7-Bromo-1,4-dibutyl-1,2,3,4-tetra­hydro­pyrido[2,3-b]pyrazine-2,3-dione

Hjouji, M.Y.; Mague, J.T.; Kandri Rodi, Y.; Ouzidan, Y.; Ouazzani Chahdi, F.; Essassi, E.M.

doi:10.1107/S2414314617009841

organic compounds

IUCrDATA

ISSN: 2414-3146

Volume 2| Part 7| July 2017| x170984

https://doi.org/10.1107/S2414314617009841

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access

7-Bromo-1,4-dibutyl-1,2,3,4-tetrahydropyrido[2,3-b]pyrazine-2,3-dione

Mohammed Yassin Hjouji,^a Joel T. Mague,^b Youssef Kandri Rodi,^a Younes Ouzidan,^a ^* Fouad Ouazzani Chahdi ^a and El Mokhtar Essassi ^c

^aLaboratoire de Chimie Organique Appliquée, Université Sidi Mohamed Ben Abdallah, Faculté des Sciences et Techniques, Route d'Imouzzer, BP 2202, Fez, Morocco, ^bDepartment of Chemistry, Tulane University, New Orleans, LA 70118, USA, and ^cLaboratoire de Chimie Organique Hétérocyclique URAC 21, Pôle de, Compétence Pharmacochimie, Av. Ibn Battouta, BP 1014, Faculté des Sciences, Université Mohammed V, Rabat, Morocco
^*Correspondence e-mail: younes.ouzidan@usmba.ac.ma

Edited by O. Blacque, University of Zürich, Switzerland (Received 30 June 2017; accepted 3 July 2017; online 7 July 2017)

In the title compound, C₁₅H₂₀BrN₃O₂, the butyl substituents are in extended conformations on opposite sides of the bicyclic core. In the crystal, oblique stacks of molecules, formed by offset π-stacking interactions between pyridine and pyrazine rings in adjacent molecules, extend along the b-axis direction. The stacks are associated through a combination of C—H⋯O hydrogen bonds and C—Br⋯π(ring) interactions.

Keywords: crystal structure; pyridopyrazine; hydrogen bond; π-stacking.

CCDC reference: 1559996

Structure description

Pyrido-pyrazine derivatives are a versatile class of nitrogen-containing heterocyclic compounds and they constitute useful intermediates in organic synthesis and medicinal chemistry. They possess a broad spectrum of biological activities including anti-cancer (Gong et al., 2011 ), anti-inflammatory (Hodgetts et al., 2010 ) and antimalarial (Richter et al., 2006 ). They are also used as inhibitors of anaplastic lymphoma kinase (Milkiewicz et al., 2010 ). As a continuation of our research in the field of substituted pyrido[2,3-b]pyrazine derivatives (Hjouji et al., 2014 ), we report here the synthesis of the title compound by the condensation of butyl bromide and 7-bromopyrido[2,3-b]pyrazine-2,3(1H,4H)-dione.

In the title molecule, the n-butyl substituents are both in extended conformations with one extending above and the other below the pyrazine ring (Fig. 1). The bicyclic core is planar within experimental error.

Figure 1
The title molecule with labeling scheme and 50% probability ellipsoids.

In the crystal, the molecules form oblique stacks along the b-axis direction through offset π-stacking interactions between the pyridine portion of one molecule and the pyrazine portion of the next (Fig. 2). In the stack, the centroid–centroid distance of the respective rings is 3.695 (5) Å and they are parallel within experimental error. Assisting these interactions in forming the stacks are π interactions between the C7O2 carbonyl group and the pyridine ring in adjacent molecules (Fig. 2) where the distance between the mid-point of the double bond and the ring centroid is 3.268 (8) Å. Finally, the stacks are associated through a combination of C2—H2⋯O1 and C12—H12A⋯O1 hydrogen bonds (Table 1 and Fig. 3) and C3—Br1⋯π(ring)ⁱ interactions (Fig. 2) where Br1⋯Cg2 = 3.701 (4) Å, C3—Br1⋯Cg2 = 118.1 (3)° and Cg2 is the centroid of the pyrazine portion of the molecule with symmetry code (i) $[{1\over 2}]$ − x, − $[{1\over 2}]$ + y, − $[{1\over 2}]$ + z).

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
C2—H2⋯O1ⁱ	0.95	2.33	3.253 (12)	164
C12—H12A⋯O1ⁱ	0.99	2.55	3.447 (11)	150
Symmetry code: (i) $[-x+{\script{1\over 2}}, y-{\script{1\over 2}}, z-{\script{1\over 2}}]$ .

Figure 2
Detail of the intermolecular interactions (dashed lines), viewed along the a-axis direction (C—H⋯O hydrogen bonds (black), π-stacking (purple), C=O⋯π(ring) (green), C—Br⋯π(ring) (orange)).

Figure 3
Packing viewed along the b-axis direction, with C—H⋯O hydrogen bonds shown as black dashed lines.

Synthesis and crystallization

Butyl bromide (0.2 ml, 1.82 mmol) was added to a solution of 7-bromopyrido[2,3-b]pyrazine-2,3(1H,4H)-dione (0.2 g, 0.83 mmol), K₂CO₃ (0.28 g, 2.07 mmol) and tetra-n-butyl ammonium bromide (0.03 g, 0.1 mmol) in DMF (10 ml). The mixture was then stirred for 6 h at room temperature. The solvent was evaporated under reduced pressure and the product isolated by chromatography on a silica gel column with ethyl acetate/hexane (1/2) as eluent. The compound forms pale-blue plate-shaped crystals in 77% yield and was recrystallized from a solvent mixture (ethanol/dichloromethane: 2/1).

Refinement

Crystal and refinement details are presented in Table 2. The hydrogen atoms were included as riding contributions in idealized positions.

Table 2
Experimental details

Crystal data
Chemical formula	C₁₅H₂₀BrN₃O₂
M_r	354.25
Crystal system, space group	Orthorhombic, Pna2₁
Temperature (K)	100
a, b, c (Å)	22.569 (6), 5.1097 (13), 13.497 (3)
V (Å³)	1556.5 (7)
Z	4
Radiation type	Mo Kα
μ (mm⁻¹)	2.65
Crystal size (mm)	0.22 × 0.21 × 0.02

Data collection
Diffractometer	Bruker SMART APEX CCD
Absorption correction	Multi-scan (SADABS; Bruker, 2016 )
T_min, T_max	0.50, 0.96
No. of measured, independent and observed [I > 2σ(I)] reflections	13413, 3859, 2916
R_int	0.091
(sin θ/λ)_max (Å⁻¹)	0.676

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.068, 0.156, 1.06
No. of reflections	3859
No. of parameters	192
No. of restraints	94
H-atom treatment	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	2.33, −1.38
Absolute structure	Flack x determined using 1091 quotients [(I⁺)−(I⁻)]/[(I⁺)+(I⁻)] (Parsons et al., 2013 )
Absolute structure parameter	0.038 (15)
Computer programs: APEX3 and SAINT (Bruker, 2016), SHELXT (Sheldrick, 2015a ), SHELXL2014 (Sheldrick, 2015b ), DIAMOND (Brandenburg & Putz, 2012 ) and SHELXTL (Sheldrick, 2008 ).

Structural data

CCDC reference: 1559996

Crystal structure: contains datablock I. DOI: https://doi.org/10.1107/S2414314617009841/zq4022sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2414314617009841/zq4022Isup2.hkl

Supporting information file. DOI: https://doi.org/10.1107/S2414314617009841/zq4022Isup3.cdx

Supporting information file. DOI: https://doi.org/10.1107/S2414314617009841/zq4022Isup4.cml

checkCIF report

Computing details top

Data collection: APEX3 (Bruker, 2016); cell refinement: SAINT (Bruker, 2016); data reduction: SAINT (Bruker, 2016); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015b); molecular graphics: DIAMOND (Brandenburg & Putz, 2012); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

7-Bromo-1,4-dibutyl-1,2,3,4-tetrahydropyrido[2,3-b]pyrazine-2,3-dione top

Crystal data top

C₁₅H₂₀BrN₃O₂	D_x = 1.512 Mg m⁻³
M_r = 354.25	Mo Kα radiation, λ = 0.71073 Å
Orthorhombic, Pna2₁	Cell parameters from 3677 reflections
a = 22.569 (6) Å	θ = 2.4–26.1°
b = 5.1097 (13) Å	µ = 2.65 mm⁻¹
c = 13.497 (3) Å	T = 100 K
V = 1556.5 (7) Å³	Plate, pale blue
Z = 4	0.22 × 0.21 × 0.02 mm
F(000) = 728

Data collection top

Bruker SMART APEX CCD diffractometer	3859 independent reflections
Radiation source: fine-focus sealed tube	2916 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.091
Detector resolution: 8.3333 pixels mm^-1	θ_max = 28.7°, θ_min = 1.8°
ω scans	h = −29→29
Absorption correction: multi-scan (SADABS; Bruker, 2016)	k = −6→6
T_min = 0.50, T_max = 0.96	l = −18→17
13413 measured 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)] = 0.068	H-atom parameters constrained
wR(F²) = 0.156	w = 1/[σ²(F_o²) + 6.1176P] where P = (F_o² + 2F_c²)/3
S = 1.06	(Δ/σ)_max < 0.001
3859 reflections	Δρ_max = 2.33 e Å⁻³
192 parameters	Δρ_min = −1.38 e Å⁻³
94 restraints	Absolute structure: Flack x determined using 1091 quotients [(I⁺)-(I^-)]/[(I⁺)+(I^-)] (Parsons et al., 2013)
Primary atom site location: structure-invariant direct methods	Absolute structure parameter: 0.038 (15)

Special details top

Experimental. The diffraction data were collected in three sets of 363 frames (0.5° width in ω) at φ = 0, 120 and 240°. A scan time of 80 sec/frame was used.

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.

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 > 2sigma(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. H-atoms attached to carbon were placed in calculated positions (C—H = 0.95 - 0.99 Å). All were included as riding contributions with isotropic displacement parameters 1.2 - 1.5 times those of the attached atoms.

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

	x	y	z	U_iso*/U_eq
Br1	0.30633 (4)	−0.15545 (15)	0.26770 (7)	0.0209 (2)
O1	0.3211 (3)	0.8535 (14)	0.7274 (5)	0.0249 (16)
O2	0.2233 (3)	0.9499 (14)	0.6151 (5)	0.0249 (16)
N1	0.3741 (4)	0.1719 (16)	0.5238 (6)	0.0203 (16)
N2	0.3449 (4)	0.5053 (15)	0.6305 (5)	0.0180 (16)
N3	0.2463 (4)	0.6115 (14)	0.5112 (6)	0.0158 (15)
C1	0.2854 (4)	0.4141 (18)	0.4835 (7)	0.0168 (18)
C2	0.2764 (5)	0.259 (2)	0.4013 (7)	0.0191 (19)
H2	0.2422	0.2821	0.3612	0.023*
C3	0.3178 (4)	0.0675 (19)	0.3780 (6)	0.0193 (19)
C4	0.3653 (4)	0.0215 (19)	0.4425 (6)	0.0184 (19)
H4	0.3920	−0.1180	0.4289	0.022*
C5	0.3348 (5)	0.3572 (18)	0.5437 (6)	0.0172 (17)
C6	0.3103 (4)	0.7138 (17)	0.6563 (7)	0.0168 (18)
C7	0.2560 (5)	0.7737 (18)	0.5913 (7)	0.0185 (18)
C8	0.3954 (4)	0.4332 (19)	0.6961 (7)	0.021 (2)
H8A	0.4020	0.2419	0.6924	0.026*
H8B	0.3854	0.4776	0.7655	0.026*
C9	0.4524 (5)	0.576 (2)	0.6662 (7)	0.024 (2)
H9A	0.4622	0.5355	0.5963	0.029*
H9B	0.4464	0.7678	0.6717	0.029*
C10	0.5036 (5)	0.493 (2)	0.7330 (8)	0.028 (2)
H10A	0.4924	0.5245	0.8029	0.033*
H10B	0.5103	0.3028	0.7248	0.033*
C11	0.5613 (5)	0.637 (2)	0.7113 (10)	0.036 (3)
H11A	0.5737	0.6012	0.6431	0.055*
H11B	0.5921	0.5770	0.7572	0.055*
H11C	0.5552	0.8256	0.7198	0.055*
C12	0.1912 (4)	0.661 (2)	0.4538 (7)	0.0200 (19)
H12A	0.1987	0.6253	0.3827	0.024*
H12B	0.1799	0.8473	0.4605	0.024*
C13	0.1406 (4)	0.4884 (19)	0.4902 (7)	0.021 (2)
H13A	0.1526	0.3026	0.4847	0.026*
H13B	0.1331	0.5262	0.5610	0.026*
C14	0.0842 (5)	0.530 (2)	0.4327 (7)	0.025 (2)
H14A	0.0908	0.4771	0.3630	0.031*
H14B	0.0743	0.7186	0.4331	0.031*
C15	0.0318 (5)	0.377 (2)	0.4738 (8)	0.030 (2)
H15A	−0.0025	0.3997	0.4302	0.046*
H15B	0.0221	0.4411	0.5403	0.046*
H15C	0.0420	0.1905	0.4774	0.046*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Br1	0.0338 (5)	0.0191 (4)	0.0099 (3)	0.0011 (4)	0.0012 (6)	−0.0072 (5)
O1	0.035 (4)	0.027 (4)	0.012 (3)	−0.005 (3)	0.002 (3)	−0.013 (3)
O2	0.035 (4)	0.020 (4)	0.020 (4)	0.003 (3)	0.004 (3)	−0.002 (3)
N1	0.024 (4)	0.022 (4)	0.015 (3)	0.002 (3)	−0.002 (3)	0.000 (3)
N2	0.029 (4)	0.016 (4)	0.009 (3)	−0.003 (3)	−0.001 (3)	−0.005 (3)
N3	0.025 (4)	0.012 (3)	0.010 (3)	−0.001 (3)	0.001 (3)	−0.003 (3)
C1	0.023 (4)	0.016 (4)	0.011 (4)	0.001 (3)	0.002 (3)	−0.002 (3)
C2	0.028 (5)	0.019 (4)	0.010 (4)	−0.001 (4)	0.000 (4)	0.000 (3)
C3	0.030 (5)	0.021 (4)	0.007 (4)	−0.001 (3)	0.004 (3)	−0.002 (3)
C4	0.028 (5)	0.018 (4)	0.010 (4)	0.001 (4)	0.001 (3)	−0.002 (3)
C5	0.025 (4)	0.018 (4)	0.009 (3)	0.000 (3)	0.000 (3)	−0.002 (3)
C6	0.029 (4)	0.010 (4)	0.011 (3)	−0.004 (3)	0.001 (3)	−0.002 (3)
C7	0.029 (5)	0.018 (4)	0.009 (3)	−0.001 (3)	0.001 (3)	0.000 (3)
C8	0.030 (6)	0.025 (5)	0.009 (4)	−0.002 (4)	−0.002 (4)	−0.005 (4)
C9	0.031 (6)	0.023 (5)	0.017 (5)	0.000 (4)	−0.002 (4)	0.001 (4)
C10	0.036 (6)	0.024 (5)	0.023 (5)	−0.001 (4)	−0.005 (4)	−0.001 (4)
C11	0.031 (6)	0.031 (6)	0.047 (7)	0.001 (5)	−0.009 (5)	0.003 (5)
C12	0.023 (5)	0.025 (5)	0.013 (4)	0.002 (4)	−0.005 (4)	−0.001 (4)
C13	0.026 (6)	0.022 (5)	0.016 (5)	0.005 (4)	0.000 (4)	0.006 (4)
C14	0.034 (6)	0.025 (5)	0.017 (5)	0.002 (5)	0.001 (4)	−0.002 (4)
C15	0.031 (6)	0.036 (6)	0.024 (5)	−0.003 (5)	0.001 (5)	−0.006 (5)

Geometric parameters (Å, º) top

Br1—C3	1.892 (9)	C9—C10	1.527 (15)
O1—C6	1.221 (11)	C9—H9A	0.9900
O2—C7	1.207 (12)	C9—H9B	0.9900
N1—C5	1.325 (12)	C10—C11	1.523 (15)
N1—C4	1.354 (12)	C10—H10A	0.9900
N2—C6	1.365 (12)	C10—H10B	0.9900
N2—C5	1.414 (11)	C11—H11A	0.9800
N2—C8	1.489 (12)	C11—H11B	0.9800
N3—C7	1.379 (12)	C11—H11C	0.9800
N3—C1	1.392 (11)	C12—C13	1.525 (14)
N3—C12	1.487 (12)	C12—H12A	0.9900
C1—C2	1.380 (13)	C12—H12B	0.9900
C1—C5	1.408 (14)	C13—C14	1.506 (14)
C2—C3	1.388 (14)	C13—H13A	0.9900
C2—H2	0.9500	C13—H13B	0.9900
C3—C4	1.402 (14)	C14—C15	1.523 (15)
C4—H4	0.9500	C14—H14A	0.9900
C6—C7	1.537 (14)	C14—H14B	0.9900
C8—C9	1.535 (14)	C15—H15A	0.9800
C8—H8A	0.9900	C15—H15B	0.9800
C8—H8B	0.9900	C15—H15C	0.9800

C5—N1—C4	118.2 (8)	C8—C9—H9B	109.6
C6—N2—C5	122.4 (8)	H9A—C9—H9B	108.1
C6—N2—C8	118.7 (7)	C11—C10—C9	113.5 (9)
C5—N2—C8	118.9 (8)	C11—C10—H10A	108.9
C7—N3—C1	123.1 (8)	C9—C10—H10A	108.9
C7—N3—C12	116.0 (8)	C11—C10—H10B	108.9
C1—N3—C12	120.9 (8)	C9—C10—H10B	108.9
C2—C1—N3	122.6 (9)	H10A—C10—H10B	107.7
C2—C1—C5	117.5 (9)	C10—C11—H11A	109.5
N3—C1—C5	119.7 (8)	C10—C11—H11B	109.5
C1—C2—C3	119.2 (9)	H11A—C11—H11B	109.5
C1—C2—H2	120.4	C10—C11—H11C	109.5
C3—C2—H2	120.4	H11A—C11—H11C	109.5
C2—C3—C4	119.5 (9)	H11B—C11—H11C	109.5
C2—C3—Br1	120.6 (7)	N3—C12—C13	111.1 (8)
C4—C3—Br1	119.5 (7)	N3—C12—H12A	109.4
N1—C4—C3	121.3 (9)	C13—C12—H12A	109.4
N1—C4—H4	119.3	N3—C12—H12B	109.4
C3—C4—H4	119.3	C13—C12—H12B	109.4
N1—C5—C1	124.0 (8)	H12A—C12—H12B	108.0
N1—C5—N2	116.3 (8)	C14—C13—C12	112.7 (8)
C1—C5—N2	119.7 (8)	C14—C13—H13A	109.0
O1—C6—N2	122.8 (9)	C12—C13—H13A	109.0
O1—C6—C7	119.4 (8)	C14—C13—H13B	109.0
N2—C6—C7	117.8 (8)	C12—C13—H13B	109.0
O2—C7—N3	124.0 (9)	H13A—C13—H13B	107.8
O2—C7—C6	118.9 (8)	C13—C14—C15	113.4 (9)
N3—C7—C6	117.0 (8)	C13—C14—H14A	108.9
N2—C8—C9	111.6 (8)	C15—C14—H14A	108.9
N2—C8—H8A	109.3	C13—C14—H14B	108.9
C9—C8—H8A	109.3	C15—C14—H14B	108.9
N2—C8—H8B	109.3	H14A—C14—H14B	107.7
C9—C8—H8B	109.3	C14—C15—H15A	109.5
H8A—C8—H8B	108.0	C14—C15—H15B	109.5
C10—C9—C8	110.3 (8)	H15A—C15—H15B	109.5
C10—C9—H9A	109.6	C14—C15—H15C	109.5
C8—C9—H9A	109.6	H15A—C15—H15C	109.5
C10—C9—H9B	109.6	H15B—C15—H15C	109.5

C7—N3—C1—C2	−177.7 (9)	C5—N2—C6—O1	−174.9 (9)
C12—N3—C1—C2	1.5 (14)	C8—N2—C6—O1	4.6 (14)
C7—N3—C1—C5	6.0 (14)	C5—N2—C6—C7	4.5 (13)
C12—N3—C1—C5	−174.8 (9)	C8—N2—C6—C7	−176.1 (8)
N3—C1—C2—C3	179.3 (9)	C1—N3—C7—O2	178.2 (9)
C5—C1—C2—C3	−4.3 (14)	C12—N3—C7—O2	−1.1 (14)
C1—C2—C3—C4	5.2 (15)	C1—N3—C7—C6	−5.1 (13)
C1—C2—C3—Br1	178.5 (7)	C12—N3—C7—C6	175.7 (8)
C5—N1—C4—C3	3.2 (13)	O1—C6—C7—O2	−3.9 (14)
C2—C3—C4—N1	−4.7 (14)	N2—C6—C7—O2	176.8 (9)
Br1—C3—C4—N1	−178.1 (7)	O1—C6—C7—N3	179.2 (9)
C4—N1—C5—C1	−2.4 (14)	N2—C6—C7—N3	−0.1 (12)
C4—N1—C5—N2	178.5 (8)	C6—N2—C8—C9	−89.8 (10)
C2—C1—C5—N1	3.0 (15)	C5—N2—C8—C9	89.6 (10)
N3—C1—C5—N1	179.5 (9)	N2—C8—C9—C10	−178.6 (8)
C2—C1—C5—N2	−177.9 (9)	C8—C9—C10—C11	−177.1 (9)
N3—C1—C5—N2	−1.4 (14)	C7—N3—C12—C13	−93.8 (10)
C6—N2—C5—N1	175.3 (8)	C1—N3—C12—C13	87.0 (10)
C8—N2—C5—N1	−4.1 (12)	N3—C12—C13—C14	−179.1 (8)
C6—N2—C5—C1	−3.8 (13)	C12—C13—C14—C15	−175.0 (8)
C8—N2—C5—C1	176.8 (9)

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
C2—H2···O1ⁱ	0.95	2.33	3.253 (12)	164
C12—H12A···O1ⁱ	0.99	2.55	3.447 (11)	150

Symmetry code: (i) −x+1/2, y−1/2, z−1/2.

Acknowledgements

JTM thanks Tulane University for support of the Tulane Crystallography Laboratory.

References

Brandenburg, K. & Putz, H. (2012). DIAMOND. Crystal Impact GbR, Bonn, Germany. Google Scholar
Bruker (2016). APEX3, SADABS and SAINT. Bruker AXS Inc., Madison, Wisconsin, USA. Google Scholar
Gong, Y. D., Dong, M. S., Lee, S. B., Kim, N., Bae, M. S. & Kang, N. S. (2011). Bioorg. Med. Chem. 19, 5639–5647. Web of Science CrossRef CAS PubMed Google Scholar
Hjouji, M. Y., Kandri Rodi, Y., Misbahi, K., Ouazzani Chahdi, F., Akhazzane, M. & Essassi, E. M. (2014). J. Mar. Chim. Heterocycl, 13, 65–71. Google Scholar
Hodgetts, K. J., Blum, C. A., Caldwell, T., Bakthavatchalam, R., Zheng, X., Capitosti, S., Krause, J. E., Cortright, D., Crandall, M., Murphy, B. A., Boyce, S., Jones, A. B. & Chenard, B. L. (2010). Bioorg. Med. Chem. Lett. 20, 4359–4363. Web of Science CrossRef CAS PubMed Google Scholar
Milkiewicz, K. L., Weinberg, L. R., Albom, M. S., Angeles, T. S., Cheng, M., Ghose, A. K., Roemmele, R. C., Theroff, J. P., Underiner, T. L., Zificsak, C. A. & Dorsey, B. D. (2010). Bioorg. Med. Chem. 18, 4351–4362. Web of Science CrossRef CAS PubMed Google Scholar
Parsons, S., Flack, H. D. & Wagner, T. (2013). Acta Cryst. B69, 249–259. Web of Science CrossRef CAS IUCr Journals Google Scholar
Richter, H. G. F., Adams, D. R., Benardeau, A., Bickerdike, M. J., Bentley, J. M., Blench, T. J., Cliffe, I. A., Dourish, C., Hebeisen, P., Kennett, G. A., Knight, A. R., Malcolm, C. S., Mattei, P., Misra, A., Mizrahi, J., Monck, N. J. T., Plancher, J.-M., Roever, S., Roffey, J. R. A., Taylor, S. & Vickers, S. P. (2006). Bioorg. Med. Chem. Lett. 16, 1207–1211. Web of Science CSD CrossRef PubMed CAS 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

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