Dehydro­brachylaenolide: an eudesmane-type sesquiterpene lactone

Rademeyer, M.; Heerden, F.R. van; Merwe, M. M. van der

doi:10.1107/S1600536808042402

organic compounds

CRYSTALLOGRAPHIC
COMMUNICATIONS

ISSN: 2056-9890

Volume 65| Part 1| January 2009| Page o196

doi:10.1107/S1600536808042402

Open

access

Dehydrobrachylaenolide: an eudesmane-type sesquiterpene lactone

M. Rademeyer,^a ^* F. R. van Heerden ^b and M. M. van der Merwe ^c

^aDepartment of Chemistry, University of Pretoria, Pretoria 0002, South Africa, ^bSchool of Chemistry, University of KwaZulu-Natal, Pietermaritzburg Campus, Private Bag X01, Scotsville 3209, South Africa, and ^cBiosciences, CSIR, Pretoria, South Africa, and, School of Chemistry, University of KwaZulu-Natal, Pietermaritzburg Campus, Private Bag X01, Scotsville 3209, South Africa
^*Correspondence e-mail: melanie.rademeyer@up.ac.za

(Received 26 November 2008; accepted 12 December 2008; online 24 December 2008)

The three-ring eudesmanolide, C₁₅H₁₆O₃, is a natural product isolated from Dicoma anomala Sond. (Asteraceae). The compound contains an endo–exo cross conjugated methylenecyclohexenone ring with an envelope conformation trans-fused with cyclohexane and trans-annelated with an α-methylene γ-lactone. The absolute structure was assigned by optical rotation measurements compared to those from the synthetic compound with known stereochemistry. The crystal packing is consolidated by C—H⋯O interactions.

Related literature

For NMR studies of this compound, see: Bohlmann & Zdero, (1982 ); Grass et al. (2004 ). For the chemical synthesis and confirmation of the absolute structure, see: Higuchi et al. (2003 ).

Experimental

Crystal data

C₁₅H₁₆O₃
M_r = 244.28
Orthorhombic, P 2₁ 2₁ 2₁
a = 9.5648 (6) Å
b = 11.1631 (6) Å
c = 11.5542 (6) Å
V = 1233.67 (12) Å³
Z = 4
Mo Kα radiation
μ = 0.09 mm⁻¹
T = 150 (2) K
0.50 × 0.50 × 0.40 mm

Data collection

Oxford Diffraction Excalibur2 CCD diffractometer
Absorption correction: multi-scan (Blessing, 1995 ) T_min = 0.909, T_max = 0.963
12604 measured reflections
2294 independent reflections
1988 reflections with I > 2σ(I)
R_int = 0.016

Refinement

R[F² > 2σ(F²)] = 0.033
wR(F²) = 0.095
S = 1.05
2294 reflections
163 parameters
H-atom parameters constrained
Δρ_max = 0.31 e Å⁻³
Δρ_min = −0.21 e Å⁻³

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
C6—H6⋯O1ⁱ	0.98	2.39	3.360 (2)	171
C14—H14A⋯O1ⁱ	0.96	2.57	3.393 (2)	143
Symmetry code: (i) $[-x+{\script{3\over 2}}, -y, z-{\script{1\over 2}}]$ .

Data collection: CrysAlis CCD (Oxford Diffraction, 2006 $[Oxford Diffraction (2006). CrysAlis CCD and CrysAlis RED. Oxford Diffraction Ltd, Abingdon, England.]$ ); cell refinement: CrysAlis RED (Oxford Diffraction, 2006 $[Oxford Diffraction (2006). CrysAlis CCD and CrysAlis RED. Oxford Diffraction Ltd, Abingdon, England.]$ ); data reduction: CrysAlis RED; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 ); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: Mercury (Macrae et al., 2006 ); software used to prepare material for publication: PLATON (Spek, 2003 ) and WinGX (Farrugia, 1999 ).

Supporting information

Crystal structure: contains datablocks global, I. DOI: 10.1107/S1600536808042402/bi2330sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S1600536808042402/bi2330Isup2.hkl

checkCIF report

Comment top

The title compound, a sesquiterpene lactone dehydrobrachylaenolide, was isolated from Dicoma anomala Sond (Asteraceae). These bi-functional exo-endo cross conjugated dienones are of importance as synthetic intermediates in the preparation of biologically active natural products (Higuchi et al., 2003). NMR studies of the compound have been reported previously (Bohlmann & Zdero, 1982; Grass et al., 2004) and the absolute stereochemistry has been confirmed as 3-oxoeudesma-1,4(15),11 (13)-triene-12,6a-olide by chemical synthesis (Higuchi et al., 2003). Here we report the crystal structure. Although the absolute structure could not be elucidated by X-ray diffraction, unambiguous assignment of stereochemistry was made on the basis of the value of optical rotation ([α] ²⁴_D+68° (c 1/2, CHCl₃)) which is identical to that of the synthetic compound ([α] ²⁴_D+67.9° (c 0.16, CHCl₃)) for which the stereochemistry is known (Higuchi et al., 2003) and very close to the value for the naturally isolated material ([α]²⁴_D+67° (c 0.16, CHCl₃)) (Bohlmann & Zdero, 1982).

The molecular geometry and labelling scheme are shown in Fig. 1. The methylenecyclohexenone ring adopts an envelope conformation, with the C5 atom out of the plane of the ring by approximately 0.7 Å. The γ-lactone ring is twisted on C6—C7, while the cyclohexane ring adopts a chair conformation. An axial position is occupied by methyl group C14, and the methylene carbon atom C15 is in the equatorial position. A weak intramolecular interaction is formed between C15—H15B···O2. Fig. 2 illustrates the molecular packing viewed down the c axis. Weak intermolecular hydrogen bonds are present between atoms C6—H6···O1ⁱ and and C14—H14···O1ⁱ [symmetry code (i): 1/2 - x, 1 - y, 1/2 + z].

Related literature top

For NMR studies of this compound, see: Bohlmann & Zdero, (1982); Grass et al. (2004). For the chemical synthesis and confirmation of the absolute structure, see: Higuchi et al. (2003).

Experimental top

The compound was isolated from Dicoma anomala Sond (Asteraceae), and recrystallized from propanol at room temperature.

Computing details top

Data collection: CrysAlis CCD (Oxford Diffraction, 2006); cell refinement: CrysAlis RED (Oxford Diffraction, 2006); data reduction: CrysAlis RED (Oxford Diffraction, 2006); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: Mercury (Macrae et al., 2006); software used to prepare material for publication: PLATON (Spek, 2003) and WinGX (Farrugia, 1999).

Figures top

Fig. 1. Molecular structure showing displacement ellipsoids at 50% probability for all atoms.

Dehydrobrachylaenolide top

Crystal data top

C₁₅H₁₆O₃	F(000) = 520
M_r = 244.28	D_x = 1.315 Mg m⁻³
Orthorhombic, P2₁2₁2₁	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: P 2ac 2ab	Cell parameters from 8167 reflections
a = 9.5648 (6) Å	θ = 4.0–31.8°
b = 11.1631 (6) Å	µ = 0.09 mm⁻¹
c = 11.5542 (6) Å	T = 150 K
V = 1233.67 (12) Å³	Block, colourless
Z = 4	0.50 × 0.50 × 0.40 mm

Data collection top

Oxford Diffraction Excalibur2 CCD diffractometer	2294 independent reflections
Radiation source: fine-focus sealed tube	1988 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.016
ω scans	θ_max = 31.9°, θ_min = 4.0°
Absorption correction: multi-scan (Blessing, 1995)	h = −13→13
T_min = 0.909, T_max = 0.963	k = −15→16
12604 measured reflections	l = −16→17

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)] = 0.033	Hydrogen site location: inferred from neighbouring sites
wR(F²) = 0.095	H-atom parameters constrained
S = 1.05	w = 1/[σ²(F_o²) + (0.0708P)²] where P = (F_o² + 2F_c²)/3
2294 reflections	(Δ/σ)_max = 0.001
163 parameters	Δρ_max = 0.31 e Å⁻³
0 restraints	Δρ_min = −0.21 e Å⁻³

Crystal data top

C₁₅H₁₆O₃	V = 1233.67 (12) Å³
M_r = 244.28	Z = 4
Orthorhombic, P2₁2₁2₁	Mo Kα radiation
a = 9.5648 (6) Å	µ = 0.09 mm⁻¹
b = 11.1631 (6) Å	T = 150 K
c = 11.5542 (6) Å	0.50 × 0.50 × 0.40 mm

Data collection top

Oxford Diffraction Excalibur2 CCD diffractometer	2294 independent reflections
Absorption correction: multi-scan (Blessing, 1995)	1988 reflections with I > 2σ(I)
T_min = 0.909, T_max = 0.963	R_int = 0.016
12604 measured reflections

Refinement top

R[F² > 2σ(F²)] = 0.033	0 restraints
wR(F²) = 0.095	H-atom parameters constrained
S = 1.05	Δρ_max = 0.31 e Å⁻³
2294 reflections	Δρ_min = −0.21 e Å⁻³
163 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
C4	0.86180 (14)	0.08184 (11)	1.24094 (11)	0.0237 (2)
O2	0.87541 (10)	0.20733 (7)	1.00141 (7)	0.0245 (2)
C7	1.04877 (12)	0.07144 (11)	0.94161 (10)	0.0210 (2)
H7	1.1253	0.1211	0.9714	0.025*
C8	1.10043 (14)	−0.05782 (12)	0.94001 (11)	0.0246 (3)
H8A	1.1798	−0.0657	0.8883	0.029*
H8B	1.0268	−0.1109	0.9136	0.029*
C9	1.14347 (13)	−0.08979 (11)	1.06503 (11)	0.0245 (2)
H9A	1.1717	−0.1732	1.0675	0.029*
H9B	1.2238	−0.0416	1.0866	0.029*
O3	0.85244 (12)	0.31953 (9)	0.84156 (9)	0.0372 (3)
C14	0.90759 (14)	−0.16252 (11)	1.13671 (12)	0.0270 (3)
H14A	0.8694	−0.1540	1.0603	0.041*
H14B	0.8354	−0.1493	1.1930	0.041*
H14C	0.9447	−0.2419	1.1458	0.041*
C6	0.92565 (13)	0.08521 (10)	1.02492 (10)	0.0197 (2)
H6	0.8525	0.0278	1.0037	0.024*
C3	0.90807 (15)	0.04878 (12)	1.36063 (11)	0.0274 (3)
O1	0.84841 (13)	0.08529 (10)	1.44767 (9)	0.0385 (3)
C11	0.98993 (14)	0.13415 (11)	0.83767 (11)	0.0234 (2)
C1	1.08111 (15)	−0.08814 (12)	1.27617 (12)	0.0279 (3)
H1	1.1553	−0.1409	1.2867	0.033*
C5	0.97025 (12)	0.06224 (10)	1.14800 (10)	0.0197 (2)
H5	1.0491	0.1156	1.1647	0.024*
C10	1.02589 (13)	−0.06954 (10)	1.15444 (10)	0.0213 (2)
C15	0.73174 (15)	0.12080 (12)	1.22442 (14)	0.0321 (3)
H15A	0.6708	0.1272	1.2868	0.039*
H15B	0.7019	0.1416	1.1505	0.039*
C13	1.00061 (16)	0.11111 (13)	0.72528 (11)	0.0303 (3)
H13A	0.9501	0.1564	0.6722	0.036*
H13B	1.0586	0.0497	0.6996	0.036*
C12	0.89936 (14)	0.23105 (11)	0.88715 (11)	0.0263 (3)
C2	1.02875 (16)	−0.03277 (13)	1.36888 (11)	0.0304 (3)
H2	1.0696	−0.0460	1.4408	0.036*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
C4	0.0291 (6)	0.0164 (5)	0.0257 (5)	−0.0021 (5)	0.0069 (5)	0.0005 (4)
O2	0.0309 (5)	0.0174 (4)	0.0253 (4)	0.0031 (3)	0.0029 (4)	0.0023 (3)
C7	0.0202 (5)	0.0206 (5)	0.0221 (5)	−0.0010 (4)	0.0013 (4)	−0.0030 (4)
C8	0.0239 (6)	0.0254 (6)	0.0244 (5)	0.0037 (5)	0.0003 (5)	−0.0048 (5)
C9	0.0200 (5)	0.0245 (6)	0.0289 (6)	0.0036 (5)	−0.0014 (5)	−0.0027 (4)
O3	0.0499 (7)	0.0262 (5)	0.0356 (5)	0.0061 (5)	0.0000 (5)	0.0081 (4)
C14	0.0277 (6)	0.0168 (5)	0.0366 (6)	−0.0020 (5)	−0.0027 (5)	0.0014 (5)
C6	0.0195 (5)	0.0148 (5)	0.0248 (5)	0.0006 (4)	0.0012 (4)	−0.0001 (4)
C3	0.0330 (6)	0.0233 (6)	0.0259 (5)	−0.0079 (5)	0.0076 (5)	0.0017 (5)
O1	0.0493 (6)	0.0366 (6)	0.0297 (5)	−0.0062 (5)	0.0165 (5)	−0.0008 (4)
C11	0.0233 (6)	0.0207 (5)	0.0261 (5)	−0.0040 (4)	0.0015 (5)	0.0001 (4)
C1	0.0288 (6)	0.0252 (6)	0.0297 (6)	0.0015 (5)	−0.0043 (5)	0.0036 (5)
C5	0.0202 (5)	0.0171 (5)	0.0217 (5)	−0.0007 (4)	0.0021 (4)	−0.0002 (4)
C10	0.0212 (5)	0.0186 (5)	0.0241 (5)	0.0011 (4)	−0.0022 (4)	0.0000 (4)
C15	0.0310 (7)	0.0263 (6)	0.0391 (7)	0.0033 (5)	0.0129 (6)	0.0040 (6)
C13	0.0314 (6)	0.0337 (7)	0.0257 (6)	−0.0050 (6)	0.0028 (6)	0.0006 (5)
C12	0.0303 (6)	0.0219 (6)	0.0266 (6)	−0.0028 (5)	−0.0002 (5)	0.0019 (4)
C2	0.0358 (7)	0.0301 (6)	0.0253 (6)	−0.0048 (5)	−0.0011 (5)	0.0048 (5)

Geometric parameters (Å, º) top

C4—C15	1.332 (2)	C14—H14B	0.960
C4—C3	1.4982 (18)	C14—H14C	0.960
C4—C5	1.5090 (16)	C6—C5	1.5067 (16)
O2—C12	1.3659 (15)	C6—H6	0.980
O2—C6	1.4708 (14)	C3—O1	1.2260 (16)
C7—C11	1.4997 (17)	C3—C2	1.473 (2)
C7—C8	1.5253 (18)	C11—C13	1.3278 (18)
C7—C6	1.5287 (16)	C11—C12	1.4991 (18)
C7—H7	0.980	C1—C2	1.334 (2)
C8—C9	1.5439 (18)	C1—C10	1.5167 (17)
C8—H8A	0.970	C1—H1	0.930
C8—H8B	0.970	C5—C10	1.5662 (15)
C9—C10	1.5437 (17)	C5—H5	0.980
C9—H9A	0.970	C15—H15A	0.930
C9—H9B	0.970	C15—H15B	0.930
O3—C12	1.2059 (16)	C13—H13A	0.930
C14—C10	1.5490 (17)	C13—H13B	0.930
C14—H14A	0.960	C2—H2	0.930

C15—C4—C3	119.26 (12)	O1—C3—C2	121.14 (13)
C15—C4—C5	125.99 (12)	O1—C3—C4	122.53 (13)
C3—C4—C5	114.71 (11)	C2—C3—C4	116.32 (11)
C12—O2—C6	107.66 (9)	C13—C11—C12	123.86 (13)
C11—C7—C8	123.60 (10)	C13—C11—C7	131.60 (13)
C11—C7—C6	99.69 (10)	C12—C11—C7	104.38 (10)
C8—C7—C6	110.63 (10)	C2—C1—C10	123.39 (12)
C11—C7—H7	107.3	C2—C1—H1	118.3
C8—C7—H7	107.3	C10—C1—H1	118.3
C6—C7—H7	107.3	C6—C5—C4	116.89 (10)
C7—C8—C9	107.08 (10)	C6—C5—C10	107.51 (9)
C7—C8—H8A	110.3	C4—C5—C10	109.63 (9)
C9—C8—H8A	110.3	C6—C5—H5	107.5
C7—C8—H8B	110.3	C4—C5—H5	107.5
C9—C8—H8B	110.3	C10—C5—H5	107.5
H8A—C8—H8B	108.6	C1—C10—C9	110.29 (10)
C10—C9—C8	113.46 (10)	C1—C10—C14	106.58 (10)
C10—C9—H9A	108.9	C9—C10—C14	110.22 (10)
C8—C9—H9A	108.9	C1—C10—C5	106.91 (10)
C10—C9—H9B	108.9	C9—C10—C5	110.69 (9)
C8—C9—H9B	108.9	C14—C10—C5	112.02 (9)
H9A—C9—H9B	107.7	C4—C15—H15A	120.0
C10—C14—H14A	109.5	C4—C15—H15B	120.0
C10—C14—H14B	109.5	H15A—C15—H15B	120.0
H14A—C14—H14B	109.5	C11—C13—H13A	120.0
C10—C14—H14C	109.5	C11—C13—H13B	120.0
H14A—C14—H14C	109.5	H13A—C13—H13B	120.0
H14B—C14—H14C	109.5	O3—C12—O2	121.23 (12)
O2—C6—C5	115.13 (9)	O3—C12—C11	129.75 (13)
O2—C6—C7	103.21 (9)	O2—C12—C11	109.00 (10)
C5—C6—C7	111.05 (10)	C1—C2—C3	121.90 (12)
O2—C6—H6	109.1	C1—C2—H2	119.1
C5—C6—H6	109.1	C3—C2—H2	119.1
C7—C6—H6	109.1

C11—C7—C8—C9	−176.53 (11)	C15—C4—C5—C10	−124.67 (13)
C6—C7—C8—C9	−58.75 (13)	C3—C4—C5—C10	52.90 (13)
C7—C8—C9—C10	55.27 (14)	C2—C1—C10—C9	151.41 (13)
C12—O2—C6—C5	153.05 (11)	C2—C1—C10—C14	−88.95 (15)
C12—O2—C6—C7	31.88 (12)	C2—C1—C10—C5	31.01 (17)
C11—C7—C6—O2	−39.39 (11)	C8—C9—C10—C1	−173.06 (11)
C8—C7—C6—O2	−171.01 (9)	C8—C9—C10—C14	69.52 (13)
C11—C7—C6—C5	−163.29 (9)	C8—C9—C10—C5	−54.96 (13)
C8—C7—C6—C5	65.10 (12)	C6—C5—C10—C1	175.55 (10)
C15—C4—C3—O1	−20.6 (2)	C4—C5—C10—C1	−56.42 (12)
C5—C4—C3—O1	161.68 (12)	C6—C5—C10—C9	55.39 (12)
C15—C4—C3—C2	158.27 (13)	C4—C5—C10—C9	−176.57 (10)
C5—C4—C3—C2	−19.48 (16)	C6—C5—C10—C14	−68.06 (12)
C8—C7—C11—C13	−19.4 (2)	C4—C5—C10—C14	59.98 (12)
C6—C7—C11—C13	−142.26 (15)	C6—O2—C12—O3	170.70 (12)
C8—C7—C11—C12	155.99 (11)	C6—O2—C12—C11	−10.45 (13)
C6—C7—C11—C12	33.14 (12)	C13—C11—C12—O3	−21.0 (2)
O2—C6—C5—C4	58.69 (14)	C7—C11—C12—O3	163.19 (14)
C7—C6—C5—C4	175.49 (10)	C13—C11—C12—O2	160.33 (12)
O2—C6—C5—C10	−177.59 (9)	C7—C11—C12—O2	−15.53 (13)
C7—C6—C5—C10	−60.79 (12)	C10—C1—C2—C3	2.2 (2)
C15—C4—C5—C6	−2.05 (18)	O1—C3—C2—C1	169.28 (13)
C3—C4—C5—C6	175.53 (10)	C4—C3—C2—C1	−9.6 (2)

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
C6—H6···O1ⁱ	0.98	2.39	3.360 (2)	171
C14—H14A···O1ⁱ	0.96	2.57	3.393 (2)	143

Symmetry code: (i) −x+3/2, −y, z−1/2.

Experimental details

Crystal data
Chemical formula	C₁₅H₁₆O₃
M_r	244.28
Crystal system, space group	Orthorhombic, P2₁2₁2₁
Temperature (K)	150
a, b, c (Å)	9.5648 (6), 11.1631 (6), 11.5542 (6)
V (Å³)	1233.67 (12)
Z	4
Radiation type	Mo Kα
µ (mm⁻¹)	0.09
Crystal size (mm)	0.50 × 0.50 × 0.40

Data collection
Diffractometer	Oxford Diffraction Excalibur2 CCD diffractometer
Absorption correction	Multi-scan (Blessing, 1995)
T_min, T_max	0.909, 0.963
No. of measured, independent and observed [I > 2σ(I)] reflections	12604, 2294, 1988
R_int	0.016
(sin θ/λ)_max (Å⁻¹)	0.743

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.033, 0.095, 1.05
No. of reflections	2294
No. of parameters	163
H-atom treatment	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	0.31, −0.21

Computer programs: CrysAlis CCD (Oxford Diffraction, 2006), CrysAlis RED (Oxford Diffraction, 2006), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), Mercury (Macrae et al., 2006), PLATON (Spek, 2003) and WinGX (Farrugia, 1999).

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
C6—H6···O1ⁱ	0.98	2.39	3.360 (2)	171
C14—H14A···O1ⁱ	0.96	2.57	3.393 (2)	143

Symmetry code: (i) −x+3/2, −y, z−1/2.

Acknowledgements

We thank the National Drug Development Platform (NDDP) and the NRF for funding.

References

Blessing, R. H. (1995). Acta Cryst. A51, 33–38. CrossRef CAS Web of Science IUCr Journals Google Scholar
Bohlmann, F. & Zdero, C. (1982). Phytochemistry, 21, 647–651. CrossRef CAS Web of Science Google Scholar
Farrugia, L. J. (1999). J. Appl. Cryst. 32, 837–838. CrossRef CAS IUCr Journals Google Scholar
Grass, S., Zidorn, C., Ellmerer, E. P. & Stuppner, H. (2004). Chem. Biodivers. 1, 353–360. Web of Science CrossRef PubMed CAS Google Scholar
Higuchi, Y., Shimota, F., Koyanagi, R., Suda, K., Mitsui, T., Kataoka, T., Nagai, K. & Ando, M. (2003). J. Nat. Prod. 66, 588–594. Web of Science CrossRef PubMed CAS Google Scholar
Macrae, C. F., Edgington, P. R., McCabe, P., Pidcock, E., Shields, G. P., Taylor, R., Towler, M. & van de Streek, J. (2006). J. Appl. Cryst. 39, 453–457. Web of Science CrossRef CAS IUCr Journals Google Scholar
Oxford Diffraction (2006). CrysAlis CCD and CrysAlis RED. Oxford Diffraction Ltd, Abingdon, England. Google Scholar
Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122. Web of Science CrossRef CAS IUCr Journals Google Scholar
Spek, A. L. (2003). J. Appl. Cryst. 36, 7–13. Web of Science CrossRef CAS IUCr Journals Google Scholar