Crystallographic and spectroscopic characterization of 5-chloro­pyridine-2,3-di­amine

Sulovari, A.; Tanski, J.M.

doi:10.1107/S2056989017010489

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Volume 73| Part 8| August 2017| Pages 1213-1217

https://doi.org/10.1107/S2056989017010489

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Crystallographic and spectroscopic characterization of 5-chloropyridine-2,3-diamine

Aron Sulovari ^a and Joseph M. Tanski ^a ^*

^aDepartment of Chemistry, Vassar College, Poughkeepsie, NY 12604, USA
^*Correspondence e-mail: jotanski@vassar.edu

Edited by L. Fabian, University of East Anglia, England (Received 21 June 2017; accepted 14 July 2017; online 21 July 2017)

The two ortho-amino groups of the title compound, C₅H₆ClN₃, twist out of the plane of the molecule to minimize intramolecular interaction between the amino hydrogen atoms. In the crystal, the amino groups and the pyridine N atom engage in intermolecular hydrogen bonding. The molecules pack into spiral hydrogen-bonded columns with offset face-to-face π-stacking.

Keywords: crystal structure; hydrogen bonding; π-stacking.

CCDC reference: 1562267

1. Chemical context

The title compound, 5-chloropyridine-2,3-diamine, is a trisubstituted pyridine featuring ortho-amino groups and a chlorine atom. While all of the sixteen isomers of 5-chloropyridine-2,3-diamine are commercially available, none of their crystal structures have been reported in the literature. 5-Chloropyridine-2,3-diamine may be produced by nitrating 2-amino-5-chloropyridine with nitric acid to give 2-amino-3-nitro-5-chloropyridine, which is then reduced with sodium dithionite (Israel & Day, 1959 ). The reduction may also be accomplished with hydrogen gas and Pd/C (Xie et al., 2016 ). 5-Chloropyridine-2,3-diamine has proven useful as a reagent in complex syntheses, such as in the synthesis of aldose reductase inhibitors with antioxidant activity (Han et al., 2016 ), the regioselective functionalization of imidazopyridines via alkenylation catalyzed by a Pd/Cu catalyst (Baladi et al., 2016 ), the preparation of amino acid oxidase inhibitors (Xie et al., 2016), the preparation of β-glucuronidase inhibitors (Taha et al., 2016 ), the preparation of imidazopyridine derivatives with activity against MCF-7 breast adenocarcinoma (Püsküllü et al., 2015 ) and the preparation of dihydroxyarene-substituted benzimidazoles, quinazolines and larger rings via cyclocondensation of diamines (Los et al., 2012 ).

2. Structural commentary

The molecular structure of the title compound 5-chloropyridine-2,3-diamine (Fig. 1) shows that the molecule is nearly planar with r.m.s deviation from the mean plane of all non-hydrogen atoms of 0.013 (3) Å. The amino groups ortho and meta to the pyridine nitrogen atom twist out of the plane of the molecule in such a way as to minimize contact with one another, with NH₂ plane to molecular plane angles of 45 (3) and 34 (3)° for N2 and N3, respectively. It is notable that the achiral title compound crystallizes in a non-enantiogenic (Söhncke) space group, although not a polar space group.

Figure 1
A view of 5-chloropyridine-2,3-diamine (I)

with the atom-numbering scheme. Displacement ellipsoids are shown at the 50% probability level.

3. Supramolecular features

Notable intermolecular interactions observed in the structure of 5-chloropyridine-2,3-diamine (I) include N_amine—H⋯N_pyr and N_amine—H⋯N_amine hydrogen bonding interactions and offset face-to-face π-stacking. The molecules connect into a one-dimensional strip running parallel to the crystallographic b axis (Fig. 2) with long N3_amine—H21⋯N2ⁱⁱ_amine [symmetry code (ii) −x + 2, y − $[{1\over 2}]$ , −z + $[{3\over 2}]$ ] and N3_amine—H22⋯N1ⁱⁱⁱ_pyr [symmetry code (iii) −x + 2, y + $[{1\over 2}]$ , −z + $[{3\over 2}]$ ] hydrogen bonding interactions with donor–acceptor distances of 3.250 (4) and 3.075 (4) Å, respectively (Table 1). A third N_amine—H⋯N_pyr hydrogen-bonding contact and offset face-to-face π-stacking can be seen to extend along the crystallographic a axis (Fig. 3), acting to link the one-dimensional strips into two-dimensional sheets. The N2_amine—H12⋯N1_pyrⁱ [symmetry code (i) −x + 1, y + $[{1\over 2}]$ , −z + $[{3\over 2}]$ ] contact exhibits a donor–acceptor distance 3.264 (3) Å. The π-stacking is characterized by a centroid-to-centroid distance of 3.756 (1) Å, plane-to-plane distances of 3.414 (2) Å and a ring offset of 1.568 (3) Å (Hunter & Saunders, 1990 ; Lueckheide et al., 2013 ). Alternatively, the three hydrogen-bonding contacts and the π-stacking taken together can be seen to form a spiral of 5-chloropyridine-2,3-diamine (I) molecules extending along the a-axis direction (Fig. 4).

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
N2—H12⋯N1ⁱ	0.86 (2)	2.48 (3)	3.264 (3)	151 (3)
N3—H21⋯N2ⁱⁱ	0.89 (2)	2.38 (2)	3.250 (4)	166 (3)
N3—H22⋯N1ⁱⁱⁱ	0.90 (2)	2.19 (2)	3.075 (4)	167 (3)
Symmetry codes: (i) $[-x+1, y+{\script{1\over 2}}, -z+{\script{3\over 2}}]$ ; (ii) $[-x+2, y-{\script{1\over 2}}, -z+{\script{3\over 2}}]$ ; (iii) $[-x+2, y+{\script{1\over 2}}, -z+{\script{3\over 2}}]$ .

Figure 2
A view of the intermolecular N3_amine—H21⋯N2ⁱⁱ_amine and N3_amine—H22⋯N1ⁱⁱⁱ_pyr one-dimensional hydrogen bonding in 5-chloropyridine-2,3-diamine (I)

. [Symmetry codes: (ii) −x + 2, y − $[{1\over 2}]$ , −z + $[{3\over 2}]$ ; (iii) −x + 2, y + $[{1\over 2}]$ , −z + $[{3\over 2}]$ .]

Figure 3
A view of the packing in 5-chloropyridine-2,3-diamine (I)

indicating hydrogen bonding connecting the one-dimensional strips into two-dimensional sheets along with offset face-to-face π-stacking.

Figure 4
A view of the spiral hydrogen-bonded chain in 5-chloropyridine-2,3-diamine (I)

highlighting the N2_amine—H12⋯N1ⁱ_pyr contact. [Symmetry code: (i) −x + 1, y + $[{1\over 2}]$ , −z + $[{3\over 2}]$ .]

4. Database survey

The Cambridge Structural Database (Groom et al., 2016 ) contains about fifty structurally similar compounds to 5-chloropyridine-2,3-diamine (I), with 2-amino-5-chloropyridine (AMCLPY12) (Pourayoubi et al., 2007 ) and 2-amino-3-chloropyridine (URAXER) (Hu et al., 2011 ) being the most chemically and structurally similar. The C—Cl bond length in the title compound, with distance 1.748 (3) Å, is comparable to those in 2-amino-5-chloropyridine (AMCLPY12) and 2-amino-3-chloropyridine (URAXER), with distances 1.7404 (14) and 1.735 (3) Å, respectively. The C—N_amine distances in the title compound, 1.406 (4) and 1.385 (4) Å, however, are somewhat longer than in 2-amino-5-chloropyridine (AMCLPY12) [1.3602 (19) Å] and 2-amino-3-chloropyridine (URAXER) [1.351 (4) Å]. 2-amino-5-chloropyridine (AMCLPY12), which does not have the meta-NH₂ substitution of the title compound, packs in a herringbone formation featuring centrosymmetric head-to-tail N_amine—H⋯N_pyr hydrogen bonding dimers with donor–acceptor distance 3.031 (2) Å. 2-Amino-3-chloropyridine (URAXER), has a meta-Cl substitution in place of the meta-NH₂ in the title compound. Like 2-amino-5-chloropyridine (AMCLPY12), 2-amino-3-chloropyridine (URAXER) features a herringbone packing with centrosymmetric head-to-tail N_amine—H⋯N_pyr hydrogen-bonded dimer with a similar donor–acceptor distance of 3.051 (5) Å. The similar hydrogen-bonding motif in these two related compounds differs from the title compound, which does not exhibit centrosymmetric hydrogen-bonding dimerization. 2-Amino-3-chloropyridine (URAXER) also has short intermolecular Cl⋯Cl interactions of 3.278 (3) Å, where no such short halogen–halogen contact was observed in 2-amino-5-chloropyridine (AMCLPY12) or the title compound.

5. Synthesis and crystallization

5-Chloropyridine-2,3-diamine (97%) was purchased from Aldrich Chemical Company, USA. A single crystal suitable for analysis was selected from the purchased sample and used as received.

6. Analytical data

¹H NMR (Bruker Avance 400 MHz, DMSO d₆): δ 4.99 (br s, 2 H, NH₂), 5.55 (br s, 2 H, NH₂), 6.69 (d, 1 H, J = 2.3 Hz, C_arylH), 7.21 (d, 1 H, J = 2.3 Hz, C_arylH). ¹³C NMR (¹³C{¹H}, 100.6 MHz, DMSO d₆): δ 116.58 (C_arylH), 118.38 (C_aryl), 131.32 (C_aryl), 131.66 (C_arylH), 147.10 (C_aryl). IR (Thermo Nicolet iS50, ATR, cm⁻¹): 3392 (m, N—H str), 3309 (m, N—H str), 3172 (m, aryl C—H str), 1637 (s, aryl C=C str), 1572 (m), 1472 (s), 1421 (m), 1347 (w), 1307 (w), 1280 (w), 1240 (m), 1068 (m), 939 (w), 887 (w), 861 (m), 792 (m), 770 (m), 680 (s), 630 (s), 568 (s), 490 (s), 449 (s). GC/MS (Hewlett-Packard MS 5975/GC 7890): M⁺ = 143 (calc. exact mass = 143.03).

7. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms on carbon were included in calculated positions and refined using a riding model with C—H = 0.95 Å and U_iso(H) = 1.2U_eq(C) of the aryl C-atoms. The positions of the four amino hydrogen atoms were found in the difference map and they were refined semi-freely using a distance restraint d(N—H) = 0.91 Å, and U_iso(H) = 1.2U_eq(N).

Table 2
Experimental details

Crystal data
Chemical formula	C₅H₆ClN₃
M_r	143.58
Crystal system, space group	Orthorhombic, P2₁2₁2₁
Temperature (K)	125
a, b, c (Å)	3.7565 (8), 8.7002 (17), 18.350 (4)
V (Å³)	599.7 (2)
Z	4
Radiation type	Mo Kα
μ (mm⁻¹)	0.53
Crystal size (mm)	0.10 × 0.05 × 0.04

Data collection
Diffractometer	Bruker APEXII CCD
Absorption correction	Multi-scan (SADABS; Bruker, 2013 )
T_min, T_max	0.75, 0.98
No. of measured, independent and observed [I > 2σ(I)] reflections	14989, 1845, 1477
R_int	0.087
(sin θ/λ)_max (Å⁻¹)	0.714

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.041, 0.084, 1.07
No. of reflections	1845
No. of parameters	94
No. of restraints	4
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement
Δρ_max, Δρ_min (e Å⁻³)	0.37, −0.38
Absolute structure	Flack x determined using 511 quotients [(I⁺)−(I⁻)]/[(I⁺)+(I⁻)] (Parsons et al., 2013 )
Absolute structure parameter	0.01 (6)
Computer programs: APEX2 and SAINT (Bruker, 2013), SHELXT2014 (Sheldrick, 2015a ), SHELXL2014 (Sheldrick, 2015b ), SHELXTL2014 (Sheldrick, 2008 ), OLEX2 (Dolomanov et al., 2009 ) and Mercury (Macrae et al., 2008 ).

Supporting information

CCDC reference: 1562267

Crystal structure: contains datablocks global, I. DOI: https://doi.org/10.1107/S2056989017010489/fy2122sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2056989017010489/fy2122Isup2.hkl

Supporting information file. DOI: https://doi.org/10.1107/S2056989017010489/fy2122Isup3.cml

checkCIF report

Computing details top

Data collection: APEX2 (Bruker, 2013); cell refinement: SAINT (Bruker, 2013); data reduction: SAINT (Bruker, 2013); program(s) used to solve structure: SHELXT2014 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015b); molecular graphics: SHELXTL2014 (Sheldrick, 2008); software used to prepare material for publication: SHELXTL2014 (Sheldrick, 2008), OLEX2 (Dolomanov et al., 2009) and Mercury (Macrae et al., 2008).

5-Chloropyridine-2,3-diamine top

Crystal data top

C₅H₆ClN₃	D_x = 1.590 Mg m⁻³
M_r = 143.58	Mo Kα radiation, λ = 0.71073 Å
Orthorhombic, P2₁2₁2₁	Cell parameters from 4306 reflections
a = 3.7565 (8) Å	θ = 2.6–29.7°
b = 8.7002 (17) Å	µ = 0.53 mm⁻¹
c = 18.350 (4) Å	T = 125 K
V = 599.7 (2) Å³	Plate, colourless
Z = 4	0.10 × 0.05 × 0.04 mm
F(000) = 296

Data collection top

Bruker APEXII CCD diffractometer	1845 independent reflections
Radiation source: fine-focus sealed tube	1477 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.087
Detector resolution: 8.3333 pixels mm^-1	θ_max = 30.5°, θ_min = 2.2°
φ and ω scans	h = −5→5
Absorption correction: multi-scan (SADABS; Bruker, 2013)	k = −12→12
T_min = 0.75, T_max = 0.98	l = −26→26
14989 measured reflections

Refinement top

Refinement on F²	Hydrogen site location: mixed
Least-squares matrix: full	H atoms treated by a mixture of independent and constrained refinement
R[F² > 2σ(F²)] = 0.041	w = 1/[σ²(F_o²) + (0.0308P)² + 0.2158P] where P = (F_o² + 2F_c²)/3
wR(F²) = 0.084	(Δ/σ)_max < 0.001
S = 1.07	Δρ_max = 0.37 e Å⁻³
1845 reflections	Δρ_min = −0.38 e Å⁻³
94 parameters	Absolute structure: Flack x determined using 511 quotients [(I⁺)-(I^-)]/[(I⁺)+(I^-)] (Parsons et al., 2013)
4 restraints	Absolute structure parameter: 0.01 (6)

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
Cl1	0.5035 (2)	0.46707 (8)	0.47194 (3)	0.01872 (16)
N1	0.8538 (6)	0.4647 (3)	0.67658 (12)	0.0148 (5)
N2	0.5894 (7)	0.8717 (3)	0.67694 (14)	0.0153 (5)
H11	0.464 (9)	0.928 (3)	0.6466 (14)	0.018*
H12	0.530 (10)	0.872 (3)	0.7224 (12)	0.018*
N3	0.8976 (7)	0.6490 (3)	0.76777 (13)	0.0154 (5)
H21	1.032 (9)	0.580 (3)	0.7905 (15)	0.018*
H22	0.955 (10)	0.748 (3)	0.7772 (16)	0.018*
C1	0.7608 (8)	0.4231 (3)	0.60820 (16)	0.0157 (6)
H1	0.7977	0.3199	0.5931	0.019*
C2	0.6148 (7)	0.5258 (4)	0.56011 (14)	0.0138 (5)
C3	0.5497 (7)	0.6759 (3)	0.58152 (14)	0.0121 (6)
H3	0.445	0.747	0.5486	0.015*
C4	0.6388 (7)	0.7209 (3)	0.65122 (15)	0.0122 (6)
C5	0.8035 (7)	0.6100 (3)	0.69716 (15)	0.0125 (6)

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Cl1	0.0204 (3)	0.0220 (3)	0.0138 (3)	−0.0021 (4)	−0.0034 (3)	−0.0035 (3)
N1	0.0154 (11)	0.0144 (11)	0.0145 (11)	0.0014 (11)	−0.0007 (9)	−0.0001 (11)
N2	0.0182 (14)	0.0137 (12)	0.0140 (11)	0.0036 (9)	−0.0003 (9)	0.0000 (9)
N3	0.0162 (14)	0.0152 (12)	0.0148 (11)	−0.0009 (10)	−0.0036 (9)	0.0004 (10)
C1	0.0176 (15)	0.0126 (14)	0.0171 (14)	0.0008 (11)	0.0014 (12)	−0.0019 (11)
C2	0.0101 (12)	0.0202 (13)	0.0112 (11)	−0.0036 (12)	0.0002 (9)	−0.0016 (12)
C3	0.0061 (14)	0.0157 (12)	0.0146 (12)	−0.0022 (10)	0.0009 (10)	0.0040 (10)
C4	0.0064 (12)	0.0142 (14)	0.0160 (14)	−0.0003 (10)	0.0026 (11)	0.0004 (11)
C5	0.0066 (13)	0.0184 (15)	0.0125 (13)	−0.0017 (11)	0.0021 (10)	0.0011 (11)

Geometric parameters (Å, º) top

Cl1—C2	1.748 (3)	N3—H22	0.90 (2)
N1—C5	1.333 (4)	C1—C2	1.371 (4)
N1—C1	1.352 (4)	C1—H1	0.95
N2—C4	1.406 (4)	C2—C3	1.385 (4)
N2—H11	0.88 (2)	C3—C4	1.379 (4)
N2—H12	0.86 (2)	C3—H3	0.95
N3—C5	1.385 (4)	C4—C5	1.423 (4)
N3—H21	0.89 (2)

C5—N1—C1	118.7 (3)	C1—C2—Cl1	120.1 (2)
C4—N2—H11	112 (2)	C3—C2—Cl1	119.7 (2)
C4—N2—H12	111 (2)	C4—C3—C2	119.2 (3)
H11—N2—H12	118 (3)	C4—C3—H3	120.4
C5—N3—H21	115 (2)	C2—C3—H3	120.4
C5—N3—H22	118 (2)	C3—C4—N2	123.0 (3)
H21—N3—H22	114 (3)	C3—C4—C5	117.5 (3)
N1—C1—C2	121.8 (3)	N2—C4—C5	119.4 (3)
N1—C1—H1	119.1	N1—C5—N3	117.5 (3)
C2—C1—H1	119.1	N1—C5—C4	122.5 (2)
C1—C2—C3	120.2 (3)	N3—C5—C4	119.9 (3)

C5—N1—C1—C2	−0.5 (4)	C1—N1—C5—N3	179.3 (2)
N1—C1—C2—C3	−1.7 (4)	C1—N1—C5—C4	3.3 (4)
N1—C1—C2—Cl1	179.3 (2)	C3—C4—C5—N1	−4.0 (4)
C1—C2—C3—C4	1.0 (4)	N2—C4—C5—N1	179.0 (3)
Cl1—C2—C3—C4	−180.0 (2)	C3—C4—C5—N3	−179.8 (3)
C2—C3—C4—N2	178.5 (3)	N2—C4—C5—N3	3.2 (4)
C2—C3—C4—C5	1.7 (4)

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
N2—H12···N1ⁱ	0.86 (2)	2.48 (3)	3.264 (3)	151 (3)
N3—H21···N2ⁱⁱ	0.89 (2)	2.38 (2)	3.250 (4)	166 (3)
N3—H22···N1ⁱⁱⁱ	0.90 (2)	2.19 (2)	3.075 (4)	167 (3)

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

Acknowledgements

This work was supported by Vassar College. X-ray facilities were provided by the US National Science Foundation (grants Nos. 0521237 and 0911324 to JMT). NMR facilities were provided by the US National Science Foundation (grant No. 1526982 to JMT and TG). We acknowledge the Salmon Fund of Vassar College for funding publication expenses.

Funding information

Funding for this research was provided by: National Science Foundation, Directorate for Mathematical and Physical Sciences (grant No. 0521237 to Joseph M. Tanski); National Science Foundation, Directorate for Mathematical and Physical Sciences (grant No. 0911324 to Joseph M. Tanski); National Science Foundation, Directorate for Mathematical and Physical Sciences (grant No. 1526982 to Joseph M. Tanski, Teresa A. Garrett).

References

Baladi, T., Granzhan, A. & Piguel, S. (2016). Eur. J. Org. Chem. pp. 2421–2434. CrossRef Google Scholar
Bruker (2013). SAINT, SADABS and APEX2. Bruxer AXS Inc., Madison, Wisconsin, USA. Google Scholar
Dolomanov, O. V., Bourhis, L. J., Gildea, R. J., Howard, J. A. K. & Puschmann, H. (2009). J. Appl. Cryst. 42, 339–341. Web of Science CrossRef CAS IUCr Journals Google Scholar
Groom, C. R., Bruno, I. J., Lightfoot, M. P. & Ward, S. C. (2016). Acta Cryst. B72, 171–179. Web of Science CSD CrossRef IUCr Journals Google Scholar
Han, Z., Hao, X., Ma, B. & Zhu, C. (2016). Eur. J. Med. Chem. 121, 308–317. CrossRef CAS PubMed Google Scholar
Hu, Z.-N., Yang, H.-B., Luo, H. & Li, B. (2011). Acta Cryst. E67, o1138. Web of Science CSD CrossRef IUCr Journals Google Scholar
Hunter, C. A. & Sanders, J. K. M. (1990). J. Am. Chem. Soc. 112, 5525–5534. CrossRef CAS Web of Science Google Scholar
Israel, M. & Day, A. R. (1959). J. Org. Chem. 24, 1455–1460. CrossRef CAS Google Scholar
Los, R., Wesołowska-Trojanowska, M., Malm, A., Karpińska, M. M., Matysiak, J., Niewiadomy, A. & Głaszcz, U. (2012). Heteroat. Chem. 23, 265–275. CrossRef CAS Google Scholar
Lueckheide, M., Rothman, N., Ko, B. & Tanski, J. M. (2013). Polyhedron, 58, 79–84. Web of Science CSD 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
Pourayoubi, M., Ghadimi, S. & Ebrahimi Valmoozi, A. A. (2007). Acta Cryst. E63, o4631. Web of Science CSD CrossRef IUCr Journals Google Scholar
Püsküllü, M. O., Karaaslan, C., Bakar, F. & Göker, H. (2015). Chem. Heterocycl. Compd. 51, 723–733. Google Scholar
Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122. Web of Science 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
Taha, M., Ismail, N. H., Imran, S., Rashwan, H., Jamil, W., Ali, S., Kashif, S. M., Rahim, F., Salar, U. & Khan, K. M. (2016). Bioorg. Chem. 65, 48–56. CrossRef CAS PubMed Google Scholar
Xie, D., Lu, J., Xie, J., Cui, J., Li, T.-F., Wang, Y.-C., Chen, Y., Gong, N., Li, X.-Y., Fu, L. & Wang, Y.-X. (2016). Eur. J. Med. Chem. 117, 19–32. CrossRef CAS PubMed Google Scholar

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