Significant steroids: Effective and general synthesis of 4α- and 4β-amino-5α-androstanes

Article abstract of DOI:10.1039/b817910g

4α-Aminosteroids were synthesized by the substitution of a 2α-bromo ketone using K₂CO₃ as an activator; 4β-aminosteroids were synthesized in excellent yields by a highly regioselective and trans-stereospecific ring opening of a steroidal 3,4α-epoxide using ZnCl₂-H₂O as a catalyst. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b817910g

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Significant steroids: effective and general synthesis of 4a- and
4b-amino-5a-androstanesw
Xianbing Ke, Hao Hu, Keda Zhang, Wenjin Xu, Qifeng Zhu, Lamei Wu and Xianming Hu*
Received (in College Park, MD, USA) 13th October 2008, Accepted 29th December 2008
First published as an Advance Article on the web 27th January 2009
DOI: 10.1039/b817910g
4a-Aminosteroids were synthesized by the substitution of a
2a-bromo ketone using K2CO3 as an activator; 4b-aminosteroids
were synthesized in excellent yields by a highly regioselective
and trans-stereospecific ring opening of a steroidal 3,4a-epoxide
using ZnCl2–H2O as a catalyst.
Aminosteroids serve as the basis for a wide range of bio-
logically active natural and synthetic products. A series of
2b-morpholinyl steroids has been shown to possess anesthetic
activity and some have been selected for development as
potential water-soluble steroidal intravenous anesthetics.1
Recently a steroid possessing a methylpiperazine nucleus was
reported to inhibit the proliferation of HL-60 or WEHI-3B cell
lines and is a promising potential new drug for the treatment
of leukemia.2 The rigid steroidal skeleton, decorated with
amino groups at different positions, provides an excellent
means of studying structure–activity relationships within a
series of neuromuscular blocking agents.3 These especially
important biological properties of the A-ring amino moiety
have urged medicinal chemists to synthesize and test a great
number of novel molecules. Considering that insertion of the
amino moiety in new positions of a steroid skeleton is an
important synthetic strategy in drug discovery, we desired to
prepare a large number of 4-aminosteroid derivatives with
established configurations for the evaluation of their biological
activities. However, effective and general methods for the
preparation of 4-aminosteroids have never been reported.
We have addressed the synthesis of both 4a- and 4b-amino-
5a-androstanes and report methods to achieve this goal.
The nucleophilic displacement of diverse steroidal bromides
generally leads to single isomers as stable end products, such
as 2a-, 3b-, 11a- and 16b-derivatives.4 These results suggest
that thermodynamic control strongly prefers the formation of
the equatorial isomers at different positions of the steroidal
skeleton. It is potentially a facile strategy to synthesize
4a-aminosteroids via the corresponding 4a- or 4b-bromo-3-
ketones. Previous work has shown, however, that the bro-
mination of 3-keto-5a-steroids only results in the formation of
the 2a-bromo ketone because of preferred enolization toward
C-2.5 On the other hand, it is interesting to note that the
dehydrobromination of a 2a-bromo-3-ketone gives a mixture
of regioisomers of 1-ene and 4-ene a,b-unsaturated ketones.6
Scheme 1 Proposed synthesis of 4a-aminosteroids.
Several studies6h,i suggest that basic conditions induced by
sodium carbonate or various pyridines could facilitate the
formation of the 4-ene product. We postulated that such basic
conditions could promote the formation of a 3,4-enolate, and
the following SN20 displacement of the bromide by the amine
would likely give 4a-amino-3-ketone 3 as a main product
(Scheme 1) which can be easily converted to the generally
active form of an amino alcohol.
This hypothesis was sustained by a series of experiments.
Studies were carried out to determine the optimum base
activator and solvent. We found the combination of K2CO3
and CH3CN was optimal in terms of yield. In order to
demonstrate the general applicability of the displacement of
2a-halo ketone 1 using the K2CO3–CH3CN system, a series of
amines were subjected to the reaction, in which the
corresponding aminosteroids 3 were obtained (Table 1). The
structures were unambiguously confirmed by X-ray crystallo-
graphy of 3a and 3e.z
The activator K2CO3 was important for increasing the
reactivity of this reaction. As for bulky amines such as
diisopropylamine and dicyclohexylamine (Table 1, entries 4
and 10), if no activator was added, no product was detected
even when the temperature was elevated and the reaction time
was lengthened. When piperidine, morpholine and 4,4-(ethyl-
enedioxy)-piperidine were used as nucleophiles (Table 1,
entries 6, 7 and 9), a small quantity (less than a 10% yield)
of the corresponding 2-amino-5a-androst-1-ene-3,17-dione
was isolated. These 1-ene products most likely arose from
the 2a-amino ketones 2 which underwent air oxidation.7 In the
azole series (Table 1, entries 11–15), small amounts (less than
an 8% yield) of 2a-azole ketones 2 were obtained.z When
2-methyl pyridine and 2,4,6-trimethyl pyridine were used as
the activator, dehydrobromination was observed in a quanti-
tative yield which probably went via an E2 elimination.
The preparation of 4b-aminosteroids requires a different
approach. An attractive route is the direct aminolysis of an
epoxide. Ring opening of epoxides is a practical and widely
State Key Laboratory of Virology, College of Pharmacy, Wuhan
University, Wuhan, P. R. China. E-mail: xmhu@whu.edu.cn;
Fax: +86-27-68754297; Tel: +86-27-68753532
w Electronic supplementary information (ESI) available: Synthetic
procedures, analytical data, X-ray and NMR spectra. CCDC
682757–682758, 695664–695665. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/b817910g
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Chem. Commun., 2009, 1037–1039 | 1037

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Table 1 Synthesis of 4a-aminosteroids 3 by the reaction of 2a-bromo
ketone 1 and various aminesa
with amines has been performed with a catalytic amount of
Gd(OTf)3 in toluene in a sealed tube at high temperature and
under anhydrous conditions.15 There are only a few reported
examples for the cleavage of 3,4-epoxy steroids. A 3a-hydroxy-
4b-nitrato steroid has been reported as the only product
with 25% yield in the reaction of 3,4a-epoxy steroid with
(NH4)2Ce(NO3)6.16 4b-Aminocholestan-3-ol is obtained from
the 3,4a-epoxide by reaction with ethanolic ammonia at
180 1C.17 Clearly there is not an efficient and universal method
for the ring opening of 3,4a-steroidal epoxides.
We first examined the aminolysis of 4 in the presence of water
according to the classical method.2c,11 This was found to be
unsatisfactory because the yields of aminosteroids were less than
10% despite heating for a week. Refluxing 4 with amines in
other protic solvents such as ethanol or 1,2-ethanediol did not
work. No reaction occurred after 2 days when (C2H5)2OꢁBF3
was used as the reagent. These poor results are not too surpris-
ing in view of the negative results of others also cited above.
Subsequently, methods including the use of catalysts such as
Lewis acids were developed to enhance the reactivity. Success
was finally achieved by extended heating of the reactants in a
ZnCl2–H2O system. Good reactivity and regioselectivity of
ring opening by amines as nucleophiles were observed. Using
the mild conditions, we treated 4 with various aliphatic and
aromatic amines and azoles and obtained the 4b-amino-3a-ols
5 in surprisingly high yields (Table 2). The structures
and absolute configurations were confirmed by X-ray
Entry
Amine
Time/h
Product
Yield (%)b
1
2
3
4
NHMe2
NHEt2
NHPr2
NHiPr2
3
4
7
12
3a
3b
3c
3d
87
70
59
36
5
6
7
8
9
4
5
5
4
6
3e
3f
3g
3h
3i
84
72
69
78
61
10
11
12
13
24
8
3j
29
76
81
83
3k
3l
8
8
3m
14
12
12
3n
3o
64
53
15
a
Conditions for reaction: 1 (10 mmol), amine (50 mmol), K2CO3
b
Table
2 Synthesis of 4b-aminosteroids 5 by the regioselective
(5 mmol), acetonitrile (80 mL), 75 1C. Yield of isolated product.
aminolysis of epoxide 4 with various aminesa
Entry
Amine
Time/h
12
Product
Yield (%)b
used route for the synthesis of b-amino alcohols.8 The syn-
thesis of epoxides from corresponding olefins in an optically
pure form is facile.9 The 3,4a-steroidal epoxide required was
not available until the synthesis and purification of 3-en-5a-
androstane and its epoxide were reported with detailed
procedures.10 We were therefore very interested in the ring
opening of 3,4a-steroidal epoxide 4 with various amines to
prepare axial 4b-aminosteroids 5 (Scheme 2).
Unfortunately, aminolysis of steroidal epoxides is hindered
by shortcomings such as low yields, long reaction times, low
reactivity for aromatic amines, poisonous reagents, high
pressure, and moisture/air sensitive and costly catalysts. The
reported classical method2c,11 involves treatment of epoxides
with amines while heating and using a catalytic amount of
water. The solvent 1,2-ethanediol has been used in the syn-
thesis of 2-alkylamino-3-ol derivatives.1d,12 Ionic liquids have
been used both as solvents and catalysts in the ring opening of
2,3-epoxy steroids with aromatic amines.13 The aminolysis of
5,6a- and 5,10a-epoxy steroids in the presence of (C2H5)2Oꢁ
BF3 has been reported.14 The opening of 2,3a-epoxy steroids
1
2
5a
5b
98
96
15
3
4
5
6
7
20
20
20
20
20
5c
5d
5e
5f
93
92
96
92
91
5g
8
18
24
24
24
24
24
15
15
5h
5i
97
94
90
92
91
89
96
94
9
10
11
12
13
14
5j
5k
5l
5m
5n
5o
15
a
Conditions for reaction: 4 (10 mmol), amine (30 mmol), ZnCl2
b
(20 mmol), H2O (10 mL), 95 1C. Yield of isolated product.
Scheme 2 Proposed synthesis of 4b-aminosteroids.
1038 | Chem. Commun., 2009, 1037–1039
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crystallography of 5e.z As anticipated, regioselective trans-
diaxial epoxide opening led to the formation of 4b-amino-3a-ols
5 and none of the 3b-amino-4a-ols were detected.
2a-(1H-pyrazol-1-yl)-5a-androstan-3,17-dione: 22H30N2O2,
C
M
c
=
=
354.48, orthorhombic, 8.1193(9), b = 11.3485(12),
a
=
21.282(2) A, V = 1961.0(4) A3, T = 298(2) K, space group P212121,
Z = 4, final R indices [I 4 2s(I)]: R1 = 0.0497, wR2 = 0.1169;
R indices (all data): R1 = 0.0584, wR2 = 0.1223, CCDC 695665. For
5e: C23H37NO3, M = 375.54, orthorhombic, a = 8.3064(5), b =
11.8271(7), c = 21.4585(14) A, V = 2108.1(2) A3, T = 298(2) K,
space group P212121, Z = 4, final R indices [I 4 2s(I)]: R1 = 0.0488,
wR2 = 0.1236; R indices (all data): R1 = 0.0663, wR2 = 0.1334,
CCDC 695664.w
Using water as a solvent, zinc catalysts were more effective
than some other water-soluble Lewis acids such as AlCl3 and
SnCl4 with respect to yield or reaction time. We observed that
Zn(ClO4)2 was as potent as ZnCl2 although we do not
recommend the former owing to the danger of explosion.
When ZnCl2 was not added and/or H2O was replaced with
nonpolar or highly dipolar solvents (diethyl ether, toluene,
THF and DMF), it resulted in very little or no product
formation. Therefore, the ZnCl2–H2O system appears to
be crucial for the reaction outcome. The best yields of 5 were
obtained when 3 eq. of amine and 2 eq. of ZnCl2 in water were
used. With this catalytic system the reactions were carried out
readily. In most cases, the desired products were directly
extracted from the aqueous phase with CH2Cl2.
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regioselectivity of b-face attachment to the 4-position should
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In summary, we have demonstrated general and efficient
methods for the preparation of 4-aminosteroids. The present
method using K2CO3 as the activator for 4a-aminosteroids is a
simple one-step reaction involving easily prepared starting
materials. The described method using the ZnCl2–H2O system
for the preparation of 4b-aminosteroids offers specific advan-
tages with operational simplicity, simple purification proce-
dure, high yields and complete regioselectivity. The present
work offers interesting and new investigational insights into
designing novel, improved compounds for further research
and development of aminosteroidal drugs.
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Notes and references
z Crystal data for 3a: C21H33NO2, M = 331.48, orthorhombic, a =
5.9317(4), b = 12.8974(9), c = 24.7747(17) A, V = 1895.4(2) A3, T =
294(2) K, space group P212121, Z = 4, final R indices [I 4 2s(I)]:
R1 = 0.0566, wR2 = 0.1178; R indices (all data): R1 = 0.0956,
wR2 = 0.1316, CCDC 682757. For 3e: C23H35NO2, M = 357.52,
orthorhombic, a = 7.7287(7), b = 8.6066(8), c = 29.9778(16) A, V =
1994.1(3) A3, T = 296(2) K, space group P212121, Z = 4, final R
22 (a) H. C. Kolb, M. G. Finn and K. B. Sharpless, Angew. Chem.,
Int. Ed., 2001, 40, 2004; (b) J. J. Gajewski, Acc. Chem. Res., 1997,
30, 219.
indices [I 4 2s(I)]: R1 = 0.0529, wR2 = 0.1207; R indices
(all data): R1 = 0.0867, wR2 = 0.1350, CCDC 682758. For
ꢀc
This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 1037–1039 | 1039