Synthesis and crystal structure of [1,2,4]-triazolo-annulated 3-aza-A-homocholestane derivative: A novel pentaheterocyclic ring system

Article abstract of DOI:10.1007/s10870-010-9880-x

A novel pentaheterocyclic ring system derived from (5α)-cholestan-3- one, i.e. [1R-[1α(R*),3aβ,3bα,5aβ,12aα, 12bβ,14aα]]-1-(1,5-dimethylhexyl)-1,2,3,3a,3b,4,5,11,12,12a,12b,13, 14,14a-tetradecahydro-8,12a,14a-trimethyl-9-(2,4,6-trichlorophenyl) -cyclopenta[5,6]naphtho[2,1-d][1,2,4]triazolo[1,5-a]azepinium hexachloroantimonate (6) has been synthesized via the reverse-electron-demand 1,3-dipolar cycloaddition of the 1-aza-2-azoniaallene cation 4 to the triple bond of acetonitrile followed by ring enlargement. The structure of 6 was determined by NMR, IR and high-resolution mass spectra, and unequivocally confirmed by X-ray crystallographic analysis. The title compound crystallizes in monoclinic class under the space group P2-1 with a = 8.163(3) A, b = 11.214(4) A, c = 24.191(9) A, α = 90°, β = 97.740(5)°, and γ = 90°. The five-membered triazole ring is essentially planar and aromatic, while the seven-membered azepine ring is not planar, but adopts a chair-like conformation.

Full text of DOI:10.1007/s10870-010-9880-x

J Chem Crystallogr (2011) 41:316–321
DOI 10.1007/s10870-010-9880-x
ORIGINAL PAPER
Synthesis and Crystal Structure of [1,2,4]-Triazolo-annulated 3-
Aza-A-homocholestane Derivative: A Novel Pentaheterocyclic
Ring System
•
•
•
He-Xiang Bai Jing-Mei Wang Shi-Jing Xia
Quan-Rui Wang
Received: 14 January 2010 / Accepted: 25 August 2010 / Published online: 15 September 2010
Ó Springer Science+Business Media, LLC 2010
Abstract A novel pentaheterocyclic ring system derived
from (5a)-cholestan-3-one, i.e. [1R-[1a(R*),3ab,3ba,5a-
b,12aa,12bb,14aa]]-1-(1,5-dimethylhexyl)-1,2,3,3a,3b,4,5,
11,12,12a,12b,13,14,14a-tetradecahydro-8,12a,14a-trimethyl-
9-(2,4,6-trichlorophenyl)-cyclopenta[5,6]naphtho[2,1-d][1,2,4]
triazolo[1,5-a]azepinium hexachloroantimonate (6) has been
synthesized via the reverse-electron-demand 1,3-dipolar
cycloaddition of the 1-aza-2-azoniaallene cation 4 to the triple
bond of acetonitrile followed by ring enlargement. The struc-
ture of 6 was determined by NMR, IR and high-resolution
mass spectra, and unequivocally confirmed by X-ray
crystallographic analysis. The title compound crystallizes
in monoclinic class under the space group P2-1 with a =
azepines with an additional triazole ring annulated to the
seven-membered ring have been extensively investigated
due to their ubiquitous feature of many pharmaceutical
products [8–10]. In general, the incorporation of a triazole
ring plays a fundamental role in enhancing the affinities for
receptors.
On the other hand, many steroidal heterocycles with high
biological activities have attracted widespread attention
[11–13]. The replacement of one or more carbon atoms of a
steroid molecule by a heteroatom, especially nitrogen often
results in useful alterations to its biological activity [14]. The
easily accessible steroidal ketone (5a)-cholestan-3-one 1 has
been widely employed as a substrate for preparation of
thiazolyl and thieno cholestane derivatives with potent
antiinflammatory effects, [15], 3-aza-A-homocholestanes
and 3-aza-A-homo-5a-cholestano[3,4-d]tetrazole analogues
[16, 17]. It is therefore worth developing new routes to
steroidal heterocyclic derivatives [18]. A literature survey
revealed that methods for achieving the expansion of A-ring
to azepine ring seem to be not as plentiful. The well
exploited strategies are the Schmidt reaction [19, 20],
Beckmann rearrangement [21, 22].
˚
˚
˚
8.163(3) A, b = 11.214(4) A, c = 24.191(9) A, a = 90°,
b = 97.740(5)°, and c = 90°. The five-membered triazole
ring is essentially planar and aromatic, while the seven-
membered azepine ring is not planar, but adopts a chair-
like conformation.
Keywords 1,2,4-Triazole ꢀ Azepine ꢀ 3-Aza-A-
Homocholestane ꢀ Synthesis ꢀ Crystal Structure
We are currently engaged in a program aimed at syn-
thesizing novel triazolo annulated heterocycles. In this
regard, we have established a reliable general synthetic
pathway to attain [1,2,4]-triazoloazepines [23–25]. Working
on this line, it appeared to us to be of interest to combine the
triazole ring to the 3-aza-A-homo-cholestane motif. In this
contribution, we report the synthesis and crystal structure of
an unprecedented pentacyclic compound, [1R-[1a(R*),3ab,
3ba,5ab,12aa,12bb,14aa]]-1-(1,5-dimethylhexyl)-1,2,3,3a,
3b,4,5,11,12,12a,12b,13,14,14a-tetradecahydro-8,12a,14a-
trimethyl-9-(2,4,6-trichlorophenyl)-cyclopenta[5,6]naphtho
[2,1-d][1,2,4]triazolo[1,5-a]azepinium hexachloroantimo-
nate (6).
Introduction
[1,2,4]-Triazoles and related fused heterocycles belong to a
class of privileged structures in the design and discovery of
new physiologically active compounds [1–7]. For instance,
H.-X. Bai ꢀ J.-M. Wang ꢀ S.-J. Xia ꢀ Q.-R. Wang (&)
Department of Chemistry, Fudan University,
Shanghai 200433, People’s Republic of China
e-mail: qrwang@fudan.edu.cn
123

J Chem Crystallogr (2011) 41:316–321
Experimental
317
product 3 was unstable, the crude material was used in the
next step without further purification. Orange oil. Yield:
1
Instruments
97%. IR (KBr pellets, cm -1): m 1720, 1552, 1382. H
NMR (CDCl3): d 0.66–2.70 (m, 46H, steroidal H), 7.20 (s,
2H, aromatic H).
The infrared spectrum IR spectra were recorded using KBr
pellets on a Nicolet-360 IR spectrometer, and absorptions
1
are given in wavenumbers (cm-1). The H and 13C NMR
[1R-[1a(R*),3ab,3ba,5ab,12aa,12bb,14aa]]-1-(1,5-
dimethylhexyl)-1,2,3,3a,3b,4,5,11,12,12a,12b,13,14,
14a-tetradecahydro-8,12a,14a-trimethyl-9-(2,4,6-
trichlorophenyl)-cyclopenta[5,6]naphtho
[2,1-d][1,2,4]triazolo[1,5-a]azepinium
spectra were measured on a JEOL ECA 400 spectrometer
with TMS as an internal reference and CDCl3 as the
solution, and the chemical shifts are given in d (ppm).
Coupling constants (J) are reported in Hz. High-resolution
mass spectrum was recorded on a SHIMADZU LCMS-IT-
TOF mass spectrometer with ESI ionization. Melting
points are uncorrected and expressed in °C.
hexachloroantimonate (6)
A mixture of compound 3 (0.61 g, 1 mmol) and acetonitrile
(1 mL, *19 mmol) was prepared and cooled to about
-60 °C, to which a solution of SbCl5 (0.45 g, 1.5 mmol) in
dry CH2Cl2 (5 mL) was carefully dropped in a period of
30 min. The mixture was stirred at this temperature for 2 h,
then allowed gradually to warm to 30 °C (bath temperature)
and was stirred further for 1.5 h. After concentrating the
mixture to about 2 mL by evaporating the volatiles, the
crude product was precipitated by slow addition of cold Et2O
(20 mL). The crude product was purified by recrystallization
from MeOH-MeCN-Et2O (1:1:1) to afford 0.85 g of pure
compound 6 as brownish crystals. Yield: 90%. m.p.
222–224 °C. Anal. Required for C35H51Cl9N3Sb: C: 44.04;
H, 5.38; N, 4.40%. Found C: 43.84; H, 5.38; N, 4.80%. IR
(KBr pellets, cm-1): m 2491, 2865, 1570, 1556, 1458, 1389.
1H NMR (CDCl3): d 0.65 (s, 3H, CH3), 0.85–2.00 (m, 38H,
steroidal H), 2.42 (d, J = 12.2 Hz, 1H), 2.50 (s, 3H, CH3),
2.99–3.01 (m, 1H, one of C(6)–H), 3.35 (m, 1H, one of C(6)–
H)), 4.10 (m, 1H, one of C(11)–H), 4.38 (m, 1H, one of
C(11)-H), 7.76 (s, 2H, aromatic H). 13C NMR (CDCl3): d
11.9, 12.1, 18.7, 21.3, 22.6, 22.8, 23.8, 24.1, 28.0, 28.2, 30.4,
31.2, 31.5, 34.9, 35.8, 36.1, 39.5, 39.7, 39.8, 42.2, 43.8, 45.7,
52.6, 56.2, 56.3 (steroidal CH3), 122.4, 130.9, 131.2, 135.7,
136.6, 143.1 (Cl3C6H2), 159.5, 164.0 (C=N). HRMS (ESI):
m/z calcd for the cation C35H51Cl3N3?: 618.3143; found:
618.3173.
Single crystal X-ray data were collected on a BRUKER
SMART APEX-CCD diffractometer equipped with a
graphite-monochromated MoKa radiation. The structure
was solved by direct Fourier methods. Full-matrix least-
squares refinement was based on F2 with SHELXL-97 [26].
Synthesis of the Compound
All chemicals used for this synthesis were of reagent grade
quality and used as received. Solvents were dried by
standard methods and distilled prior to use.
(5a)-Cholestan-3-one (2,4,6-trichlorophenyl)hydrazone (2)
A mixture of cholestan-3-one 1 (1.93 g, 5 mmol) and 2,4,6-
trichlorophenylhydrazine (1.05 g, 5 mmol) containing
0.5 mL of glacial acetic acid were refluxed in ethanol for
3 h. After cooling, the precipitates were obtained by filtra-
tion, which was recrystallized from hot 95% ethanol to
provide 2.50 g of the pure hydrazone 2 as a white solid.
Yield: 86%. m.p. 118–120 °C. Anal. required for
C33H49Cl3N2: C, 68.32; H, 8.51; N, 4.83%. Found: C, 68.23;
H, 8.45; N, 4.95%. IR (KBr pellets, cm -1): m 3422, 3222,
2963, 2932, 2852, 1550, 1467, 1445, 1377. 1H NMR
(CDCl3): d 0.67–3.05 (m, 46H, steroidal H), 6.78 (s, 1H,
NH), 7.28 (s, 2H, aromatic H).
A fully completed Crystallographic Information File
deposited with the CCDC is available (Deposition CCDC
No. 754601).
3-Chloro-3-(2,4,6-trichlorophenyl)azo-(5a)-cholestane (3)
The reaction was performed in a nitrogen atmosphere. To a
solution of hydrazone 2 (1.16 g, 2.0 mmol) in CH2Cl2
(20 mL) under external ice-water cooling was added
dropwise over 15 min a solution of t-BuOCl (0.32 g,
3 mmol) in CH2Cl2 (5 mL). The mixture was stirred for
30 min, the progress of the reaction was monitored by
TLC. Upon full conversion of the hydrazone 2, the mixture
was dried over CaCl2, filtered and concentrated under
reduced pressure to remove volatiles. Since the resulting
Results and Discussion
The required starting material, (5a)-cholestan-3-one 1, was
easily prepared via a two-step procedure from cholesterol.
Hydrogenation of the 5-ene moiety of cholesterol under
Pd/C catalysis in tetrahydrofuran afforded 5a-dihydrocho-
lesterol in 90% yield [27] which was oxidized with CrO3 in
acetic acid to provide 1 in 91% yield [28].
123

318
J Chem Crystallogr (2011) 41:316–321
The synthetic route of the title compound 6 is illustrated
ambient temperature, ring enlargement via hetero-Wagner-
Meerwein rearrangement of the primary cycloadduct 5
proceeds with insertion of the nitrogen atom into the car-
bon skeleton to furnish the isolated product 6. Evidence
supporting this selectivity was initially deduced from the
observed strong downfield shift between 4.1 and 4.4 ppm
for the two C(11)–methylene protons attached to the tria-
zole ring.
in Scheme 1, which was commenced with the condensation
of 1 with (2,4,6-trichlorophenyl)hydrazine in refluxing
ethanol in the presence of catalytic amount of acetic acid to
provide the corresponding hydrazone 2. Upon treatment
with 1.5 equivalents of t-BuOCl under external ice-water
cooling, the hydrazone 2 was readily transformed to the
a-chloroazo compounds 3 as a yellow to red oil. Since
compound 3 is very prone to decomposition upon keeping,
it should be used freshly as obtained for further step.
Next, a solution of 1.5 equivalent SbCl5 in anhydrous
CH2Cl2 was slowly dropped to a cold (-60 °C) solution of
3 and largely excessive amount of acetonitrile in anhydrous
CH2Cl2. The mixture was stirred further at this temperature
for about 1 h and then allowed to warm to 30 °C in a
period of another hour. The mixture was allowed to stir at
the same temperature for additional 1.5 h. Usual work-up
afforded the [1,2,4]-triazolo fused 3-aza-A-homo-choles-
tane salt derivative 6 as white crystals in 90% yield.
The synthesis shares the same mechanistic scenario as
reported for other triazolo fused bi- or tricyclic compounds
[23–25]. As shown in detail in Scheme 1, SbCl5-asssited
departure of a chloride ion from 3 generated the tetracyclic
1-aza-2-azoniaallenium salt 4, which belong to a class of
highly reactive intermediates. In general, hetero-allenium
ions like 4 functions well as a kind of positively charged
1,3-dipoles [29]. Thus, in the presence of acetonitrile, 4 is
intercepted by polar [3??2]-cycloadditon to the triple bond
of acetonitrile to produce the 3-sipro-substituted 3H-1,2,4-
triazolium salt 5. Finally, on elevating the temperature to
Noteworthy is the exclusive migratory aptitude observed
in the ring enlargement step which led to the isolation of 6
as sole product. The C(2) of the cholestane migrates in
preference to the C(4), and exclusively to N(2) of the
triazolium unit rather than N(4).
1
The data of IR, H NMR, 13C NMR, HRMS and ele-
mental analysis for the product are in good agreement of
the proposed structure of compound 6. In preceding papers
[23–25], we have revealed that the migratory aptitude of
the substituents in the rearrangement 5 ? 6 tends to par-
allel their ability to accommodate the respective carbo-
cation. Based on this rule, one would expect that the final
ring expansion should give the regioisomeric product 7
(see Fig. 1). Considering this controversy, the structural
establishment required indisputable proofs. As such, the
crystal of 6 was developed and subjected to X-ray dif-
fraction analysis. Table 1 contains crystallographic data of
compound 6, and Table 2 lists the selected bond lengths
and angles. Table 3 gives the selected torsional angles. The
X-ray structure for 6 is shown in Fig. 2.
Compound 6 crystallized in the chiral space group P2(1).
The five-membered triazole ring (C2–N3–N1–C1–N2) is
Scheme 1 Procedure of
(CH2)3CH(CH3)2
H
preparing the title compound 6.
Reagents and conditions (i)
(2,4,6-
H
i
ii
Cl
H
Trichlorophenyl)hydrazine,
AcOH/EtOH, reflux, 3 h; (ii) t-
BuOCl/CH2Cl2, 0 °C, 30 min;
(iii, iv) SbCl5, CH2Cl2, -60 °C;
MeCN/CH2Cl2, -60–30 °C,
totally 2 h; (v) 30 °C, 1.5 h
N
H
H
N=N
H
Ar
N
Ar
H
N
O
H
1
2
3
iii
iv
v
N
Ar
N
Ar
N
N
H
H
SbCl6
5
−
SbCl6
−
H3C
4
(CH2)3CH(CH3)2
(CH2)3CH(CH3)2
H
H
H
H
Ar
Ar
N
H
H
H
H
N
N
N
−
−
SbCl6
SbCl6
H
H
H3C
N
H3C
N
6
6'
Ar = 2,4,6-trichlorophenyl
123

J Chem Crystallogr (2011) 41:316–321
319
˚
Table 2 Selected bond lengths (A) and angles (°) of compound 6
(CH2)3CH(CH3)2
H
Bond lengths
H
N1–C1
1.343(13)
1.321(13)
1.443(13)
1.506(13)
1.583(15)
1.510(13)
1.506(13)
N1–N3
N3–C2
N2–C1
C2–C3
1.378(11)
1.328(12)
1.328(13)
1.462(14)
1.600(15)
1.456(13)
1.440(15)
Ar
H
H
N2–C2
N
−
N
SbCl6
N1–C19
H
7
H3C
N
N3–C18
C3–C4
C4–C16
C17–C18
C1–C25
Fig. 1 Structure of a hypothetical isomeric product 7 from the
reaction
C16–C17
C18–N3
Bond angles
C1–N1–N3
N3–N1–C19
C2–N3–N1
N1–N3–C18
N2–C1–C25
N2–C2–N3
N3–C2–C3
Table 1 Crystal data and structure refinement for compound 6
104.8(8)
121.9(8)
107.1(9)
122.1(8)
125.2(10)
110.9(9)
124.5(12)
C1–N1–C19
C2–N2–C1
132.6(10)
105.8(9)
Empirical formula
Formula weight
CCDC deposit no.
Temperature
C35H51Cl9N3Sb
954.59
C2)–N(3)–C18
N2–C1–N1
N1–C1–C25
N2–C2–C3
130.8(10)
111.3(11)
123.3(10)
124.6(10)
112.4(8)
754601
293(2) K
˚
0.71073 A
Wavelength
Crystal system
Space group
Monoclinic
P2(1)
N3–C18–C17
˚
a = 8.163(3) A
Unit cell dimensions
˚
b = 11.214(4) A
Table 3 Selected torsional angles (°) for compound 6
˚
c = 24.191(9) A
C18–N3–C2–C3
1.25(1.69) C16–C17–C18–
N3
-69.83(1.42)
a = 90°
b = 97.740(5)°
c = 90°
N3–C2–C3–C4
C2–C3–C4–C16
C2–N3–C18–C17
-55.33(1.41) C2–N3–C18–C17
74.02(1.25) C1–N1–N3–C18
50.76(1.48) C1–N2–C2–C3
50.76(1.48)
176.73(0.92)
-177.82(1.04)
95.68(1.39)
3
˚
Volume
2194.2(14) A
Z
2
C3–C4–C16–C17 -60.53(1.17) C1–N1–C19–C24
Density (calculated)
Absorption coefficient
F(000)
1.445 Mg m-3
1.205 mm-1
C4–C16–C17–
C18
62.35(1.33) N3–N1–C19–C20
90.85(1.20)
972
0.15 9 0.08 9 0.05 mm3
Crystal size
h range for data collection
Index ranges
1.70–26.01°
-10 B h B 9
-11 B k B 13
-26 B l B 29
9990
However, it is slightly shorter than the N–N single bond
˚
value of 1.401 A reported by Allen [30]. In a [1, 3, 4]-
˚
thiadiazole system a N–N distance of 1.373(3) A has been
Reflections collected
found by Song [31] which is quite close to our value. In
addition, Schantl has reported a stable five-membered azo-
methine imine in which a much shorter N–N distance of
Independent reflections
Reflections observed ([2r)
Data completeness
6689 [Rint = 0.0538]
3185
˚
1.303(2) A was disclosed [32]. On the other hand, the other
0.991
four bond lengths of N1–C1, N2–C2, N3–C2 and N2–C1 are
nearly equal as the value ranges between 1.321(13) and
˚
1.343(13) A, which refer to a partial double bond, and are
Absorption correction
Max. and min. transmission
Refinement method
Semi-empirical from equivalents
0.9422 and 0.8400
Full-matrix least-squares on F2
6689/1/439
almost identical to those found in related delocalized system
˚
such as 1.338 A in pyridine [33]. All above data suggest that
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I [ 2r(I)]
R indices (all data)
0.820
there is a delocalized system in this five-membered ring, in
which the positive charge is expected to reside mainly on the
N(1) and N(3) atoms. In line with this assumption, two main
resonance hybrids 6 and 60 may be drawn (Scheme 1).
Due to hindered rotation about the N1–C19 bond, the
trichlorophenyl ring at N1 is nearly perpendicular to the
triazole ring. The dihedral angle between these two planes
is found to be 87.26° (0.31°). This view was reinforced by
the dihedral angles of 95.68(1.39)° for C1–N1–C19–C24 as
R1 = 0.0514, wR2 = 0.1254
R1 = 0.1126, wR2 = 0.1435
-0.01(3)
Absolute structure parameter
Largest diff. peak and hole
-3
˚
0.579 and -0.446 e A
essentially planar (Rms deviation of fitted atoms = 0.0156).
The ring contains a significantly long N(1)–N(3) distance of
˚
1.378(11) A, which represents a N–N single bond.
123

320
J Chem Crystallogr (2011) 41:316–321
Fig. 2 Molecular structure of
compound 6 showing the atomic
labeling. Atoms are given by
thermal ellipsoids at 50%
probability. H atoms are omitted
for clarity
Acknowledgment This study was carried out with the financial
assistance of the National Natural Science Foundation of China
(Project 20372015).
well as 90.85(1.20)° for N3–N1–C19–C20. Besides, the
˚
rather long N1–C19 bond distance [1.443(13) A] is indic-
ative of a C(sp2)–N single bond. Therefore, we conclude
that no conjugation occurs between the two rings.
The seven-membered azepine ring is not planar, but adopts
a chair-like conformation. This may be described in terms of
the seven torsion angles inside the ring (Table 3). It can also
be seen from the torsion angles of C1–N1–N3–C18
[176.73(0.92)°] and C1–N2–C2–C3 [-177.82(1.04)°] that
C18 and C3 atoms are situated almost in the plan of the tria-
zole ring. Aside from the normal C(sp3)–C(sp3) single bonds
found for C16–C17 and C17–C18, the other two C(sp3)–
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CCDC-754601 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
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