6β,8β-Dihy­droxy­eremophil-7(11)-en-8α,12-olide

Zhang, Z.-X.; Fei, D.-Q.

doi:10.1107/S160053681104150X

organic compounds

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ISSN: 2056-9890

Volume 67| Part 11| November 2011| Page o2957

doi:10.1107/S160053681104150X

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6β,8β-Dihydroxyeremophil-7(11)-en-8α,12-olide

Zhan-Xin Zhang ^a ^* and Dong-Qing Fei ^a

^aSchool of Pharmacy, Lanzhou University, Lanzhou 730000, People's Republic of China
^*Correspondence e-mail: zhangzhx@lzu.edu.cn

(Received 28 September 2011; accepted 8 October 2011; online 12 October 2011)

The title compound, C₁₅H₂₂O₄, an eremophilane sesquiternoid, was isolated from the roots of Ligularia virgaurea. Both six-membered rings (A and B) adopt chair conformations and the five-membered ring is almost planar (r.m.s. deviation = 0.016 Å). The two methyl and two hydroxy groups adopt a syn conformation and the A/B ring junction is cis-fused. An intramolecular O—H⋯O hydrogen bond generates an S(6) ring. In the crystal, O—H⋯O hydrogen bonds link the molecules into [100] chains.

Related literature

For further information on the isolation of the title compound, see Moriyama & Takahashi (1976 ); Zhang et al. (2008 ).

Experimental

Crystal data

C₁₅H₂₂O₄
M_r = 266.33
Orthorhombic, P 2₁ 2₁ 2₁
a = 9.8627 (7) Å
b = 10.5674 (7) Å
c = 13.1565 (9) Å
V = 1371.21 (16) Å³
Z = 4
Mo Kα radiation
μ = 0.09 mm⁻¹
T = 296 K
0.38 × 0.33 × 0.29 mm

Data collection

Bruker APEXII CCD diffractometer
Absorption correction: multi-scan (SADABS; Bruker, 2005 ) T_min = 0.966, T_max = 0.974
7478 measured reflections
2680 independent reflections
2306 reflections with I > 2σ(I)
R_int = 0.028

Refinement

R[F² > 2σ(F²)] = 0.034
wR(F²) = 0.087
S = 1.06
2680 reflections
178 parameters
H-atom parameters constrained
Δρ_max = 0.15 e Å⁻³
Δρ_min = −0.12 e Å⁻³

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O4—H4A⋯O3	0.82	2.11	2.8061 (19)	142
O3—H3⋯O1ⁱ	0.82	1.93	2.7502 (18)	174
Symmetry code: (i) $[x+{\script{1\over 2}}, -y-{\script{1\over 2}}, -z]$ .

Data collection: APEX2 (Bruker, 2005); cell refinement: SAINT (Bruker, 2005); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 ); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL.

Supporting information

Crystal structure: contains datablocks I, global. DOI: 10.1107/S160053681104150X/hb6428sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S160053681104150X/hb6428Isup2.hkl

checkCIF report

Comment top

The title compound, 6β,8β-dihydroxyeremophil-7(11)-en-8α,12-olide (Fig. 1), was originally isolated from the Ligularia fauriei (Moriyama et al., 1976). With the present compound, which was isolated from the roots of L. virgaurea (Zhang et al., 2008). The title compound is composed of three rings, two six-membered and one five-membered. The two six-membered rings A and B adopt chair conformations with pucking paramaters Q = 0.559 (2) Å, θ = 176.0 (2)^o, ϕ = 101 (3)° and Q = 0.5527 (18) Å, θ = 1.20 (19)^o, ϕ = 127 (9)°, respectively. The five-membered ring C is almost planar with a mean torsion angle of 1.65 (8)°. The A/B ring junction is cis- fused. In the crystal, O—H···O hydrogen bonds occur.

Related literature top

For further information on the isolation of the title compound, see Moriyama & Takahashi (1976); Zhang et al. (2008).

Experimental top

The air-dried roots of L. virgaurea (3.8 kg) were pulverized and extracted with petroleum ether (60–90°C)—Et₂O-MeOH (1: 1: 1) (6 days × 3 times) at room temperature. The extract was concentrated under reduced pressure giving a residue (256 g), which was chromatographed on a silica gel column (200–300 mesh) with a gradient of PE-acetone (AC) (30: 1; 15: 1; 8: 1; 5: 1; 3: 1; 1: 1 and 0: 1). According to TLC analysis, seven crude fractions (Fr. A—Fr. G) were collected. Fr. F was further fractionated on a silica gel column to obtain a mixture of the title compound and other compounds, which were purified by preparative TLC using PE—AC (2: 1) to give pure the title compound. Colourless blocks of the title compound were obtained after slow evaporation of a methanolic solution at room temperature.

Computing details top

Data collection: APEX2 (Bruker, 2005); cell refinement: SAINT (Bruker, 2005); data reduction: SAINT (Bruker, 2005); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

Figures top

Fig. 1. The molecular structure of the title compound showing 30% probability displacement ellipsoids.

6β,8β-Dihydroxyeremophil-7(11)-en-8α,12-olide top

Crystal data top

C₁₅H₂₂O₄	D_x = 1.290 Mg m⁻³
M_r = 266.33	Mo Kα radiation, λ = 0.71073 Å
Orthorhombic, P2₁2₁2₁	Cell parameters from 2527 reflections
a = 9.8627 (7) Å	θ = 2.5–24.7°
b = 10.5674 (7) Å	µ = 0.09 mm⁻¹
c = 13.1565 (9) Å	T = 296 K
V = 1371.21 (16) Å³	Block, colourless
Z = 4	0.38 × 0.33 × 0.29 mm
F(000) = 576

Data collection top

Bruker APEXII CCD diffractometer	2680 independent reflections
Radiation source: fine-focus sealed tube	2306 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.028
ϕ and ω scans	θ_max = 26.0°, θ_min = 2.5°
Absorption correction: multi-scan (SADABS; Bruker, 2005)	h = −12→6
T_min = 0.966, T_max = 0.974	k = −13→12
7478 measured reflections	l = −16→15

Refinement top

Refinement on F²	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F² > 2σ(F²)] = 0.034	H-atom parameters constrained
wR(F²) = 0.087	w = 1/[σ²(F_o²) + (0.043P)² + 0.0843P] where P = (F_o² + 2F_c²)/3
S = 1.06	(Δ/σ)_max < 0.001
2680 reflections	Δρ_max = 0.15 e Å⁻³
178 parameters	Δρ_min = −0.12 e Å⁻³
0 restraints	Extinction correction: SHELXTL (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc²λ³/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.043 (3)

Crystal data top

C₁₅H₂₂O₄	V = 1371.21 (16) Å³
M_r = 266.33	Z = 4
Orthorhombic, P2₁2₁2₁	Mo Kα radiation
a = 9.8627 (7) Å	µ = 0.09 mm⁻¹
b = 10.5674 (7) Å	T = 296 K
c = 13.1565 (9) Å	0.38 × 0.33 × 0.29 mm

Data collection top

Bruker APEXII CCD diffractometer	2680 independent reflections
Absorption correction: multi-scan (SADABS; Bruker, 2005)	2306 reflections with I > 2σ(I)
T_min = 0.966, T_max = 0.974	R_int = 0.028
7478 measured reflections

Refinement top

R[F² > 2σ(F²)] = 0.034	0 restraints
wR(F²) = 0.087	H-atom parameters constrained
S = 1.06	Δρ_max = 0.15 e Å⁻³
2680 reflections	Δρ_min = −0.12 e Å⁻³
178 parameters

Special details top

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
C1	0.52336 (19)	0.0280 (2)	0.02370 (15)	0.0528 (5)
H1A	0.6033	0.0428	0.0650	0.063*
H1B	0.5488	−0.0277	−0.0317	0.063*
C2	0.4742 (2)	0.1530 (2)	−0.01946 (17)	0.0589 (6)
H2A	0.4024	0.1371	−0.0682	0.071*
H2B	0.5482	0.1941	−0.0550	0.071*
C3	0.4214 (2)	0.24061 (19)	0.06327 (16)	0.0573 (5)
H3A	0.3821	0.3150	0.0318	0.069*
H3B	0.4970	0.2682	0.1049	0.069*
C4	0.31514 (18)	0.17838 (17)	0.13139 (14)	0.0432 (4)
H4	0.2371	0.1584	0.0881	0.052*
C5	0.36613 (16)	0.05137 (17)	0.17644 (13)	0.0380 (4)
C6	0.24978 (17)	−0.01496 (17)	0.23600 (12)	0.0401 (4)
H6	0.2093	0.0468	0.2825	0.048*
C7	0.14293 (16)	−0.05820 (15)	0.16381 (12)	0.0335 (4)
C8	0.01487 (16)	−0.02563 (16)	0.14955 (12)	0.0360 (4)
C9	−0.03401 (18)	−0.09426 (16)	0.05925 (13)	0.0405 (4)
C10	0.18821 (16)	−0.14622 (15)	0.08141 (13)	0.0349 (4)
C11	0.29893 (16)	−0.08367 (15)	0.01959 (12)	0.0359 (4)
H11A	0.2612	−0.0128	−0.0178	0.043*
H11B	0.3347	−0.1438	−0.0292	0.043*
C12	0.41454 (16)	−0.03650 (17)	0.08853 (13)	0.0373 (4)
H12	0.4563	−0.1111	0.1197	0.045*
C13	0.2660 (3)	0.2734 (2)	0.21109 (18)	0.0673 (6)
H13A	0.1857	0.2415	0.2434	0.101*
H13B	0.2459	0.3527	0.1787	0.101*
H13C	0.3355	0.2858	0.2612	0.101*
C14	0.4830 (2)	0.0732 (2)	0.25188 (15)	0.0559 (5)
H14A	0.5526	0.1226	0.2198	0.084*
H14B	0.5198	−0.0069	0.2725	0.084*
H14C	0.4495	0.1175	0.3104	0.084*
C15	−0.07474 (19)	0.06430 (19)	0.20584 (15)	0.0498 (5)
H15A	−0.0367	0.0811	0.2716	0.075*
H15B	−0.1632	0.0276	0.2137	0.075*
H15C	−0.0819	0.1420	0.1684	0.075*
O1	−0.14548 (13)	−0.09203 (13)	0.02165 (11)	0.0576 (4)
O2	0.06687 (12)	−0.16515 (11)	0.01970 (9)	0.0429 (3)
O3	0.22670 (13)	−0.26232 (11)	0.12267 (9)	0.0471 (3)
H3	0.2694	−0.3028	0.0802	0.071*
O4	0.29776 (15)	−0.11964 (14)	0.29448 (10)	0.0555 (4)
H4A	0.3033	−0.1824	0.2580	0.083*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
C1	0.0351 (9)	0.0668 (13)	0.0565 (11)	−0.0029 (9)	0.0062 (9)	−0.0042 (11)
C2	0.0502 (11)	0.0631 (13)	0.0635 (13)	−0.0217 (10)	0.0032 (11)	0.0153 (11)
C3	0.0551 (12)	0.0445 (11)	0.0723 (13)	−0.0125 (9)	−0.0128 (11)	0.0068 (10)
C4	0.0401 (10)	0.0368 (9)	0.0526 (10)	0.0005 (7)	−0.0124 (8)	−0.0072 (8)
C5	0.0328 (9)	0.0441 (10)	0.0369 (9)	0.0036 (8)	−0.0076 (7)	−0.0035 (8)
C6	0.0416 (10)	0.0486 (10)	0.0299 (8)	0.0076 (8)	−0.0010 (7)	−0.0035 (8)
C7	0.0347 (9)	0.0335 (9)	0.0322 (8)	−0.0004 (7)	0.0052 (7)	0.0024 (7)
C8	0.0349 (9)	0.0327 (9)	0.0406 (9)	−0.0038 (7)	0.0043 (8)	−0.0021 (7)
C9	0.0377 (9)	0.0339 (9)	0.0498 (10)	−0.0049 (8)	−0.0017 (8)	−0.0012 (8)
C10	0.0362 (9)	0.0331 (8)	0.0355 (8)	0.0005 (7)	0.0015 (7)	−0.0024 (7)
C11	0.0406 (9)	0.0345 (8)	0.0326 (8)	0.0018 (7)	0.0023 (7)	−0.0027 (7)
C12	0.0320 (8)	0.0398 (9)	0.0400 (9)	0.0069 (7)	0.0001 (7)	0.0024 (8)
C13	0.0713 (15)	0.0482 (12)	0.0823 (16)	0.0058 (11)	−0.0124 (12)	−0.0203 (11)
C14	0.0453 (11)	0.0706 (14)	0.0519 (10)	0.0003 (10)	−0.0178 (9)	−0.0061 (11)
C15	0.0436 (10)	0.0480 (11)	0.0577 (11)	0.0047 (9)	0.0100 (9)	−0.0039 (9)
O1	0.0394 (7)	0.0547 (8)	0.0786 (9)	−0.0039 (6)	−0.0148 (7)	−0.0134 (8)
O2	0.0402 (7)	0.0426 (7)	0.0458 (7)	−0.0028 (5)	−0.0015 (6)	−0.0104 (6)
O3	0.0542 (8)	0.0359 (7)	0.0511 (7)	0.0070 (6)	0.0127 (6)	0.0047 (5)
O4	0.0637 (9)	0.0639 (9)	0.0390 (7)	0.0101 (7)	−0.0063 (6)	0.0115 (6)

Geometric parameters (Å, º) top

C1—C2	1.517 (3)	C8—C15	1.494 (2)
C1—C12	1.531 (2)	C9—O1	1.206 (2)
C1—H1A	0.9700	C9—O2	1.350 (2)
C1—H1B	0.9700	C10—O3	1.3943 (19)
C2—C3	1.521 (3)	C10—O2	1.460 (2)
C2—H2A	0.9700	C10—C11	1.514 (2)
C2—H2B	0.9700	C11—C12	1.540 (2)
C3—C4	1.528 (3)	C11—H11A	0.9700
C3—H3A	0.9700	C11—H11B	0.9700
C3—H3B	0.9700	C12—H12	0.9800
C4—C13	1.531 (3)	C13—H13A	0.9600
C4—C5	1.551 (2)	C13—H13B	0.9600
C4—H4	0.9800	C13—H13C	0.9600
C5—C14	1.538 (2)	C14—H14A	0.9600
C5—C12	1.558 (2)	C14—H14B	0.9600
C5—C6	1.556 (2)	C14—H14C	0.9600
C6—O4	1.428 (2)	C15—H15A	0.9600
C6—C7	1.490 (2)	C15—H15B	0.9600
C6—H6	0.9800	C15—H15C	0.9600
C7—C8	1.323 (2)	O3—H3	0.8200
C7—C10	1.497 (2)	O4—H4A	0.8200
C8—C9	1.473 (2)

C2—C1—C12	111.83 (15)	O1—C9—O2	121.66 (16)
C2—C1—H1A	109.2	O1—C9—C8	128.29 (17)
C12—C1—H1A	109.2	O2—C9—C8	110.06 (14)
C2—C1—H1B	109.2	O3—C10—O2	108.61 (12)
C12—C1—H1B	109.2	O3—C10—C7	110.25 (13)
H1A—C1—H1B	107.9	O2—C10—C7	104.09 (12)
C1—C2—C3	111.80 (17)	O3—C10—C11	113.40 (14)
C1—C2—H2A	109.3	O2—C10—C11	110.63 (13)
C3—C2—H2A	109.3	C7—C10—C11	109.46 (13)
C1—C2—H2B	109.3	C10—C11—C12	111.04 (13)
C3—C2—H2B	109.3	C10—C11—H11A	109.4
H2A—C2—H2B	107.9	C12—C11—H11A	109.4
C2—C3—C4	113.11 (16)	C10—C11—H11B	109.4
C2—C3—H3A	109.0	C12—C11—H11B	109.4
C4—C3—H3A	109.0	H11A—C11—H11B	108.0
C2—C3—H3B	109.0	C1—C12—C11	109.57 (14)
C4—C3—H3B	109.0	C1—C12—C5	111.28 (15)
H3A—C3—H3B	107.8	C11—C12—C5	113.78 (13)
C3—C4—C13	109.68 (16)	C1—C12—H12	107.3
C3—C4—C5	111.97 (15)	C11—C12—H12	107.3
C13—C4—C5	114.13 (15)	C5—C12—H12	107.3
C3—C4—H4	106.9	C4—C13—H13A	109.5
C13—C4—H4	106.9	C4—C13—H13B	109.5
C5—C4—H4	106.9	H13A—C13—H13B	109.5
C14—C5—C4	111.07 (15)	C4—C13—H13C	109.5
C14—C5—C12	109.80 (14)	H13A—C13—H13C	109.5
C4—C5—C12	109.35 (14)	H13B—C13—H13C	109.5
C14—C5—C6	107.17 (14)	C5—C14—H14A	109.5
C4—C5—C6	110.06 (14)	C5—C14—H14B	109.5
C12—C5—C6	109.34 (14)	H14A—C14—H14B	109.5
O4—C6—C7	109.88 (14)	C5—C14—H14C	109.5
O4—C6—C5	112.07 (14)	H14A—C14—H14C	109.5
C7—C6—C5	109.79 (13)	H14B—C14—H14C	109.5
O4—C6—H6	108.3	C8—C15—H15A	109.5
C7—C6—H6	108.3	C8—C15—H15B	109.5
C5—C6—H6	108.3	H15A—C15—H15B	109.5
C8—C7—C6	133.31 (15)	C8—C15—H15C	109.5
C8—C7—C10	110.12 (14)	H15A—C15—H15C	109.5
C6—C7—C10	116.18 (14)	H15B—C15—H15C	109.5
C7—C8—C9	107.39 (15)	C9—O2—C10	108.29 (12)
C7—C8—C15	131.31 (15)	C10—O3—H3	109.5
C9—C8—C15	121.29 (15)	C6—O4—H4A	109.5

C12—C1—C2—C3	54.1 (2)	C7—C8—C9—O2	−1.19 (19)
C1—C2—C3—C4	−52.5 (2)	C15—C8—C9—O2	177.43 (15)
C2—C3—C4—C13	−179.04 (17)	C8—C7—C10—O3	−118.86 (15)
C2—C3—C4—C5	53.2 (2)	C6—C7—C10—O3	67.41 (18)
C3—C4—C5—C14	67.24 (19)	C8—C7—C10—O2	−2.55 (17)
C13—C4—C5—C14	−58.1 (2)	C6—C7—C10—O2	−176.28 (13)
C3—C4—C5—C12	−54.10 (18)	C8—C7—C10—C11	115.75 (15)
C13—C4—C5—C12	−179.45 (15)	C6—C7—C10—C11	−57.99 (18)
C3—C4—C5—C6	−174.23 (14)	O3—C10—C11—C12	−70.31 (17)
C13—C4—C5—C6	60.42 (19)	O2—C10—C11—C12	167.39 (12)
C14—C5—C6—O4	−48.17 (19)	C7—C10—C11—C12	53.25 (17)
C4—C5—C6—O4	−169.07 (14)	C2—C1—C12—C11	69.93 (19)
C12—C5—C6—O4	70.79 (16)	C2—C1—C12—C5	−56.8 (2)
C14—C5—C6—C7	−170.58 (14)	C10—C11—C12—C1	−179.08 (14)
C4—C5—C6—C7	68.52 (17)	C10—C11—C12—C5	−53.80 (18)
C12—C5—C6—C7	−51.62 (18)	C14—C5—C12—C1	−66.08 (19)
O4—C6—C7—C8	121.9 (2)	C4—C5—C12—C1	56.03 (18)
C5—C6—C7—C8	−114.4 (2)	C6—C5—C12—C1	176.60 (13)
O4—C6—C7—C10	−66.24 (18)	C14—C5—C12—C11	169.56 (15)
C5—C6—C7—C10	57.46 (19)	C4—C5—C12—C11	−68.34 (17)
C6—C7—C8—C9	174.56 (17)	C6—C5—C12—C11	52.23 (17)
C10—C7—C8—C9	2.30 (18)	O1—C9—O2—C10	179.36 (15)
C6—C7—C8—C15	−3.9 (3)	C8—C9—O2—C10	−0.46 (18)
C10—C7—C8—C15	−176.13 (17)	O3—C10—O2—C9	119.19 (14)
C7—C8—C9—O1	179.00 (18)	C7—C10—O2—C9	1.73 (16)
C15—C8—C9—O1	−2.4 (3)	C11—C10—O2—C9	−115.75 (14)

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
O4—H4A···O3	0.82	2.11	2.8061 (19)	142
O3—H3···O1ⁱ	0.82	1.93	2.7502 (18)	174

Symmetry code: (i) x+1/2, −y−1/2, −z.

Experimental details

Crystal data
Chemical formula	C₁₅H₂₂O₄
M_r	266.33
Crystal system, space group	Orthorhombic, P2₁2₁2₁
Temperature (K)	296
a, b, c (Å)	9.8627 (7), 10.5674 (7), 13.1565 (9)
V (Å³)	1371.21 (16)
Z	4
Radiation type	Mo Kα
µ (mm⁻¹)	0.09
Crystal size (mm)	0.38 × 0.33 × 0.29

Data collection
Diffractometer	Bruker APEXII CCD diffractometer
Absorption correction	Multi-scan (SADABS; Bruker, 2005)
T_min, T_max	0.966, 0.974
No. of measured, independent and observed [I > 2σ(I)] reflections	7478, 2680, 2306
R_int	0.028
(sin θ/λ)_max (Å⁻¹)	0.617

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.034, 0.087, 1.06
No. of reflections	2680
No. of parameters	178
H-atom treatment	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	0.15, −0.12

Computer programs: APEX2 (Bruker, 2005), SAINT (Bruker, 2005), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), SHELXTL (Sheldrick, 2008).

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
O4—H4A···O3	0.82	2.11	2.8061 (19)	142
O3—H3···O1ⁱ	0.82	1.93	2.7502 (18)	174

Symmetry code: (i) x+1/2, −y−1/2, −z.

Acknowledgements

This work was kindly supported by the National Natural Science Foundation of China (No. 21102065) and the Fundational Research Funds for the Central Universities (No. lzujbky-2009–144).

References

Bruker (2005). APEX2, SAINT and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA. Google Scholar
Moriyama, Y. & Takahashi, T. (1976). Chem. Pharm. Bull. 24, 360–362. CrossRef CAS Google Scholar
Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122. Web of Science CrossRef CAS IUCr Journals Google Scholar
Zhang, Z.-X., Fei, D.-Q. & Jia, Z.-J. (2008). Helv. Chim. Acta, 91, 1045–1052. Web of Science CrossRef CAS Google Scholar