Synthesis of 6-aryl-substituted cholesterol derivatives from 3-acetoxy-6-iodocholest-5-ene and arylboronic acids promoted by palladium-catalyzed cross-coupling

Article abstract of DOI:10.1055/s-0029-1218557

3-Acetoxy-6-iodocholest-5-ene was prepared with a convenient method from cholesterol and used as a key intermediate for the synthesis of 6-arylated cholesterol derivatives using a Suzuki-Miyaura cross-coupling. This versatile and efficient synthesis way can widely be used in the synthesis of other 6-aryl or 6-heteroaryl steroids as potential drugs.

Full text of DOI:10.1055/s-0029-1218557

LETTER
215
Synthesis of 6-Aryl-Substituted Cholesterol Derivatives from
3b-Acetoxy-6-iodocholest-5-ene and Arylboronic Acids Promoted by
Palladium-Catalyzed Cross-Coupling
S
ynthesis of 6-Ary
o
l-Substitute
d
m
Cholesterol
D
eriva
i
tive
s
nique Brossard,a Laïla El Kihel,*a Mohamed Khalid,b Sylvain Raulta
a
Centre d’Etudes et de Recherche sur le Médicament de Normandie, UFR des Sciences Pharmaceutiques,
Boulevard Becquerel, 14032 Caen Cedex, France
Fax +33(2)31566803; E-mail: laila.elkihel@unicaen.fr
Université Hassan Premier, Faculté des Sciences et Techniques, Km 3, Route de Casablanca, BP 577, 26000 Settat, Morocco
b
Received 20 October 2009
7-positions in the ring B are the most sensitive sites for
reactions involved in various biological mechanisms.9
Abstract: 3b-Acetoxy-6-iodocholest-5-ene was prepared with a
convenient method from cholesterol and used as a key intermediate
for the synthesis of 6-arylated cholesterol derivatives using a
Suzuki–Miyaura cross-coupling. This versatile and efficient syn-
thesis way can widely be used in the synthesis of other 6-aryl or 6-
heteroaryl steroids as potential drugs.
By analogy to oxysterols, we have developed in previous
studies, a series of aminosterols which have demonstrated
promising activities.10 There is an increasing interesting
development of new strategies to introduce other groups
into specific positions of steroidal nuclei to modify their
biological properties. Two strategies have been described
in the literature. The first is the introduction of an hetero-
atom on the steroidal skeleton,11 and the second is the
functionalization of the steroid which was used of various
steroids with potential therapeutic applications.12
Key words: cross-coupling, palladium, 6-aryl steroids, Suzuki
reaction, 6-iodovinyl steroid
Cholesterol is most widely known for its association with
arteriosclerotic heart disease; however, it is a necessary
constituent of all our cells. Cholesterol plays a diverse ar-
ray of functions in mammalian tissues. This sterol is a ma-
jor lipid constituent of cellular membrane, and is involved
in cell-signaling pathways and gene transcription.1 A pre-
vious study attests to the vital roles of cholesterol in nor-
mal mammalian central nervous system development and
function, and in a host of inherited and acquired neurolog-
ical conditions.2 Brain cholesterol is mainly produced in
situ, and its metabolism is regulated independently of that
in peripheral tissues.3 Cholesterol is an essential precursor
for steroid hormones and bile acids. A third group of its
metabolites, the oxysterols, are oxidized forms of choles-
terol.4 These oxysterols, synthesized in the brain (called
neurosterols) and in peripheral tissues, mediated feedback
regulation of cholesterol biosynthesis rather than choles-
terol itself.5 Many studies have focused on the effects of
oxysterols and showed diverse biological properties.6
A series of 17-pyridyl-androstene was prepared from 3b-
acetoxy-17-triflyloxy-5a-androst-16-ene (2-, 3-, and 4-
pyridyl) as inhibitors of human cytochrome P45017a (17a-
hydroxylas-C17,20-lyase), which is a key enzyme in an-
drogen hormone biosynthesis.13 Some other heterocyclic
groups were introduced to the C-17 position of 3b-
hydroxy-androsta-5,16-diene.14 Herein we focused on the
incorporation of aryl substituent on the C-6 position.
However, to the best of our knowledge, the ring B of ste-
roids has received little attention, and it was also noticed
that few aryl or heterocyclic groups were introduced in
this ring via C–C coupling reaction. The coupling of 17-
acetoxy-6-chloro-pregna-4,6-diene-3,20-dione (commer-
cial starting material) and arylboronic acids was carried
out by the Suzuki–Miyaura cross-coupling reaction.15
As part of our ongoing medicinal chemistry program, the
aim of our project is based on the introduction of an aryl
group on the C-6 position of 3b-acetoxycholest-5-ene
skeleton. The main goal of our work is to develop a con-
venient synthetic method of 3b-acetoxy-6-iodocholest-5-
ene, used as a key intermediate, which is explored as a
building block for the synthesis of 6-arylated cholesterol
derivatives by Suzuki–Miyaura cross-coupling reaction
(Scheme 1).
Cholesterol is composed of three regions: a ring-structure
region with four hydrocarbon rings (A, B, C, and D), a
side chain on ring D, and a b-hydroxyl group. Oxidation
in cells can occur on the ring B of the sterol, particularly
on the C-7 position (7-ketocholesterol, 7a-hydroxycho-
lesterol, and 7b-hydroxycholesterol), or on the side chain
[25-hydroxycholesterol, 20(S)-hydroxycholesterol, 22(R)-
hydroxycholesterol, 24(S)-hydroxycholesterol, 27-hy-
droxycholesterol].7 The double bond D5–6 can be also the
target of free radical attacks (5,6a- and 5,6b-epoxy-
cholestanol).8 These structures showed that the 5-, 6-, and
In the literature and to our knowledge, 6-iodocholest-5-
en-3b-ol has been described in one paper from cholesterol
in four steps via 6-acetoxymercuricholest-5-ene interme-
diate. However, this method leads to a mixture of com-
pounds and that proceeds with low yields (2–20%).16
Furthermore, the high toxicity of organomercuric com-
pounds has been observed and widely studied.17
SYNLETT 2010, No. 2, pp 0215–0218
x
x.
x
x.
2
0
1
0
Advanced online publication: 09.12.2009
DOI: 10.1055/s-0029-1218557; Art ID: G35109ST
© Georg Thieme Verlag Stuttgart · New York

216
D. Brossard et al.
LETTER
O
O
O
i
O
H
H
O
N
H2N
1
2
ii
O
O
O
iii
O
Ar
I
4a–j
3
Scheme 1 Reagents and conditions: (i) N2H4·xH2O, AcOH, EtOH, reflux, 12 h; (ii) I2, 1,1,3,3-tetramethylguanidine, anhyd THF, 50 °C,
60 h; (iii) aryl boronic acid, Pd(OAc)2, Ph3P, K2CO3, DMF, 150 °C, 12–18 h.
For these reasons, this method cannot be applicable to the The hydrazone 2 is oxidized by iodine to the diazo com-
development of biological compounds. On the other hand, pound 2a which reacts further to give the iodo derivative
an improved preparation of vinyl iodide on the C-17 posi- 2b. A loss of nitrogen leads to the key iodo carbocation 2c.
tion of the steroid skeleton was described by Barton.18 We Elimination of proton on C-5 position of steroid skeleton
explore here this method for the synthesis of 3b-acetoxy- gives vinyl iodide 3.
6-iodocholest-5-ene (3) which was prepared as depicted
in Scheme 1.
The second goal of our work is to prepare the 6-aryl-sub-
stituted cholesteryl acetate 4a–j by palladium-mediated
3b-Acetoxy-5a-cholestan-6-one (1) has been prepared reaction of 3b-acetoxy-6-iodocholest-5-ene (3) with vari-
from cholesterol according to our reported procedure.10b ous arylboronic acids. Though the Suzuki–Miyaura cou-
The hydrazone derivative 2 has been obtained in a good pling is a universal and widely employed tool in organic
yield from the latter ketone 1, using hydrazine hydrate and synthesis, a protocol adjustment is often needed to obtain
acetic acid in ethanol.19 This hydrazone 2 was then oxi- the optimal performance with the catalytic system. The
dized by iodine in the presence of tetramethylguanidine step reaction iii (Scheme 1) has been improved by explor-
base leading to 3b-acetoxy-6-iodocholest-5-ene (3).20 The ing various reagents and conditions (Table 1). The
mechanism proposed for the transformation of hydrazone progress of the reaction was followed by thin-layer chro-
to iodovinyl is indicated in Scheme 2.
matography. These conditions have also been carried out
O
O
I2
oxidation
O
O
H
H
N
N+
2
2a
I+
N
H2N
O
O
+
O
O
H
I
H
I
N+
2c
2b
N
O
O
3
I
Scheme 2 Mechanism proposed of ketone hydrazone oxidation by iodine
Synlett 2010, No. 2, 215–218 © Thieme Stuttgart · New York

LETTER
Synthesis of 6-Aryl-Substituted Cholesterol Derivatives
217
Table 1 Optimization of Suzuki Coupling Conditions with p-Methoxyphenylboronic Acid
Catalyst
Base
Solvent
DME
Temp (°C)
70
Time (h)
Yield (%)
Pd(PPh3)2Cl2
CuI, DIPA
K2CO3
K2CO3
CsF, CuI
K2CO3
28
24
24
24
12
–
Pd(PPh3)2Cl2
dioxane–H2O (3:1)
DMF
100
–
Pd(OAc)2, P((OMe)3Ph)3
Pd(OAc)2, P((OMe)3Ph)3
Pd(OAc)2, Ph3P
150
<10
<10
50
DMF
150
DMF
150
under microwave irradiation but none improvement in In summary, we have reported a convenient synthetic
yield reactions was achieved.
method of 3b-acetoxy-6-iodocholest-5-ene as a key inter-
mediate for C–C cross-coupling reaction. This versatile
method can be used to prepare other various 6-iodovinyl-
steroids, which were coupled with various aryl or het-
eroaryl boronic acids in order to prepare scaffolds for
potent chemical drugs libraries.
The best conditions found were palladium acetate, triphe-
nylphosphine, and potassium carbonate in DMF with con-
ventional heating (12–18 h). Results are presented in
Table 2.
Table 2 Yields of 3b-Acetoxy-6-arylcholest-5-ene21
Acknowledgment
Arylboronic acid
Time (h) Product 4a–j Yield (%)
This work was financially supported by Conseil Régional Basse-
Normandie. The authors thank Dr Rémi Legay for his assistance in
the measurement of mass spectra.
12
12
12
4a
4b
4c
50
50
42
(HO)2B
OMe
(HO)2B
References and Notes
(1) (a) Simons, K.; Ikonen, E. Science 2000, 290, 1721.
(b) Maxfield, F. R.; Tabas, I. Nature (London) 2005, 438,
612.
(HO)2B
t-Bu
(2) Dietschy, J. M.; Turley, S. D. J. Lipid Res. 2004, 45, 1375.
(3) Andersson, M.; Elmberger, P. G.; Edlund, C.; Kristensson,
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Jessup, W. Redox Rep. 2006, 11, 255.
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Y.; Karu, K.; Griffiths, W. J. Biochimie 2007, 89, 182.
(c) Wang, Y.; Griffiths, W. J. Neurochem. Int. 2008, 52, 506.
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(b) Volle, D. H.; Lobaccaro, J.-M. A. Mol. Cell. Endocrinol.
2007, 265. (c) Javitt, N. B. Steroids 2008, 73, 149.
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M.; Roussakis, C.; El Kihel, L. Tetrahedron 2004, 60,
11477. (c) Bazin, M. A.; Loiseau, P. M.; Bories, C.;
Letourneux, Y.; Rault, S.; El Kihel, L. Eur. J. Med. Chem.
2006, 41, 1109. (d) Bazin, M. A.; El Kihel, L.; Boulouard,
M.; Bouët, V.; Rault, S. Steroids 2009, 74, 931.
(11) Dubis, A. T.; Lotowski, Z.; Siergiejezyk, L.; Wilezewska,
A. Z.; Morzycki, J. W. J. Chem. Res. 1998, 170.
(12) El Kihel, L.; Soustre, I.; Karst, F.; Letourneux, Y. FEMS
Microbiol. Lett. 1994, 120, 163.
OMe
18
4d
67
(HO)2B
OMe
OMe
18
18
18
4e
4f
50
46
79
(HO)2B
(HO)2B
OCF3
SMe
O
4g
(HO)2B
(HO)2B
OMe
18
4h
64
F
COOEt
CN
18
15
4i
4j
62
29
(HO)2B
(HO)2B
Our method has been applied to diverse arylboronic acids,
which have various functions (such as protected alcohol,
ester, cyano group, ketone, sulfur product, and trifluo-
romethoxy group) with correct yields.
Synlett 2010, No. 2, 215–218 © Thieme Stuttgart · New York

218
D. Brossard et al.
LETTER
(13) Jarman, M.; Barrie, S. E.; Llera, J. M. J. Med. Chem. 1998,
41, 5375.
NaHCO3, sat. Na2S2O3, H2O, and dried over MgSO4. The
crude product was purified by column chromatography
(silica gel, cyclohexane–EtOAc = 9:1) to give the product as
a yellow oil (4.55 g, 88%). IR (KBr): n = 2935 (CH alkane),
1735 (C=O ester), 1030 (C=CI) cm–1. 1H NMR (400 MHz,
CDCl3): d = 4.64–4.58 (m, 1 H, C3H), 2.94 and 2.93 (each 1
H, m, C4aH and C4bH), 2.63 and 2.60 (each 1 H, m, C7aH and
C7bH), 2.05 (s, 3 H, CH3COO), 1.07 (s, 3 H, 19-Me), 0.91 (d,
J = 6.83 Hz, 3 H, 21-Me), 0.87 and 0.85 (each 3 H, d,
J = 1.96 Hz, 26-Me and 27-Me), 0.66 (s, 3 H, 18-Me) ppm.
13C NMR (100 MHz, CDCl3): d = 170.3 (CH3COO), 142.8
(C-5), 101.3 (C-6), 72.7 (C-3), 55.9 (C-14, C-17), 49.6 (C-
9), 42.4 (C-13), 41.2 (C-12), 39.5 (C-7, C-24), 36.8 (C-4),
36.1 (C-1), 35.7 (C-22), 35.3 (C-20), 34.4 (C-10), 28.2 (C-
16), 27.9 (C-25), 27.7 (C-2), 24.1 (C-15), 23.8 (C-23), 22.8
(C-27), 22.5 (C-26), 21.4 (C-11), 21.2 (CH3COO), 19.7 (C-
19), 18.7 (C-21), 11.8 (C-18) ppm.
(14) (a) Njar, V. C. O.; Kato, K.; Nnane, I. P.; Grigoryev, D. N.;
Long, B. J.; Brodie, A. M. H. J. Med. Chem. 1998, 41, 902.
(b) Ling, Y. Z.; Li, J. S.; Liu, Y.; Kato, K.; Klus, G. T.;
Brodie, A. J. Med. Chem. 1997, 40, 3297. (c) Zhu, N.; Ling,
Y.; Lei, X.; Handratta, V.; Brodie, A. M. H. Steroids 2003,
68, 603.
(15) Lukashev, N. V.; Latyshehev, G. V.; Donez, P. A.; Skryabin,
G. A.; Beletskaya, I. P. Synthesis 2006, 3, 533.
(16) Flagan, R. J.; Charleson, F. P.; Synnes, E. I. Can. J. Chem.
1985, 63, 2853.
(17) Gibicar, D.; Logar, M.; Horvat, N.; Marn-Pernat, A.;
Ponikvar, R.; Horvat, M. Anal. Bioanal. Chem. 2007, 388,
329.
(18) Barton, D. H. R.; Bashiardes, G.; Fourrey, J.-L. Tetrahedron
Lett. 1983, 24, 1605.
(19) Synthesis of 3b-Acetoxy-5a-cholestane-6-hydrazone (2)
Hydrazine hydrate (0.42 mL, 13.5 mmol) was added to a
solution of 3b-acetoxy-5a-cholestan-6-one (1, 3.00 g, 6.8
mmol) and AcOH (0.01 mL, 0.2 mmol) in EtOH (70 mL).
The mixture was refluxed for 24 h. The reaction mixture was
concentrated, diluted with H2O (70 mL) and then extracted
with CH2Cl2 (4 × 30 mL). The combined CH2Cl2 extracts
were dried over MgSO4 and evaporated under reduced
pressure to give compound 2 as a white powder (3.06 g,
99%); mp 56 °C. IR (KBr): n = 3480–3300 (NH2
hydrazone), 2950–2875 (CH alkane), 1727 (C=O ester),
1651 (C=N) cm-1. 1H NMR (400 MHz, CDCl3): d = 4.96 (s,
2 H, NH2 D2O exchange), 4.69–4.66 (m, 1 H, C3H), 2.92–
2.97 (dd, 1 H, J = 13.28, 4.48 Hz, C5H), 2.04–2.10 (each 1
H, m, C7aH and 7b-CH), 2.06 (s, 3 H,CH3COO), 0.92–0.89
(m, 3 H, 19-Me), 0.87 and 0.86 (each 3 H, d, J = 1.96 Hz, 26-
Me and 27-Me), 0.63 (s, 3 H, 18-Me) ppm.13C NMR (100
MHz, CDCl3): d = 170.6 (CH3COO), 154.1 (C-6), 73.7 (C-
3), 56.4 (C-14), 56.1 (C-17), 50.5 (C-9), 48.8 (C-13), 42.9
(C-5), 39.4 (C-12), 38.8 (C-10), 36.0 (C-1, C-20, C-22), 35.6
(C-7, C-8), 29.3 (C-4), 28.0 (C-16, C-25), 28.9 (C-2), 24.0
(C-19), 23.7 (C-15, C-23), 22.7 (C-26), 22.5 (C-27), 21.3
(C-11), 21.3 (CH3COO), 19.6 (C-21), 11.9 (C-18) ppm.
(20) Synthesis of 3b-Acetoxy-6-iodocholest-5-ene (3)
A 100 mL round-bottomed flask equipped with a magnetic
stirring bar was charged with iodine (4.76 g, 18.8 mmol) and
dry THF (5 mL) and flushed with nitrogen. The reactor was
pushed into an ice bath (0 °C), and then 1,1,3,3-tetramethyl-
guanidine (6 mL, 46.9 mmol) was added. Compound 5 was
then dissolved into dry THF (30 mL) and added dropwise,
and the solution was kept under stirring at 0 °C for 2 h. The
mixture was filtered and the solvent evaporated. The
solution was then stirred at 50 °C for 48 h. The reaction
mixture was dissolved in Et2O, washed with 1 N HCl, 5%
(21) General Procedure for the Coupling of 6-Iodovinyl-
steroid with Boronic Acids
In a vial with a screw cap, compound 3 (0.30 g, 0.5 mmol),
boronic acid (1.1 mmol), Pd(OAc)2 (0.01 g, 0.05 mmol),
Ph3P (0.03 g, 0.1 mmol), and K2CO3 (0.19 g, 1.4 mmol) were
mixed in DMF (6 mL) under Ar atmosphere. The mixture
was stirred at 150 °C for 12–18 h, then diluted with H2O (30
mL), and extracted with Et2O (4 × 15 mL). The combined
organic layers were dried over MgSO4. The crude product
was purified by column chromatography on deactivated
alumina (6% H2O), eluting by cyclohexane–EtOAc (99:1).
3b-Acetoxy-6-(4-methoxy)phenylcholest-5-ene (4a):
White amorphous solid. Yield: 48 mg (50%). IR (KBr): n =
2928 (CH alkane), 1716 (C=O ester), 1607–1509 (C=C)
cm–1. 1H NMR (400 MHz, CDCl3): d = 7.01 (d, 2 H, J = 8.79
Hz, H-ar), 6.85 (d, 2 H, J = 7.79 Hz, H-ar),4.62–4.50 (m, 1
H, C3H), 3.80 (s, 3 H, OCH3), 2.54 and 2.50 (each 1 H, m,
C4aH and C4bH), 2.20 and 2.16 (each 1 H, m, C7aH and
C7bH), 1.95 (s, 3 H, CH3COO), 1.10 (s, 3 H, 19-Me), 0.92 (d,
3 H, J = 6.83 Hz, 21-Me), 0.87 and 0.85 (each 3 H, d,
J = 1.96 Hz, 26-Me and 27-Me), 0.71 (s, 3 H, 18-Me) ppm.
13C NMR (100 MHz, CDCl3): d = 170.4 (CH3COO), 157.9
(C-4ar), 136.4 (C-6), 134.7 (C-5), 133.6 (C-1-ar), 129.2 (C-
2 and C-2¢-ar), 113.5 (C3 and C-3¢-ar), 73.9 (C-3), 56.6 (C-
14), 56.1 (C-17), 55.2 (OCH3), 49.9 (C-9), 42.3 (C-13), 39.8
(C-24), 39.6 (C-12), 39.5 (C-1), 37.3 (C-22), 36.9 (C-20),
36.2 (C-7), 35.8 (C-10), 32.8 (C-8), 32.0 (C-4), 28.3 (C-16),
27.9 (C-25), 27.7 (C-2), 24.2 (C-15), 23.8 (C-23), 22.8 (C-
26 and C27), 21.4 (C-11), 21.1 (CH3COO-), 19.5 (C-19),
18.7 (C-21), 11.9 (C-18). MS (30eV, IE): m/z = 534.3 (8)
[M+], 474.3 (100) [M+ – CH3COOH], 459.3 (8), 368.3 (25).
Anal. Calcd for C36H54O3 (534.81): C, 80.85; H, 10.18.
Found: C, 80.42; H, 9.77.
Synlett 2010, No. 2, 215–218 © Thieme Stuttgart · New York

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