Short synthesis of 2-methoxyestradiol and 2-hydroxyestradiol

Article abstract of DOI:10.1016/S0039-128X(03)00035-7

The estrogen metabolite 2-methoxyestradiol was synthesized from estradiol bis-THP-ether which was 2-hydroxylated using the superbase LIDAKOR, trimethyl borate, and H₂O₂, then methylated and deprotected to obtain 2-methoxyestradiol in three steps and 61% yield. 2-Hydroxyestradiol was obtained by deprotecting the 2-hydroxyestradiol bis-THP-ether from the first step.

Full text of DOI:10.1016/S0039-128X(03)00035-7

Steroids 68 (2003) 373–375
Short synthesis of 2-methoxyestradiol and 2-hydroxyestradiol
Paula S. Kiuru, Kristiina Wähälä∗
Department of Chemistry, Organic Chemistry Laboratory, University of Helsinki, A. I. Virtasen aukio 1,
P.O. Box 55, FIN-00014 Helsinki, Finland
Received 15 January 2003; received in revised form 19 February 2003; accepted 25 February 2003
Abstract
Theestrogenmetabolite2-methoxyestradiolwassynthesizedfromestradiolbis-THP-etherwhichwas2-hydroxylatedusingthesuperbase
LIDAKOR, trimethyl borate, and H2O2, then methylated and deprotected to obtain 2-methoxyestradiol in three steps and 61% yield.
2-Hydroxyestradiol was obtained by deprotecting the 2-hydroxyestradiol bis-THP-ether from the first step.
© 2003 Elsevier Science Inc. All rights reserved.
Keywords: Steroid; Estrogens; Synthesis; Metabolites; 2-Methoxyestradiol; 2-Hydroxyestradiol
1. Introduction
sized from 2-aminoestradiol via reduction of the derived
o-quinone [12].
We report here the 2-hydroxylation of estradiol bis-THP
ether (3), which affords the estrogen metabolite 2-methoxy-
estradiol (1) in three steps and 61% overall yield (Scheme 1).
2-Methoxyestradiol (1) has become a target of interest in
biological studies due to its cancer preventing properties. It
has been proven to inhibit the proliferation of various cancer
cell lines, e.g. breast cancer, it inhibits tubulin polymeriza-
tion, and it is antiangiogenic [1].
2. Experimental
2-Methoxyestradiol (1) has been prepared by the
methoxylation of 2-iodo or 2-bromoestradiol using various
catalysts [2–5], but a limiting step in this route is the lack of
viable regioselective halogenation procedures. Le Bras et al.
have used a somewhat complicated “Cp∗Ir” organometal-
lic approach to the synthesis of 2-methoxyestradiol (1) in
five steps and 60% overall yield [6]. Another route (four
steps, 66% overall) to (1) relies on the formylation of
MOM-protected 2-lithioestradiol with DMF at −78 ◦C
in 86% yield [7,8], followed by Baeyer–Villiger oxi-
dation, methylation, and MOM cleavage [9]. Estradiol
diacetate undergoes a selective ZrCl4-mediated fries re-
arrangement to give the 2-acetyl derivative which fur-
nishes 2-methoxyestradiol (1) after 3-OH protection as
the benzyl ether, Baeyer–Villiger oxidation, hydrolysis of
the resulting 2-acetate, methylation, and benzyl cleavage
(49% overall from estradiol in seven steps) [10]. Earlier a
similar route was used for the Friedel–Crafts 2-acylation
of 3-methoxyestradiol 17-acetate (26% overall yield in
eight steps) [11]. 2-Hydroxyestradiol (2) has been synthe-
2.1. General methods
2-Methoxyestradiol and 2-hydroxyestradiol were charac-
1
terized by H, 13C, and 2D (GHSQC, GHMQC, RelayH,
and NOESY) NMR. NMR spectra were recorded on Varian
GEMINI 2000 (200 MHz) or Varian Inova 300 WB spec-
trometers. Chemical shifts are given in δ and J values in
Hz. CDCl3 was used as a solvent and tetramethylsilane was
used as an internal standard. Mass spectra were obtained
with a JEOL JMS SX102 mass spectrometer at 70 eV. Melt-
ing points were determined in open capillary tubes with a
Büchi B-545 melting point apparatus, and are uncorrected.
TLC was conducted on Merck silica gel 60 F254 plates and
visualized with UV light and with a mixture of isovanillin
and sulfuric acid in ethanol. Estradiol was purchased from
Steraloids. THF was obtained from JT Baker and dried with
continuous reflux with sodium and benzophenone as indi-
cator. DMF was kept over CaH2 and distilled prior to use.
2.1.1. 2-Hydroxyestradiol bis-THP ether (4)
n-BuLi (1.3 M, 72 ml, 14 Eq.) followed by potassium
1,1-dimethylpropoxide in THF/hexane (2.5 M, 11 ml, 4 Eq.)
∗
Corresponding author. Tel.: +358-9-191-50356;
fax: +358-9-191-50357.
E-mail address: kristiina.wahala@helsinki.fi (K. Wähälä).
0039-128X/03/$ – see front matter © 2003 Elsevier Science Inc. All rights reserved.
doi:10.1016/S0039-128X(03)00035-7

374
P.S. Kiuru, K. Wähälä / Steroids 68 (2003) 373–375
Scheme 1. Synthesis of 2-methoxyestradiol (1).
and diisopropylamine (3.8 ml, 4 Eq.) were added to 50 ml of
dry THF at −78 ◦C under Ar. After 5 min, 3 g (6.8 mmol) of
estradiol bis-THP ether (3) [13] in 10 ml hexane was added
with stirring. After 3 h, 10.7 ml of trimethyl borate was
added and the flask placed in an ice bath for 2 h. H2O2 (30%,
21 ml) was added and the reaction mixture was allowed
to reach ambient temperature. After 1 h, 100 ml of 10%
aqueous Na2S2O3 was added and the mixture was extracted
with ethyl acetate. The aqueous phase was extracted twice
with ethyl acetate, the combined organic phases washed
with water, dried (anhydrous Na2SO4) and the solvent re-
moved to furnish the crude (4) (an oil, 3.1 g, 99%), used
for the next step without purification. 1H NMR (CDCl3,
200 MHz) δ: 0.79, 0.81 (2s, 3H, 18-CH3), 1.18–2.2 (25H),
2.72 (m, 2H, H-6), 3.4–3.8 (m, 3H), 3.8–4.1 (m, 2H), 4.65
(m, 1H, CH–O–C(17)), 5.10 (m, 1H, CH–O–C(3)), 6.77 (s,
1H, H-4), 6.88, 6.90 (2s, 1H, H-1).
acetyl chloride was heated slowly to 60 ◦C. After 1 h the
hydrolysis was complete according to TLC. The mixture was
poured into water and extracted with diethyl ether. Combined
ether layers were washed with water, dried with Na2SO4
and evaporated. The crude product (96%) was purified with
flash chromatography (CHCl3–acetone 40:1) to give (1) in
64% yield. (1) was recrystallized from CHCl3–acetone to
afford white crystals. Melting point: 186–187 ◦C (lit. mp
188–191 ◦C [14]). H NMR (CDCl3, 300 MHz) δ: 0.79 (s,
1
3H, CH3), 1.2 (m, 1H, H-14), 1.3 (m), 1.35 (2H, m, 15␣),
1.4 (m, 1H, H-8), 1.5 (m, 1H), 1.55 (m, 1H), 1.7 (m, 1H),
1.85 (m, 1H), 1.9 (qd, 1H, H-12␣), 2.1 (m, 1H), 2.2 (m,
1H, H-9), 2.3 (m, 1H), 2.77 (m, 2H, H-6), 3.74 (dd, 1H,
H-17␣), 3.86 (s, 3H, OCH3), 5.43 (bs, 1H, ArOH), 6.64 (s,
1H, H-4), 6.80 (s, 1H, H-1). 13C NMR (CDCl3, 300 MHz)
δ: 11.1 (C-18), 23.1 (C-15), 26.6 (C-11), 27.3 (C-7), 29.0
(C-6), 30.7 (C-16), 36.8 (C-12), 38.8 (C-8), 43.3 (C-13),
44.3 (C-9), 50.1 (C-14), 56.1 (Ar–OCH3), 81.9 (C-17), 108.1
(C-1), 114.6 (C-4), 129.6 (C-5), 131.8 (C-10), 143.5 (C-3),
144.6 (C-2). HRMS calculated for C19H26O3: 302.1873;
found: 302.1882.
2.1.2. 2-Methoxyestradiol bis-THP ether (5)
2-Hydroxyestradiol bis-THP ether (4) (2.0 g, 4.4 mmol)
was dissolved in 25 ml of dry DMF and 6 g (44 mmol) of
anhydrous potassium carbonate and 32 mg of tetra-n-butyl-
ammonium iodide were added. The solution was stirred
for 15 min under argon followed by addition of 10.1 ml
(65.8 mmol) of methyl iodide. The stirring was contin-
ued for 17 h and then the mixture was poured into water,
extracted with diethyl ether and washed with water. The
organic phase was dried with anhydrous Na2SO4 and evap-
orated. The crude product (5) was an oil (1.99 g, 97%)
and it was used without purification. 1H NMR (CDCl3,
200 MHz) δ: 0.80, 0.82 (2s, 3H, 18-CH3), 1.18–2.2 (25H),
2.76 (m, 2H, H-6), 3.4–3.8 (m, 3H), 3.83 (s, 3H, OCH3),
3.84–4.1 (m, 2H), 4.66 (m, 1H, CH-(C17)), 5.37 (m, 1H,
CH–O–(C3)), 6.83 (s, 1H, H-4), 6.97, 6.98 (2s, 1H, H-1).
2.1.4. 2-Hydroxyestradiol (2)
2-Hydroxyestradiol (2) was obtained from (4) as detailed
for the conversion of 2-methoxyestradiol bis-THP-ether (5)
into 2-methoxyestradiol (1). The crude product 96% was pu-
rified by flash chromatography CHCl3/MeOH (95:5) to give
(2) in 58% yield. Melting point: 185–186 ◦C, recrystallized
from ethyl acetate–benzene–pentane (lit. mp 186–190 ◦C
1
[15]). H NMR (CDCl3, 200 MHz) δ: 0.78 (s, 3H, CH3),
2.77 (m, 2H, H-6), 3.50 (s, 1H, OH), 3.75 (t, 1H, H-17␣),
5.18 (bs, 1H, ArOH), 6.58 (s, 1H, H-4), 6.81 (s, 1H, H-1).
13C NMR (CDCl3, 300 MHz) δ: 11.1 (C-18), 23.1 (C-15),
26.4 (C-11), 27.3 (C-7), 28.9 (C-6), 30.6 (C-16), 36.7 (C-12),
38.7 (C-8), 43.2 (C-13), 43.9 (C-9), 49.9 (C-14), 81.9 (C-17),
112.4 (C-1), 115.4 (C-4), 129.1 (C-5), 132.9 (C-10), 141.2
(C-3 and C-2). HRMS calculated for C18H24O3: 288.1725;
found: 288.1712.
2.1.3. 2-Methoxyestradiol (1)
A solution of 2-methoxyestradiol bis-THP ether (5) (2 g,
4.3 mmol) in 60 ml of methanol and 4.5 ml (63.6 mmol) of

P.S. Kiuru, K. Wähälä / Steroids 68 (2003) 373–375
375
Scheme 2. Deprotection of (4) produces 2-hydroxyestradiol (2).
3. Results and discussion
[2] Rao PN, Burdett Jr JE. A novel, two-step synthesis of 2-methoxy-
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[3] Numazawa M, Ogura Y. Efficient synthesis of 2-methoxy- and
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Steroids 1986;47:63–6.
[6] Le Bras J, Rager MN, Besace Y, Amouri H, Vaissermann J. Activation
and regioselective ortho-functionalization of the A-ring of ␤-estradiol
promoted by “Cp∗Ir”: an efficient organometallic procedure for
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71.
The hydroxylation at C-2 to give (4) was performed us-
ing the superbase LIDAKOR [13] (potassium 1,1-dimethyl-
propoxide, n-BuLi, and diisopropylamine) with a 3.5-fold
excess of n-BuLi, followed by trimethylborate and H2O2.
This excess of n-BuLi in LIDAKOR was crucial for the hy-
droxylation at C-2. Reaction with a 14-fold excess of n-BuLi
in the absence of LIDAKOR produced just unreacted starting
material. Also reaction with LDA (LIDAKOR without potas-
sium 1,1-dimethylpropoxide) was unsuccessful. Deprotec-
tion of (4) gave 2-hydroxyestradiol (2) (Scheme 2) while
methylation of the 2-hydroxy group of (4) and deprotection
yielded 2-methoxyestradiol (1) (Scheme 1).
[7] Pert DJ, Ridley DD. Formylation of oestrogens. Aust J Chem
1989;42:405–19.
LIDAKOR has been used for the hydroxylation of the C-6
position of estrogens [13]. However, we observed no oxida-
tion of (3) at C-6 with LIDAKOR with a 3.5-fold excess of
n-BuLi. This was also proved by quenching the reaction with
D2O after 3 h which showed no deuteration at C-6. Traces
of 4-hydroxyestradiol and unreacted estradiol were found.
[8] Lovely CJ, Gilbert NE, Liberto MM, Sharp DW, Lin YC,
Brueggemeier RW. 2-(Hydroxyalkyl)estradiols: synthesis and bio-
logical evaluation. J Med Chem 1996;39:1917–23.
[9] Wang Z, Cushman M. An optimized synthesis of 2-methoxyestradiol,
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estradiols. Steroids 2002;67:1065–70.
[11] Nambara T, Honma S, Akiyama S. Studies on steroid conjugates. III.
New synthesis of 2-methoxyestrogens. Chem Pharm Bull 1970;18:
474–80.
Acknowledgments
[12] Stubenrauch G, Knuppen R. Convenient large scale preparation of
catechol estrogens. Steroids 1976;28:733–41.
[13] Tedesco R, Fiaschi R, Napolitano E. 6-Oxoestradiols from estra-
diols: exploiting site selective metalation of arylalkylsystems with
superbases. Synthesis 1995:1493–5.
We thank Mr Seppo Kaltia for running the 2-D NMR
spectra. Financial support to P.K. from Bioorganic Graduate
School is gratefully acknowledged.
[14] Fishman J. Synthesis of 2-methoxyestrogens. J Am Chem Soc
1958;80:1213–6.
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