Electrochemical A-ring bromination of estrogens

Article abstract of DOI:10.1246/bcsj.80.407

A-ring bromination of estrogens has been achieved by constant current electrolysis of the solutions of these substrates and Et₄NBr in appropriate solvents. Thus, electrolysis consuming 2 F mol^-1 charge gave mixtures of 2- and 4-estrogens (1:1.1-2.5; up to 97%), whereas 4 F mol ^-1 charge experiments yielded 2,4-dibromoestrogens as the sole products.

Full text of DOI:10.1246/bcsj.80.407

Short Articles
Bull. Chem. Soc. Jpn. Vol. 80, No. 2, 407–409 (2007)
407
R1
R2
Electrochemical A-Ring
Bromination of Estrogens
X
RO
Y
1a (R = R = X = Y = H; R = OH)
Ivan Damljanovi c´ , Mirjana Vuki c´ evi c´ ,
2
1
ꢀ
and Rastko D. Vuki c´ evi c´
1
2
1
2
2
b (R = X = Y = H; R , R = O)
2
1
a (R = R = Y = H; R = OH; X = Br)
Department of Chemistry, Faculty of Science, University of
Kragujevac, R. Domanovi c´ a 12, 34000 Kragujevac, Serbia
1
2
b (R = Y = H; R , R = O; X = Br)
2
1
3a (R = R = X = H; R = OH;Y = Br)
1
2
3
4
5
6
6
b (R = X = H; R , R = O;Y = Br)
1
2
a (R = Ac; R = OAc; R = Y = H; X = Br)
Received May 10, 2006; E-mail: vuk@kg.ac.yu
1
2
a (R = Ac; R = OAc; R = X = H;Y = Br)
2
1
a (R = R = H; R = OH; X = Y = Br)
1
2
b (R = H; R , R = O; X = Y = Br)
A-ring bromination of estrogens has been achieved by
constant current electrolysis of the solutions of these sub-
strates and Et4NBr in appropriate solvents. Thus, electrolysis
Scheme 1.
cally changes if Br2 or PBPB in AcOH are used for bromina-
tion (1:1.27 and 1:0.85, respectively). All later reports con-
cerning this reaction mainly confirm these results.
In continuation of our investigations of electrochemically
ꢁ1
consuming 2 F mol charge gave mixtures of 2- and 4-estro-
gens (1:1.1–2.5; up to 97%), whereas 4 F mol charge ex-
periments yielded 2,4-dibromoestrogens as the sole products.
ꢁ1
15,17,18,24,25
generated agents and electrochemical halogenation of ste-
we decided to subject ꢁ-estradiol (1a) and estrone
roids,2
6,27
(1b) to reaction conditions, in which bromine is generated at
One of the major metabolism pathways of estrogens, which
are important mammalian hormones, is A-ring functionali-
zation to give products, which are themselves active com-
the anode. This technique has already been used in the bromi-
28
1
nation of organic compounds, among which phenols have
29
pounds, characterized by different biological properties.2
A-ring halogenated estrogens, which are not naturally occur-
ring compounds, have been used in a variety of medical appli-
–5
been used as substrates. Although oxidation potentials of
phenols strongly depend on the number and types of substitu-
ents,30 the oxidation of bromides is possible using potential
6
29,30
cations, such as diagnostic radio-imaging agents, inhibitors of
7
uncontrolled electrolysis.
Our focus has been to examine
steroid metabolizing enzymes, and metabolic probes of estro-
whether this methodology is applicable to the synthesis of bro-
minated estrogens.
Bearing in mind the reaction conditions of classical bromi-
nations of estrogens with bromine described in the litera-
20,22
8
,9
gen carcinogenesis. From a synthetic point of view, these
compounds are interesting because they are good starting ma-
terials for the synthesis of the other A ring mono- and disubsti-
10–12
tuted derivatives. Thus, deuterium-labeled
and tritiated es-
ture,
we started with constant current electrolysis (20 mA)
1
3
ꢁ1
trogens have been prepared starting from the corresponding
brominated compounds. Naturally occurring A-ring-oxygenat-
ed compounds, which are metabolites of estrogens, have been
of the substrate 1a in a 0.05 mol L glacial acetic acid solu-
tion of tetraethylammonium bromide. When this electrolysis
was performed in an undivided electrolytic cell using a plati-
num anode and a graphite cathode, consuming 2 F mol
charge, which generates one equivalent of bromine, a complex
mixture of products was obtained. On the basis of the position
of aromatic protons signals in H NMR spectra, this mixture
contained unconsumed 1a, both 2- (2a) and 4-bromoestradiol
(3a) (Scheme 1), and some unknown products. Approximate
calculations from the H NMR spectrum showed that more
than 50% of 1a remained. That prompted us to perform an ex-
synthesized by halogen–oxygen exchange reactions,1
4–18
while
ꢁ1
the bromo–nitrogen exchange reaction had been used to syn-
thesize 2-aminoestrogens.19 Therefore, there is strong interest
in the synthesis of A-ring-brominated estrogens.
The first well documented direct bromination of an estrogen
has been reported by Woodward,20 who obtained 2,4-dibromo-
1
1
ꢀ
-estradiol in a 68% yield by treating ꢀ-estradiol with N-bro-
moacetamide (NBA) in ethanol (EtOH). A better yield has
been achieved in a similar dibromination of estriol by using
N-bromosuccinimide (NBS).12 Selective bromination of estro-
gens at the 2- or 4-position and dibromination with elemental
bromine in acetic acid (AcOH) has been reported in 1962.2
ꢁ1
periment with 4 F mol charge consumption. However, even
then considerable amounts of 1a remained.
In the next experiment substrate 1a was electrolyzed under
the same conditions, but in a divided electrolytic cell with
1
2
2
ꢁ1
However, it has not been confirmed in other laboratories.
a ceramic membrane with 2 F mol charge consumption. In
1
2
3
Wilbur and O’Brien have tested several systems for bromina-
tion of estradiol, such as NCS/NaBr/EtOH, NCS/LiBr/THF,
NBA/EtOH, NBS/EtOH, pyridinium bromide perbromide
the H NMR spectrum of the obtained crude reaction mixture,
compounds 2a and 3a and only negligible amounts of 1a were
observed. Since monobromo derivatives have very similar Rf
values in many solvents and in mixtures, the mixture was treat-
ed with acetyl chloride and pyridine, and the obtained diace-
tates (4a and 5a) were separated by column chromatography
(SiO2/ethyl acetate–petrol ether 99:1). After hydrolysis, 2a
and 3a were obtained as pure compounds in the ratio of
(PBPB)/EtOH, PBPB/THF, PBPB/AcOH, and Br2/AcOH.
Their investigations have shown that brominations with one
equivalent of NCS and NBS independent of the solvent used
give 2- and 4-bromo derivatives in a similar ratio (1:2.38–
2
.80), along with 2,4-dibromide. However, this ratio dramati-
Published on the web February 13, 2007; doi:10.1246/bcsj.80.407

4
08 Bull. Chem. Soc. Jpn. Vol. 80, No. 2 (2007)
Ó 2007 The Chemical Society of Japan
Table 1. Bromination of Estrogens
lation. A Uniwatt Beha Labor-Netzger a¨ t (NG 394) was used as a
direct current source for the electrolysis. A cylindrical glass vessel
equipped with a magnetic stirrer, a cylindrical platinum foil as the
anode (ꢂ ¼ 2:5 cm), a ceramic tube as the membrane (ꢂ ¼ 1:5
cm), and a graphite stick as the cathode (ꢂ ¼ 0:5 cm) was as-
sembled as the divided electrochemical cell. NMR spectra were
recorded on a Varian Gemini (200 MHz) spectrometer, using
(CD3)2SO as the solvent. Chemical shifts are expressed in ꢃ
(ppm). IR measurements were carried out with a Perkin-Elmer
457 grating FT instrument in KBr tablets. For TLC, silica gel
60 on Al plates, layer thickness 0.2 mm (Merck), was used.
General Procedure for Electrochemical Bromination,
Methods A and B. Substrate 1a or 1b (100 mg, ꢂ0:37 mmol)
and a 0.05 M solution of Et4NBr in the corresponding solvent
(20 mL) were placed in the anodic compartment of the cell (out-
side the ceramic tube). The same solution of Et4NBr (2.5 mL)
was used as the catholyte. Constant current electrolysis (20 mA)
was stopped after 60 (method A) or 120 min (method B) in order
a)
Substrate Solvent Method
Products Yield
Ratio
(2:3)
1:1.16b)
/%
1
a
AcOH
DMSO
AcOH
A
B
A
B
A
B
C
D
A
B
C
D
2a þ 3a
6a
2a þ 3a
6a
2b þ 3b
6b
2b þ 3b
2b þ 3b
2b þ 3b
6b
94
95
97
92
93
90
87
91
96
91
90
96
b)
1:2.15
1b
1:1.09c)
c)
1:1.20
1:1.18
c)
c)
DMSO
1:2.50
c)
2b þ 3b
1:2.30
1:2.60
c)
2b þ 3b
a) See experimental. b) Based on the isolated bromides after
1
acetylation/hydrolysis procedure. c) Estimated from H NMR
spectra of the mixture.
to provide 2 or 4 F mol 1 charge. If acetic acid was used the sol-
vent was removed by evaporation, H2O (20 mL) was added to the
residue, and the mixture was extracted with three portions of ether
ꢁ
1
:1.16 (94%), respectively.
On the other hand, when electrolysis was carried out under
(
3 ꢃ 30 mL). The organic layers were first collected and washed
ꢁ
1
with a solution of NaHSO3 (10%, 40 mL) to remove residual
amounts of bromine, then with saturated NaHCO3 (40 mL), brine
the same experimental conditions, but with 4 F mol charge
consumption, 2,4-dibromoestradiol (6a, Scheme 1) was ob-
tained as the sole product (95%), which could be identified
(
40 mL), and H2O (40 mL). After drying over anhydrous Na2SO4
overnight, the solvent was evaporated, and the crude reaction mix-
ture was subjected to column chromatography on SiO2 (5–10 g).
The elution (dichloromethane) was monitored by TLC. Monobro-
mides 2 and 3 were eluted as one fraction, whereas dibromides 6a
and 6b were isolated as the pure compounds. In the case of sub-
strate 1a the two isomeric monobromides were separated through
an acetylation/hydrolysis procedure (see below).
1
13
by H and C NMR spectroscopies.
Similar results were obtained with estrone (1b), as shown in
Table 1 and Scheme 1. However, in this case, regioisomers 2b
and 3b could not been separated, even after acetylation.
Since in classical bromination of estrogens with elemental
bromine,20,22 the solvent affects the distribution of 2- and 4-
bromo products, several additional experiments were carried
out using different solvents. The addition 5–10% of water or
If DMSO was used as the solvent, the mixture was diluted after
electrolysis with water and extracted with Et O, then worked up as
it is described above.
2
2
0–30% of acetic anhydride to acetic acid did not affect either
ꢁ
1
the overall yield or the ratio of 2 and 3 in the 2 F mol charge
consumption experiments. Electrolysis in dichloromethane al-
so gave similar results. However, the use of dimethyl sulfoxide
as the solvent significantly changed the ratio of the 2- and 4-
bromo isomers, although the overall yield remained almost
the same (see Table 1). This solvent effect is not specific for
the electrochemical reaction, and it has been found in chemical
bromination of estrone with elemental bromine using the same
solvents with or without Et4NBr (see Table 1).
In conclusion, we have shown that ꢁ-estradiol and estrone
can be smoothly brominated by anodically generated bromine
from tetraethylammonium bromide dissolved in acetic acid or
DMSO, in a divided electrolytic cell. Controlling the charge
consumption, this method allows for the preparation of the cor-
responding 2- and 4-monobromo- and 2,4-dibromoestrogens in
high yields. The regiochemistry of the monobrominated prod-
uct depended on the solvent used. The process compares favor-
ably in yield and ease of operation with classic bromination re-
actions but avoids the use of hazardous brominating reagents.
Since the necessary equipment is very simple, inexpensive,
and readily available, this methodology should replace classi-
cal ones.
General Procedure for Chemical Bromination, Methods C
and D. To a solution of Br2 (60 mg, 0.37 mmol) in 20 mL of
the corresponding solvent 1b (100 mg, 0.37 mmol) was added,
and the mixture was stirred 2 h. Work up was the same as previous
experiments (method C). For method D, Et4NBr (210 mg) was
added.
Acetylation/Hydrolysis Procedure for Separation of Mono-
bromoestradiols. To a dichloromethane solution of crude reac-
ꢁ
1
tion mixture obtained after the 2 F mol charge consumption
electrolysis of ꢁ-estradiol (ꢂ130 mg/10 mL), acetyl chloride (0.1
mL) and pyridine (0.1 mL) were added, and the resulting mixture
was stirred vigorously overnights with a magnetic stirrer. The
mixture was diluted with dichloromethane (20 mL), washed with
ꢁ1
water (30 mL), 1 mol L HCl, saturated NaHCO3, brine and wa-
ter subsequently and dried over anhydrous Na2SO4. After evapo-
ration of the solvent, the mixture was separated by column chro-
matography (SiO2/petrol ether–ethyl acetate 99:1) to give pure 4a
and 5a. The separated diacetates were dissolved in 10 mL of
ꢁ1
0
.5 mol L KOH in methanol and the obtained solution was
refluxed 2 h. After evaporation of the methanol, to the resulting
residue was added 20 mL of water, and the mixture extracted with
ether (3 ꢃ 20 mL). Evaporation of the solvent gave pure com-
pounds 2a and 3a.
ꢄ
2
97–198, 191–193, 200–207 C ); IR (cm ): 3248, 2955,
-Bromo-ꢀ-estradiol (2a): White solid; mp 189–192 C (lit.:
Experimental
22
23
ꢄ 31 ꢁ1
1
1604, 1500, 1414, 1342, 1255, 1053, 1026, 1008, 884, 730;
General. All chemicals were commercially available and used
as received, except for the solvents which were purified by distil-
1
H NMR: ꢃ 0.65 (s, 3H, 18-Me), 4.50 (d, J ¼ 4:7 Hz, 1H, C17H),

I. Damljanovi c´ et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 2 (2007)
409
6
1
1
.62 and 7.26 (two s, 2H, C1 and C4), 9.81 (s, 3-OH); 13C NMR: ꢃ
1.4, 23.0, 26.2, 26.9, 28.8, 30.1, 36.7, 39.5, 43.0, 43.4, 49.6, 80.2,
06.6, 116.3, 129.5, 132.9, 137.0, 151.7.
3
J. G. Liehr, Endocr. Rev. 2000, 21, 40.
P. Ball, R. Knuppen, Acta Endocrinol., Suppl. 1980,
4
232, 1.
5
1
2- and 4-Bromoestrone (2b and 3b): H NMR: ꢃ 0.80 (s, 3H,
N. J. Macluskey, F. Naftolin, L. C. Krey, S. Franks, J.
1
8-Me of both isomers), 6.64 and 7.27 (two s, 2H, C1H and C4H
Steroid Biochem. 1981, 15, 111.
J. A. Spicer, D. F. Preston, R. J. Baranczuk, E. Harvey,
M. M. Guffey, D. L. Bradshaw, R. G. Robinson, J. Nucl. Med.
1979, 20, 761.
of 2a), 6.75 and 7.10 (two d, J ¼ 8:5 Hz, 2H, C1H and C2H of
6
3
a), 9.55 (s, 1H, OH of 2a), 9.97 (s, 1H, OH of 3a).
ꢄ
4
-Bromo-ꢀ-estradiol (3a): White solid; mp 205–207 C (lit.:
13–215, 207–208.5, 209–211,31 209–212 C ); IR (cm ):
2
2
23
ꢄ
32
ꢁ1
7
111.
8
M. Numazawa, S. Satoh, J. Steroid Biochem. 1989, 33,
2
3
1
399, 3153, 2924, 2854, 1467, 1378, 1292, 976, 790; H NMR:
ꢃ 0.64 (s, 3H, 18-Me), 4.50 (d, J ¼ 4:7 Hz, 1H, C17H), 6.74 and
J. G. Liehr, Mol. Phamacol. 1983, 23, 278.
J. J. Li, R. H. Purdy, E. H. Appelman, J. K. Klicka, S. A.
7
.08 (two d, J ¼ 8:4 Hz, 2H, C1H and C2H), 9.83 (s, 1H, 3-OH);
C NMR: ꢃ 11.4, 22.9, 26.4, 27.2, 30.1, 31.0, 36.7, 37.8, 42.9,
3.9, 49.6, 80.2, 112.7, 113.3, 125.1, 132.9, 136.5, 152.0.
9
Li, Mol. Pharmacol. 1985, 27, 559.
13
4
10 J. Pinkus, D. Charles, S. C. Chattoraj, J. Biol. Chem. 1971,
246, 633.
11 J. Fishman, H. Guzik, L. Hellman, Biochemistry 1970, 9,
1593.
2
65–168 C (lit.: 166–168 C ); IR (cm ): 3067, 2964, 2927,
-Bromo-3,17ꢀ-estradiol Diacetate (4a): White solid; mp
ꢄ
ꢄ
22
ꢁ1
1
1
1
770, 1728, 1486, 1375, 1255, 1197, 1042, 910; H NMR: ꢃ 0.78
(
s, 3H, 18-Me), 2.00 (s, 3H, COMe), 2.28 (s, 3H, COMe), 4.60 (t,
12 B. H. Albrecht, D. D. Hagerman, Steroids 1972, 19, 177.
13 M. M. Coombs, H. R. Roderick, Steroids 1968, 11, 925.
14 P. N. Rao, J. E. Burdett, Jr., Synthesis 1977, 168.
15 X. Zheng, W. Wang, Z. Zhong, Z. Xu, H. Zhao, Steroids
1982, 40, 121.
J ¼ 8:1 Hz, 1H, C17H), 6.96 and 7.49 (two s, 2H, C1H and C4H);
1
3
C NMR: ꢃ 12.1, 20.8, 21.1, 23.0, 25.8, 26.4, 27.4, 28.6, 36.6,
7.7, 42.7, 43.4, 49.2, 82.0, 112.5, 124.1, 129.8, 138.0, 140.2,
45.6, 168.8, 170.7.
3
1
4
68–171 C (lit.: 175.5–177.5, 171–173 C ); IR (cm ): 2968,
-Bromo-3,17ꢀ-estradiol Diacetate (5a): White solid; mp
16 M. Numazawa, Y. Ogura, K. Kimura, M. Nagaoka, J.
Chem. Res., Synop. 1985, 348.
17 D. J. Pert, D. D. Ridley, Aust. J. Chem. 1989, 42, 421.
18 A. J. Lee, S. J. Walter, W. E. Cotham, T. B. Zhu, Steroids
2004, 69, 61.
19 M. Numazawa, K. Kimura, Steroids 1983, 41, 675.
20 R. B. Woodward, J. Am. Chem. Soc. 1940, 62, 1625.
21 W. R. Slaunwhite, Jr., L. Neely, J. Org. Chem. 1962, 27,
1749.
22 T. Utne, R. B. Jobson, F. W. Landraf, J. Org. Chem. 1968,
33, 1654.
23 D. S. Wilbur, H. A. O’Brien, J. Org. Chem. 1982, 47, 359.
24 Z. Szendi, G. Dombi, I. Vincze, Steroids 1991, 56, 392.
25 P. C. Bulman Page, F. Hussain, N. M. Bonham, P. Morgan,
J. L. Maggs, B. K. Park, Tetrahedron 1991, 47, 2871.
26 S. Milisavljevi c´ , R. D. Vuki c´ evi c´ , J. Serb. Chem. Soc.
2004, 69, 941.
27 S. S. Milisavljevi c´ , K. Wurst, G. Laus, M. D. Vuki c´ evi c´ ,
R. D. Vuki c´ evi c´ , Steroids 2005, 70, 867.
28 Organic Electrochemistry an Introduction and Guide, 3rd
ed., ed. by H. Lund, M. M. Baizer, Marcel Dekker, New Yurk,
1991.
ꢄ
22
ꢄ
32
ꢁ1
1
2
1
(
(
2
4
1
930, 2874, 2852, 1769, 1721, 1470, 1370, 1261, 1197, 1043,
1
019; H NMR: ꢃ 0.77 (s, 3H, 18-Me), 2.01 (s, 3H, COMe), 2.29
s, 3H, COMe), 4.62 (t, J ¼ 8:0 Hz, 1H, C17H), 7.04 and 7.37
13
two d, J ¼ 8:4 Hz, 2H, C1H and C2H); C NMR: ꢃ 12.1, 20.1,
1.1, 23.0, 26.1, 26.8, 27.4, 29.2, 30.9, 36.6, 37.1, 37.7, 42.6,
3.8, 49.2, 82.1, 118.8, 120.9, 125.8, 137.4, 140.3, 146.1, 168.8,
70.7.
ꢄ
2
,4-Dibromo-ꢀ-estradiol (6a): White solid; mp 219–222 C
2
2
23
ꢄ ꢁ1
33
(
3
1
(
3
1
lit.: 223–226, 225–226, 220–222 C ); IR (cm ): 3585,
1
293, 1542, 1464, 1269, 1180, 1014, 761; H NMR: ꢃ 0.62 (s, 3H,
8-Me), 4.52 (d, J ¼ 4:7 Hz, 1H, C17H), 7.37 (s, 1H, C1H), 9.50
13
s, 1H, 3-OH); C NMR: ꢃ 11.3, 22.9, 26.3, 27.0, 30.1, 31.2, 36.6,
7.6, 42.8, 43.5, 49.5, 80.1, 108.8, 116.6, 128.5, 135.5, 136.5,
48.5.
ꢄ
2,4-Dibromoestrone (6b): White solid; mp 224–226 C (lit.:
20–226, 235–237 C ); IR (cm ): 3270, 2937, 2869, 1723,
3
3
ꢄ
34
ꢁ1
2
1
7
2
1
1
545, 1464, 1275, 1171, 762; H NMR: ꢃ 0.79 (s, 1H, 18-Me),
1
3
.40 (s, 1H, C1H), 9.53 (s, 1H, OH); C NMR: ꢃ 13.6, 21.2, 25.6,
6.1, 31.0, 31.3, 35.5, 36.8, 43.4, 47.3, 49.5, 108.9, 115.6, 128.6,
34.9, 135.4, 148.6, 219.6.
2
30 M. M. Ngundi, O. A. Sadik, T. Yamaguchi, S. Suye,
Electrochem. Commun. 2003, 5, 61, and literature cited there.
9
T. Bejerano, E. Gileadi, Electrochim. Acta 1976, 21, 231.
This work was supported by the Ministry of Science and En-
vironmental Protection of the Republic of Serbia (grant 2042).
3
1
A. J. Lee, J. W. Sowell, W. E. Cotham, B. T. Zhu, Steroids
004, 69, 61.
A. D. Cross, E. Denot, R. Acevedo, R. Urquiza, A. Bowers,
2
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