Molecular association via halogen bonding and other weak interactions in the crystal structures of 11-bromo-12-oxo-5β-cholan derivatives

Article abstract of DOI:10.1016/j.molstruc.2008.05.051

Methyl 3α,7α-diacetoxy-12-oxo-5β-cholan-24-oate 2, methyl 11α-bromo-3α,7α-diacetoxy-12-oxo-5β-cholan-24-oate 3, methyl 11β-bromo-3α,7α-diacetoxy-12-oxo-5β-cholan-24-oate 4 and methyl 11,11-dibromo-3α,7α-diacetoxy-12-oxo-5β-cholan-24-oate 5 were synthesized. The crystal structures of these molecules were resolved to study the effect of bulky bromine atom in the steroid skeleton of cholic acid with different stereo-chemical orientations at C-11 on the two-dimensional arrangement of molecules and solid-state properties. All the molecules associate only via weak intermolecular interactions in their crystal structures, notable one being the Halogen Bonded assembly (C-Br...O) in 5.

Full text of DOI:10.1016/j.molstruc.2008.05.051

Journal of Molecular Structure 892 (2008) 246–253
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Molecular association via halogen bonding and other weak interactions
in the crystal structures of 11-bromo-12-oxo-5b-cholan derivatives
b
a
b,
*
Deepak B. Salunke a, Braja G. Hazra a, , Rajesh G. Gonnade , Vandana S. Pore , Mohan M. Bhadbhade
*
a Organic Chemistry Division, National Chemical Laboratory, Pune 411 008, India
b Center for Materials Characterization, National Chemical Laboratory, Pune 411 008, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 May 2008
Received in revised form 26 May 2008
Accepted 27 May 2008
Available online 12 June 2008
Methyl 3
cholan-24-oate 3, methyl 11b-bromo-3
dibromo-3 ,7 -diacetoxy-12-oxo-5b-cholan-24-oate 5 were synthesized. The crystal structures of these
molecules were resolved to study the effect of bulky bromine atom in the steroid skeleton of cholic acid
with different stereo-chemical orientations at C-11 on the two-dimensional arrangement of molecules
and solid-state properties. All the molecules associate only via weak intermolecular interactions in their
crystal structures, notable one being the Halogen Bonded assembly (C–Br. . .O) in 5.
Ó 2008 Elsevier B.V. All rights reserved.
a
,7
a
-diacetoxy-12-oxo-5b-cholan-24-oate 2, methyl 11
a-bromo-3a,7a-diacetoxy-12-oxo-5b-
a
,7a-diacetoxy-12-oxo-5b-cholan-24-oate 4 and methyl 11,11-
a
a
Keywords:
11-Bromosteroids
Halogen bonding
Crystal structure
Cholic acid
1. Introduction
and to construct environmentally responsive molecules [8,9].
Herein, we would like to report the synthesis and supramolecular
Stereoselective C-11 functionalization in steroids is one of the
challenging targets for synthetic organic chemists as it involves
severe steric interactions caused due to C-18 and C-19 angular
as well as spectral properties of methyl 3
5b-cholan-24-oate 2, methyl 11 -bromo-3
oxo-5b-cholan-24-oate 3, methyl 11b-bromo-3
12-oxo-5b-cholan-24-oate and methyl 11
3a,7a-diacetoxy-12-oxo-5b-cholan-24-oate 5, respectively. The
synthesis of compound 5 and the unique halogen bonding of this
novel molecule in steroid have been reported herein for the first
time.
a
,7
a
a a
-diacetoxy-12-oxo-
,7 -diacetoxy-12-
,7 -diacetoxy-
,11b-dibromo-
a
a
a
a
methyl groups. Introduction of C-11
a-hydroxyl functionality via
4
microbial hydroxylation by Syntex group [1] and via long-range
chemical functionalization by Breslow [2] are well documented.
Steroids with C-11 functionality are well known for biological
activity and are present in a number of naturally occurring mol-
ecules such as cortisone, hydrocortisone and corticosterone
[3,4]. Much more potent synthetic corticosteroids such as dexa-
methasone, triamcinolone and fluticasone also possess C-11 hy-
droxy functionality [5]. We planned to study the effect of bulky
halogen atom (bromine) at C-11 in the steroid skeleton of cholic
acid 1 with different stereo-chemical orientations on the crystal
structure and two-dimensional arrangement of molecules. Cholic
acid 1, a main bile acid, is a biosurfactant involved in the diges-
tion of dietary lipids [6]. It has unique distribution of three hydro-
2. Experimental
2.1. Synthesis
All chemicals were commercially available and used as re-
ceived, except that the solvents were dried and purified by distilla-
tion. Flash column chromatography was carried out using 230–400
mesh silica gel. For TLC analysis, precoated plates of silica gel 60
F254 (Merck) were used. Spots were visualized with UV light and/
or with dipping in a phosphomolybdic acid solution and charring
on a hot plate.
philic hydroxyl groups in the
a-face, and the b-face is
hydrophobic consisting of only hydrocarbons. Because of this cho-
lic acid is often considered to be a facial amphiphile. Due to its
interesting structural features and commercial availability, cholic
acid is a popular building block in supramolecular chemistry [7]
2.1.1. Methyl 3
Compound 2 was synthesized in an overall 88% yield start-
ing from cholic acid as reported by us earlier [10,11]
a,7a-diacetoxy-12-oxo-5b-cholan-24-oate (2)
1
* Corresponding authors. Tel.: +91 20 25902336; fax: +91 20 25902624
(B.G. Hazra), tel.: +91 20 25902290; fax: +91 20 25902642 (M.M. Bhadbhade).
E-mail addresses: bg.hazra@ncl.res.in (B.G. Hazra), mm.bhadbhade@ncl.res.in
(M.M. Bhadbhade).
(Scheme 1). White solid, mp 175–176 °C (EtOAc); IR and
NMR spectroscopic data was consistent with that reported in
the literature.
0022-2860/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2008.05.051

D.B. Salunke et al. / Journal of Molecular Structure 892 (2008) 246–253
247
O
O
O
O
OH
O
R1
OH
a/b/c
OMe
R2
H
OMe
d
AcO
OAc
HO
OH
AcO
OAc
H
H
1
3, R1 = H, R2 = Br
4. R1 = Br, R2 = H
5, R1 = Br, R2 = Br
2
Scheme 1. Reagents and conditions. (a) CH3OH, TsOH, 28 °C, 24 h, 98%; (b) Ac2O, DMAP, Et3N, DCM, 28 °C, 4–5 h, 92%; (c) CrO3, H2SO4, Acetone, 10 °C, 5 min, 98%; (d) Br2,
Benzene, 28 °C, 6 days, 88%.
2.1.2. Methyl 11
(3), Methyl 11b-bromo-3
(4), Methyl 11,11-dibromo-3
oate (5)
a
-bromo-3
a
a
,7
-diacetoxy-12-oxo-5b-cholan-24-oate
,7 -diacetoxy-12-oxo-5b-cholan-24-
a
-diacetoxy-12-oxo-5b-cholan-24-oate
respectively. High resolution mass spectra were obtained on a Kra-
tos MS-80 spectrometer. Elemental analyses were performed by
CHNS-O EA 1108-Elemental analyser, Carloerba Instrument (Italy)
and were within 0.4% of calculated values.
a
,7
a a
To a solution of 2 (1.008 g, 2 mmol) in benzene (10 mL), a bro-
mine solution (1 mL, 2 M in benzene) was slowly added with stir-
ring, at 30 °C in dark. After 6 days, TLC analysis showed the total
consumption of the starting material. The reaction mixture was di-
luted with 150 mL of ethyl acetate and poured in a 250 mL separ-
atory funnel. The organic layer was washed with 10% Na2S2O5
(2 ꢀ 10 mL), cold H2O (2 ꢀ 10 mL), brine (2 ꢀ 10 mL) and dried
over Na2SO4. Solvent was evaporated under reduced pressure to af-
ford crude product. The residue (1.35 g) was chromatographed on
silica gel (EtOAc/Pet Ether, 1:4) to yield the dibromo compound 5
2.3. Single-crystal structure determination: X-ray measurements
Single crystals of 2, 3, 4 and 5 were grown from a hot saturated
filtered solution of these compounds in ethyl acetate. Suitable crys-
tals were obtained by slow evaporation of the solvent at room tem-
perature (RT). Compound 2 was crystallized as colorless long
needles where as crystals of compounds 3, 4 and 5 were obtained
as thin plates. The best amongst them were selected using Leica
polarizing microscope. X-ray diffraction data of all the compounds
were collected on a Bruker SMART APEX CCD diffractometer with
(0.05 g, 4%), mp 181–182 °C (EtOAc); ½a D25
ꢁ
+40.00 (c 1.1, CHCl3);
IR (KBr) 2955, 1737, 1716 cmꢂ1
;
1H NMR (200 MHz, CDCl3) d
omega and phi scan mode, kMo K = 0.71073 Å at T = 297(2) K. All
a
1.02 (d, J = 6.3 Hz, 3H), 1.37 (s, 3H), 1.54 (s, 3H), 2.04 (s, 3H), 2.05
(s, 3H), 3.04 (bd, J = 15.3 Hz, 3 Hz), 3.38 (d, J = 11.4 Hz, 1H), 3.67
(s, 3H), 4.67 (m, 1H), 4.97 (m, 1H); 13C NMR (50 MHz, CDCl3) d
15.6, 18.4, 21.4, 21.4, 24.0, 26.6, 27.0, 29.7, 30.4, 31.1, 31.3, 34.5,
35.5, 36.2, 39.6, 40.3, 46.3, 49.0, 49.5, 51.5, 54.1, 54.7, 71.4, 73.6,
74.8, 169.9, 170.7, 174.5, 194.4; MS (LCMS) m/z: 685.1 [M+Na]+;
HRESIMS m/z 685.1152 [M+Na]+ (C29H42Br2NaO7; calcd.
685.1174). Anal. Calcd. for C29H42Br2O7: C, 52.58; H, 6.39. Found:
C, 52.60; H, 6.04. On further elution with the same solvent system
furnished b-bromo compound 4 (0.22 g, 19%), mp 183–184 °C
the data were corrected for Lorentzian, polarization, and absorp-
tion effects using Bruker’s SAINT and SADABS programs. The crys-
tal structures were solved by direct methods using SHELXS-97 and
the refinement was performed by full matrix least squares on F2
using SHELXL-97 [13]. Hydrogen atoms were included in the
refinement as per the riding model. Molecular graphics were from
Mercury (http://www.ccdc.cam.ac.uk/prods/mercury). Selected
details about the crystal structure, experiment and structure solu-
tion and refinement are given in Table 2. Geometrical parameters
for intermolecular interactions are included in Table 3. The crystal-
lographic-information-files (CIFs) have been deposited at the Cam-
(EtOAc); and followed by a-bromo compound 3 (0.76 g, 65%), mp
202–203 °C (EtOAc). IR and NMR spectroscopic data for com-
bridge Crystallographic Database Centre as a supplementary
pounds 3 and 4 are consistent with that reported in literature [12].
publication. The respective CCDC numbers are given in Table 2.
2.2. Methods
3. Results and discussion
Melting points were obtained with Buchi Melting Point appara-
tus B-540. Optical rotations were obtained on Bellingham & Stan-
3.1. Synthesis of compounds (2), (3), (4) and (5)
ley ADP-220 Polarimeter. Specific rotations ([a]D) are reported in
In the course of our studies on the synthesis of various bile
acid conjugates [12] we have recently synthesized [10] C-11 azi-
do/amino functionalized novel cholic acid derivatives. These C-11
functionalized cholic acid derivatives induced host cell fusion
during the progression of HIV-1 infection and produced multinu-
cleated giant cells [11]. We were interested in the investigation
of the effect of bulky bromine atom in steroid skeleton of cholic
acid with different stereo-chemical orientations at C-11 on the
two-dimensional arrangement of molecules and solid-state prop-
erties. Bromination of 12-oxo steroids has been widely explored
[14]. However, there is no report on the study of crystal struc-
ture properties of these 11-bromo compounds. A stereoselective
high-yield bromination of compound 2 to compound 3 was dem-
onstrated [14b] by Yanuka et al. and reproduced by us [11]. Re-
deg/dm, and the concentration (c) is given in g/100 mL in the spe-
cific solvent. Ultraviolet spectroscopy was performed using Perkin–
Elemer instrument, Lambda 35 UV/VIS Spectrometer. CD spectra
were taken on spectropolarimeter, Jasco J-715 at 25 °C using solu-
tions of the products in methanol exhibiting absorbance values in
the range 0.1–0.2 at 220 nm. Fourier transform infrared (FTIR)
spectra were recorded on Schimadzu 8400 series FTIR instrument.
For the IR spectrum 1 mg of sample was dispersed in 300 mg of KBr
and the pellets were prepared with a mini-press at 10 tonnes after
grinding manually in a mortar. IR spectra were acquired accumu-
lating 40 scans at 8 cmꢂ1 resolution. Only diagnostic bands are re-
ported on cmꢂ1 scale. 1H and 13C NMR spectra were recorded on a
Bruker DRX-500 spectrometer at 500.00 and 125.78 MHz, respec-
tively. The chemical shifts are given in ppm relative to tetrameth-
ylsilane. Mass spectra were recorded on LC-MS/MS-TOF API QSTAR
PULSAR spectrometer, samples introduced by infusion method
using Electrospray Ionisation Technique. EI and CI mass spectra
were recorded on an AEI MS-50 and AEI MS-9 spectrometer,
cently we have elaborated cholic acid 1 to its 11a-bromo and
11b-bromo derivatives 3 and 4 via 12-oxo methyl cholate 2
using bromine in benzene [10]. Detailed investigation of the bro-
mination reaction on compound 2 with excess bromine and
longer reaction period led to the isolation of hitherto unknown

248
D.B. Salunke et al. / Journal of Molecular Structure 892 (2008) 246–253
Table 1
Some representative 1H, 13C NMR chemical shifts and IR spectroscopy data
Compound
1H NMR, d ppm and coupling constants J in Hz
Methyl group Proton (H) at
C-12 carbonyl
13C NMR d ppm
IR mmax cmꢂ1
C-18
1.03
C-19
C-21
C-9
–
C-11
–
C-1e
–
2
3
4
5
1.03
1.21
1.38
1.52
0.85
d, J = 5.8
213.3
202.3
203.4
194.3
1701
1718
1697
1716
1.03
1.36
1.36
0.86
d, J = 5.8
2.79
dd, J = 10.7
5.01
d, J = 10.7
2.90
dt, J = 15.4 and 3.0
0.93
d, J = 6.6
2.64
4.42
d, J = 5.9
–
dd, J = 11.7 and 5.9
1.00
3.35
–
3.02
d, J = 5.9
d, J = 11.4
dt, J = 15.2 and 3.1
Table 2
Crystal data for compounds 2, 3, 4 and 5
Compound No.
2
3
4
5
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
C29H44O7
C29H43BrO7
583.54
297(2)
0.71073
Orthorhombic
P212121
10.059(2)
11.392(3)
25.629(6)
90
C29H43BrO7
C29H42Br2O7
662.45
297(2)
0.71073
Monoclinic
P21
504.64
297(2)
0.71073
Monoclinic
C2
27.845(15)
6.284(4)
18.143(10)
90
583.54
297(2)
0.71073
Monoclinic
P21
10.575(11)
6.541(7)
20.83(2
90
8.825(5)
b (Å)
c (Å)
11.778(6)
15.053(8)
90
a
(°)
b (°)
122.219(8)
90
2686(3)
4, 1.248
0.088
90
90
97.966(18)
90
1427(3)
2, 1.358
1.484
101.755(8)
90
1531.8(13)
2, 1.436
2.688
684
0.87 ꢀ 0.45 ꢀ 0.12
2.21–25.00
ꢂ10 <= h <= 10
ꢂ13 <= k <= 13
ꢂ17 <= l <= 17
10,530/5087
[R(int) = 0.0296]
99.8
c
(°)
Volume (Å3)
2936.9(12)
4, 1.320
1.442
Z, Calculated density (g/cm3)
Absorption coefficient (mmꢂ1
F(000)
)
1096
1232
616
Crystal size (mm3)
0.45 ꢀ 0.05 ꢀ 0.03
0.58 ꢀ 0.36 ꢀ 0.17
2.39–24.99
ꢂ5 <= h <= 11
ꢂ12 <= k <= 13
ꢂ30< = l <= 30
14,791/5145
[R(int) = 0.0346]
99.7
0.99 ꢀ 0.24 ꢀ 0.04
Theta range (data collection) (°)
Index ranges
2.27–25.00
1.97–25.00
ꢂ32 <= h <= 32
ꢂ7 <= k <= 7
ꢂ21 <= l <= 21
12,738/4690
[R(int) = 0.1103]
99.7
0.9972 and 0.9616
4690/1/332
1.003
ꢂ12 <= h <= 12
ꢂ7 <= k <= 7
ꢂ24 <= l <= 24
10,137/4796
[R(int) = 0.0466]
99.9
0.9430 and 0.3212
4796/1/340
0.994
Reflections collected/unique
Completeness to h = 25.00 (%)
Max. and min. transmission
Data/restraints/parameters
Goodness-of-fit on F2
0.7916 and 0.4884
5145/0/340
1.017
0.7351 and 0.2038
5087/1/349
0.990
Final R indices [I > 2r(I)]
R1
wR2
0.0768
0.1703
0.0540
0.1429
0.0630
0.1623
0.0383
0.0898
R indices (all data)
R1
0.1248
0.0698
0.1012
0.0484
wR2
0.1968
0.1531
0.2031
0.0965
Absolute structure parameter
Extinction coefficient
Largest diff. peak and hole (Åꢂ3
CCDC No.
3(2)
0.039(12)
None
ꢂ0.010(18)
ꢂ0.005(8)
0.0054(10)
0.276 and ꢂ0.218
676282
None
None
)
0.690 and ꢂ0.369
0.531 and ꢂ0.346
676284
0.363 and ꢂ0.235
676285
676283
C-11 dibrominated product, namely methyl
11 ,11b-dibromo-12-oxo-5b-cholan-24-oate
3
a
,7
a
-diacetoxy-
methyl group in 11a-bromo compound 3 in comparison with
a
5
in 4% yield
12-oxo compound 2, while deshielding effect of ꢃ0.33 to
0.35 d ppm on both C-18 and C-19 methyl’s was observed in
11b-bromo compound 4 in comparison with 12-oxo compound
2. Similarly, deshielding effect of ꢃ0.14 d ppm was observed
on C-19 methyl group in 11-dibromo compound 5 in compar-
ison with 11b-bromo compound 4, while deshielding effect of
about 0.31–0.32 d ppm on both C-19 and C-18 methyl’s were
(Scheme 1). The high resolution mass spectra of 5 showed the
expected molecular ion peak and in the 13C NMR spectra a qua-
ternary C-11 carbon appeared at d 74.76 ppm. The absolute
structure of 12-oxo compound 2 and bromo ketones 3, 4, 5
has been established by single-crystal X-ray analysis (Fig. 1).
3.2. Spectroscopic discussion
observed in 11-dibromo compound 5 in comparison with 11
a-
bromo compound 3.
Some interesting observations can be made on the 1H, 13C
NMR and IR spectra of the bromo ketones (3, 4 and 5) with
respect to the starting 12-oxo compound 2 (Table 1, Fig. 2).
Thus, C-11
0.18 d ppm of only C-19 methyl protons where as C-11b-bro-
mine contributes for the deshielding (ꢃ0.31–0.35 ppm of
both C-18 and C-19 methyl protons. The C-11 -bromine also
a-bromine contributes for the deshielding (ꢃ0.14–
d
Deshielding effect of ꢃ0.18
d
ppm was observed on C-19
a

D.B. Salunke et al. / Journal of Molecular Structure 892 (2008) 246–253
249
Table 3
Geometrical parameters for intermolecular interactions
Compound
2
D–H. . .
A
d(D–H)
d(H. . .A)
d(D. . .A)
h(D–H. . .A)
C(3)–H(3B). . .O(2)[a]
C(15)–H(15A). . .O(4)[b]
C(29)–H(29B). . .O(4)[c]
C(29)–H(29A). . .O(1)[d]
C(18)–H(18C). . .O(5)[e]
C(19)–H(19B). . .O(5)[e]
C(8)–H(8B). . .O(5)[e]
0.98
0.97
0.96
0.96
0.96
0.96
0.98
2.61
2.73
2.55
2.82
2.63
2.52
2.46
3.522(7)
3.558(6)
3.412(8)
3.732(8)
3.398(6)
3.439(6)
3.415(6)
155
144
149
159
138
159
165
3
4
5
C(26)–H(26A). . .Br(1)[f]
C(28)–H(28B). . .O(5)[g]
C(29)–H(29B). . .O(2)[g]
C(26)–H(26C). . .O(6)[g]
C(19)–H(19A). . .O(6)[h]
C(5)–H(5). . .O(6)[h]
0.96
0.96
0.96
0.96
0.96
0.98
0.97
0.96
3.15
2.74
2.81
2.66
2.77
2.79
2.82
2.70
3.856(6)
3.477(6)
3.748(12)
3.483(15)
3.628(14)
3.734(12)
3.601(8)
3.651(7)
132
134
165
145
149
161
139
172
C(2)–H(2A). . .O(7)[i]
C(21)–H(21A). . .O(1)[i]
C(26)–H(26C). . .O(2)[j]
C(18)–H(18C). . .O(5)[k]
C(28)–H(28A). . .O(6)[l]
C(15)–H(15A). . .O(6)[l]
C(23)–H(23A). . .O(4)[m]
C(3)–H(3B). . .O(2)[n]
C(19)–H(19B). . .O(5)[k]
C(8)–H(8B). . .O(5)[k]
C(11)–Br(2) . . .O(5)[o]
C(14)–H(14A) . . .Br(2)[p]
C(28)–H(28A). . .O(5)[p]
C(15)–H(15B). . .Br(1)[q]
C(26)–H(26B). . .O(4)[r]
0.96
0.96
0.96
0.97
0.97
0.98
0.96
0.98
2.68
2.92
2.71
2.56
2.60
2.44
2.74
2.47
3.503(10)
3.720(10)
3.22(2)
3.393(19)
3.476(17)
3.366(9)
3.698(10)
3.382(9)
144
141
114
144
151
159
173
154
1.967(3)
0.98
0.96
0.97
0.96
3.256(3)
3.11
2.46
2.99
2.51
–
139.3(4)
158
134
161
171
4.032(4)
3.198(6)
3.920(5)
3.461(8)
Symmetry codes: [a] ꢂx + 1/2, y ꢂ 1/2, ꢂz; [b] ꢂx + 1/2, y + 1/2, ꢂz + 1; [c] x ꢂ 1/2, y + 1/2, z; [d] x ꢂ 1/2, yꢂ1/2, z; [e] x, y ꢂ 1, z ; [f] ꢂx + 1, y + 1/2, ꢂz + 3/2; [g] x ꢂ 1/2, ꢂy + 1/
2, ꢂz + 2; [h] ꢂx + 3/2, ꢂy + 1, z ꢂ 1/2; [i] x + 1/2, ꢂy + 1/2, ꢂz + 2; [j] ꢂx + 2, y + 1/2, ꢂz; [k] x, y ꢂ 1, z; [l] ꢂx + 1, y ꢂ 1/2, ꢂz + 1; [m] x ꢂ 1, y + 1, z; [n] ꢂx + 2, y ꢂ 1/2, ꢂz; [o]
y + 1/2, ꢂz, [p] ꢂx + 1, y ꢂ 1/2, ꢂz; [q] x + 1, y, z; [r] x ꢂ 1, y, z.
Fig. 1. ORTEP views of compounds 2, 3, 4 and 5.
contributes for the deshielding of C-1 equatorial (C-1e) proton
Carbonyl resonance of C-12 in compound 2 appeared at
through space [15] this can be observed in compounds 3 and
213.3 ppm (Table 1). As expected [16], marked shielded shift
(shielding) of about 10 ppm was observed for carbonyl resonances
in 11-bromo-12-oxo compounds 3 (202.3) and 4 (203.4). This
clearly suggests that irrespective of the stereochemistry at C-11
position the bromo substitution accounts for shielding of about
10 ppm, accordingly carbonyl resonance of C-12 in C-11-gem-di-
bromo compound 5 appeared at 194.3 ppm. In this compound
5, in which there is a presence of C-11a-bromine. These obser-
vations demonstrate well-defined effect of bulky bromine
a
atom through space on the chemical shifts of C-18, C-19 angu-
lar methyl protons and C-1e proton. Earlier we have demon-
strated such kind of anisotropy by C-11 azido and amino
functionalities [10].

250
D.B. Salunke et al. / Journal of Molecular Structure 892 (2008) 246–253
Fig. 2. Partial 1H NMR spectra of 2, 3, 4 and 5 showing the effect of C-11 substitution on C-18, C-19 and C-21 methyl groups.
two bromine atoms showed an additive effect of about 19 ppm on
C-12 carbonyl resonance.
teristic of the equatorial bromine substituent. These findings are
consistent with the octant rule [18].
An equatorial
the infrared carbonyl stretching frequency (about 17 cmꢂ1 in case
of 11
-bromo-12-oxo compound 3 and 15 cmꢂ1 in case of C-11-
a-bromo substituent produced a marked shift in
In conclusion 1H NMR, IR and CD spectroscopy showed a con-
stant and pronounced effect of bulky bromine atom through space
which depends on the axial or equatorial orientation of bulky bro-
mine atom.
a
gem-dibromo-12-oxo compound 5) relative to that of the parent
ketone 2. An axial halogen has a negligible effect on the carbonyl
stretching frequency and this can be observed in case of com-
3.3. Single crystal X-ray diffraction studies
pounds 4 and 5. This is in accordance [17] with equatorial a-bromo
ketone.
The effects of sequential replacement of methylene-H atoms at
the C11 position of 2 by the bromine atom on molecular organiza-
tion in crystals were studied. The effect of classical H-bonding
interaction on the molecular packing of bile acid skeleton was re-
cently reported by Zhao [19] and Tato et al. [20]. In order to study
the interplay of weak intermolecular interactions on the molecular
aggregation of C11 functionalized bile acid skeleton in the absence
of conventional H-bonding, the C3 and C7 hydroxyl groups as well
The circular dichroism (CD) curves for the three bromoketones
3, 4 and 5 are compared with that of the parent 12-oxo compound
2 (Fig. 3). The axial nature of the bromine atom in compounds 4
and 5 was clearly evident from its strong negative Cotton effects,
indicating a very high degree of asymmetry in these compounds.
The similarity of the circular dichroism absorption (positive sign)
of the 11
a
-bromo ketone 3 and the parent compound 2 is charac-
2
0.2 mmol of 5
40
20
0
3
4
5
0.1 mmol of 5
0.05 mmol of 5
40
20
0
-20
-40
-20
200
250
300
350
250
300
350
400
Wavelength (nm)
Wavelength (nm)
Fig. 3. (A) CD of compounds 2, 3, 4 and 5; (B) CD of 5 at variable concentrations.

D.B. Salunke et al. / Journal of Molecular Structure 892 (2008) 246–253
251
as C24 carboxylic acid was masked with acetate and methyl ester
functionalities, respectively. As expected, the molecular packing
in all the four structures was dominated by C–H. . .O interactions
(Table 3).
these chains as compare to 2 giving more stability to the associa-
tion. These layers are zipped along the 21-screw axis via bifurcated
weak C–H. . .O interactions (C28–H28A. . .O6 and C15–H15A. . .O6)
between acetate group at C7 and side chain at C17. Molecular
packing viewed down the b-axis shows the dimeric assemblies of
layers, which are connected via C23–H23A. . .O4 contact (Fig. 6B).
It is interesting to note that Br atom in this molecule does not make
any short contact.
In 5, replacement of both the H-atoms at C-11 by Br atoms
causes an interesting effect on molecular organization. Molecules
in 5 are helically assembled around the crystallographic 21-screw
axis via halogen bonding (Br. . .O) and C–H. . .O contacts (Fig. 7A).
Beta bromine atom at C-11 interacts with the carbonyl oxygen
(O5) of the 21-screw axis molecule via C11–Br2. . .O5 contact. The
same carbonyl oxygen (O5) accepts H atom from C28-H28A of C7
acetate group of the next 21-screw axis molecule to make
C–H. . .O interaction (Table 3). In continuing this pattern, each suc-
cessive molecule gets a twist of 180° to generate helicity along the
b-axis, which coincides with the crystallographic twofold screw
axis. The Br2. . .O5 distance (3.256 Å) is much less than the sum
of their van der Waals radii (3.35 Å) although the angle of approach
deviates from linearity (hC11–Br2. . .O5 = 139.33°). The same is the
case with C28–H28A. . .O5 contact (H28A. . .O5 = 2.455 Å and hC28–
H28A. . .O5 = 133.93°). These successive helices are held together
along the a-axis through C26–H26C. . .O4 and C15–H15B. . .Br1
(Fig. 7B).
In 2, molecules make trifurcated C–H. . .O interactions with
unit-translated molecules along b-axis forming a molecular chain
involving carbonyl oxygen O5 and the methyl hydrogen from
C18, C19 and C8 atoms (Fig. 4A). These chains are zipped by 21-
screw axis via C3B–H3B. . .O2 interactions bringing the head
groups in close proximity. These layers make cohesions through
C29–H29B. . .O4 and C29–H29A. . .O1 interactions along the a-axis
and via C15–H15A. . .O4 contacts along the c-axis (Table 3,
Fig. 4B). The molecular organization does not exhibit typical
‘head-to-tail’ organization generally observed in steroids [19].
In 3, bromine at the C-11 alpha position breaks the trifurcated
C–H. . .O linked molecular assembly seen in 2. Molecules along a-
axis are linked by C28–H28B. . .O5 contacts across crystallographic
21-screw axis forming helical assembly (Fig. 5A). In addition, four
other C–H. . .O interactions (C2–H2A. . .O7, C29–H29B. . .O2, C21–
H21A. . .O1 and C26–H26C. . .O6, Table 3) are made between the
21 related molecules along the helix that brings the C3-acatate
group and the side chain in close proximity. These helices are
stitched together by C5–H5. . .O6, C19–H19A. . .O6 and C26–
H26A. . .Br1 interactions (Fig. 5B). The helices along the a-axis are
discretely packed by creating well-guided tunnel.
In 4 with Br atom at b position of C11, the organization of the
molecule is very similar to 2 along b-axis (Fig. 6A). Molecules made
trifurcated C–H. . .O assembly with longer C18–H18C. . .O5, C19–
H19B. . .O5 (Table 3) interactions but similar C8–H8B. . .O5 interac-
tion as compared to 2 (Fig. 4). Again here, these molecular chains
are glued through C3–H3B. . .O2 interactions along crystallographic
twofold screw axis by bringing the heads together. In addition, one
more C–H. . .O interaction (C26–H26C. . .O2) is involved in bridging
The molecular assembly via ‘Halogen bonding’, a non covalent
interaction between the halogen atoms (as acceptors of electron
density) and lone-pair possessing atoms (mostly O and N) was
vastly studied since last decade due to its application in the field
of crystal engineering, molecular recognition, solid-state synthesis
[21]. To the best of our knowledge, structure of 5 is the first exam-
ple of halogen bonding recognized in steroid structures. Since we
Fig. 4. Association of molecules in 2 showing (A) formation of molecular chain via trifurcated C–H. . .O interactions and (B) bridging of the layers viewed down b-axis.

252
D.B. Salunke et al. / Journal of Molecular Structure 892 (2008) 246–253
Fig. 5. View of molecular packing in 3 showing (A) helical assembly along the a-axis and (B) interlinking of the helices via C–H. . .O interactions.
Fig. 7. (A) Helical self-assembly through C–Br. . .O and C–H. . .O interactions in 5
along the 21-axis and (B) interlinking of helices along a-axis via C–H. . .O and C–
H. . .Br contacts.
groups in steroid structures, 7 structures showed C–X. . .O interac-
tion. In all the seven structures, only carbonyl oxygen was involved
in the short contact with Cl, Br and I, but no hits were found for F.
This weak interaction was considered to have a role in binding of
halo-steroids to the carbonyl of the peptide chain of the receptor
protein that offers optimum desired binding affinity, as observed
in the crystal structure of an inhibitor-protein complex between
the inhibitor 4,5,6,7-tetra-bromobenzotriazole and phosphor-
CDK2-cyclin A [23].
The best fit of the four steroids 2, 3, 4 and 5 (Fig. 8) reveals the
large conformational differences, maximum seen in the side chains
at C-3 and C-17. It is note worthy to see some amount of flexibility
upon bulky substitution at C11 in the steroid skeleton itself, which
is normally taken as a rigid framework.
Fig. 6. Molecular packing viewed down a-axis in 4 (A) viewed down b-axis (B)
association of the molecules via C–H. . .O interactions.
observed halogen bonding contact for the first time in steroids, we
carried out a CSD search to examine its occurrence and preferred
geometries in other steroid structures [22]. The constrains applied
were R < 0.10, distances 6 sum of van der Waals radii and angles in
the range 130–180°. All searches were carried out with error-free
coordinates and restricted entries of disordered, ionic, polymeric
and powdered structures. The CSD search included halogens
(X = F, Cl, Br, I) and differently hybridized oxygen atoms (carbonyl
and ether). Out of 104 structures containing halogen atoms and CO
4. Conclusion
Steroids with C-11 functionality are well known for their bio-
logical activity. Keeping this in mind the effect of bulky bromine
atom at C-11 in steroid skeleton of cholic acid with different ste-
reo-chemical orientations on the molecular organization in solid-

D.B. Salunke et al. / Journal of Molecular Structure 892 (2008) 246–253
253
Acknowledgements
DBS thanks Council of Scientific and Industrial Research (CSIR),
New Delhi for the Senior Research Fellowship. Emeritus Scientist
Scheme awarded to BGH by the CSIR, New Delhi is gratefully
acknowledged.
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Crystallographic data for the structural analysis of 2, 3, 4 and 5
have been deposited with the Cambridge Crystallographic Data
Centre, CCDC No. 676282, 676283, 676284 and 676285, respec-
tively. Copies of this information can be obtained free of charge
from The Director, CCDC, 12 Union road, Cambridge CB2 1EZ, UK
(fax: C44 1223 336 033; e-mail: deposit@ccdc.cam.ac.uk) or via
www.ccdc.cam.ac.uk/conts/retrieving.html.