6-Hydroxy­methyl-4-meth­oxy-2H-pyran-2-one (Opuntiol)

Abbasi, M.A.; Shahzadi, T.; Akkurt, M.; Aziz-ur-Rehman; Riaz, T.

doi:10.1107/S1600536809053860

organic compounds

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ISSN: 2056-9890

Volume 66| Part 1| January 2010| Page o199

doi:10.1107/S1600536809053860

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6-Hydroxymethyl-4-methoxy-2H-pyran-2-one (Opuntiol)

Muhammad Athar Abbasi,^a ‡ Tayyaba Shahzadi,^a Mehmet Akkurt,^b ^* Aziz-ur-Rehman ^a and Tauheeda Riaz ^a

^aDepartment of Chemistry, Center for Natural Product Drug Development, Government College University, Lahore 54000, Pakistan, and ^bDepartment of Physics, Faculty of Arts and Sciences, Erciyes University, 38039 Kayseri, Turkey
^*Correspondence e-mail: akkurt@erciyes.edu.tr

(Received 9 December 2009; accepted 14 December 2009; online 19 December 2009)

The title compound, C₇H₈O₄, isolated from Opuntia dillenii Haw (Cactaceae), is almost planar [maximum deviation of 0.027 (2) Å] except for the H atoms of the methylene and methyl groups. The crystal packing is stabilized by C—H⋯O and O—H⋯O intermolecular hydrogen bonds, resulting in the formation of a three-dimensional network.

Related literature

For the use of the stem and fruit of Opuntia dillenii Haw (Cactaceae) in folk medicine, see: Chang et al. (2008 ). For phytochemical investigations of this plant, see: Qiu et al. (2002 ). For comparitive bond lengths, see: Allen et al. (1987 ).

Experimental

Crystal data

C₇H₈O₄
M_r = 156.13
Monoclinic, P 2₁ /c
a = 4.0499 (5) Å
b = 18.101 (2) Å
c = 9.4743 (13) Å
β = 96.720 (7)°
V = 689.76 (15) Å³
Z = 4
Mo Kα radiation
μ = 0.13 mm⁻¹
T = 296 K
0.34 × 0.25 × 0.19 mm

Data collection

Bruker Kappa APEXII CCD area-detector diffractometer
6516 measured reflections
1268 independent reflections
882 reflections with I > 2σ(I)
R_int = 0.046

Refinement

R[F² > 2σ(F²)] = 0.037
wR(F²) = 0.094
S = 1.02
1268 reflections
105 parameters
1 restraint
H atoms treated by a mixture of independent and constrained refinement
Δρ_max = 0.16 e Å⁻³
Δρ_min = −0.16 e Å⁻³

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O4—H1⋯O1ⁱ	0.836 (14)	2.47 (2)	3.1840 (19)	144.2 (17)
O4—H1⋯O2ⁱ	0.836 (14)	2.073 (13)	2.8678 (18)	158.6 (18)
C7—H7B⋯O4ⁱⁱ	0.96	2.58	3.494 (2)	159
C7—H7C⋯O2ⁱⁱⁱ	0.96	2.57	3.504 (2)	164
Symmetry codes: (i) $[x-1, -y+{\script{1\over 2}}, z-{\script{1\over 2}}]$ ; (ii) -x, -y, -z; (iii) -x+1, -y, -z+1.

Data collection: APEX2 (Bruker, 2007 ); cell refinement: SAINT (Bruker, 2007); data reduction: SAINT; program(s) used to solve structure: SIR97 (Altomare et al., 1999 ); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008 ); molecular graphics: ORTEP-3 (Farrugia, 1997 ); software used to prepare material for publication: WinGX (Farrugia, 1999 ) and PLATON (Spek, 2009 ).

Supporting information

Crystal structure: contains datablocks global, I. DOI: 10.1107/S1600536809053860/rk2184sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S1600536809053860/rk2184Isup2.hkl

checkCIF report

Comment top

Opuntia dillenii Haw (Cactaceae) usually grows in semi-desert regions in the tropics and subtropics. The stem and fruit of this plant are used in a folk medicine for reducing cholesterol levels, treatment of gastric ulcers, inflammation, diabetes and several other diseases (Chang et al., 2008). The phytochemical investigations on this plant have led to the isolation of various oxygenated constituents namely opuntiol, opuntioside-I, p-hydroxybenzoic acid, ethyl 3,4-dihydroxybenzoate, 3,4-dihydroxybenzoic acid, L-(-)-malic acid, (E)-ferulic acid, 4-ethoxy-6-hydroxymethyl-α-pyrone, 1-heptanecanol, vanillic acid, isorhamnetin, isorhamnetin-3-O-rutinoside, rutin, quercetin, 3,3'-dimethyl quercetin, 3-O-methyl quercetin, 3-O-methyl quercetin 7-O-β-D-glucopyranoside, kaempferol, kaempferol 7-O-β-D-glucopyranoside, kaempferol 7-O-β-D-glucopyranosyl-(1→4)- β-D-glucopyranoside, kaempferide, β-sitosterol, and manghaslin (Qiu et al., 2002). In the present study we first time report the crystal structure of opuntiol using single-crystal XRD.

In the title molecule, I, shown in Fig. 1, bond lengths and angles display normal values (Allen et al., 1987). Except the H atoms of the methlene and methyl groups, the molecule of I is almost planar, with maximum deviations of 0.027 (2)Å for O4, 0.023 (2)Å for C7, 0.020 (1)Å for O2 and -0.019 (2)Å for C2. For the methoxy and hydroxy methyl groups at the C3 and C5 positions in I, the C2–C3–O3–C7 and C4–C3–O3–C7 torsion angles are 0.9 (2)° and -178.94 (15)°, the O1–C5–C6–O4 and C4–C5–C6–O4 torsion angles are 177.77 (14)° and -2.9 (3)°, respectively.

In the crystal structure of I, there are non-classical C–H···O and classical O–H···O intermolecular hydrogen bonds (Table 1), forming a three-dimensional network (Figs. 2 and 3).

Related literature top

For the use of the stem and fruit of Opuntia dillenii Haw (Cactaceae) in folk medicine, see: Chang et al. (2008). For phytochemical investigations of this plant, see: Qiu et al. (2002). For comparitive bond lengths, see: Allen et al. (1987).

Experimental top

Plant Material: Opuntia dillenii Haw (whole plant) was collected from the areas of Mar Balochan, Sangla Hill, Distt. Nankana, Pakistan, in April 2008, and identified by Muhammad Ajaib (Taxonomist), Department of Botany, Government College University, Lahore.

Extraction and isolation: The shade-dried ground whole plant (7 kg) of Opuntia dillenii Haw was exhaustively extracted with methanol (10L ×4) at room temperature. The extract was evaporated to yield the residue (1.1 kg), which was dissolved in distilled water (2.0 L) and partitioned with n-hexane (2L ×4), chloroform (2L × 4), ethyl acetate (2L × 4) and n-butanol (2L × 4) respectively. The chloroform soluble extract (157 g) was subjected to column chromatography using hexane with gradient of CHCl₃ and followed by methanol up to 100%. Fifteen fractions (Fr. 1-15) were collected. The Fr. 14 was loaded on flash silica gel and eluted with MeOH : CHCl₃ (2 : 98) to get purified crystals of opuntiol (84.7 mg).

Computing details top

Data collection: APEX2 (Bruker, 2007); cell refinement: SAINT (Bruker, 2007); data reduction: SAINT (Bruker, 2007); program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 (Farrugia, 1997); software used to prepare material for publication: WinGX (Farrugia, 1999) and PLATON (Spek, 2009).

Figures top

	Fig. 1. Molecular structure of the title compound with the atom numbering scheme. Displacement ellipsoids for non-H atoms are drawn at the 50% probability level. H atoms are presented as a small spheres of arbitrary radius.
	Fig. 2. The packing and hydrogen bonding of the title compound viewed down a axis. Hydrogen atoms not involved in hydrogen bonding have been omitted for clarity.
	Fig. 3. The packing and hydrogen bonding of the title compound viewed down b axis. Hydrogen atoms not involved in hydrogen bonding have been omitted for clarity.

6-Hydroxymethyl-4-methoxy-2H-pyran-2-one top

Crystal data top

C₇H₈O₄	F(000) = 328
M_r = 156.13	D_x = 1.503 Mg m⁻³
Monoclinic, P2₁/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 1306 reflections
a = 4.0499 (5) Å	θ = 2.3–23.4°
b = 18.101 (2) Å	µ = 0.13 mm⁻¹
c = 9.4743 (13) Å	T = 296 K
β = 96.720 (7)°	Needle, colourless
V = 689.76 (15) Å³	0.34 × 0.25 × 0.19 mm
Z = 4

Data collection top

Bruker Kappa APEXII CCD area-detector diffractometer	882 reflections with I > 2σ(I)
Radiation source: sealed tube	R_int = 0.046
Graphite monochromator	θ_max = 25.5°, θ_min = 2.4°
ϕ– and ω–scans	h = −4→4
6516 measured reflections	k = −21→21
1268 independent reflections	l = −11→11

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)] = 0.037	H atoms treated by a mixture of independent and constrained refinement
wR(F²) = 0.094	w = 1/[σ²(F_o²) + (0.0406P)² + 0.1053P] where P = (F_o² + 2F_c²)/3
S = 1.02	(Δ/σ)_max < 0.001
1268 reflections	Δρ_max = 0.16 e Å⁻³
105 parameters	Δρ_min = −0.16 e Å⁻³
1 restraint	Extinction correction: SHELXL97 (Sheldrick, 2008), FC^*=KFC[1+0.001XFC²Λ³/SIN(2Θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.0080 (19)

Crystal data top

C₇H₈O₄	V = 689.76 (15) Å³
M_r = 156.13	Z = 4
Monoclinic, P2₁/c	Mo Kα radiation
a = 4.0499 (5) Å	µ = 0.13 mm⁻¹
b = 18.101 (2) Å	T = 296 K
c = 9.4743 (13) Å	0.34 × 0.25 × 0.19 mm
β = 96.720 (7)°

Data collection top

Bruker Kappa APEXII CCD area-detector diffractometer	882 reflections with I > 2σ(I)
6516 measured reflections	R_int = 0.046
1268 independent reflections

Refinement top

R[F² > 2σ(F²)] = 0.037	1 restraint
wR(F²) = 0.094	H atoms treated by a mixture of independent and constrained refinement
S = 1.02	Δρ_max = 0.16 e Å⁻³
1268 reflections	Δρ_min = −0.16 e Å⁻³
105 parameters

Special details top

Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'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	0.6125 (3)	0.16542 (6)	0.30332 (12)	0.0376 (4)
O2	0.9490 (3)	0.11978 (7)	0.48000 (14)	0.0495 (5)
O3	0.3187 (3)	−0.03651 (6)	0.14748 (13)	0.0392 (4)
O4	0.0594 (4)	0.22405 (7)	0.00555 (16)	0.0558 (5)
C1	0.7497 (4)	0.10447 (9)	0.3778 (2)	0.0362 (6)
C2	0.6491 (4)	0.03392 (9)	0.32627 (18)	0.0336 (6)
C3	0.4283 (4)	0.02721 (9)	0.20778 (18)	0.0303 (5)
C4	0.2914 (4)	0.09149 (9)	0.13544 (18)	0.0334 (6)
C5	0.3878 (4)	0.15778 (9)	0.18521 (18)	0.0325 (6)
C6	0.2807 (5)	0.23160 (9)	0.1300 (2)	0.0413 (6)
C7	0.4484 (5)	−0.10405 (10)	0.2121 (2)	0.0437 (7)
H1	−0.003 (6)	0.2662 (7)	−0.022 (2)	0.0840*
H2	0.73440	−0.00810	0.37370	0.0400*
H4	0.13750	0.08710	0.05500	0.0400*
H6A	0.17300	0.25800	0.20100	0.0500*
H6B	0.47320	0.25990	0.11000	0.0500*
H7A	0.68640	−0.10410	0.21640	0.0660*
H7B	0.35940	−0.14530	0.15650	0.0660*
H7C	0.38600	−0.10780	0.30650	0.0660*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
O1	0.0434 (7)	0.0266 (7)	0.0405 (8)	−0.0032 (5)	−0.0045 (6)	−0.0040 (6)
O2	0.0590 (9)	0.0408 (8)	0.0437 (8)	−0.0065 (7)	−0.0149 (7)	−0.0057 (7)
O3	0.0503 (8)	0.0224 (7)	0.0420 (8)	−0.0014 (6)	−0.0073 (6)	−0.0020 (6)
O4	0.0688 (10)	0.0331 (8)	0.0596 (10)	0.0072 (7)	−0.0169 (8)	0.0056 (7)
C1	0.0385 (10)	0.0342 (11)	0.0347 (11)	−0.0020 (8)	−0.0002 (8)	0.0000 (9)
C2	0.0385 (10)	0.0256 (10)	0.0355 (11)	−0.0003 (8)	−0.0008 (8)	0.0019 (8)
C3	0.0333 (9)	0.0238 (9)	0.0335 (10)	−0.0023 (7)	0.0026 (8)	−0.0030 (8)
C4	0.0371 (10)	0.0312 (10)	0.0305 (10)	0.0010 (8)	−0.0021 (8)	0.0002 (8)
C5	0.0349 (10)	0.0274 (10)	0.0347 (10)	−0.0006 (8)	0.0022 (8)	0.0007 (8)
C6	0.0459 (11)	0.0267 (10)	0.0500 (12)	0.0014 (8)	0.0000 (9)	0.0015 (9)
C7	0.0548 (12)	0.0240 (10)	0.0501 (13)	0.0006 (8)	−0.0028 (10)	0.0008 (9)

Geometric parameters (Å, º) top

O1—C1	1.390 (2)	C4—C5	1.331 (2)
O1—C5	1.364 (2)	C5—C6	1.481 (2)
O2—C1	1.218 (2)	C2—H2	0.9300
O3—C3	1.340 (2)	C4—H4	0.9300
O3—C7	1.439 (2)	C6—H6A	0.9700
O4—C6	1.402 (2)	C6—H6B	0.9700
O4—H1	0.836 (14)	C7—H7A	0.9600
C1—C2	1.410 (2)	C7—H7B	0.9600
C2—C3	1.356 (2)	C7—H7C	0.9600
C3—C4	1.429 (2)

O1···O4ⁱ	3.1840 (19)	C2···H7C	2.7800
O2···O4ⁱ	2.8678 (18)	C2···H7A	2.7200
O2···C6ⁱ	3.258 (2)	C7···H2	2.5100
O3···C4ⁱⁱ	3.415 (2)	C7···H7A^vii	3.0900
O4···O2ⁱⁱⁱ	2.8678 (18)	C7···H6A^x	2.9900
O4···O1ⁱⁱⁱ	3.1840 (19)	C7···H6B^x	2.9800
O1···H1ⁱ	2.47 (2)	H1···H6B^vii	2.5900
O2···H2^iv	2.6900	H1···O1ⁱⁱⁱ	2.47 (2)
O2···H7C^v	2.5700	H1···O2ⁱⁱⁱ	2.073 (13)
O2···H1ⁱ	2.073 (13)	H1···C1ⁱⁱⁱ	2.676 (15)
O3···H4^vi	2.6700	H2···C7	2.5100
O4···H7B^vi	2.5800	H2···H7A	2.2800
O4···H4	2.5400	H2···H7C	2.3300
O4···H6B^vii	2.7500	H2···O2^iv	2.6900
C1···C4^viii	3.365 (2)	H4···O4	2.5400
C1···C5^viii	3.470 (2)	H4···O3^vi	2.6700
C2···C3^viii	3.473 (2)	H6A···C7^ix	2.9900
C2···C4^viii	3.496 (2)	H6B···O4^viii	2.7500
C3···C2^vii	3.473 (2)	H6B···H1^viii	2.5900
C4···C2^vii	3.496 (2)	H6B···C7^ix	2.9800
C4···C1^vii	3.365 (2)	H6B···H7C^ix	2.5700
C4···C7ⁱⁱ	3.580 (3)	H7A···C2	2.7200
C4···O3ⁱⁱ	3.415 (2)	H7A···C7^viii	3.0900
C5···C1^vii	3.470 (2)	H7A···H2	2.2800
C6···C7^ix	3.451 (3)	H7B···O4^vi	2.5800
C6···O2ⁱⁱⁱ	3.258 (2)	H7C···C2	2.7800
C7···C6^x	3.451 (3)	H7C···H2	2.3300
C7···C4ⁱⁱ	3.580 (3)	H7C···H6B^x	2.5700
C1···H1ⁱ	2.676 (15)	H7C···O2^v	2.5700

C1—O1—C5	121.64 (13)	C1—C2—H2	120.00
C3—O3—C7	117.64 (14)	C3—C2—H2	120.00
C6—O4—H1	108.3 (14)	C3—C4—H4	121.00
O1—C1—O2	114.29 (14)	C5—C4—H4	121.00
O1—C1—C2	117.44 (15)	O4—C6—H6A	110.00
O2—C1—C2	128.25 (16)	O4—C6—H6B	110.00
C1—C2—C3	120.23 (15)	C5—C6—H6A	110.00
O3—C3—C2	125.70 (15)	C5—C6—H6B	110.00
C2—C3—C4	120.35 (15)	H6A—C6—H6B	108.00
O3—C3—C4	113.95 (14)	O3—C7—H7A	109.00
C3—C4—C5	118.87 (15)	O3—C7—H7B	109.00
O1—C5—C4	121.46 (15)	O3—C7—H7C	109.00
C4—C5—C6	128.79 (16)	H7A—C7—H7B	109.00
O1—C5—C6	109.75 (14)	H7A—C7—H7C	109.00
O4—C6—C5	109.94 (14)	H7B—C7—H7C	109.00

C5—O1—C1—O2	179.27 (15)	C1—C2—C3—O3	178.79 (16)
C5—O1—C1—C2	0.3 (2)	C1—C2—C3—C4	−1.1 (3)
C1—O1—C5—C4	−0.4 (2)	O3—C3—C4—C5	−178.94 (15)
C1—O1—C5—C6	179.00 (15)	C2—C3—C4—C5	0.9 (2)
C7—O3—C3—C2	−0.9 (2)	C3—C4—C5—O1	−0.2 (2)
C7—O3—C3—C4	178.94 (15)	C3—C4—C5—C6	−179.49 (17)
O1—C1—C2—C3	0.4 (2)	O1—C5—C6—O4	177.77 (14)
O2—C1—C2—C3	−178.35 (18)	C4—C5—C6—O4	−2.9 (3)

Symmetry codes: (i) x+1, −y+1/2, z+1/2; (ii) −x+1, −y, −z; (iii) x−1, −y+1/2, z−1/2; (iv) −x+2, −y, −z+1; (v) −x+1, −y, −z+1; (vi) −x, −y, −z; (vii) x−1, y, z; (viii) x+1, y, z; (ix) −x+1, y+1/2, −z+1/2; (x) −x+1, y−1/2, −z+1/2.

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
O4—H1···O1ⁱⁱⁱ	0.836 (14)	2.47 (2)	3.1840 (19)	144.2 (17)
O4—H1···O2ⁱⁱⁱ	0.836 (14)	2.073 (13)	2.8678 (18)	158.6 (18)
C7—H7B···O4^vi	0.96	2.58	3.494 (2)	159
C7—H7C···O2^v	0.96	2.57	3.504 (2)	164

Symmetry codes: (iii) x−1, −y+1/2, z−1/2; (v) −x+1, −y, −z+1; (vi) −x, −y, −z.

Experimental details

Crystal data
Chemical formula	C₇H₈O₄
M_r	156.13
Crystal system, space group	Monoclinic, P2₁/c
Temperature (K)	296
a, b, c (Å)	4.0499 (5), 18.101 (2), 9.4743 (13)
β (°)	96.720 (7)
V (Å³)	689.76 (15)
Z	4
Radiation type	Mo Kα
µ (mm⁻¹)	0.13
Crystal size (mm)	0.34 × 0.25 × 0.19

Data collection
Diffractometer	Bruker Kappa APEXII CCD area-detector diffractometer
Absorption correction	–
No. of measured, independent and observed [I > 2σ(I)] reflections	6516, 1268, 882
R_int	0.046
(sin θ/λ)_max (Å⁻¹)	0.605

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.037, 0.094, 1.02
No. of reflections	1268
No. of parameters	105
No. of restraints	1
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement
Δρ_max, Δρ_min (e Å⁻³)	0.16, −0.16

Computer programs: APEX2 (Bruker, 2007), SAINT (Bruker, 2007), SIR97 (Altomare et al., 1999), SHELXL97 (Sheldrick, 2008), ORTEP-3 (Farrugia, 1997), WinGX (Farrugia, 1999) and PLATON (Spek, 2009).

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
O4—H1···O1ⁱ	0.836 (14)	2.47 (2)	3.1840 (19)	144.2 (17)
O4—H1···O2ⁱ	0.836 (14)	2.073 (13)	2.8678 (18)	158.6 (18)
C7—H7B···O4ⁱⁱ	0.9600	2.5800	3.494 (2)	159.00
C7—H7C···O2ⁱⁱⁱ	0.9600	2.5700	3.504 (2)	164.00

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

Footnotes

‡Additional corresponding author, e-mail: atrabbasi@yahoo.com.

Acknowledgements

The authors are grateful to the Higher Education Commission for providing financial support. Professor Islam Ullah Khan and Mr Shahzad Sharif are also gratefully acknowledged for providing single-crystal X-ray diffraction facilities at the Materials Chemistry Laboratory, GC University Lahore.

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. CrossRef Web of Science Google Scholar
Altomare, A., Burla, M. C., Camalli, M., Cascarano, G. L., Giacovazzo, C., Guagliardi, A., Moliterni, A. G. G., Polidori, G. & Spagna, R. (1999). J. Appl. Cryst. 32, 115–119. Web of Science CrossRef CAS IUCr Journals Google Scholar
Bruker (2007). APEX2 and SAINT. Bruker AXS Inc., Madison, Wisconsin, USA. Google Scholar
Chang, S. F., Hsieh, C. L. & Yen, G. C. (2008). Food Chem. 106, 569–575. Web of Science CrossRef CAS Google Scholar
Farrugia, L. J. (1997). J. Appl. Cryst. 30, 565. CrossRef IUCr Journals Google Scholar
Farrugia, L. J. (1999). J. Appl. Cryst. 32, 837–838. CrossRef CAS IUCr Journals Google Scholar
Qiu, Y., Chen, Y., Pei, Y., Matsuda, H. & Yoshikawa, M. (2002). Chem. Pharm. Bull. 50, 1507–1510. Web of Science CrossRef PubMed CAS 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