2-Cyano­quinolin-1-ium nitrate

Loh, W.-S.; Hemamalini, M.; Fun, H.-K.

doi:10.1107/S1600536810039243

organic compounds

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ISSN: 2056-9890

Volume 66| Part 11| November 2010| Pages o2726-o2727

https://doi.org/10.1107/S1600536810039243

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2-Cyanoquinolin-1-ium nitrate

Wan-Sin Loh,^a ‡ Madhukar Hemamalini ^a and Hoong-Kun Fun ^a ^*§

^aX-ray Crystallography Unit, School of Physics, Universiti Sains Malaysia, 11800 USM, Penang, Malaysia
^*Correspondence e-mail: hkfun@usm.my

(Received 30 September 2010; accepted 30 September 2010; online 9 October 2010)

A proton is transferred from the nitric acid to the N atom of 2-cyanoquinoline during crystallization, resulting in the formation of the title salt, C₁₀H₇N₂⁺·NO₃⁻. The quinolinium ring system is approximately planar, with a maximum deviation of 0.013 (3) Å. In the crystal, a very asymmetric bifurcated N—H⋯(O,O) hydrogen bond to two O atoms of an adjacent nitrate anion occurs, generating an R₂¹(4) ring motif. C—H⋯O hydrogen bonds link the ions into sheets stacking along the a axis.

Related literature

For background to and the biological activities of quinoline derivatives, see: Loh, Quah et al. (2010a ,b ); Loh et al. (2010 ); Sasaki et al. (1998 ); Reux et al. (2009 ); Morimoto et al. (1991 ); Michael (1997 ); Markees et al. (1970 ); Campbell et al. (1988 ). For the hydrogen-bond motif, see: Bernstein et al. (1995 ). For related structures, see: Loh, Quah et al. (2010a,b); Loh et al. (2010). For the stability of the temperature controller used for the data collection, see: Cosier & Glazer (1986 ). For bond-length data, see: Allen et al. (1987 ).

Experimental

Crystal data

C₁₀H₇N₂⁺·NO₃⁻
M_r = 217.19
Monoclinic, P 2₁ /c
a = 3.6969 (1) Å
b = 17.7031 (3) Å
c = 14.6029 (2) Å
β = 95.802 (1)°
V = 950.81 (3) Å³
Z = 4
Mo Kα radiation
μ = 0.12 mm⁻¹
T = 100 K
0.44 × 0.18 × 0.07 mm

Data collection

Bruker SMART APEXII CCD diffractometer
Absorption correction: multi-scan (SADABS; Bruker, 2009 ) T_min = 0.951, T_max = 0.992
16184 measured reflections
2758 independent reflections
2103 reflections with I > 2σ(I)
R_int = 0.041

Refinement

R[F² > 2σ(F²)] = 0.045
wR(F²) = 0.122
S = 1.05
2758 reflections
170 parameters
H atoms treated by a mixture of independent and constrained refinement
Δρ_max = 0.37 e Å⁻³
Δρ_min = −0.29 e Å⁻³

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
N1—H1N1⋯O1	0.95	2.56	3.2376 (15)	128
N1—H1N1⋯O3	0.95	1.60	2.5432 (14)	172
C5—H5A⋯O3ⁱ	0.943 (16)	2.497 (16)	3.2835 (16)	141.0 (15)
C7—H7A⋯O2ⁱⁱ	0.976 (16)	2.473 (16)	3.3641 (16)	151.8 (12)
C8—H8A⋯O2ⁱⁱⁱ	0.977 (18)	2.391 (19)	3.3355 (17)	162.4 (15)
Symmetry codes: (i) $[-x+1, y+{\script{1\over 2}}, -z+{\script{3\over 2}}]$ ; (ii) $[-x, y+{\script{1\over 2}}, -z+{\script{3\over 2}}]$ ; (iii) $[x-1, -y+{\script{1\over 2}}, z-{\script{1\over 2}}]$ .

Data collection: APEX2 (Bruker, 2009); cell refinement: SAINT (Bruker, 2009); data reduction: SAINT; program(s) used to solve structure: SHELXTL (Sheldrick, 2008 ); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL and PLATON (Spek, 2009 ).

Supporting information

Crystal structure: contains datablocks global, I. DOI: https://doi.org/10.1107/S1600536810039243/hb5669sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S1600536810039243/hb5669Isup2.hkl

checkCIF report

Comment top

Recently, hydrogen-bonding patterns involving quinoline and its derivatives with organic acid have been investigated (Loh at al., 2010a,b; Loh et al., 2010). Syntheses of the quinoline derivatives were discussed earlier (Sasaki et al., 1998; Reux et al., 2009). Quinolines and their derivatives are very important compounds because of their wide occurrence in natural products (Morimoto et al., 1991; Michael, 1997) and biologically active compounds (Markees et al., 1970; Campbell et al., 1988). Heterocyclic molecules containing cyano group are useful as drug intermediates. Herein we report the synthesis of 2-cyanoquinolin-1-ium nitrate.

The asymmetric unit of the title compound (Fig. 1) consists of one 2-cyanoquinolin-1-ium cation (C1–C10/N1/N2) and one nitrate anion (N3/O1–O3). One proton is transferred from the hydroxyl group of nitrate to the atom N1 of 2-cyanoquinoline during the crystallization, resulting in the formation of salt. The quinoline ring system (C1–C9/N1) is approximately planar with a maximum deviation of 0.013 (3) Å at atom C6. The R₂¹(4) ring motif (Fig. 1; Bernstein et al., 1995) indicates a bifurcated hydrogen bond from N1–H1N1 to the two acceptors (O1/O3). Bond lengths (Allen et al., 1987) and angles are within the normal ranges and are comparable to the related structures (Loh et al., 2010a,b; Loh et al., 2010).

In the crystal packing (Fig. 2), intermolecular N1—H1N1···O1, N1—H1N1···O3, C5—H5A···O3, C7—H7A···O2 and C8—H8A···O2 hydrogen bonds (Table 1) link the molecules into two-dimensional planes stacking along the a axis.

Related literature top

For background to and the biological activities of quinoline derivatives, see: Loh, Quah et al. (2010a,b); Loh et al. (2010); Sasaki et al. (1998); Reux et al. (2009); Morimoto et al. (1991); Michael (1997) Markees et al. (1970); Campbell et al. (1988). For the hydrogen-bond motif, see: Bernstein et al. (1995). For related structures, see: Loh et al. (2010a,b); Loh et al. (2010). For the stability of the temperature controller used for the data collection, see: Cosier & Glazer (1986). For bond-length data, see: Allen et al. (1987).

Experimental top

A few drops of nitric acid were added to a hot methanol solution (20 ml) of quinoline-2-carbonitrile (39 mg, Aldrich) which had been warmed over a magnetic stirrer hotplate for a few minutes. The resulting solution was allowed to cool slowly to room temperature. Colourless plates of (I) appeared after a few days.

Structure description top

For background to and the biological activities of quinoline derivatives, see: Loh, Quah et al. (2010a,b); Loh et al. (2010); Sasaki et al. (1998); Reux et al. (2009); Morimoto et al. (1991); Michael (1997) Markees et al. (1970); Campbell et al. (1988). For the hydrogen-bond motif, see: Bernstein et al. (1995). For related structures, see: Loh et al. (2010a,b); Loh et al. (2010). For the stability of the temperature controller used for the data collection, see: Cosier & Glazer (1986). For bond-length data, see: Allen et al. (1987).

Computing details top

Data collection: APEX2 (Bruker, 2009); cell refinement: SAINT (Bruker, 2009); data reduction: SAINT (Bruker, 2009); program(s) used to solve structure: SHELXTL (Sheldrick, 2008); program(s) used to refine structure: SHELXTL (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008) and PLATON (Spek, 2009).

Figures top

	Fig. 1. The molecular structure of the title compound showing 50% probability displacement ellipsoids for non-H atoms. The R₂¹(4) ring motif which indicates the bifurcated hydrogen bond is shown.
	Fig. 2. The crystal structure of the title compound, viewed along the a axis.

2-Cyanoquinolin-1-ium nitrate top

Crystal data top

C₁₀H₇N₂⁺·NO₃⁻	F(000) = 448
M_r = 217.19	D_x = 1.517 Mg m⁻³
Monoclinic, P2₁/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 3861 reflections
a = 3.6969 (1) Å	θ = 2.7–31.0°
b = 17.7031 (3) Å	µ = 0.12 mm⁻¹
c = 14.6029 (2) Å	T = 100 K
β = 95.802 (1)°	Plate, colourless
V = 950.81 (3) Å³	0.44 × 0.18 × 0.07 mm
Z = 4

Data collection top

Bruker SMART APEXII CCD diffractometer	2758 independent reflections
Radiation source: fine-focus sealed tube	2103 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.041
φ and ω scans	θ_max = 30.0°, θ_min = 1.8°
Absorption correction: multi-scan (SADABS; Bruker, 2009)	h = −5→5
T_min = 0.951, T_max = 0.992	k = −24→24
16184 measured reflections	l = −20→20

Refinement top

Refinement on F²	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F² > 2σ(F²)] = 0.045	Hydrogen site location: inferred from neighbouring sites
wR(F²) = 0.122	H atoms treated by a mixture of independent and constrained refinement
S = 1.05	w = 1/[σ²(F_o²) + (0.0614P)² + 0.2227P] where P = (F_o² + 2F_c²)/3
2758 reflections	(Δ/σ)_max < 0.001
170 parameters	Δρ_max = 0.37 e Å⁻³
0 restraints	Δρ_min = −0.29 e Å⁻³

Crystal data top

C₁₀H₇N₂⁺·NO₃⁻	V = 950.81 (3) Å³
M_r = 217.19	Z = 4
Monoclinic, P2₁/c	Mo Kα radiation
a = 3.6969 (1) Å	µ = 0.12 mm⁻¹
b = 17.7031 (3) Å	T = 100 K
c = 14.6029 (2) Å	0.44 × 0.18 × 0.07 mm
β = 95.802 (1)°

Data collection top

Bruker SMART APEXII CCD diffractometer	2758 independent reflections
Absorption correction: multi-scan (SADABS; Bruker, 2009)	2103 reflections with I > 2σ(I)
T_min = 0.951, T_max = 0.992	R_int = 0.041
16184 measured reflections

Refinement top

R[F² > 2σ(F²)] = 0.045	0 restraints
wR(F²) = 0.122	H atoms treated by a mixture of independent and constrained refinement
S = 1.05	Δρ_max = 0.37 e Å⁻³
2758 reflections	Δρ_min = −0.29 e Å⁻³
170 parameters

Special details top

Experimental. The crystal was placed in the cold stream of an Oxford Cryosystems Cobra open-flow nitrogen cryostat (Cosier & Glazer, 1986) operating at 100.0 (1) K.

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq
O1	0.1851 (3)	0.21342 (6)	0.92466 (7)	0.0314 (3)
O2	0.4106 (3)	0.10064 (5)	0.91484 (7)	0.0255 (3)
O3	0.4296 (3)	0.17922 (5)	0.80150 (6)	0.0258 (3)
N1	0.2524 (3)	0.30942 (6)	0.73844 (7)	0.0144 (2)
H1N1	0.3241	0.2630	0.7672	0.079 (8)*
N2	−0.1335 (4)	0.17939 (7)	0.58610 (8)	0.0265 (3)
N3	0.3373 (3)	0.16413 (6)	0.88321 (7)	0.0196 (3)
C1	0.3582 (3)	0.37566 (7)	0.78076 (8)	0.0142 (3)
C2	0.5593 (4)	0.37429 (7)	0.86850 (8)	0.0161 (3)
C3	0.6627 (4)	0.44133 (7)	0.90990 (9)	0.0176 (3)
C4	0.5697 (4)	0.51140 (7)	0.86660 (9)	0.0181 (3)
C5	0.3739 (4)	0.51378 (7)	0.78226 (9)	0.0175 (3)
C6	0.2637 (3)	0.44559 (7)	0.73637 (8)	0.0148 (3)
C7	0.0675 (4)	0.44419 (7)	0.64838 (9)	0.0176 (3)
C8	−0.0316 (4)	0.37627 (7)	0.60703 (9)	0.0175 (3)
C9	0.0658 (4)	0.30972 (7)	0.65509 (8)	0.0158 (3)
C10	−0.0406 (4)	0.23670 (7)	0.61661 (9)	0.0184 (3)
H2A	0.619 (4)	0.3267 (9)	0.8975 (11)	0.020 (4)*
H3A	0.805 (5)	0.4406 (9)	0.9717 (12)	0.023 (4)*
H4A	0.649 (5)	0.5595 (10)	0.8975 (11)	0.028 (4)*
H5A	0.304 (5)	0.5598 (9)	0.7531 (11)	0.024 (4)*
H7A	0.002 (4)	0.4918 (9)	0.6174 (11)	0.020 (4)*
H8A	−0.166 (5)	0.3735 (10)	0.5461 (13)	0.030 (4)*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
O1	0.0486 (7)	0.0188 (5)	0.0295 (6)	0.0018 (5)	0.0167 (5)	−0.0018 (4)
O2	0.0375 (6)	0.0142 (5)	0.0234 (5)	−0.0007 (4)	−0.0040 (4)	0.0047 (4)
O3	0.0438 (7)	0.0173 (5)	0.0170 (5)	0.0075 (4)	0.0072 (4)	0.0026 (4)
N1	0.0169 (6)	0.0120 (5)	0.0142 (5)	0.0006 (4)	0.0010 (4)	0.0000 (4)
N2	0.0331 (7)	0.0217 (6)	0.0237 (6)	−0.0029 (5)	−0.0020 (5)	−0.0014 (5)
N3	0.0255 (6)	0.0153 (5)	0.0172 (5)	−0.0027 (4)	−0.0015 (4)	−0.0003 (4)
C1	0.0146 (6)	0.0126 (6)	0.0159 (6)	0.0002 (4)	0.0035 (5)	−0.0005 (4)
C2	0.0180 (6)	0.0145 (6)	0.0156 (6)	0.0004 (5)	0.0013 (5)	0.0005 (4)
C3	0.0164 (6)	0.0187 (6)	0.0177 (6)	−0.0017 (5)	0.0020 (5)	−0.0024 (5)
C4	0.0180 (7)	0.0151 (6)	0.0216 (6)	−0.0035 (5)	0.0044 (5)	−0.0037 (5)
C5	0.0200 (7)	0.0128 (6)	0.0204 (6)	0.0002 (5)	0.0053 (5)	0.0002 (5)
C6	0.0146 (6)	0.0140 (6)	0.0160 (6)	0.0009 (4)	0.0027 (5)	0.0008 (4)
C7	0.0185 (7)	0.0166 (6)	0.0177 (6)	0.0030 (5)	0.0023 (5)	0.0023 (5)
C8	0.0179 (6)	0.0192 (6)	0.0153 (6)	0.0016 (5)	0.0008 (5)	0.0009 (5)
C9	0.0157 (6)	0.0159 (6)	0.0158 (6)	−0.0003 (5)	0.0017 (5)	−0.0016 (4)
C10	0.0190 (7)	0.0192 (6)	0.0166 (6)	0.0004 (5)	0.0003 (5)	0.0005 (5)

Geometric parameters (Å, º) top

O1—N3	1.2301 (15)	C3—H3A	0.998 (17)
O2—N3	1.2347 (14)	C4—C5	1.3646 (19)
O3—N3	1.3015 (14)	C4—H4A	0.993 (18)
N1—C9	1.3370 (16)	C5—C6	1.4203 (17)
N1—C1	1.3642 (15)	C5—H5A	0.943 (17)
N1—H1N1	0.9481	C6—C7	1.4104 (18)
N2—C10	1.1466 (18)	C7—C8	1.3784 (18)
C1—C2	1.4152 (17)	C7—H7A	0.976 (17)
C1—C6	1.4243 (16)	C8—C9	1.3998 (18)
C2—C3	1.3686 (18)	C8—H8A	0.977 (18)
C2—H2A	0.959 (16)	C9—C10	1.4480 (18)
C3—C4	1.4188 (18)

C9—N1—C1	120.45 (10)	C3—C4—H4A	119.9 (10)
C9—N1—H1N1	120.2	C4—C5—C6	120.01 (12)
C1—N1—H1N1	119.3	C4—C5—H5A	122.1 (10)
O1—N3—O2	123.76 (12)	C6—C5—H5A	117.9 (10)
O1—N3—O3	118.74 (11)	C7—C6—C5	122.77 (11)
O2—N3—O3	117.50 (11)	C7—C6—C1	118.63 (11)
N1—C1—C2	119.72 (11)	C5—C6—C1	118.60 (12)
N1—C1—C6	119.68 (11)	C8—C7—C6	120.25 (12)
C2—C1—C6	120.60 (11)	C8—C7—H7A	120.5 (10)
C3—C2—C1	118.87 (12)	C6—C7—H7A	119.2 (10)
C3—C2—H2A	121.7 (10)	C7—C8—C9	118.10 (12)
C1—C2—H2A	119.4 (10)	C7—C8—H8A	122.1 (10)
C2—C3—C4	121.13 (12)	C9—C8—H8A	119.8 (10)
C2—C3—H3A	119.1 (9)	N1—C9—C8	122.87 (11)
C4—C3—H3A	119.8 (9)	N1—C9—C10	116.40 (11)
C5—C4—C3	120.79 (12)	C8—C9—C10	120.72 (12)
C5—C4—H4A	119.3 (10)	N2—C10—C9	178.33 (15)

C9—N1—C1—C2	−179.23 (11)	C2—C1—C6—C7	179.11 (11)
C9—N1—C1—C6	1.04 (18)	N1—C1—C6—C5	179.37 (11)
N1—C1—C2—C3	−179.94 (12)	C2—C1—C6—C5	−0.36 (18)
C6—C1—C2—C3	−0.21 (19)	C5—C6—C7—C8	179.86 (12)
C1—C2—C3—C4	0.39 (19)	C1—C6—C7—C8	0.40 (19)
C2—C3—C4—C5	0.0 (2)	C6—C7—C8—C9	0.44 (19)
C3—C4—C5—C6	−0.6 (2)	C1—N1—C9—C8	−0.16 (19)
C4—C5—C6—C7	−178.69 (12)	C1—N1—C9—C10	−178.81 (11)
C4—C5—C6—C1	0.76 (19)	C7—C8—C9—N1	−0.6 (2)
N1—C1—C6—C7	−1.15 (18)	C7—C8—C9—C10	177.99 (12)

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1N1···O1	0.95	2.56	3.2376 (15)	128
N1—H1N1···O3	0.95	1.60	2.5432 (14)	172
C5—H5A···O3ⁱ	0.943 (16)	2.497 (16)	3.2835 (16)	141.0 (15)
C7—H7A···O2ⁱⁱ	0.976 (16)	2.473 (16)	3.3641 (16)	151.8 (12)
C8—H8A···O2ⁱⁱⁱ	0.977 (18)	2.391 (19)	3.3355 (17)	162.4 (15)

Symmetry codes: (i) −x+1, y+1/2, −z+3/2; (ii) −x, y+1/2, −z+3/2; (iii) x−1, −y+1/2, z−1/2.

Experimental details

Crystal data
Chemical formula	C₁₀H₇N₂⁺·NO₃⁻
M_r	217.19
Crystal system, space group	Monoclinic, P2₁/c
Temperature (K)	100
a, b, c (Å)	3.6969 (1), 17.7031 (3), 14.6029 (2)
β (°)	95.802 (1)
V (Å³)	950.81 (3)
Z	4
Radiation type	Mo Kα
µ (mm⁻¹)	0.12
Crystal size (mm)	0.44 × 0.18 × 0.07

Data collection
Diffractometer	Bruker SMART APEXII CCD
Absorption correction	Multi-scan (SADABS; Bruker, 2009)
T_min, T_max	0.951, 0.992
No. of measured, independent and observed [I > 2σ(I)] reflections	16184, 2758, 2103
R_int	0.041
(sin θ/λ)_max (Å⁻¹)	0.703

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.045, 0.122, 1.05
No. of reflections	2758
No. of parameters	170
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement
Δρ_max, Δρ_min (e Å⁻³)	0.37, −0.29

Computer programs: APEX2 (Bruker, 2009), SAINT (Bruker, 2009), SHELXTL (Sheldrick, 2008) and PLATON (Spek, 2009).

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1N1···O1	0.95	2.56	3.2376 (15)	128
N1—H1N1···O3	0.95	1.60	2.5432 (14)	172
C5—H5A···O3ⁱ	0.943 (16)	2.497 (16)	3.2835 (16)	141.0 (15)
C7—H7A···O2ⁱⁱ	0.976 (16)	2.473 (16)	3.3641 (16)	151.8 (12)
C8—H8A···O2ⁱⁱⁱ	0.977 (18)	2.391 (19)	3.3355 (17)	162.4 (15)

Symmetry codes: (i) −x+1, y+1/2, −z+3/2; (ii) −x, y+1/2, −z+3/2; (iii) x−1, −y+1/2, z−1/2.

Footnotes

‡Thomson Reuters ResearcherID: C-7581-2009.

§Thomson Reuters ResearcherID: A-3561-2009.

Acknowledgements

The authors thank Universiti Sains Malaysia (USM) for the Research University Grant (grant No. 1001/PFIZIK/811160). WSL thanks USM for the award of a USM fellowship. MH thanks USM for the award of a postdoctoral fellowship.

References

Allen, F. H., Kennard, O., Watson, D. G., Brammer, L., Orpen, A. G. & Taylor, R. (1987). J. Chem. Soc. Perkin Trans. 2, pp. S1–19. CSD CrossRef Web of Science Google Scholar
Bernstein, J., Davis, R. E., Shimoni, L. & Chang, N.-L. (1995). Angew. Chem. Int. Ed. Engl. 34, 1555–1573. CrossRef CAS Web of Science Google Scholar
Bruker (2009). APEX2, SAINT and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA. Google Scholar
Campbell, S. F., Hardstone, J. D. & Palmer, M. J. (1988). J. Med. Chem. 31, 1031–1035. CrossRef CAS PubMed Web of Science Google Scholar
Cosier, J. & Glazer, A. M. (1986). J. Appl. Cryst. 19, 105–107. CrossRef CAS Web of Science IUCr Journals Google Scholar
Loh, W.-S., Hemamalini, M. & Fun, H.-K. (2010). Acta Cryst. E66, o2709. Web of Science CSD CrossRef IUCr Journals Google Scholar
Loh, W.-S., Quah, C. K., Hemamalini, M. & Fun, H.-K. (2010a). Acta Cryst. E66, o2357. Web of Science CSD CrossRef IUCr Journals Google Scholar
Loh, W.-S., Quah, C. K., Hemamalini, M. & Fun, H.-K. (2010b). Acta Cryst. E66, o2396. Web of Science CSD CrossRef IUCr Journals Google Scholar
Markees, D. G., Dewey, V. C. & Kidder, G. W. (1970). J. Med. Chem. 13, 324–326. CrossRef CAS PubMed Web of Science Google Scholar
Michael, J. P. (1997). Nat. Prod. Rep. 14, 605–608. CrossRef CAS Web of Science Google Scholar
Morimoto, Y., Matsuda, F. & Shirahama, H. (1991). Synlett, 3, 202–203. CrossRef Google Scholar
Reux, B., Nevalainen, T., Raitio, K. H. & Koskinen, A. M. P. (2009). Bioorg. Med. Chem. 17, 4441–4447. Web of Science CrossRef PubMed CAS Google Scholar
Sasaki, K., Tsurumori, A. & Hirota, T. (1998). J. Chem. Soc. Perkin Trans. 1, pp. 3851–3856. Web of Science CrossRef Google Scholar
Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122. Web of Science CrossRef CAS IUCr Journals Google Scholar
Spek, A. L. (2009). Acta Cryst. D65, 148–155. Web of Science CrossRef CAS IUCr Journals Google Scholar