Crystal structure of 2,4,6-tri­methyl­benzoic anhydride

Land, M.A.; Robertson, K.N.; Clyburne, J.A.C.

doi:10.1107/S2056989017014670

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Volume 73| Part 11| November 2017| Pages 1735-1738

https://doi.org/10.1107/S2056989017014670

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Crystal structure of 2,4,6-trimethylbenzoic anhydride

Michael A. Land,^a Katherine N. Robertson ^a and Jason A. C. Clyburne ^a ^*

^aAtlantic Centre for Green Chemistry, Department of Chemistry, Saint Mary's University, 923 Robie Street, Halifax, Nova Scotia, B3H 3C3 Canada
^*Correspondence e-mail: jason.clyburne@smu.ca

Edited by W. T. A. Harrison, University of Aberdeen, Scotland (Received 29 September 2017; accepted 10 October 2017; online 20 October 2017)

The title compound, C₂₀H₂₂O₃, was formed in the reaction between 2,4,6-trimethylbenzoic acid and N,N-diisopropylethylamine in the presence of 1,3-dichloro-1,3-bis(dimethylamino)propenium hydrogen dichloride, and was recrystallized from diethyl ether solution. It is the first exclusively alkyl-substituted benzoic anhydride to have been structurally characterized. The asymmetric unit consists of a half molecule, the other half of which is generated by twofold rotation symmetry; the dihedral angle between the symmetry-related aromatic rings is 54.97 (3)°. The geometric parameters of the aromatic ring are typical of those for 2,4,6-trimethylphenyl substituted groups. The C=O and C—O bond lengths are 1.1934 (12) and 1.3958 (11) Å, respectively, and the angle between these three atoms (O=C—O) is 121.24 (9)°. In the crystal, molecules are linked by weak C—H⋯O hydrogen bonds and C—H⋯π interactions. The packing features wavy chains that extend parallel to [001].

Keywords: crystal structure; 2,4,6-trimethylbenzoic anhydride; mesitoic anhydride.

CCDC reference: 1579203

1. Chemical context

Benzoic anhydrides have traditionally been used in synthetic organic chemistry for the preparation of aromatic esters, amides and carboxylic acids. Aromatic anhydrides have also been shown to be effective acylating agents (Shiina, 2004 ; Shiina & Nakata, 2007 ). The title compound has been used to trap deprotonated 3,4-epoxy-2,3,4,5-tetrahydrothiophene 1,1-dioxide, forming 3-(2,4,6-trimethylbenzoyloxy)-2,3-dihydrothiophene 1,1-dioxide (Alonso et al., 2004 , 2005 ). The synthesis of the compound we report here, 2,4,6-trimethylbenzoic anhydride (common name: mesitoic anhydride), was first published in 1941, where it was formed in the reaction between (2,4,6-trimethylphenyl)sodium and 2,4,6-trimethylbenzoic acid in the presence of pyridine (Fuson et al., 1941 ). Recently, several new approaches for the syntheses of symmetric acid anhydrides, including the title compound, have been reported (Kazemi et al., 2004 ; Li et al., 2012 ; McCallum & Barriault, 2015 ). The most recent report involves the in situ generation of a Vilsmeier–Haack reagent for the coupling of symmetric carboxylic acids (McCallum & Barriault, 2015). Due to the structural similarities between this reagent and the 1,3-dichloro-1,3-bis(dimethylamino)propenium salt used here, it is possible that the title compound was formed via a similar method in our reaction. The crystal structure that we report is the first example of a benzoic anhydride where the aryl rings are substituted with only alkyl groups, although several other substituted benzoic anhydrides are known.

2. Structural commentary

The molecular structure of the title compound is shown in Fig. 1. It crystallizes in the monoclinic space group C2/c with one half of the molecule uniquely present in the asymmetric unit. The two C—O bond lengths are significantly different, as would be expected for anhydrides, with lengths of 1.1934 (12) (C1—O2) and 1.3958 (11) Å (C1—O1). The C1—C2 distance is normal for an sp²–sp² bond, with a length of 1.4873 (13) Å. The second half of the molecule, which is generated by rotation about the twofold axis passing through O1 (0, y, 0.25), forms a dihedral angle of 54.97 (3)° between the equivalent aromatic rings. If the planes of the two overlapping CO₂ groups are chosen instead, the dihedral angle becomes 59.30 (11)°. The C—C bonds in the aromatic ring are not all statistically equivalent. Unsurprisingly, the longest C—C bonds in the ring are adjacent to the electron-withdrawing anhydride group, C2—C3 [1.4032 (13) Å] and C2—C7 [1.4059 (13) Å]. The remaining C—C bonds are statistically equivalent, averaging 1.3942 (8) Å. All of the C—CH₃ bond lengths are statistically equivalent with an average length of 1.5102 (8) Å.

Figure 1
The molecular structure of the title compound. Only half of the molecule is crystallographically unique (labelled atoms). Displacement ellipsoids are drawn at the 50% probability level.

3. Supramolecular features

The packing of the molecules, when viewed in projection down the a axis, forms wavy chains that run parallel to the c-axis direction (Fig. 2). Within the chains, the molecules are oriented in a alternating up and down fashion, shifting by $[1\over 2]$ along [001] each time, such that they overlap slightly. There are no close stacking interactions between the phenyl rings in various planes. However, if the packing is viewed down the b axis, the C5—C6—C7—C10 fragment of one trimethylphenyl group lies directly above/below the same fragment running in the opposite direction, C10—C7—C6—C5, in the plane above/below it.

Figure 2
Packing diagram of the title compound, viewed in projection down [100], showing wavy [001] chains.

There are short intramolecular contacts between the aromatic H atoms H4 (2.37 Å to H8A and 2.37 to H9C) and H6 (2.39 Å to H10A), and the designated methyl H atoms, which close five-membered rings in the molecule.

In the crystal, molecules are linked by weak intermolecular C—H⋯O hydrogen bonds and C—H⋯π contacts (Table 1, Fig. 3). It is notable that these contacts involve one H atom from each of the three methyl substituents on the phenyl ring. All of these contacts occur between the chains that run parallel to the c axis and not within the individual chains, thus consolidating the overall structure.

Table 1
Hydrogen-bond geometry (Å, °)

Cg1 is the centroid of the C2–C7 ring.

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
C8—H8A⋯O2ⁱ	0.98	2.48	3.4612 (13)	176
C9—H9A⋯O2ⁱⁱ	0.98	2.66	3.5842 (14)	157
C10—H10B⋯Cg1ⁱⁱⁱ	0.98	2.96	3.5255 (14)	118
Symmetry codes: (i) $[-x+{\script{1\over 2}}, y-{\script{1\over 2}}, -z+{\script{1\over 2}}]$ ; (ii) $[x, -y+1, z-{\script{1\over 2}}]$ ; (iii) $[x+{\script{1\over 2}}, y+{\script{3\over 2}}, z]$ .

Figure 3
Short intermolecular contacts (defined in the text and close to the sum of the van der Waals radii; shown as heavy dotted lines), with only donors from the central molecule included (Table 1

4. Database survey

A survey of the Cambridge Structural Database (CSD, Version 5.38; Groom et al., 2016 ), performed on 24 July, 2017, located 25 substituted benzoic anhydrides, all of which were symmetric. Interestingly, there were no other benzoic anhydrides identified that were substituted exclusively with alkyl groups. The only other structurally characterized alkyl-substituted benzoic anhydrides are 2-acetoxy-5-methylbenzoic anhydride (CSD refcode IBOCOT; Solanko & Bond, 2011 ) and 2-methyl-3-nitrobenzoic anhydride (QUFTIW; Moreno-Fuquen et al., 2015 ). Most of the examples found in the CSD contain various substitution patterns involving halogens (Cl, Br or I). There are also several structures that contain aromatic activating groups, such as ethers or amines. The parent compound, benzoic anhydride (ZZZQRI; van Alen & Krauze, 1964 ), is known, as is the precursor to the title compound, 2,4,6-trimethylbenzoic acid (TMBZAC; Florencio & Smith, 1970 ), which can be overlaid with the asymmetric unit of the title compound reasonably well.

5. Synthesis and crystallization

The title compound was isolated from the following reaction mixture, although more convenient synthetic methods are known (Fuson et al., 1941; Kazemi et al., 2004). 2,4,6-Trimethylbenzoic acid (1.11 g, 6.78 mmol) and N,N-diisopropylethylamine (1.21 ml, 6.94 mmol) were added to a chloroform solution (25 ml) of 1,3-dichloro-1,3-bis(dimethylamino)propenium hydrogen dichloride (0.92 g, 3.42 mmol), which had been prepared following the known literature method (Janousek & Viehe, 1971 ), in chloroform (25 ml). The mixture was stirred at reflux for 18 h under nitrogen. After cooling to room temperature, a saturated KOH (aqueous) solution (∼2 ml) and water (70 ml) were added to the mixture. The organic layer was extracted and the aqueous phase was washed with chloroform (two 25 ml portions). The combined organic extracts were washed with brine and dried with MgSO₄, and the solvent was removed in vacuo. The resulting material was washed with ice-cold water (25 ml) and then isolated via vacuum filtration. The off-white solid was further purified by recrystallization through slow evaporation of a saturated diethyl ether solution. After 3 h, clear and colourless thin plate-like crystals were obtained [yield: 0.98 g, 3.15 mmol, 93%; m.p. 374–375 K (literature = 375–377 K)]. Elemental analysis, calculated for C₂₀H₂₂O₃ (%): C 77.39, H 7.14, N 0.00; found (%): C 77.26, H 7.13, N 0.01. The ¹H and ¹³C{¹H} NMR and IR spectroscopic data for the title compound are identical to those previously reported (Kazemi et al., 2004).

6. Refinement

Crystal data, data collection, and structure refinement details are summarized in Table 2. H atoms were included in calculated positions (C—H = 0.95–0.98 Å) and refined as riding with U_iso(H) = 1.2 or 1.5U_eq(C).

Table 2
Experimental details

Crystal data
Chemical formula	C₂₀H₂₂O₃
M_r	310.37
Crystal system, space group	Monoclinic, C2/c
Temperature (K)	125
a, b, c (Å)	16.080 (2), 7.9997 (12), 14.308 (2)
β (°)	114.094 (2)
V (Å³)	1680.1 (4)
Z	4
Radiation type	Mo Kα
μ (mm⁻¹)	0.08
Crystal size (mm)	0.55 × 0.26 × 0.25

Data collection
Diffractometer	Bruker APEXII CCD area-detector
Absorption correction	Multi-scan (SADABS; Bruker, 2008 )
T_min, T_max	0.683, 0.746
No. of measured, independent and observed [I > 2σ(I)] reflections	9832, 2095, 1857
R_int	0.019
(sin θ/λ)_max (Å⁻¹)	0.679

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.037, 0.107, 1.05
No. of reflections	2095
No. of parameters	108
H-atom treatment	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	0.34, −0.19
Computer programs: APEX2 and SAINT (Bruker, 2008), SHELXT2014 (Sheldrick, 2015a ), SHELXL2014 (Sheldrick, 2015b ) and Mercury CSD 3.9 (Macrae et al., 2008 ).

Supporting information

CCDC reference: 1579203

Crystal structure: contains datablocks I, global. DOI: https://doi.org/10.1107/S2056989017014670/hb7710sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2056989017014670/hb7710Isup2.hkl

Supporting information file. DOI: https://doi.org/10.1107/S2056989017014670/hb7710Isup3.cml

checkCIF report

Computing details top

Data collection: APEX2 (Bruker, 2008); cell refinement: SAINT (Bruker, 2008); data reduction: SAINT (Bruker, 2008); program(s) used to solve structure: SHELXT2014 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015b); molecular graphics: Mercury (Macrae et al., 2008) [OK?]; software used to prepare material for publication: SHELXL2014 (Sheldrick, 2015b).

2,4,6-Trimethylbenzoic anhydride top

Crystal data top

C₂₀H₂₂O₃	F(000) = 664
M_r = 310.37	D_x = 1.227 Mg m⁻³
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
a = 16.080 (2) Å	Cell parameters from 6207 reflections
b = 7.9997 (12) Å	θ = 2.8–28.7°
c = 14.308 (2) Å	µ = 0.08 mm⁻¹
β = 114.094 (2)°	T = 125 K
V = 1680.1 (4) Å³	Wedge shaped (cut from a large block), colourless
Z = 4	0.55 × 0.26 × 0.25 mm

Data collection top

Bruker APEXII CCD area-detector diffractometer	2095 independent reflections
Radiation source: sealed tube	1857 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.019
φ and ω scans	θ_max = 28.9°, θ_min = 2.8°
Absorption correction: multi-scan (SADABS; Bruker, 2008)	h = −21→21
T_min = 0.683, T_max = 0.746	k = −10→10
9832 measured reflections	l = −19→19

Refinement top

Refinement on F²	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F² > 2σ(F²)] = 0.037	H-atom parameters constrained
wR(F²) = 0.107	w = 1/[σ²(F_o²) + (0.0543P)² + 0.9964P] where P = (F_o² + 2F_c²)/3
S = 1.05	(Δ/σ)_max < 0.001
2095 reflections	Δρ_max = 0.34 e Å⁻³
108 parameters	Δρ_min = −0.19 e Å⁻³
0 restraints

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
O1	0.0000	0.31743 (12)	0.2500	0.0206 (2)
O2	0.08932 (5)	0.53524 (10)	0.24991 (6)	0.0272 (2)
C1	0.05461 (6)	0.40599 (12)	0.21238 (7)	0.0193 (2)
C2	0.06727 (6)	0.31349 (12)	0.12897 (7)	0.0190 (2)
C3	0.15548 (7)	0.26116 (12)	0.14551 (7)	0.0206 (2)
C4	0.16788 (7)	0.18095 (13)	0.06542 (8)	0.0229 (2)
H4	0.2269	0.1425	0.0759	0.028*
C5	0.09582 (8)	0.15592 (13)	−0.02937 (8)	0.0237 (2)
C6	0.00895 (7)	0.20904 (13)	−0.04303 (8)	0.0245 (2)
H6	−0.0406	0.1912	−0.1073	0.029*
C7	−0.00725 (7)	0.28740 (12)	0.03480 (8)	0.0213 (2)
C8	0.23514 (7)	0.28879 (15)	0.24696 (8)	0.0275 (2)
H8A	0.2859	0.2167	0.2514	0.041*
H8B	0.2170	0.2615	0.3028	0.041*
H8C	0.2542	0.4061	0.2526	0.041*
C9	0.11059 (9)	0.07659 (15)	−0.11716 (9)	0.0310 (3)
H9A	0.1003	0.1600	−0.1709	0.047*
H9B	0.0679	−0.0164	−0.1449	0.047*
H9C	0.1732	0.0347	−0.0926	0.047*
C10	−0.10199 (7)	0.34744 (15)	0.01547 (8)	0.0268 (2)
H10A	−0.1368	0.3649	−0.0581	0.040*
H10B	−0.0980	0.4530	0.0519	0.040*
H10C	−0.1326	0.2634	0.0401	0.040*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
O1	0.0218 (5)	0.0187 (5)	0.0253 (5)	0.000	0.0138 (4)	0.000
O2	0.0251 (4)	0.0256 (4)	0.0340 (4)	−0.0059 (3)	0.0153 (3)	−0.0060 (3)
C1	0.0149 (4)	0.0210 (5)	0.0220 (5)	0.0018 (3)	0.0074 (4)	0.0031 (4)
C2	0.0187 (5)	0.0190 (4)	0.0200 (5)	−0.0001 (3)	0.0086 (4)	0.0023 (3)
C3	0.0200 (5)	0.0211 (5)	0.0212 (5)	0.0013 (4)	0.0089 (4)	0.0026 (4)
C4	0.0242 (5)	0.0219 (5)	0.0257 (5)	0.0023 (4)	0.0133 (4)	0.0025 (4)
C5	0.0328 (6)	0.0182 (5)	0.0232 (5)	−0.0022 (4)	0.0146 (4)	0.0011 (4)
C6	0.0271 (5)	0.0238 (5)	0.0199 (5)	−0.0043 (4)	0.0067 (4)	0.0013 (4)
C7	0.0200 (5)	0.0208 (5)	0.0222 (5)	−0.0014 (4)	0.0076 (4)	0.0037 (4)
C8	0.0194 (5)	0.0369 (6)	0.0243 (5)	0.0060 (4)	0.0068 (4)	−0.0018 (4)
C9	0.0439 (7)	0.0273 (5)	0.0270 (5)	−0.0031 (5)	0.0197 (5)	−0.0032 (4)
C10	0.0188 (5)	0.0311 (6)	0.0267 (5)	−0.0002 (4)	0.0053 (4)	0.0039 (4)

Geometric parameters (Å, º) top

O1—C1ⁱ	1.3958 (11)	C6—C7	1.3921 (15)
O1—C1	1.3958 (11)	C6—H6	0.9500
O2—C1	1.1934 (12)	C7—C10	1.5111 (14)
C1—C2	1.4873 (13)	C8—H8A	0.9800
C2—C3	1.4032 (13)	C8—H8B	0.9800
C2—C7	1.4059 (13)	C8—H8C	0.9800
C3—C4	1.3968 (14)	C9—H9A	0.9800
C3—C8	1.5095 (14)	C9—H9B	0.9800
C4—C5	1.3924 (15)	C9—H9C	0.9800
C4—H4	0.9500	C10—H10A	0.9800
C5—C6	1.3953 (16)	C10—H10B	0.9800
C5—C9	1.5101 (14)	C10—H10C	0.9800

C1ⁱ—O1—C1	119.00 (11)	C6—C7—C10	120.03 (9)
O2—C1—O1	121.24 (9)	C2—C7—C10	121.97 (9)
O2—C1—C2	126.87 (9)	C3—C8—H8A	109.5
O1—C1—C2	111.76 (8)	C3—C8—H8B	109.5
C3—C2—C7	121.65 (9)	H8A—C8—H8B	109.5
C3—C2—C1	118.24 (8)	C3—C8—H8C	109.5
C7—C2—C1	120.03 (9)	H8A—C8—H8C	109.5
C4—C3—C2	118.14 (9)	H8B—C8—H8C	109.5
C4—C3—C8	120.45 (9)	C5—C9—H9A	109.5
C2—C3—C8	121.40 (9)	C5—C9—H9B	109.5
C5—C4—C3	121.63 (9)	H9A—C9—H9B	109.5
C5—C4—H4	119.2	C5—C9—H9C	109.5
C3—C4—H4	119.2	H9A—C9—H9C	109.5
C4—C5—C6	118.69 (9)	H9B—C9—H9C	109.5
C4—C5—C9	121.31 (10)	C7—C10—H10A	109.5
C6—C5—C9	119.99 (10)	C7—C10—H10B	109.5
C7—C6—C5	121.91 (9)	H10A—C10—H10B	109.5
C7—C6—H6	119.0	C7—C10—H10C	109.5
C5—C6—H6	119.0	H10A—C10—H10C	109.5
C6—C7—C2	117.96 (9)	H10B—C10—H10C	109.5

C1ⁱ—O1—C1—O2	35.04 (7)	C8—C3—C4—C5	−179.10 (10)
C1ⁱ—O1—C1—C2	−148.96 (8)	C3—C4—C5—C6	−1.62 (15)
O2—C1—C2—C3	59.63 (14)	C3—C4—C5—C9	177.01 (9)
O1—C1—C2—C3	−116.09 (9)	C4—C5—C6—C7	0.66 (15)
O2—C1—C2—C7	−117.38 (11)	C9—C5—C6—C7	−177.99 (9)
O1—C1—C2—C7	66.89 (11)	C5—C6—C7—C2	0.47 (15)
C7—C2—C3—C4	−0.20 (14)	C5—C6—C7—C10	178.17 (9)
C1—C2—C3—C4	−177.17 (9)	C3—C2—C7—C6	−0.70 (14)
C7—C2—C3—C8	−179.70 (9)	C1—C2—C7—C6	176.21 (9)
C1—C2—C3—C8	3.33 (14)	C3—C2—C7—C10	−178.35 (9)
C2—C3—C4—C5	1.39 (15)	C1—C2—C7—C10	−1.44 (14)

Symmetry code: (i) −x, y, −z+1/2.

Hydrogen-bond geometry (Å, º) top

Cg1 is the centroid of the C2–C7 ring.

D—H···A	D—H	H···A	D···A	D—H···A
C8—H8A···O2ⁱⁱ	0.98	2.48	3.4612 (13)	176
C9—H9A···O2ⁱⁱⁱ	0.98	2.66	3.5842 (14)	157
C10—H10B···Cg1^iv	0.98	2.96	3.5255 (14)	118

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

Acknowledgements

Financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC) (Discovery Grant to JACC), the Canada Foundation for Innovation (CFI), the Nova Scotia Research and Innovation Trust Fund, and the Faculty of Graduate Studies and Research of Saint Mary's University (FGSR Fellowship to MAL) is gratefully acknowledged.

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