Synthesis of phosphinate analogues of the phospholipid anti-tumour agent hexadecylphosphocholine (miltefosine)

Article abstract of DOI:10.1016/j.tetlet.2011.03.107

Efficient synthesis of phosphinate analogues (in six steps and 68-69% overall yields) of the anti-tumour agent miltefosine are reported, which involve a radical hydrophosphinylation addition reaction followed by conversion to the P(III) silyloxy intermediate and Michael-type addition as the key steps.

Full text of DOI:10.1016/j.tetlet.2011.03.107

Tetrahedron Letters 52 (2011) 2954–2956
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of phosphinate analogues of the phospholipid anti-tumour agent
hexadecylphosphocholine (miltefosine)
⇑
Marios S. Markoulides, Andrew C. Regan
School of Chemistry, The University of Manchester, Oxford Road, Manchester M13 9PL, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Efficient synthesis of phosphinate analogues (in six steps and 68–69% overall yields) of the anti-tumour
agent miltefosine are reported, which involve a radical hydrophosphinylation addition reaction followed
by conversion to the P(III) silyloxy intermediate and Michael-type addition as the key steps.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 3 December 2010
Revised 16 March 2011
Accepted 23 March 2011
Available online 1 April 2011
Keywords:
Phosphinic acids
Phospholipid analogues
Miltefosine
Anti-tumour agents
Carbon–phosphorus bonds
Silyl phosphonites
Radical hydrophosphinylation reaction
Alkylphosphocholine (APC) analogues (prototype: miltefosine 1,
phospholipid analogue 2, which should be resistant to hydrolysis
by phospholipid-metabolising enzymes (Fig. 1).
Fig. 1) of natural phospholipids are a family of anti-cancer drugs
1
with a wide range of pharmacological behaviour. There is a grow-
It is a characteristic of phosphinic acids that the carbon–
phosphorus bond is strongly resistant to hydrolytic cleavage. In
addition, their wide range of activity has been attributed to the
physical and structural similarity with biologically important
ing interest in synthetic anti-cancer phospholipids owing to their
selectivity against tumours, which has resulted in a much lower
toxicity as compared with some other classical anti-cancer chemo-
1
c
4
therapeutic agents.
phosphate esters and peptides.
Many conventional anti-tumour agents run the risk of damag-
ing healthy fast-proliferating tissues via a direct effect on cellular
DNA. Alkylphosphocholines display two important advantages in
contrast to common anti-tumour agents: (a) they target the plasma
membrane rather than directly interacting with cellular DNA, and
Carbon–phosphorus bonds have traditionally been constructed
2
by addition reactions of a phosphorus-centred radical to an sp car-
5
bon, or through interaction between a trivalent phosphorus atom
and an electrophilic centre (e.g., the Arbuzov and Pudovik reac-
6
7
tions). Our previous work in this latter field suggested the possi-
bility of preparing phosphinate analogue 2 from hypophosphorous
acid 5 by Michael-type addition to acrylonitrile 6, followed by
Arbuzov-type alkylation of the resulting 2-cyanoethylphosphinic
acid 4 with hexadecyl iodide 3 (Scheme 1, path A).
1b,2
(
b) they reveal a strong apoptosis-inducing ability.
A remark-
able feature of APCs lies in the fact that, whereas malignant cells
are highly sensitive to their lethal action, normal cells remain
relatively unaffected, illustrating the potentially selective anti-
tumour properties of this class of compounds.1
c
A second retrosynthetic analysis on the target molecule 2
(Scheme 1, path B) revealed the possibility of a potentially shorter
approach, by the sequential radical addition reaction of hypophos-
However, due to the presence of the phosphate diester func-
tionality, alkylphosphocholines are prone to biodegradation by
phospholipid-metabolising enzymes such as phospholipases C
1
b,3
and D.
In order to address this problem, replacement of both
of the O–P bonds of the phosphate diester in miltefosine 1 with
two C–P bonds would result in a sterically similar phosphinate–
O
15O
O
(
)
P
N
15P
O
N
(
)
O
O
1
: Miltefosine
2
⇑
Corresponding author. Tel./fax: +44 161 275 4617.
E-mail address: andrew.regan@manchester.ac.uk (A.C. Regan).
Figure 1. Structures of miltefosine 1 and phosphinate analogue 2.
0
040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.03.107

M. S. Markoulides, A. C. Regan / Tetrahedron Letters 52 (2011) 2954–2956
2955
PATH A:
Care should be taken in handling bis(trimethylsilyl) phosphonite
1, owing to its pyrophoric nature exhibited on exposure to air
1
Arbuzov-type
alkylation
O
Michael-type
addition
O
or moisture. Michael-type addition of the silyl phosphonite 11 to
acrylonitrile 6, followed by acidic hydrolysis of the silyl groups,
afforded the intermediate 2-cyanoethylphosphinic acid 4 in 81%
yield. Attempts to react the mono-substituted phosphinic acid 4
with hexadecyl iodide 3 using our silyl phosphonite alkylation
)15P
N
)15 I
P
(
(
H
CN
O
HO
7
b
methodology proved to be unsuccessful. Owing to the disap-
pointing results in obtaining 12 via the Arbuzov-type alkylation
of 4, it was then decided to change the order of the reactions. How-
ever, once again, attempts to alkylate 11 with hexadecyl iodide 3 to
give the mono-substituted phosphinic acid 7 were not successful.
Attention was then turned to free radical reactions (Scheme 3).
Mono-substituted hexadecyl- and octadecyl-phosphinic acids 7
and 14 were conveniently prepared under free radical conditions
from sodium phosphinate 13 and the appropriate terminal alkenes
2
3
4
O
CN
P
H
H
OH
5
6
PATH B:
Radical addition
Radical addition
5
O
O
in high yields. However, subsequent hydrophosphinylation of
)15P
N
P
N,N-dimethylallylamine 8 with hexadecylphosphinic acid 7, under
free radical conditions, afforded only a 3% yield of the desired addi-
tion product, which was isolated as a phosphinate salt 15 (formed
with the starting phosphinic acid 7). Extensive experimentation in
efforts to improve this free radical step, in order to obtain synthet-
N
(
( )15
H
O
OH
2
7
8
ically useful yields, proved to be unsuccessful. Attempts included
O
P
8
the use of different radical initiators such as AIBN or Et
and also the use of acrylonitrile as the substrate.
3 2
B/O ,
(
)13
H
H
OH
5
The phosphinate salt 15 was then dissolved in hot ethyl acetate
9
and treated with concentrated hydrochloric acid to give the zwit-
3
1
terionic neutral form 16. The P (proton-decoupled mode) NMR
spectrum of 16 displayed a signal at d 53.59 for P–O( , which
moved downfield to d 56.69 for P–OH when acidified with hydro-
chloric acid.
Scheme 1. Retrosynthetic analyses of 2.
ꢀ)
phorous acid 5 to hexadecene 9 and N,N-dimethylallylamine 8 (in
either order). The involvement of hypophosphorous acid 5 in the
Efficient syntheses of phosphinate analogues of miltefosine 1
were finally realised as outlined in Scheme 4, by combining the
high-yielding radical-mediated preparation of mono-alkyl phos-
phinic acids 7 and 14 with a silyl phosphonite mediated addition
as the second step. Following a similar procedure to that described
for the preparation of 2-cyanoethylphosphinic acid 4, the desired
di-substituted phosphinic acids 12 and 19 were successfully
formed in high yields. Initial attempts at esterification of 12 and
formation of phosphinates through radical chain reactions has
been reported by Nifant’ev.5 Strategically, both routes could offer
useful flexibility over the order of assembly of the hydrophilic
polar head group and the hydrophobic tail of the phosphinate ana-
logue 2.
We describe herein an efficient synthesis of phosphinate ana-
logue 2, exploiting a combination of our Michael-type addition
protocol using silyl phosphonites together with a radical addition
step, in order to form the two key C–P bonds. This has also enabled
us to prepare further analogues for biological testing.
1
9 included the use of Cs
2 3 3 3
CO or Et N together with MeI, Et N with
ethyl chloroformate, and reaction with an alcohol with azeotropic
9
a
removal of water. However, all these attempts proved to be low
yielding (<20%). The best results were achieved upon refluxing
phosphinic acids 12 and 19 in excess trimethyl orthoformate.9b
Hydrogenation of the nitriles 20 and 21 at atmospheric pressure
in the presence of Raney-Nickel then afforded the amines 22 and
23 in 95% and 94% yields, respectively. Other reducing conditions,
Ammonium hypophosphite 10 was conveniently prepared in
quantitative yield by neutralisation of 50% aqueous hypophospho-
rous acid 5 with ammonium hydroxide solution (Scheme 2). Dou-
ble silylation of the salt 10 using hexamethyldisilazane (HMDS)
afforded, after careful fractional distillation of the reaction mixture,
7
the highly nucleophilic bis(trimethylsilyl) phosphonite (BTSP) 11.
O
O
O
P
O
P
O
P
O
TMSO
TMSO
H
a
b
c, d
81%
P
a
b
3%
P
( )15P
N
P
H
H
H
H
H
H
( )
n
H
H
99%
85%
H
CN
O
O
OH
HO
HO
OH
NH4
O
P
Na
5
10
11
4
1
3
7: (n = 15) 91%
14: (n = 17) 91%
15
H
(
)15
e, f
g-i
O
c
97.5%
O
O
P
P
O
(
)15
( )15
H
CN
H
HO
OH
P
N
(
)15
7
12
O
Scheme 2. Reagents and conditions: (a) concd NH
4
OH (aq), 0 °C, 2.5 h, toluene
Cl , 0 °C, 2 h, then rt,
Cl , 0 °C to reflux, 2
, 0 °C to rt, 2 h; (h)
16
azeotrope; (b) HMDS, 120–130 °C, 2 h; (c) acrylonitrile, CH
2
2
+
1
2 h; (d) THF–H
3
O , 0 °C to rt, 2 h; (e) hexadecyl iodide, CH
2
2
Scheme 3. Reagents and conditions: (a) terminal alkene, concd H
reflux, 1 day; (b) N,N-dimethylallylamine 8, 1,1 -azobis(cyclohexanecarbonitrile),
2 4
SO , AIBN, EtOH,
+
0
days; (f) THF–H
3
O , 0 °C to rt, 2 h; (g) TMSCl, Et
3
N, CH
2
Cl
2
+
hexadecyl iodide, rt to reflux, 5 days; (i) THF–H
3
O , 0 °C to rt, 2 h.
EtOH, reflux, 5 days; (c) concd HCl, hot EtOAc, rt, 1 h.

2
956
M. S. Markoulides, A. C. Regan / Tetrahedron Letters 52 (2011) 2954–2956
O
P
O
P
In summary, an efficient and flexible synthetic strategy has
OTMS
P
a
b, c
been developed for the synthesis of phosphinate analogues of the
anti-tumour agent hexadecylphosphocholine (miltefosine) 1, mak-
ing use of a radical hydrophosphinylation addition reaction of ter-
minal olefins to introduce the hydrophobic tail, in combination
with a Michael-type addition protocol using silyl phosphonites to
attach the hydrophilic polar head group. Overall, the synthesis of
(
)n
H
( )
n
CN
( )
n
OTMS
OH
OH
7
1
: (n = 15)
4: (n = 17)
17: (n = 15)
18: (n = 17)
12: (n = 15) 90%
19: (n = 17) 91%
d
+
+
phosphinate analogues 2 (C16, NMe
3
), 26 (C18, NMe
3
) and 28
+
(
C16, NH
3
) proceeded in six steps and 68–69% overall yields. By
O
P
O
suitable editing of the hydrophobic tail and the hydrophilic polar
head group, further nonhydrolysable analogues may be designed
in order to explore biological structure–activity relationships.
e
NH2
P
(
)
( )n
n
CN
OMe
OMe
2
2: (n = 15) 95%
20: (n = 15) 97%
21: (n = 17) 98%
Acknowledgments
23: (n = 17) 94%
We thank the University of Manchester and the A.G. Leventis
Foundation (Cyprus) for financial support, and Dr. C.I.F. Watt at
Manchester for valuable assistance and helpful discussions.
f
i
96%
O
O
P
P
N
NH3
Cl
(
)n
( )15
Supplementary data
OMe
OMe
27
I
2
2
4: (n = 15) 94%
5: (n = 17) 93%
Supplementary data (experimental procedures and character-
1
13
isation data for all new compounds along with copies of H,
C
3
1
g, h 95%
and P NMR spectra) associated with this article can be found,
in the online version, at doi:10.1016/j.tetlet.2011.03.107.
g, h
O
O
P
NH3
References and notes
(
)15
P
N
O
(
)n
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