N-[(1aR,5aR,8R,9aR)-1,1-Di­chloro-1a,5,5,7-tetra­methyl-1a,2,3,4,5,5a,8,9-octa­hydro-1H-benzo[a]cyclo­propa[b][7]annulen-8-yl]acetamide

Benharref, A.; El Ammari, L.; Saadi, M.; Taourirte, M.; Oukhrib, A.; Berraho, M.

doi:10.1107/S2414314617004928

organic compounds

IUCrDATA

ISSN: 2414-3146

Volume 2| Part 4| April 2017| x170492

https://doi.org/10.1107/S2414314617004928

Open

access

N-[(1aR,5aR,8R,9aR)-1,1-Dichloro-1a,5,5,7-tetramethyl-1a,2,3,4,5,5a,8,9-octahydro-1H-benzo[a]cyclopropa[b][7]annulen-8-yl]acetamide

Ahmed Benharref,^a ^* Lahcen El Ammari,^b Mohamed Saadi,^b Moha Taourirte,^c Abdelouahd Oukhrib ^a and Moha Berraho ^a

^aLaboratoire de Chimie des Substances Naturelles, Unité Associé au CNRST (URAC16), Faculté des Sciences Semlalia, BP 2390 Bd My Abdellah, Université Cadi Ayyad, 40000 Marrakech, Morocco, ^bLaboratoire de Chimie du Solide Appliquée, Faculty of Sciences, Mohammed V University in Rabat, Avenue Ibn Batouta BP 1014 Rabat, Morocco, and ^cLaboratoire de Chimie Bioorganique et Macromoléculaire, Faculté des Sciences et Techniques, Université Cadi Ayyad, 40000 Marrakech, Morocco
^*Correspondence e-mail: benharref@uca.ac.ma

Edited by H. Stoeckli-Evans, University of Neuchâtel, Switzerland (Received 21 March 2017; accepted 29 March 2017; online 11 April 2017)

In the title compound, C₁₈H₂₇Cl₂NO, the cyclohexene ring has an envelope conformation, with the C atom at the 9a position as the flap. The cycloheptane ring, to which it is fused, has a boat conformation. The dihedral angle between their mean planes is 60.7 (2)°. The 1,1-dichloro-cyclopropane ring is inclined to these two ring mean planes by 88.5 (3) and 28.3 (3)°, respectively. In the crystal, molecules are linked by N—H⋯O hydrogen bonds, forming 6₁ helices along the c-axis direction. The absolute configuration of the molecule in the crystal could be fully confirmed from anomalous dispersion effects [Flack parameter = 0.020 (15)].

Keywords: crystal structure; β himachalene; three fused rings; hydrogen bonding.

CCDC reference: 1540993

Structure description

The bicyclic sesquiterpene β-himachalene is the main constituent of the essential oil of the Atlas cedar (Cedrus Atlantica) (El Haib et al., 2010 ; Loubidi et al., 2014 ). The reactivity of these sesquiterpenes and their derivatives has been studied extensively by our team (El Jamili et al., 2002 ; El Haib et al., 2011 ; Benharref et al., 2015 , 2016 ) in order to prepare new products with biological properties. Indeed, these compounds have been tested, using the food-poisoning technique, for their potential antifungal activity against the phytopathogen Botrytis cinerea (Daoubi et al., 2004 ). Herein, we report on the synthesis and crystal structure of the title modified β-himachalene compound.

The structure of the title compound, Fig. 1, is built up from a cycloheptane ring (C1/C3–C8), which is fused to a cyclohexene ring (C1/C8–C12), and a cyclopropane ring (C1–C3). The six-membered ring has an envelope conformation with atom C1 (position 9a) as the flap [puckering parameters are: Q_T = 0.456 (4) Å, θ = 125.3 (6)° and φ = 173.6 (7)°], whereas the seven-membered ring displays a boat conformation [puckering parameters are: Q_T = 1.1390 (53) Å, θ = 89.19 (30)°, φ2 = 311.1 (3)°, φ3 = 24 (2)°]. The dihedral angle between their mean planes is 60.7 (2)°. The cyclopropane ring is normal to the mean plane of the cyclohexene ring, making a dihedral angle of 88.5 (3)°.

Figure 1
The molecular structure of the title compound, with the atom labelling. Displacement ellipsoids are drawn at the 30% probability level.

In the crystal, molecules are linked by N—H⋯O hydrogen bonds, forming helices running along the c-axis direction (Fig. 2 and Table 1).

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
N1—H1⋯O1ⁱ	0.86	2.20	3.059 (4)	175
Symmetry code: (i) $[y, -x+y, z-{\script{1\over 6}}]$ .

Figure 2
A view along the a axis of the crystal packing of the title compound, showing molecules linked by N—H⋯O hydrogen bonds (dashed lines; see Table 1

). For clarity, C-bound H atoms have been omitted.

Synthesis and crystallization

3 g (10 mmol) of 2,2-dichloro-9,10-epoxy-3,7,7,10- tetramethyl-tricyclo[6.4.0.0¹,³]dodecane (Sbai et al., 2002 ) was dissolved in 30 ml of CH₃CN and stirred at 273 K under argon. BF₃OEt (3% mmol) was added and the reaction mixture was stirred and monitored by TLC. After the completion of reaction, the solvent was removed and the residue obtained was chromatographed on silica, eluting with hexane–ethylacetate (90:10), which allowed the isolation of the title compound (yield 70%). It was recrystallized from ethyl acetate solution and yielded colourless prismatic crystals on slow evaporation of the solvent.

Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2. Owing to the presence of the Cl atoms, the absolute configuration could be fully confirmed from anomalous dispersion effects [Flack parameter = 0.020 (15)].

Table 2
Experimental details

Crystal data
Chemical formula	C₁₈H₂₇Cl₂NO
M_r	344.30
Crystal system, space group	Hexagonal, P6₁
Temperature (K)	296
a, c (Å)	10.5372 (4), 29.9710 (11)
V (Å³)	2881.9 (2)
Z	6
Radiation type	Mo Kα
μ (mm⁻¹)	0.34
Crystal size (mm)	0.24 × 0.2 × 0.15

Data collection
Diffractometer	Bruker X8 APEX Diffractometer
Absorption correction	Multi-scan (SADABS; Bruker, 2009 )
T_min, T_max	0.666, 0.746
No. of measured, independent and observed [I > 2σ(I)] reflections	53780, 3934, 3513
R_int	0.042
(sin θ/λ)_max (Å⁻¹)	0.625

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.039, 0.109, 1.08
No. of reflections	3934
No. of parameters	204
No. of restraints	1
H-atom treatment	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	0.45, −0.22
Absolute structure	Flack x determined using 1571 quotients [(I⁺)−(I⁻)]/[(I⁺)+(I⁻)] (Parsons et al., 2013 )
Absolute structure parameter	0.020 (15)
Computer programs: APEX2 and SAINT (Bruker, 2009), SHELXT2014/7 (Sheldrick, 2015a ), Mercury (Macrae et al., 2008 ), SHELXL2014/7 (Sheldrick, 2015b ) and publCIF (Westrip, 2010 ).

Structural data

CCDC reference: 1540993

Crystal structure: contains datablocks I, block. DOI: https://doi.org/10.1107/S2414314617004928/su4144sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2414314617004928/su4144Isup2.hkl

checkCIF report

Computing details top

Data collection: APEX2 (Bruker, 2009); cell refinement: SAINT (Bruker, 2009); data reduction: SAINT (Bruker, 2009); program(s) used to solve structure: SHELXT2014/7 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014/7 (Sheldrick, 2015b); molecular graphics: Mercury (Macrae et al., 2008); software used to prepare material for publication: SHELXL2014/7 (Sheldrick, 2015b) and publCIF (Westrip, 2010).

N-[(1aR,5aR,8R,9aR)-1,1-Dichloro-1a,5,5,7-tetramethyl-1a,2,3,4,5,5a,8,9-octahydro-1H-benzo[a]cyclopropa[b][7]annulen-8-yl]acetamide top

Crystal data top

C₁₈H₂₇Cl₂NO	D_x = 1.190 Mg m⁻³
M_r = 344.30	Mo Kα radiation, λ = 0.71073 Å
Hexagonal, P6₁	Cell parameters from 3934 reflections
a = 10.5372 (4) Å	θ = 2.3–26.4°
c = 29.9710 (11) Å	µ = 0.34 mm⁻¹
V = 2881.9 (2) Å³	T = 296 K
Z = 6	Prismatic, colourless
F(000) = 1104	0.24 × 0.2 × 0.15 mm

Data collection top

Bruker X8 APEX Diffractometer	3513 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	R_int = 0.042
φ and ω scans	θ_max = 26.4°, θ_min = 2.3°
Absorption correction: multi-scan (SADABS; Bruker, 2009)	h = −13→13
T_min = 0.666, T_max = 0.746	k = −13→13
53780 measured reflections	l = −37→37
3934 independent reflections

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)] = 0.039	H-atom parameters constrained
wR(F²) = 0.109	w = 1/[σ²(F_o²) + (0.0665P)² + 0.3403P] where P = (F_o² + 2F_c²)/3
S = 1.08	(Δ/σ)_max < 0.001
3934 reflections	Δρ_max = 0.45 e Å⁻³
204 parameters	Δρ_min = −0.22 e Å⁻³
1 restraint	Absolute structure: Flack x determined using 1571 quotients [(I⁺)-(I^-)]/[(I⁺)+(I^-)] (Parsons et al., 2013)
Primary atom site location: structure-invariant direct methods	Absolute structure parameter: 0.020 (15)

Special details top

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

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

	x	y	z	U_iso*/U_eq
C18	0.5450 (7)	0.7825 (6)	0.1505 (3)	0.128 (3)
H18A	0.5000	0.7884	0.1233	0.192*
H18B	0.6077	0.7429	0.1443	0.192*
H18C	0.6019	0.8787	0.1631	0.192*
C17	0.5129 (8)	0.6699 (7)	0.2229 (2)	0.123 (3)
H17A	0.5735	0.6318	0.2126	0.184*
H17B	0.4447	0.6044	0.2447	0.184*
H17C	0.5732	0.7644	0.2362	0.184*
Cl1	0.09118 (10)	0.37089 (13)	0.08694 (3)	0.0641 (3)
Cl2	−0.10316 (9)	0.19825 (11)	0.15688 (3)	0.0607 (3)
N1	0.1265 (4)	0.1158 (3)	0.21087 (9)	0.0511 (7)
H1	0.1080	0.0828	0.1840	0.061*
C2	0.0507 (4)	0.3694 (4)	0.14440 (11)	0.0469 (7)
C10	0.3586 (4)	0.3202 (4)	0.18513 (12)	0.0530 (8)
C9	0.3998 (4)	0.4388 (4)	0.16039 (12)	0.0520 (8)
H9	0.4794	0.4661	0.1415	0.062*
C1	0.1732 (4)	0.4450 (3)	0.17758 (10)	0.0430 (7)
C11	0.2348 (4)	0.2701 (4)	0.21848 (11)	0.0498 (8)
H11	0.2777	0.2748	0.2479	0.060*
O1	0.0812 (4)	0.0650 (4)	0.28373 (10)	0.0747 (9)
C12	0.1652 (4)	0.3679 (4)	0.22130 (10)	0.0466 (7)
H12A	0.0634	0.3083	0.2300	0.056*
H12B	0.2148	0.4410	0.2443	0.056*
C8	0.3304 (4)	0.5331 (4)	0.15991 (11)	0.0494 (8)
H8	0.3227	0.5542	0.1285	0.059*
C3	0.0687 (4)	0.5032 (4)	0.16756 (12)	0.0565 (9)
C14	0.0566 (4)	0.0260 (4)	0.24435 (12)	0.0510 (8)
C13	0.4342 (6)	0.2307 (7)	0.18241 (17)	0.0803 (13)
H13A	0.3695	0.1377	0.1689	0.120*
H13B	0.4596	0.2155	0.2119	0.120*
H13C	0.5214	0.2821	0.1647	0.120*
C15	−0.0558 (6)	−0.1280 (5)	0.23115 (18)	0.0775 (13)
H15A	−0.0320	−0.1959	0.2448	0.116*
H15B	−0.0558	−0.1376	0.1993	0.116*
H15C	−0.1510	−0.1487	0.2409	0.116*
C4	0.1314 (7)	0.6467 (6)	0.14176 (18)	0.0827 (15)
H4A	0.0515	0.6616	0.1325	0.099*
H4B	0.1795	0.6396	0.1151	0.099*
C16	−0.0474 (6)	0.4854 (6)	0.20148 (16)	0.0751 (12)
H16A	−0.1333	0.4713	0.1860	0.113*
H16B	−0.0095	0.5718	0.2196	0.113*
H16C	−0.0723	0.4020	0.2201	0.113*
C7	0.4279 (5)	0.6846 (5)	0.18313 (17)	0.0756 (13)
C5	0.2408 (8)	0.7790 (6)	0.1690 (2)	0.1000 (19)
H5A	0.1872	0.8152	0.1860	0.120*
H5B	0.3049	0.8560	0.1486	0.120*
C6	0.3382 (8)	0.7484 (6)	0.2021 (3)	0.113 (2)
H6A	0.4051	0.8399	0.2167	0.136*
H6B	0.2744	0.6820	0.2250	0.136*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
C18	0.098 (4)	0.062 (3)	0.151 (6)	−0.014 (3)	0.053 (4)	−0.003 (4)
C17	0.117 (5)	0.088 (4)	0.106 (5)	0.008 (3)	−0.042 (4)	−0.035 (4)
Cl1	0.0514 (5)	0.1048 (8)	0.0286 (4)	0.0334 (5)	0.0005 (3)	−0.0010 (4)
Cl2	0.0445 (4)	0.0679 (6)	0.0495 (5)	0.0128 (4)	0.0036 (4)	−0.0046 (4)
N1	0.0697 (19)	0.0473 (15)	0.0320 (13)	0.0262 (14)	−0.0014 (12)	0.0003 (11)
C2	0.0447 (16)	0.061 (2)	0.0299 (15)	0.0226 (15)	0.0048 (13)	0.0045 (13)
C10	0.0502 (18)	0.062 (2)	0.0375 (16)	0.0212 (16)	−0.0087 (14)	−0.0015 (15)
C9	0.0430 (16)	0.062 (2)	0.0359 (17)	0.0150 (15)	−0.0008 (13)	−0.0028 (15)
C1	0.0501 (17)	0.0399 (15)	0.0299 (14)	0.0157 (14)	0.0044 (13)	0.0001 (12)
C11	0.062 (2)	0.0530 (18)	0.0273 (14)	0.0233 (16)	−0.0061 (13)	0.0004 (13)
O1	0.091 (2)	0.093 (2)	0.0381 (14)	0.0438 (18)	0.0096 (13)	0.0121 (13)
C12	0.059 (2)	0.0438 (16)	0.0258 (13)	0.0172 (15)	0.0029 (13)	−0.0016 (12)
C8	0.0501 (17)	0.0469 (17)	0.0328 (15)	0.0105 (14)	0.0066 (14)	0.0014 (13)
C3	0.070 (2)	0.062 (2)	0.043 (2)	0.0369 (19)	0.0142 (16)	0.0122 (16)
C14	0.062 (2)	0.055 (2)	0.0431 (19)	0.0338 (17)	0.0100 (15)	0.0097 (15)
C13	0.089 (3)	0.106 (4)	0.061 (3)	0.060 (3)	−0.001 (2)	0.012 (2)
C15	0.089 (3)	0.056 (2)	0.080 (3)	0.031 (2)	0.027 (2)	0.013 (2)
C4	0.117 (4)	0.078 (3)	0.069 (3)	0.061 (3)	0.027 (3)	0.029 (2)
C16	0.095 (3)	0.086 (3)	0.064 (3)	0.059 (3)	0.024 (2)	0.010 (2)
C7	0.068 (3)	0.047 (2)	0.072 (3)	−0.0012 (19)	0.010 (2)	−0.0086 (19)
C5	0.139 (5)	0.057 (3)	0.106 (4)	0.050 (3)	0.030 (4)	0.021 (3)
C6	0.121 (5)	0.060 (3)	0.121 (5)	0.016 (3)	0.033 (4)	−0.022 (3)

Geometric parameters (Å, º) top

C18—C7	1.506 (7)	C12—H12A	0.9700
C18—H18A	0.9600	C12—H12B	0.9700
C18—H18B	0.9600	C8—C7	1.565 (5)
C18—H18C	0.9600	C8—H8	0.9800
C17—C7	1.544 (9)	C3—C4	1.524 (6)
C17—H17A	0.9600	C3—C16	1.529 (5)
C17—H17B	0.9600	C14—C15	1.506 (6)
C17—H17C	0.9600	C13—H13A	0.9600
Cl1—C2	1.772 (3)	C13—H13B	0.9600
Cl2—C2	1.760 (4)	C13—H13C	0.9600
N1—C14	1.322 (5)	C15—H15A	0.9600
N1—C11	1.464 (5)	C15—H15B	0.9600
N1—H1	0.8600	C15—H15C	0.9600
C2—C3	1.495 (5)	C4—C5	1.528 (9)
C2—C1	1.504 (5)	C4—H4A	0.9700
C10—C9	1.326 (6)	C4—H4B	0.9700
C10—C13	1.510 (7)	C16—H16A	0.9600
C10—C11	1.514 (5)	C16—H16B	0.9600
C9—C8	1.500 (6)	C16—H16C	0.9600
C9—H9	0.9300	C7—C6	1.518 (9)
C1—C12	1.522 (4)	C5—C6	1.572 (11)
C1—C8	1.532 (4)	C5—H5A	0.9700
C1—C3	1.533 (5)	C5—H5B	0.9700
C11—C12	1.537 (5)	C6—H6A	0.9700
C11—H11	0.9800	C6—H6B	0.9700
O1—C14	1.234 (5)

C7—C18—H18A	109.5	C2—C3—C4	119.2 (3)
C7—C18—H18B	109.5	C2—C3—C16	118.5 (3)
H18A—C18—H18B	109.5	C4—C3—C16	112.4 (4)
C7—C18—H18C	109.5	C2—C3—C1	59.5 (2)
H18A—C18—H18C	109.5	C4—C3—C1	117.1 (3)
H18B—C18—H18C	109.5	C16—C3—C1	120.7 (3)
C7—C17—H17A	109.5	O1—C14—N1	122.6 (4)
C7—C17—H17B	109.5	O1—C14—C15	122.1 (4)
H17A—C17—H17B	109.5	N1—C14—C15	115.4 (4)
C7—C17—H17C	109.5	C10—C13—H13A	109.5
H17A—C17—H17C	109.5	C10—C13—H13B	109.5
H17B—C17—H17C	109.5	H13A—C13—H13B	109.5
C14—N1—C11	121.5 (3)	C10—C13—H13C	109.5
C14—N1—H1	119.2	H13A—C13—H13C	109.5
C11—N1—H1	119.2	H13B—C13—H13C	109.5
C3—C2—C1	61.5 (2)	C14—C15—H15A	109.5
C3—C2—Cl2	119.0 (2)	C14—C15—H15B	109.5
C1—C2—Cl2	120.6 (2)	H15A—C15—H15B	109.5
C3—C2—Cl1	121.7 (3)	C14—C15—H15C	109.5
C1—C2—Cl1	119.9 (2)	H15A—C15—H15C	109.5
Cl2—C2—Cl1	108.06 (19)	H15B—C15—H15C	109.5
C9—C10—C13	122.2 (4)	C3—C4—C5	112.9 (4)
C9—C10—C11	121.1 (4)	C3—C4—H4A	109.0
C13—C10—C11	116.7 (4)	C5—C4—H4A	109.0
C10—C9—C8	126.3 (3)	C3—C4—H4B	109.0
C10—C9—H9	116.9	C5—C4—H4B	109.0
C8—C9—H9	116.9	H4A—C4—H4B	107.8
C2—C1—C12	118.9 (3)	C3—C16—H16A	109.5
C2—C1—C8	118.3 (3)	C3—C16—H16B	109.5
C12—C1—C8	112.0 (3)	H16A—C16—H16B	109.5
C2—C1—C3	59.0 (2)	C3—C16—H16C	109.5
C12—C1—C3	121.5 (3)	H16A—C16—H16C	109.5
C8—C1—C3	117.7 (3)	H16B—C16—H16C	109.5
N1—C11—C10	110.0 (3)	C18—C7—C6	114.7 (5)
N1—C11—C12	112.7 (3)	C18—C7—C17	104.6 (6)
C10—C11—C12	114.8 (3)	C6—C7—C17	106.0 (5)
N1—C11—H11	106.2	C18—C7—C8	107.8 (4)
C10—C11—H11	106.2	C6—C7—C8	112.4 (4)
C12—C11—H11	106.2	C17—C7—C8	111.1 (4)
C1—C12—C11	113.2 (3)	C4—C5—C6	115.0 (4)
C1—C12—H12A	108.9	C4—C5—H5A	108.5
C11—C12—H12A	108.9	C6—C5—H5A	108.5
C1—C12—H12B	108.9	C4—C5—H5B	108.5
C11—C12—H12B	108.9	C6—C5—H5B	108.5
H12A—C12—H12B	107.8	H5A—C5—H5B	107.5
C9—C8—C1	109.5 (3)	C7—C6—C5	118.0 (5)
C9—C8—C7	112.5 (3)	C7—C6—H6A	107.8
C1—C8—C7	115.1 (3)	C5—C6—H6A	107.8
C9—C8—H8	106.4	C7—C6—H6B	107.8
C1—C8—H8	106.4	C5—C6—H6B	107.8
C7—C8—H8	106.4	H6A—C6—H6B	107.1

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1···O1ⁱ	0.86	2.20	3.059 (4)	175

Symmetry code: (i) y, −x+y, z−1/6.

Acknowledgements

The authors thank the Unit of Support for Technical and Scientific Research (UATRS, CNRST) for the X-ray measurements.

References

Benharref, A., Elkarroumi, J., El Ammari, L., Saadi, M. & Berraho, M. (2015). Acta Cryst. E71, o659–o660. Web of Science CSD CrossRef IUCr Journals Google Scholar
Benharref, A., Oukhrib, A., Ait Elhad, M., El Ammari, L., Saadi, M. & Berraho, M. (2016). IUCrData, 1, x160703. Google Scholar
Bruker (2009). APEX2, SAINT and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA. Google Scholar
Daoubi, M., Durán-Patrón, R., Hmamouchi, M., Hernández-Galán, R., Benharref, A. & Collado, I. G. (2004). Pest Manag. Sci. 60, 927–932. Web of Science CrossRef PubMed CAS Google Scholar
El Haib, A., Benharref, A., Parrès-Maynadié, S., Manoury, E., Urrutigoïty, M. & Gouygou, M. (2011). Tetrahedron Asymmetry, 22, 101–108. Web of Science CrossRef CAS Google Scholar
El Jamili, H., Auhmani, A., Dakir, M., Lassaba, E., Benharref, A., Pierrot, M., Chiaroni, A. & Riche, C. (2002). Tetrahedron Lett. 43, 6645–6648. CAS Google Scholar
Haib, A. E., Benharref, A., Parrès-Maynadié, S., Manoury, E., Daran, J. C., Urrutigoïty, M. & Gouygou, M. (2010). Tetrahedron Asymmetry, 21, 1272–1277. Web of Science CSD CrossRef Google Scholar
Loubidi, M., Agustin, D., Benharref, A. & Poli, R. (2014). C. R. Chim. 17, 549–556. Web of Science CrossRef CAS Google Scholar
Macrae, C. F., Bruno, I. J., Chisholm, J. A., Edgington, P. R., McCabe, P., Pidcock, E., Rodriguez-Monge, L., Taylor, R., van de Streek, J. & Wood, P. A. (2008). J. Appl. Cryst. 41, 466–470. Web of Science CSD CrossRef CAS IUCr Journals Google Scholar
Parsons, S., Flack, H. D. & Wagner, T. (2013). Acta Cryst. B69, 249–259. Web of Science CrossRef CAS IUCr Journals Google Scholar
Sbai, F., Dakir, M., Auhmani, A., El Jamili, H., Akssira, M., Benharref, A., Kenz, A. & Pierrot, M. (2002). Acta Cryst. C58, o518–o520. Web of Science CSD CrossRef CAS IUCr Journals Google Scholar
Sheldrick, G. M. (2015a). Acta Cryst. A71, 3–8. Web of Science CrossRef IUCr Journals Google Scholar
Sheldrick, G. M. (2015b). Acta Cryst. C71, 3–8. Web of Science CrossRef IUCr Journals Google Scholar
Westrip, S. P. (2010). J. Appl. Cryst. 43, 920–925. Web of Science CrossRef CAS IUCr Journals Google Scholar

This is an open-access article distributed under the terms of the Creative Commons Attribution (CC-BY) Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original authors and source are cited.