An efficient synthesis of 2-hydroxyethyl N,N,N′ ,N′-tetrakis(2-chloroethyl)phosphorodiamidate

Article abstract of DOI:10.1021/op010208k

A process for the multikilogram preparation of 2-hydroxyethyl N,N,N′ ,N′-tetrakis(2-chloroethyl)phosphorodiamidate has been achieved in substantially pure form by a short synthetic sequence starting from phosphorus oxychloride and 2 equiv of bis(2-chloroethyl)amine. This process involves a two-step preparation of the intermediate mustard chloride in one pot, followed by the base-catalyzed reaction with excess ethylene glycol. This method has been carried out to provide 2.9 kg of this key drug substance intermediate in 52% overall yield.

Full text of DOI:10.1021/op010208k

Organic Process Research & Development 2001, 5, 442−444
An Efficient Synthesis of 2-Hydroxyethyl
N,N,N′,N′-Tetrakis(2-chloroethyl)phosphorodiamidate
R. Jason Herr,*,† Paul Zhichkin,† Pedro E. Herna´ndez-Abad,† Harold Meckler,† and Steven R. Schow‡
Chemical DeVelopment Department, Albany Molecular Research, Inc., P.O. Box 15098,
Albany, New York 12212-5098, U.S.A., and Medicinal Chemistry Department, Telik, Inc., 750 Gateway BouleVard,
South San Francisco, California 94080, U.S.A.
Scheme 1
Abstract:
A process for the multikilogram preparation of 2-hydroxyethyl
N,N,N′,N′-tetrakis(2-chloroethyl)phosphorodiamidate has been
achieved in substantially pure form by a short synthetic
sequence starting from phosphorus oxychloride and 2 equiv of
bis(2-chloroethyl)amine. This process involves a two-step prepa-
ration of the intermediate mustard chloride in one pot, followed
by the base-catalyzed reaction with excess ethylene glycol. This
method has been carried out to provide 2.9 kg of this key drug
substance intermediate in 52% overall yield.
Scheme 2
Introduction
Recently, a program was carried out to develop a synthetic
route for the future large-scale preparation of the oncolytic
drug substance TLK 286 (1).1 The literature approach1a to 1
involved the coupling of the tripeptide thiol 2 with 2-bro-
moethyl N,N,N′,N′-tetrakis(2-chloroethyl)phosphorodiamidate
(3), which was realized with a low overall yield due to
reactions of 2 with impurities carried through from the
preparation of the mustard bromide 3 (Scheme 1). In addition,
further synthetic manipulations provided products that were
inseparable from impurities derived from the manufacture
of 3. All of these problems afforded 1 in such poor quality
that the product required several purifications by reverse
phase HPLC.
The poor overall yield of the coupling reaction led to a
revised synthetic sequence whereby thiol 2 was to be coupled
with a more reactive and substantially pure mustard sulfonate
4 under milder reaction conditions. This sulfonate 4 was
prepared from phosphorus oxychloride and bis(2-chloroet-
hyl)amine by a streamlined process that was found to be
amenable to large-scale production. The coupling reaction
and final synthetic transformations were then performed to
provide multikilogram quantities of 1, the details of which
will be discussed in a forthcoming publication.
Results and Discussion
The previous reported synthesis of the mustard bromide
31a described its preparation by a one-pot process from
bromoethanol (5) and phosphorus oxychloride (Scheme 2).
However, the purification of this material required multiple
column chromatographies, providing low yields of semi-
purified 3 that was used in the subsequent synthetic sequence.
Previously, the largest lot of 3 obtained by this route was
86 g.
To optimize the coupling process in the next step, several
avenues were investigated to prepare the mustard component
3 in substantially pure form. Initial attempts to react the lithio
alkoxide of 5 with commercially available tetrakis(2-chlo-
roethyl)phosphorodiamidic chloride (6) did provide a moder-
ate yield (39%) of the clean mustard bromide 3, but this
reaction was also shown to work much better with mono-
protected ethylene glycol derivatives.2,3 This led to a plan
in which easily purified protected ethylene glycol mustard
derivatives 7 would be converted to activated mustard
sulfonate derivatives 4, which in turn would be coupled with
glutathione 2 (Scheme 3). After some optimization, it was
eventually found that an excess of ethylene glycol (>10
equiv) could react directly with chloride 6 when treated with
1 equiv of potassium tert-butoxide at room temperature in
tetrahydrofuran to provide the crude hydroxyethyl mustard
* Corresponding author. E-mail: rjasonh@albmolecular.com.
† Albany Molecular Research, Inc.
‡ Telik, Inc.
(1) (a) Lyttle, M. H.; Satyam, A.; Hocker, M. D.; Bauer, K. E.; Caldwell, C.
G.; Hui, H. C.; Morgan, A. S.; Mergia, A.; Kauvar, L. M. J. Med. Chem.
1994, 37, 1501. (b) Satyam, A.; Hocker, M. D.; Kane-Maguire, K. A.;
Morgan, A. S.; Villar, H. O.; Lyttle, M. H. J. Med. Chem. 1996, 39, 1736.
(c) See also: Lyttle, M. H.; Hocker, M. D.; Hui, H. C.; Caldwell, C. G.;
Aaron, D. T.; Engqvist-Goldstein, A° .; Flatgaard, J. E.; Bauer, K. E. J. Med.
Chem. 1994, 37, 189.
(2) R. Jason Herr, unpublished results, Albany Molecular Research, Inc., 1997.
(3) For a similar transformation, see ref 1a as well as Borch, R. F.; Valente,
R. R. J. Med. Chem. 1991, 34, 3052.
442
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10.1021/op010208k CCC: $20.00 © 2001 American Chemical Society and The Royal Society of Chemistry
Published on Web 05/17/2001

Scheme 3
7 in good yields (47-63%). Unfortunately, some of the
product was lost to the aqueous layer during workup due to
the partitioning effect of the excess ethylene glycol in the
mixed solvent system, and all attempts to recover 7 from
the aqueous wash by removal of ethylene glycol were largely
unsuccessful. Stoichiometric variations of the reaction condi-
tions showed that fewer equivalents of the diol or greater
amounts of base led to decreases in yield due to the formation
of greater amounts of inseparable dimeric by-products.
However, these by-products were easily removed after one
recrystallization from methyl tert-butyl ether to provide 7
as a white powder in excellent purity (greater than 98% by
HPLC analysis) and with minimal loss of material. Starting
with 5.2 kg of the chloride 6, a 2.9 kg lot of 2-hydroxyethyl
N,N,N′,N′-tetrakis(2-chloroethyl)phosphorodiamidate (7) was
prepared by this process in 53% isolated yield.
Due to the unavailability of commercial quantities of the
diamidic monochloride 6,4 a process was developed for the
large-scale preparation of this compound from phosphorus
oxychloride and 2 equiv of bis(2-chloroethyl)amine. Several
research groups have shown that phosphorus oxychloride
may be coupled with 1 equiv of bis(2-chloroethyl)amine to
provide bis(2-chloroethyl)phosphoroamidic dichloride, which
may be coupled with other amines to provide analogues of
6 in a separate step.5,6 While many of these preparations
involved refluxing a suspension of the mustard chloride in
excess phosphorus oxychloride, we found that phosphorus
oxychloride could be treated with 1 equiv of bis(2-chloro-
ethyl)amine hydrochloride in toluene in the presence of 2
equiv of triethylamine at room temperature to provide the
intermediate dichloride adduct in quantitative yield (Scheme
3). This in situ intermediate was then treated with a second
equivalent of the amine and an additional 2 equiv of
triethylamine, and the mixture was heated at reflux to provide
tetrakis(2-chloroethyl)phosphorodiamidic chloride (6) in
near-quantitative yields. Filtration of the cooled mixture to
remove the insoluble triethylamine hydrochloride salts was
usually all that was necessary to produce multikilogram
quantities of 6 in excellent purity (greater than 98% by proton
NMR analysis), although occasionally a slurry of the mixture
with activated charcoal followed by filtration through Celite
was useful to decolorize the product. Using 1.0 L of
phosphorus oxychloride, a 4.0 kg lot of 6 was prepared by
this two-step process in 98% isolated yield.
It is interesting to note that the first amine addition was
accompanied by a slow exothermic heating to 30 °C over a
2-3 h period, while the second amine addition required the
(endothermic) heating of the mixture to reflux to accomplish
reaction completion. When 2 equiv of the amine were added
to phosphorus oxychloride at room temperature in toluene,
complete conversion to the amidic dichloride intermediate
was observed with only a small amount of the diamidic
monochloride formed. When 2 equiv of the amine were
added to phosphorus oxychloride in toluene and heated at
reflux, the product yield was reduced in addition to the
formation of significant amounts of unidentified by-products.
It should also be mentioned that the chloride 6 was typically
not crystallized,7 but used as an oil for the coupling reaction
with ethylene glycol, as described above (Scheme 3). When
run in tandem, this process was used for the preparation of
multikilogram quantities of the mustard alcohol 7 from
phosphorus oxychloride with an overall isolated yield of 52%
and with a purity of greater than 98% (by proton NMR and
HPLC analyses).
The mustard alcohol 7 was treated with sodium hydroxide
and several sulfonyl chlorides in a water/tetrahydrofuran
solvent mixture to provide a series of mustard sulfonate
derivatives 4 (Scheme 3), which in turn were examined as
coupling partners with glutathione 2. The results of these
studies will be presented in a separate article.
(4) This work was performed in 1997. At that time, the availability of 4 from
Aldrich (the only vendor of the reagent) was $42.90/5 g at a maximum of
200 g deliverable.
(5) (a) Friedman, O. M.; Seligman, A. M. J. Am. Chem. Soc. 1954, 76, 655.
(b) Radu, V.; Nicoleta, V.; Ion, N. Romanian Patent 84,601 B, 1985; Chem.
Abstr. 1985, 103, 105152. (c) Borch, R. F.; Canute, G. W. J. Med. Chem.
1991, 34, 3044. (d) McGuigan, C.; Narashiman, P. Synthesis 1993, 311.
(e) Wan, H.; Modro, T. A. Synthesis 1996, 1227.
(6) For methods involving the bis-amination of phosphorous trichloride,
followed by oxidation to analogues of 4, see: (a) Mulcahy, R. T.; Gipp, J.
J.; Schmidt, J. P.; Joswig, C.; Borch, R. F. J. Med. Chem. 1994, 37, 1610.
(b) Ghosh, A. K.; Farquhar, D. Tetrahedron Lett. 1997, 38, 8795.
Conclusions
An efficient process for the preparation of 2-hydroxyethyl
N,N,N′,N′-tetrakis(2-chloroethyl)phosphorodiamidate (7) from
phosphorus oxychloride has been achieved in substantially
pure form by a two-step preparation of the intermediate
mustard chloride in one pot, followed by the base-catalyzed
(7) Compound 6 can be isolated as a solid, if necessary, but in this process the
crude oil was suitable for use without purification.
Vol. 5, No. 4, 2001 / Organic Process Research & Development
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reaction with excess ethylene glycol. Following a purification
by recrystallization, this process has been carried out to
provide 2.9 kg of this key drug substance intermediate in
52% overall isolated yield. The synthetic transformations that
were performed to provide multikilogram quantities of the
glutathione drug substance 1 will be discussed in a forthcom-
ing publication.
Preparation of 2-Hydroxyethyl N,N,N′,N′-Tetrakis(2-
chloroethyl)-phosphorodiamidate (7). A 72-L, three-neck,
round-bottom flask was equipped with an overhead mechan-
ical stirrer, a thermometer, and a nitrogen inlet/outlet bubbler.
The flask was charged with a solution of N,N,N′,N′-tetrakis-
(2-chloroethyl)phosphorodiamidic chloride (6, 5.18 kg, 14.2
mol) in tetrahydrofuran (17.5 L) and ethylene glycol (7.8 L,
112.9 mol). The stirrer was set to agitate at a moderate rate
(to provide a clear red solution), and the mixture was cooled
to 2 °C using an ice/water bath. To this cooled solution was
added potassium tert-butoxide (1.9 kg, 16.9 mol) in portions
over 30 min, maintaining the temperature below 8 °C. After
the addition was completed, the ice and water were removed,
and the reaction mixture was allowed to reach ambient
temperature and stirred for a total of 20 h. Hydrochloric acid
(36 L, 1.2 M) was introduced into the flask over 10 min.
The organic layer was separated, and the aqueous layer was
extracted with ethyl acetate (15 L). The combined organic
layers were washed with saturated aqueous sodium chloride
solution (2 × 6 L), and the solvents were removed under
reduced pressure (30 mmHg vacuum, final bath temperature
at 50 °C) on a rotary evaporator to provide the crude product
7 as a dark reddish-brown oil. This crude organic product
was then dissolved with warm (40 °C) methyl tert-butyl ether
(6 L) and allowed to cool to room temperature, with stirring,
for a total of 16 h. The resulting slurry was stirred for 30
min at 0 °C, after which the precipitate was collected by
vacuum filtration and washed with cold methyl tert-butyl
ether (2 L). The solid product was dried overnight (30
mmHg, 25 °C) to produce 2-hydroxyethyl N,N,N′,N′-tetrakis-
(2-chloroethyl)phosphorodiamidate (7) as an off-white pow-
der (2.90 kg, 53% yield): mp 79-81 °C; TLC Rf ) 0.50
(methanol/ethyl acetate ) 1:20); 1H NMR (CDCl3) δ 4.10-
4.20 (m, 2H), 3.80-3.90 (m, 2H), 3.60-3.70 (m, 8H), 3.40-
3.50 (m, 8H) ppm; 13C NMR (CD3OD) δ 68.7, 62.3 (d),
50.6, 43.3 ppm; IR (KBr) 3372, 2957, 1446 cm-1; MS (CI,
methane) m/z 391.07 [(C10H21Cl4N2O3P + H)+]. Anal. Calcd
for C10H21Cl4N2O3P: C, 30.79; H, 5.43; N, 7.18. Found: C,
30.89; H, 5.65; N, 7.10. A thermal analysis (DSC) showed
two exothermic decomposition ranges of 164-179 and 222-
231 °C. HPLC analysis chemical purity (tR ) 23.7 min)
showed one major peak, with a purity of 99.8%.
Experimental Section
Reagents and solvents were used as received from
vendors, and no attempts were made to purify or dry these
components further. Thin-layer chromatography was per-
formed using 1 in. × 3 in. Analtech GF 350 silica gel plates
with fluorescent indicator. Visualization of TLC plates was
made by observation in iodine vapors. The proton and carbon
magnetic resonance spectra were obtained on a Bruker AC
300 MHz nuclear magnetic resonance spectrometer, using
tetramethylsilane as an internal reference. Melting points
were obtained using an Electrothermal melting point ap-
paratus and are uncorrected. Infrared spectra were obtained
as KBr pellets and obtained on a Perkin-Elmer Spectrum
1000 FT-infrared spectrophotometer. CI mass spectroscopic
analyses were performed on a Shimadzu QP-5000 GC/mass
spectrometer (methane) by direct injection. Thermal analyses
were run on a Mettler Toledo DSC821e differential scanning
calorimeter. HPLC analysis was performed using a Zorbax
RX-SIL 5 µm column (4.6 mm × 250 mm) and a methanol/
chloroform (99:1) isocratic mobile phase (flow rate ) 1 mL/
min) with ELSD detection (drift tube, 45 °C, gas flow 1.6).
Preparation of N,N,N′,N′-Tetrakis(2-chloroethyl)phos-
phorodiamidic Chloride (6). A 72-L, three-neck, round-
bottom flask in an electric heating mantle was equipped with
an overhead mechanical stirrer, a water-cooled reflux con-
denser and a 5-L pressure-equalizing addition funnel capped
with a nitrogen inlet/outlet bubbler. The flask was charged
with phosphorus oxychloride (1.04 mL, 11.2 mol), bis(2-
chloroethyl)amine hydrochloride (2.0 kg, 11.2 mol), and
toluene (25 L). The stirrer was set to agitate at a moderate
rate (to provide a clear solution), and triethylamine (3.28 L,
23.5 mol) was added to the reaction mixture over 10 min.
The reaction mixture was then stirred for 26 h at room
temperature. To this mixture was charged bis(2-chloroethyl)-
amine hydrochloride (2.0 kg, 11.2 mol) and triethylamine
(3.3 L, 23.5 mol) over 10 min. The tan suspension was heated
to toluene reflux (110 °C) and stirred for 20 h. The mixture
was then cooled to room temperature and filtered to remove
the triethylamine hydrochloride salt. The solvent was re-
moved under reduced pressure (30 mmHg, final bath
temperature at 50 °C) on a rotary evaporator to produce
N,N,N′,N′-tetrakis(2-chloroethyl)phosphorodiamidic chloride
(6) as a reddish-brown oil (4.0 kg, 98% yield): TLC Rf )
0.71 (ethyl acetate/hexanes ) 1:1). The proton NMR
spectrum (CDCl3) was consistent with the proton NMR
spectrum of a commercially available batch of this material
(Aldrich, lot no. 11608 TZ).
Acknowledgment
We acknowledge and thank Juris Ekmanis, Ph.D., of
Albany Molecular Research, Inc. and Betsy R. Hughes of
Telik, Inc. for development of analytical methods, and Telik,
Inc. for permission to publish this body of work.
Received for review February 23, 2001.
OP010208K
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