retained 3–4% DCU in the evaporated
product filtrate. In addition, hydrolysis of the dimethyl-
diphosphonate during filtration and isolation was more
pronounced in THF. In toluene or ethyl acetate, the DCU
precipitation was complete (,0.5% DCU in the filtrate) but
coprecipitation of the dimethyldiphosphonate (after 1 h in
toluene and after 24 h in ethyl acetate) led to lower yields of
the product, if the DCU was not removed immediately by
filtration. The ideal solvent adopted for a 20 g scale
synthesis of 7 was a 10:1 mixture of ethyl acetate and
methylene chloride. Similar chemistry was utilized to
prepare the dibenzyl analog of 7 as an oil, but the
4-bromobenzyl derivative proved advantageous due to
easy purification and the exceptional storage stability of
this crystalline derivative.12
Dicyclohexylcarbodiimide-promoted self-coupling of
monoester 6 occurred rapidly at room temperature in
tetrahydrofuran, ethyl acetate, toluene, or methylene
chloride to afford dimethyldiphosphonate 7 as a 1:1 mixture
of diastereomers.11 THF and methylene chloride would
The effectiveness of dimethyldiphosphonate 7 as a phos-
phonylating agent is seen in the highly selective synthesis of
Scheme 1. Synthesis of dimethyldiphosphonate 7.

U. E. Udodong et al. / Tetrahedron 59 (2003) 2667–2672
2669
Table 1. Methylphosphonylation of ECB 1c
Entry
Basea (mol equiv.)
7 (mol equiv.)
Solvent
DMF
2:3 DMF/THF
DMF
DMF
1:1 DMF/THF
1:1 DMF/THF
2:3 DMF/THF
1:1 DMF/THF
1:1 DMF/THF
% 1cb
% 1db
% By-productsc
1
2
3
4
5
6
7
8
9
2.2 LiOH
1.0 LiOH
1.05 LIOH
2.2 LiOH
1.1 LiOTMS
1.3 LiOTMS
1.0 tBuOLi
1.18 tBuOLi
1.25 tBuOLi
1.5
2.0
2.0
2.0
1.1
1.2
1.1
1.18
1.33
15
23
,1
,1
5
3
5
,1
,1
74
74
95
80
89
90
86
94
94
3.0
0.1
1.5
16.0
1.5
1.7
1.8
1.0
1.5
Reactions were run with 0.5–2.0 g of 1c.
3 M LiOH was used.
a
b
UV area percent by HPLC.
Percent late-eluting by-products by HPLC.
c
methylphosphonate diester 1d and ultimately methyl-
phosphonate monoester 1e from ECB analog 1c. The
greater acidity of the phenolic hydroxyl in 1c should
allow selective derivatization under basic conditions;
however, the presence of eleven other acidic functional
groups suggests the potential for reaction at other sites or at
multiple sites. Typically, the phenolic hydroxyl of the ECB
was first deprotonated with lithium t-butoxide by adding a
THF solution of the base to a DMF solution of the substrate
at 08C.13 A solution of the dimethyldiphosphonate 7 in THF
or DMF was then added dropwise. The reaction was stirred
at 58C and monitored to completion by HPLC (0.5–3 h). In
some cases, additional base and dimethyldiphosphonate
were added in small portions to consume remaining starting
materials. Other bases such as lithium hydroxide, sodium
hydride, tertiary amines, and lithium trimethylsilanolate in a
variety of solvents (THF, DMF, DMAc, DME, DMSO) and
at different temperatures gave less satisfactory results
(Table 1). Aqueous lithium hydroxide required a larger
excess of 7 for complete phosphonylation, presumably due
to competing hydrolysis of the dimethyldiphosphonate.
LiOTMS gave higher levels of by-products, perhaps
resulting from multiple methylphosphonylation of the
ECB. Entry 4 shows that treatment with excess base and
excess dimethyldiphosphonate 7 gave the highest level of
by-products (multiple peaks by HPLC), suggesting that
these by-products are the result of indiscriminant reaction at
other alcohol or secondary amide sites on the molecule.
Lithium t-butoxide gave the highest yields and the lowest
levels of by-products (entry 8).
diester 1d, which was further converted to the methyl-
phosphonate monoester 1e.
4. Experimental
4.1. General
Unless otherwise noted, starting materials were obtained
from commercial suppliers and used without further
˚
purification. DMF and THF were stored over 4 A molecular
sieves. For small-scale reactions (less than 20 g), THF was
distilled from sodium benzophenone ketyl. Anhydrous
t-butyl methyl ether (MTBE), and DME were obtained
from Aldrich. Reactions using base or organometallic
reagents were run under nitrogen. Reactions were monitored
by HPLC using the conditions specified below. Thin layer
chromatography (TLC) was done using Merck plates of
1
Silica Gel 60 with a fluorescent indicator (F254). H, 13C,
and 31P NMR spectra were recorded at 300, 75, and
121 MHz respectively, using CDCl3 as solvent unless
specified otherwise, except for the ECB compounds which
required DMSO-d6. NMR chemical shifts are reported in
ppm with solvent as the internal standard on the d scale and
J values are in Hertz. IR, UV, and Mass Spec analyses were
done by Eli Lilly Physical Chemistry Laboratory. HPLC
conditions: 25 cm Zorbax RX C18 column, 60:40 CH3-
CN/H2O, with 0.1% TFA in each, 230 nm for 2 and 280 nm
for 1, 1 mL/min flow rate. Preparatory HPLC conditions for
1c and 1e: HP20SS column by step gradient elution; solvent
A—42:58 MeCN/0.1% HOAc at pH 5; solvent B—60:40
MeCN/0.1% HOAc at pH 5.
The crude methylphosphonate diester was purified by silica
gel chromatography (CH2Cl2/MeOH) to provide 1d14 in
71% yield (97% purity, HPLC). Hydrogenolytic debenzyl-
ation in 9:1 THF/DMF in the presence of triethylamine
furnished methylphosphonate monoester 1e in 82% yield
and 96% purity.7b The use of triethylamine was critical to
avoid side reactions caused by the liberated HBr.
4.1.1. Di-[(4-bromophenyl)methyl] methylphosphonate
(4).
A
solution of 4-bromobenzyl alcohol (22 g,
117.6 mmol) and 1H-tetrazole (0.34 g, 4.85 mmol) in
Et2O (300 mL) was cooled to 08C under nitrogen and
treated with diisopropylethylamine (24 mL, 137.8 mmol). A
solution of methylphosphonic dichloride (8.7 g,
65.45 mmol) in Et2O was added dropwise over 0.5 h
while maintaining the temperature between 0 and 3.58C
with an ice-salt bath. The mixture was stirred at 08C for
0.5 h and then at room temperature for 4 h until TLC (9:1
CH2Cl2/EtOAc) showed complete consumption of the
alcohol. The precipitated salt was removed by suction
filtration and rinsed with Et2O. The filtrate was concentrated
and redissolved in 10 mL of CH2Cl2. This was filtered
3. Conclusions
We have developed a practical and reliable method for
preparation of dimethyldiphosphonate 7.15 ECB analog 1c
was chemoselectively methylphosphonylated with 7 in the
presence of lithium t-butoxide to afford methylphosphonate

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U. E. Udodong et al. / Tetrahedron 59 (2003) 2667–2672
through a sintered glass funnel of silica gel (68 g, packed
with CH2Cl2) and eluted with 9.5:0.5 CH2Cl2/EtOAc to
collect 22.7 g (89% yield) of 4. Rf 0.43 (9:1 CH2Cl2/ EtOAc).
The cooling bath was removed and the mixture was
transferred to a separatory funnel and washed with 1N
HCl (2£). To the organic layer was added 150 mL of 2N
NaOH and 50 mL of H2O and the mixture was stirred
overnight to complete hydrolysis of the dimethyl-
diphosphonate. The layers were separated and the aqueous
layer was washed with CH2Cl2 (2£) to remove 4. The
aqueous layer was acidified with 22 mL of 12N HCl and
extracted with 400 mL of CH2Cl2. The organic layer was
dried (Na2SO4) and evaporated to afford 53.74 g (78% crude
yield) of a white solid. After EtOAc (110 mL) was added,
the large chunks were broken with a spatula and the
suspension was stirred vigorously for 3 h. The white solid
was collected by filtration and dried overnight in a vacuum
oven at 508C to afford 48.72 g (71% yield) of 6. IR (CHCl3)
3600–3000, 1597 cm21; 1H NMR d 1.50 (d, 3H, J¼18 Hz),
4.97 (d, 2H, J¼8 Hz), 7.23 (d, 2H, J¼8 Hz), 7.48 (d, 2H,
J¼8 Hz), 12.41 (br s, 1H); 1H NMR (DMSO-d6) d 11.8 (d,
J¼148 Hz), 65.8 (d, J¼6 Hz), 122.4, 129.4, 131.7, 135.2 (d,
J¼7 Hz). 31P NMR d 34.51. Anal. calcd for C8H10BrO3P: C,
36.25; H, 3.80. Found: C, 36.55; H, 3.86.
IR (CHCl3) 3420, 3005 cm21 1H NMR d 1.47 (d, 3H,
.
J¼18 Hz), 4.92 (dd, 2H, J¼9, 12 Hz), 4.98 (dd, 2H, J¼9,
12 Hz), 7.19 (d, 4H, J¼8 Hz), 7.46 (d, 4H, J¼8 Hz). 1H NMR
(DMSO-d6) d 10.6 (d, J¼140 Hz), 65.4 (d, J¼6 Hz), 121.2,
129.7, 131.3, 136.1 (d, J¼6 Hz). MS (FDþ) m/z 434. 31P NMR
(DMSO-d6) d 24.36. Anal. calcd for C15H15BrO3P: C, 41.51;
H, 3.48; Br, 36.82. Found: C, 41.31; H, 3.34; Br, 37.21.
4.1.2. Mono[(4-bromophenyl)methyl] methylphospho-
nate (6): from diester 4. Note. 4-Bromobenzyl iodide, the
by-product of this reaction, is a strong lachrymator. A
mixture of 4 (17 g, 39.16 mmol) and NaI (11.7 g,
˚
78.06 mmol) in acetone (20 mL, dried over 4 A molecular
sieves) was heated at reflux for 5 h until TLC (9:1
CH2Cl2/EtOAc) showed complete consumption of 4. The
mixture was allowed to cool to room temperature and then
suction filtered to collect the precipitated salt. The salt was
transferred to a flask and slurried with acetone (20 mL) to
break up the lumps and then refiltered. The solid was rinsed
with acetone to remove all yellow color. The resulting white
solid was dried under vacuum to obtain 10 g (88.9% yield)
of sodium mono[(4-bromophenyl)methyl] methylphospho-
4.1.4. Di-[(4-bromophenyl)methyl] dimethyldiphospho-
nate (7). Acid 6 (70 g, 261.1 mmol) and dicyclohexyl-
carbodiimide (DCC, 24.7 g, 132.8 mmol) were weighed
into a flask and EtOAc (700 mL) and CH2Cl2 (140 mL)
were added. The mixture was stirred for 1 h then the DCU
was removed by filtration. The cake was rinsed with 135 mL
of EtOAc and the filtrate was evaporated to a solid. Heptane
(340 mL) was added and the mixture was stirred for 20 min
to break up the large chunks. The solid was collected by
filtration and rinsed with heptane. After drying in a vacuum
oven, 64.2 g (95% yield) of 7 was isolated as a white solid
(1:1 mixture of diastereomers, NMR data complicated). IR
(CHCl3) 3012, 1596 cm21. 1H NMR d 1.55–1.80 (m, 6H);
5.21–5.01 (m, 4H), 7.23 and 7.26 (d, 4H, J¼8 Hz), 7.48 and
7.49 (d, 4H, J¼8 Hz). 1H NMR d 12.00, 12.05, 12.09, 13.99,
14.04, 14.09, 67.05, 67.10, 67.15, 67.19, 122.80, 129.72,
129.80, 131.86, 134.61. 31P NMR d 24.20, 24.12. MS (FDþ)
m/z¼508, 509, 510, 511, 512, 513, 514, 515 for 79Br and
81Br combinations. Anal. calcd for C16H18Br2O5P2: C,
37.53; H, 3.54. Found: C, 37.74; H, 3.59.
1
nate (5). IR (CHCl3) 1489, 1307 cm21; H NMR (D2O) d
1.29 (d, 3H, J¼16 Hz), 4.87 (d, 2H, J¼7 Hz), 7.36 (d, 2H,
J¼8 Hz), 7.59 (d, 2H, J¼8 Hz). MS (FABþ) m/z 287. Anal.
calcd for C8H9BrNaO3P: C, 33.48; H, 3.16. Found: C,
33.71; H, 3.11.
A solution of 5 (7 g, 24.38 mmol) in 35 mL of THF was
cooled to 08C and treated dropwise with 6 M HCl (4.2 mL,
25.2 mmol). The cooling bath was removed and the mixture
stirred for 10 min. The precipitated NaCl was removed by
filtration and the filtrate was concentrated. The resulting
solid was taken up in CH2Cl2 and dried over Na2SO4. The
solution was concentrated and dried under vacuum to obtain
6.8 g of a white solid. HPLC showed 96.3% uv purity. The
solid was dissolved in warm EtOAc and filtered through a
fritted disc to remove insoluble white particles. The filtrate
was concentrated to remove half the volume. Hexanes were
added to precipitate a white powder. The solid was collected
by filtration, washed with 80:20 hexanes/EtOAc, and dried
under vacuum to obtain 5.5 g of 6. HPLC showed 99% uv
purity and the spectral data was identical with the material
prepared by the one step method below.
4.1.5. Preparation of dideoxy ECB 1c. A suspension of
CH2Cl2 (0.77 L), 1a1 (0.235 kg, 0.207 mol, 1.0 equiv.), and
triethylsilane (0.78 kg, 6.70 mol, 30 equiv.) was cooled to
168C. Trifluoroacetic acid (0.978 kg, 8.56 mol, 35 equiv.)
was added over 3 min via an addition funnel while
maintaining the temperature at 208C. After 1.5 h, the
mixture was cooled to 258C and diluted with THF
(4.0 L). The mixture was poured into a solution of K2CO3
(0.862 kg, 8.62 mol, 38.5 equiv.) in 4.0 L of H2O. The
aqueous phase was discarded and the organic phase was
stripped to dryness to afford 0.308 kg of crude 1c. After
correction for HPLC potency (61.7%), the yield from 1a
was 83.1%. An analytical sample was purified by HPLC
chromatography. IR (CHCl3) 1636, 1517 cm21. HRMS
(FABþ) m/z calcd for C58H74N7O15: 1108.5243, Found:
1108.5265. Anal. calcd for C58H73N7O15: C, 62.85; H, 6.63;
N, 8.85. Found: C, 62.90; H, 6.49; N, 8.96.
4.1.3. Mono[(4-bromophenyl)methyl] methylphospho-
nate (6): one-step method. Methyl phosphonic dichloride
(3) (36.96 g, 0.28 mol) was rapidly poured into an oven-
dried flask under N2 and dissolved in 200 mL of CH2Cl2.
The solution was cooled in an ice/H2O bath and a solution of
4-bromobenzyl alcohol (49.4 g, 0.26 mol) and Et3N (stored
over KOH, 39 mL, 0.28 mol) in CH2Cl2 (150 mL) was
added over 90 min using an addition funnel. HPLC 10 min
after the addition was complete showed 78% 6, 19% 4, and
1.2% unreacted alcohol (little dimethyldiphosphonate
formed upon quenching into acidic HPLC eluent). An
additional 39 mL of Et3N was added, followed by 20 mL of
H2O over 1 min. An exotherm up to 278C occurred and then
an additional 30 mL of H2O was added over 5 min. HPLC
showed 55% dimethyldiphosphonate 7, 4.5% 6 and 20% 4.
4.1.6. Preparation of dideoxy ECB methylphosphonate
diester 1d. A solution of 1c (91% pure by HPLC, 5.3 g,

U. E. Udodong et al. / Tetrahedron 59 (2003) 2667–2672
2671
Chem. 1992, 35, 194. (b) Iwamoto, T.; Fujie, A.; Sakamoto,
K.; Tsurumi, Y.; Shigematsu, N.; Yamashita, M.; Hashimoto,
S.; Okuhara, M.; Kohsaka, M. J. Antibiot. 1994, 47, 1084.
(c) Fromtling, R. A. Drugs Future 1994, 19, 933.
(d) Hammond, M. L. Chemical and Structure–Activity
Studies of the Echinocandin Lipopeptides. In Cutaneous
Antifungal Agents; Rippon, J. W., Fromtling, R. A., Eds.;
Marcel Dekker: New York, 1993; pp 395–420.
4.35 mmol) in DMF (13 mL) was added dropwise to a
solution of t-BuOLi (95% pure, 0.43 g, 5.13 mmol) in DMF
(13 mL). The mixture was stirred at room temperature for
20–30 min or until a homogeneous solution (dark brown)
resulted. Upon cooling to 08C, the dimethyldiphosphonate 7
(97.7% pure, 2.69 g, 5.13 mmol) in THF (26 mL) was added
dropwise (0.4 mL/min). After the addition, the reaction was
stirred at 08C and monitored by HPLC until most of 1c was
consumed (1–2% remained). The mixture was quenched
with 2 equiv. of acetic acid (based on amount of base used)
and stirred at 08C for 10–15 min. The mixture was poured
into CH3CN (133 mL) with stirring at room temperature.
The reaction flask was rinsed with another 100 mL of
CH3CN. The precipitate was collected by filtration and
dried under vacuum. The crude product was dissolved in
methanol (10.5 mL) and the solution poured into water
(133 mL) to re-precipitate the product. The precipitate was
stirred vigorously for 10 min and filtered to obtain 4.6 g
(84% yield, corrected for 87.7% HPLC potency) of 1d. An
analytical sample was purified by silica gel chromatography
(85:15 CH2Cl2/MeOH). Rf 0.43 (90:10 CH2Cl2/MeOH). IR
4. Udodong, U. E.; Turner, W. W.; Astleford, B. A.; Brown, F.,
Jr.; Clayton, M. T.; Dunlap, S. E.; Frank, S. A.; Grutsch, J. L.;
LaGrandeur, L. M.; Verral, D. E.; Werner, J. A. Tetrahedron
Lett. 1998, 39, 6115.
5. Rodriquez, M. J.; Vasudevan, V.; Jamison, J. A.; Borromeo,
P. S.; Turner, W. W. Bioorg. Med. Chem. Lett. 1999, 9, 1863.
6. (a) Dorman, M. A.; Noble, S. A.; McBride, L. J.; Caruthers,
M. H. Tetrahedron 1984, 40, 95. (b) Worms, K. H.; Schmidt-
Dunker, M. Organo Phosphorus Compounds; Kosolapoff,
G. M., Maier, L., Eds.; Wiley: New York, 1973; Vol. 7, p 1.
(c) Clivo, P.; Fourrey, J.-L. J. Org. Chem. 1994, 59, 7273.
7. See, for example: (a) Campbell, D. A.; Bermak, J. C. J. Org.
Chem. 1994, 59, 658. (b) Saady, M.; Lebeau, L.; Mioskowski,
C. Tetrahedron Lett. 1995, 36, 2239. (c) Willems, H. A. M.;
Veenemen, G. H.; Westerduin, P. Tetrahedron Lett. 1992, 33,
2075. (d) Valentijn, A. R. P. M.; van der Marel, G. A.; Cohen,
L. H.; van Boom, J. H. Synlett 1991, 663. (e) Tawfik, D. S.;
Eshhar, Z.; Bentolila, A.; Green, B. S. Synthesis 1993, 968.
(f) Saady, M.; Lebeau, L.; Mioskowski, C. Tetrahedron Lett.
1995, 36, 4785. (g) Wozniak, L. A.; Wieczorek, M.; Pyzowski,
J.; Majzner, W.; Stec, W. J. J. Org. Chem. 1998, 63, 5395.
(h) Wozniak, L. A.; Chworos, A.; Pyzowski, J.; Stec, W. J.
J. Org. Chem. 1998, 63, 9109. (i) Lesnikowski, Z. J.;
Zabawaska, D.; Jaworska-Maslanka, M. M.; Schinazi, R. F.;
Stec, W. J. New J. Chem. 1994, 18, 1197.
(CHCl3) 1639, 1609, 1529 cm21. MS (FABþ) m/z 1356. 31
NMR (DMSO-d6) d 25.68.
P
4.1.7. Preparation of dideoxy ECB methylphosphonate
monoester 1e. A solution of 1d (97% pure, 100 mg,
0.08 mmol) in 90:10 THF/DMF (1.5 mL) was treated with
triethylamine (0.03 mL, 0.22 mmol). The solution was
hydrogenated for 3 h over 10% Pd–C (50 mg) at 1 atm of
hydrogen. The catalyst was removed by filtration through a
bed of celite and rinsed with THF (10 mL). The filtrate was
concentrated at reduced pressure to remove THF. The
residue was triturated with MeCN (10 mL) to give a white
precipitate. The solid was filtered and rinsed twice with
Et2O (3 mL) to obtain 65 mg (74%) of 1e. An analytical
sample was purified by HPLC chromatography. IR (KBr)
1634, 1507, 1436 cm21. HRMS (FABþ) m/z calcd for
C59H77N7O17P: 1186.5114. Found: 1186.5139. 31P NMR
(DMSO-d6) d 20.93.
8. Compound 8 is only observed by HPLC as monoester 6
following an aqueous quench. Selective formation of the
desired phosphonate monoester has been reported using
phenylphosphonic dichloride. With alkyl phosphonic
dichlorides, variable success has been reported. See:
(a) Zhao, K.; Landry, D. W. Tetrahedron 1993, 49, 363.
Entries 8 and 9 in Table 1. (b) Yang, G.; Zhao, K.; Landy,
D. W. Tetrahedron Lett. 1998, 39, 2449. Entries 3 and 4 in
Table 1. (c) Mlodnosky, K. L.; Holmes, H. M.; Lam, V. Q.;
Berkman, C. E. Tetrahedron Lett. 1997, 38, 8803.
Acknowledgements
9. Conversion of phosphonic chlorides to the diphosphonate with
water is known. In our process, this results in a mixture of
diphosphonate 7, diester 4 and phosphonate monoester 6. Due
to instability of 7, this mixture cannot be purified. See: Ohms,
G.; Grossmann, G.; Schwab, B.; Schiefer, H. Phosphorus
Sulfur Silicon Relat. Elem. 1992, 68, 77.
We thank the Molecular Structure Division of Eli Lilly and
Company for spectral data, Doug Dorman for spectral
characterization of 1c, 1d and 1e, Craig Mann for HPLC
purification of ECB compounds and Bill Turner for helpful
discussions.
10. The process in path b is a significant improvement over path a
and avoids the formation of 4-bromobenzyl iodide, a strong
lachrymator. See: Zervas, L.; Kilaris, I. J. Am. Chem. Soc.
1955, 77, 5354.
References
11. Khorana, H. G.; Todd, A. R. J. Am. Chem. Soc. 1953, 2257.
12. Diphosphonate 7 is stable to storage in a freezer for at least
four months.
1. Debono, M.; Turner, W. W.; LaGrandeur, L.; Burkhardt, F. J.;
Nissen, J. S.; Nichols, K. K.; Rodriguez, M. J.; Zweifel, M. J.;
Zeckner, D. J.; Gordee, R. S.; Tang, J.; Parr, T. R., Jr. J. Med.
Chem. 1995, 38, 3271.
13. Initial studies of the phosphorylation of 1a showed that lithium
bases gave the fastest reaction, the order of reactivity being
Liþ.Naþ.Kþ. See Ref. 4.
2. Turner, W. W.; Rodriguez, M. J. Curr. Pharm. Des. 1996, 2,
209, and references therein.
3. (a) Balkovec, J. M.; Black, R. M.; Hammond, M. L.; Heck,
J. V.; Zambias, R. A.; Abruzzo, G.; Bartizal, K.; Kopp, H.;
Trainor, C.; Schwartz, R. E.; McFadden, D. C.; Nollstadt,
K. H.; Pittarelli, L. A.; Powles, M. A.; Schmatz, D. M. J. Med.
14. Compound 1d is a mixture of diastereomers, but the isomers
are so similar that only one set of peaks is observed by 1H, 13
and 31P NMR spectroscopy.
15. Although toxicity data for the phosphorus compounds
C

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described here is not available, the toxicity of related esters
have been recognized. Hence, all phosphorus esters should be
handled with caution. O’Brien, R. D. Toxic Phosphorus
Esters. Chemistry, Metabolism, and Biological Effects;
Academic: London, 1960; See, for example, tetraethyl dipho-
sphate in The Merck Index; 11th ed., 1989, p 1450.