organic compounds
Ethyl (2Z)-3-oxo-2-(3,4,5-trimethoxybenzylidene)butanoate
aDepartment of Chemistry, Universidade Federal de São Carlos, 13565-905 São Carlos, SP, Brazil, bInstituto de Química, Universidade Estadual de Campinas, CP 6154, 13083-970 Campinas, SP, Brazil, cDepartment of Chemistry, University of Malaya, 50603 Kuala Lumpur, Malaysia, and dChemistry Department, Faculty of Science, King Abdulaziz University, PO Box 80203 Jeddah, Saudi Arabia
*Correspondence e-mail: julio@power.ufscar.br
In the title compound, C16H20O6, the conformation about the C=C double bond [1.344 (2) Å] is Z. With respect to this bond, the ketone is almost coplanar [C—C—C—O torsion angle = −179.60 (10)°] and the ester is almost perpendicular [C—C—C—O = 78.42 (13)°]. The methoxy substituents of the central benzene ring are either almost coplanar [C—C—O—C = 3.54 (15) and 177.70 (9)°] or perpendicular [C—C—O—C = 80.08 12)° for the central substituent]. In the crystal, the three-dimensional architecture features C—H⋯O and π–π [inter-centroid distance = 3.6283 (6) Å] interactions.
Related literature
For background to the study, see: Rodrigues et al. (2004); Zukerman-Schpector et al. (2011). For the synthesis of the title compound, see: de Paula (2012).
Experimental
Crystal data
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Data collection: CrysAlis PRO (Agilent, 2013); cell CrysAlis PRO; data reduction: CrysAlis PRO; program(s) used to solve structure: SIR92 (Altomare et al., 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012), DIAMOND (Brandenburg, 2006) and MarvinSketch (ChemAxon, 2010); software used to prepare material for publication: publCIF (Westrip, 2010).
Supporting information
10.1107/S1600536813023374/kj2231sup1.cif
contains datablocks general, I. DOI:Structure factors: contains datablock I. DOI: 10.1107/S1600536813023374/kj2231Isup2.hkl
Supporting information file. DOI: 10.1107/S1600536813023374/kj2231Isup3.cml
The description of the synthesis of compound (I) together with spectra and HRMS analyses can be found in de Paula (2012); M.pt: 381.2—381.7 K.
Carbon-bound H-atoms were placed in calculated positions (C—H = 0.93 to 0.97 Å) and were included in the
in the riding model approximation, with Uiso(H) = 1.2Ueq(C) and 1.5Ueq(methyl-C).Data collection: CrysAlis PRO (Agilent, 2013); cell
CrysAlis PRO (Agilent, 2013); data reduction: CrysAlis PRO (Agilent, 2013); program(s) used to solve structure: SIR92 (Altomare et al., 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012), DIAMOND (Brandenburg, 2006) and MarvinSketch (ChemAxon, 2010); software used to prepare material for publication: publCIF (Westrip, 2010).Fig. 1. The molecular structure of compound (I) showing the atom-labelling scheme and displacement ellipsoids at the 70% probability level. | |
Fig. 2. View in projection down the b axis of the unit-cell contents of (I). The π—π and C—H···O interactions are shown as purple and orange dashed lines, respectively. |
C16H20O6 | Z = 2 |
Mr = 308.32 | F(000) = 328 |
Triclinic, P1 | Dx = 1.341 Mg m−3 |
Hall symbol: -P 1 | Melting point = 381.2–381.7 K |
a = 8.3432 (4) Å | Cu Kα radiation, λ = 1.54184 Å |
b = 10.2446 (5) Å | Cell parameters from 3721 reflections |
c = 10.4543 (5) Å | θ = 5.1–75.8° |
α = 61.130 (5)° | µ = 0.86 mm−1 |
β = 77.450 (4)° | T = 100 K |
γ = 82.534 (4)° | Irregular, colourless |
V = 763.48 (7) Å3 | 0.35 × 0.30 × 0.25 mm |
Agilent SuperNova (Dual, Cu at zero, Atlas) diffractometer | 3101 independent reflections |
Radiation source: fine-focus sealed tube | 2943 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.012 |
Detector resolution: 10.4041 pixels mm-1 | θmax = 76.0°, θmin = 5.1° |
ω scans | h = −10→8 |
Absorption correction: multi-scan (CrysAlis PRO; Agilent, 2013) | k = −12→12 |
Tmin = 0.631, Tmax = 1.000 | l = −13→12 |
5294 measured reflections |
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)] = 0.034 | Hydrogen site location: inferred from neighbouring sites |
wR(F2) = 0.095 | H-atom parameters constrained |
S = 1.05 | w = 1/[σ2(Fo2) + (0.0524P)2 + 0.2494P] where P = (Fo2 + 2Fc2)/3 |
3101 reflections | (Δ/σ)max < 0.001 |
204 parameters | Δρmax = 0.32 e Å−3 |
0 restraints | Δρmin = −0.25 e Å−3 |
C16H20O6 | γ = 82.534 (4)° |
Mr = 308.32 | V = 763.48 (7) Å3 |
Triclinic, P1 | Z = 2 |
a = 8.3432 (4) Å | Cu Kα radiation |
b = 10.2446 (5) Å | µ = 0.86 mm−1 |
c = 10.4543 (5) Å | T = 100 K |
α = 61.130 (5)° | 0.35 × 0.30 × 0.25 mm |
β = 77.450 (4)° |
Agilent SuperNova (Dual, Cu at zero, Atlas) diffractometer | 3101 independent reflections |
Absorption correction: multi-scan (CrysAlis PRO; Agilent, 2013) | 2943 reflections with I > 2σ(I) |
Tmin = 0.631, Tmax = 1.000 | Rint = 0.012 |
5294 measured reflections |
R[F2 > 2σ(F2)] = 0.034 | 0 restraints |
wR(F2) = 0.095 | H-atom parameters constrained |
S = 1.05 | Δρmax = 0.32 e Å−3 |
3101 reflections | Δρmin = −0.25 e Å−3 |
204 parameters |
Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'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 > 2σ(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. |
x | y | z | Uiso*/Ueq | ||
O1 | 0.40766 (10) | 1.13778 (8) | 0.21253 (9) | 0.01963 (18) | |
O2 | 0.49401 (9) | 0.85742 (8) | 0.39679 (8) | 0.01745 (18) | |
O3 | 0.43618 (10) | 0.62786 (8) | 0.36141 (8) | 0.01846 (18) | |
O4 | 0.01540 (10) | 0.84612 (9) | −0.28318 (9) | 0.02070 (19) | |
O5 | 0.21960 (10) | 0.62520 (9) | −0.04434 (9) | 0.02128 (19) | |
O6 | −0.00196 (9) | 0.68398 (8) | 0.08761 (8) | 0.01854 (18) | |
C1 | 0.27109 (12) | 0.93633 (12) | 0.04388 (11) | 0.0146 (2) | |
C2 | 0.29899 (13) | 1.05493 (11) | 0.06585 (12) | 0.0157 (2) | |
H2 | 0.2687 | 1.1516 | 0.0010 | 0.019* | |
C3 | 0.37188 (13) | 1.02961 (12) | 0.18421 (12) | 0.0155 (2) | |
C4 | 0.41592 (12) | 0.88409 (12) | 0.28284 (11) | 0.0150 (2) | |
C5 | 0.38756 (13) | 0.76540 (12) | 0.26071 (11) | 0.0150 (2) | |
C6 | 0.31599 (13) | 0.79048 (11) | 0.14221 (11) | 0.0154 (2) | |
H6 | 0.2979 | 0.7110 | 0.1282 | 0.019* | |
C7 | 0.19873 (13) | 0.97298 (12) | −0.08589 (11) | 0.0153 (2) | |
H7 | 0.1953 | 1.0738 | −0.1538 | 0.018* | |
C8 | 0.13656 (13) | 0.88232 (12) | −0.12120 (11) | 0.0154 (2) | |
C9 | 0.07048 (13) | 0.93638 (12) | −0.26035 (12) | 0.0165 (2) | |
C10 | 0.07124 (14) | 1.10044 (13) | −0.36880 (12) | 0.0202 (2) | |
H10A | −0.0035 | 1.1534 | −0.3250 | 0.030* | |
H10B | 0.0377 | 1.1158 | −0.4574 | 0.030* | |
H10C | 0.1800 | 1.1366 | −0.3931 | 0.030* | |
C11 | 0.12582 (13) | 0.71652 (12) | −0.02458 (12) | 0.0160 (2) | |
C12 | −0.02291 (16) | 0.52685 (12) | 0.19624 (13) | 0.0229 (2) | |
H12A | 0.0831 | 0.4771 | 0.2147 | 0.027* | |
H12B | −0.0786 | 0.4773 | 0.1600 | 0.027* | |
C13 | −0.12367 (17) | 0.52221 (14) | 0.33569 (14) | 0.0299 (3) | |
H13A | −0.0675 | 0.5722 | 0.3699 | 0.045* | |
H13B | −0.1397 | 0.4203 | 0.4105 | 0.045* | |
H13C | −0.2284 | 0.5712 | 0.3160 | 0.045* | |
C14 | 0.35859 (15) | 1.28797 (12) | 0.11817 (13) | 0.0206 (2) | |
H14A | 0.2415 | 1.2946 | 0.1251 | 0.031* | |
H14B | 0.4105 | 1.3162 | 0.0174 | 0.031* | |
H14C | 0.3909 | 1.3537 | 0.1488 | 0.031* | |
C15 | 0.38159 (15) | 0.85960 (13) | 0.52181 (12) | 0.0225 (2) | |
H15A | 0.3099 | 0.7769 | 0.5667 | 0.034* | |
H15B | 0.3175 | 0.9512 | 0.4887 | 0.034* | |
H15C | 0.4424 | 0.8522 | 0.5932 | 0.034* | |
C16 | 0.40407 (16) | 0.50278 (12) | 0.34655 (13) | 0.0224 (2) | |
H16A | 0.4662 | 0.5097 | 0.2548 | 0.034* | |
H16B | 0.2890 | 0.5020 | 0.3470 | 0.034* | |
H16C | 0.4354 | 0.4125 | 0.4279 | 0.034* |
U11 | U22 | U33 | U12 | U13 | U23 | |
O1 | 0.0258 (4) | 0.0150 (4) | 0.0227 (4) | 0.0009 (3) | −0.0105 (3) | −0.0103 (3) |
O2 | 0.0190 (4) | 0.0215 (4) | 0.0152 (4) | 0.0011 (3) | −0.0063 (3) | −0.0103 (3) |
O3 | 0.0262 (4) | 0.0133 (4) | 0.0173 (4) | 0.0027 (3) | −0.0109 (3) | −0.0062 (3) |
O4 | 0.0226 (4) | 0.0245 (4) | 0.0189 (4) | −0.0029 (3) | −0.0050 (3) | −0.0122 (3) |
O5 | 0.0254 (4) | 0.0182 (4) | 0.0237 (4) | 0.0029 (3) | −0.0066 (3) | −0.0124 (3) |
O6 | 0.0205 (4) | 0.0147 (4) | 0.0180 (4) | −0.0011 (3) | −0.0032 (3) | −0.0056 (3) |
C1 | 0.0137 (5) | 0.0164 (5) | 0.0140 (5) | 0.0005 (4) | −0.0028 (4) | −0.0074 (4) |
C2 | 0.0161 (5) | 0.0140 (5) | 0.0160 (5) | 0.0013 (4) | −0.0039 (4) | −0.0062 (4) |
C3 | 0.0153 (5) | 0.0160 (5) | 0.0168 (5) | −0.0009 (4) | −0.0018 (4) | −0.0093 (4) |
C4 | 0.0142 (5) | 0.0187 (5) | 0.0136 (5) | 0.0005 (4) | −0.0039 (4) | −0.0083 (4) |
C5 | 0.0152 (5) | 0.0146 (5) | 0.0135 (5) | 0.0014 (4) | −0.0030 (4) | −0.0055 (4) |
C6 | 0.0173 (5) | 0.0151 (5) | 0.0158 (5) | 0.0001 (4) | −0.0042 (4) | −0.0084 (4) |
C7 | 0.0161 (5) | 0.0145 (5) | 0.0136 (5) | 0.0017 (4) | −0.0033 (4) | −0.0056 (4) |
C8 | 0.0156 (5) | 0.0161 (5) | 0.0137 (5) | 0.0015 (4) | −0.0029 (4) | −0.0069 (4) |
C9 | 0.0148 (5) | 0.0211 (5) | 0.0145 (5) | 0.0008 (4) | −0.0023 (4) | −0.0095 (4) |
C10 | 0.0242 (6) | 0.0209 (5) | 0.0157 (5) | 0.0013 (4) | −0.0077 (4) | −0.0075 (4) |
C11 | 0.0183 (5) | 0.0174 (5) | 0.0163 (5) | 0.0000 (4) | −0.0077 (4) | −0.0091 (4) |
C12 | 0.0308 (6) | 0.0144 (5) | 0.0215 (6) | −0.0050 (4) | −0.0061 (5) | −0.0054 (4) |
C13 | 0.0343 (7) | 0.0240 (6) | 0.0234 (6) | −0.0062 (5) | −0.0003 (5) | −0.0056 (5) |
C14 | 0.0254 (6) | 0.0139 (5) | 0.0244 (6) | 0.0007 (4) | −0.0072 (4) | −0.0097 (4) |
C15 | 0.0297 (6) | 0.0238 (6) | 0.0158 (5) | −0.0010 (5) | −0.0029 (4) | −0.0110 (5) |
C16 | 0.0350 (6) | 0.0140 (5) | 0.0201 (5) | 0.0014 (4) | −0.0129 (5) | −0.0067 (4) |
C1—C2 | 1.3962 (15) | C11—O5 | 1.2068 (13) |
C1—C6 | 1.4030 (14) | C11—O6 | 1.3397 (13) |
C1—C7 | 1.4667 (14) | C12—O6 | 1.4605 (13) |
C2—C3 | 1.3944 (15) | C12—C13 | 1.4984 (17) |
C2—H2 | 0.9300 | C12—H12A | 0.9700 |
C3—O1 | 1.3586 (13) | C12—H12B | 0.9700 |
C3—C4 | 1.3995 (14) | C13—H13A | 0.9600 |
C4—O2 | 1.3765 (12) | C13—H13B | 0.9600 |
C4—C5 | 1.3993 (15) | C13—H13C | 0.9600 |
C5—O3 | 1.3649 (12) | C14—O1 | 1.4332 (13) |
C5—C6 | 1.3898 (15) | C14—H14A | 0.9600 |
C6—H6 | 0.9300 | C14—H14B | 0.9600 |
C7—C8 | 1.3444 (15) | C14—H14C | 0.9600 |
C7—H7 | 0.9300 | C15—O2 | 1.4418 (13) |
C8—C9 | 1.4893 (14) | C15—H15A | 0.9600 |
C8—C11 | 1.5022 (14) | C15—H15B | 0.9600 |
C9—O4 | 1.2224 (14) | C15—H15C | 0.9600 |
C9—C10 | 1.5066 (15) | C16—O3 | 1.4274 (13) |
C10—H10A | 0.9600 | C16—H16A | 0.9600 |
C10—H10B | 0.9600 | C16—H16B | 0.9600 |
C10—H10C | 0.9600 | C16—H16C | 0.9600 |
C2—C1—C6 | 119.68 (9) | O6—C11—C8 | 110.49 (9) |
C2—C1—C7 | 117.10 (9) | O6—C12—C13 | 106.84 (9) |
C6—C1—C7 | 123.19 (9) | O6—C12—H12A | 110.4 |
C3—C2—C1 | 120.53 (9) | C13—C12—H12A | 110.4 |
C3—C2—H2 | 119.7 | O6—C12—H12B | 110.4 |
C1—C2—H2 | 119.7 | C13—C12—H12B | 110.4 |
O1—C3—C2 | 124.84 (9) | H12A—C12—H12B | 108.6 |
O1—C3—C4 | 115.31 (9) | C12—C13—H13A | 109.5 |
C2—C3—C4 | 119.84 (10) | C12—C13—H13B | 109.5 |
O2—C4—C5 | 119.72 (9) | H13A—C13—H13B | 109.5 |
O2—C4—C3 | 120.69 (9) | C12—C13—H13C | 109.5 |
C5—C4—C3 | 119.51 (9) | H13A—C13—H13C | 109.5 |
O3—C5—C6 | 123.90 (9) | H13B—C13—H13C | 109.5 |
O3—C5—C4 | 115.35 (9) | O1—C14—H14A | 109.5 |
C6—C5—C4 | 120.74 (10) | O1—C14—H14B | 109.5 |
C5—C6—C1 | 119.69 (10) | H14A—C14—H14B | 109.5 |
C5—C6—H6 | 120.2 | O1—C14—H14C | 109.5 |
C1—C6—H6 | 120.2 | H14A—C14—H14C | 109.5 |
C8—C7—C1 | 129.55 (10) | H14B—C14—H14C | 109.5 |
C8—C7—H7 | 115.2 | O2—C15—H15A | 109.5 |
C1—C7—H7 | 115.2 | O2—C15—H15B | 109.5 |
C7—C8—C9 | 123.31 (10) | H15A—C15—H15B | 109.5 |
C7—C8—C11 | 123.66 (9) | O2—C15—H15C | 109.5 |
C9—C8—C11 | 113.03 (9) | H15A—C15—H15C | 109.5 |
O4—C9—C8 | 118.94 (10) | H15B—C15—H15C | 109.5 |
O4—C9—C10 | 121.50 (9) | O3—C16—H16A | 109.5 |
C8—C9—C10 | 119.56 (9) | O3—C16—H16B | 109.5 |
C9—C10—H10A | 109.5 | H16A—C16—H16B | 109.5 |
C9—C10—H10B | 109.5 | O3—C16—H16C | 109.5 |
H10A—C10—H10B | 109.5 | H16A—C16—H16C | 109.5 |
C9—C10—H10C | 109.5 | H16B—C16—H16C | 109.5 |
H10A—C10—H10C | 109.5 | C3—O1—C14 | 117.31 (8) |
H10B—C10—H10C | 109.5 | C4—O2—C15 | 112.44 (8) |
O5—C11—O6 | 124.60 (10) | C5—O3—C16 | 117.32 (8) |
O5—C11—C8 | 124.91 (10) | C11—O6—C12 | 116.84 (9) |
C6—C1—C2—C3 | 0.35 (16) | C1—C7—C8—C11 | 1.85 (18) |
C7—C1—C2—C3 | −178.10 (9) | C7—C8—C9—O4 | −179.60 (10) |
C1—C2—C3—O1 | 178.28 (9) | C11—C8—C9—O4 | 0.17 (14) |
C1—C2—C3—C4 | −0.69 (16) | C7—C8—C9—C10 | −0.22 (16) |
O1—C3—C4—O2 | −1.71 (14) | C11—C8—C9—C10 | 179.55 (9) |
C2—C3—C4—O2 | 177.36 (9) | C7—C8—C11—O5 | −101.11 (13) |
O1—C3—C4—C5 | −178.50 (9) | C9—C8—C11—O5 | 79.13 (13) |
C2—C3—C4—C5 | 0.57 (16) | C7—C8—C11—O6 | 78.42 (13) |
O2—C4—C5—O3 | 2.47 (14) | C9—C8—C11—O6 | −101.35 (10) |
C3—C4—C5—O3 | 179.28 (9) | C2—C3—O1—C14 | 3.54 (15) |
O2—C4—C5—C6 | −176.93 (9) | C4—C3—O1—C14 | −177.45 (9) |
C3—C4—C5—C6 | −0.11 (16) | C5—C4—O2—C15 | −103.14 (11) |
O3—C5—C6—C1 | −179.57 (9) | C3—C4—O2—C15 | 80.08 (12) |
C4—C5—C6—C1 | −0.22 (16) | C6—C5—O3—C16 | −2.92 (15) |
C2—C1—C6—C5 | 0.11 (15) | C4—C5—O3—C16 | 177.70 (9) |
C7—C1—C6—C5 | 178.46 (10) | O5—C11—O6—C12 | 2.89 (15) |
C2—C1—C7—C8 | −168.73 (11) | C8—C11—O6—C12 | −176.64 (8) |
C6—C1—C7—C8 | 12.88 (18) | C13—C12—O6—C11 | 157.95 (10) |
C1—C7—C8—C9 | −178.41 (10) |
D—H···A | D—H | H···A | D···A | D—H···A |
C12—H12b···O5i | 0.97 | 2.47 | 3.4390 (16) | 174 |
C14—H14a···O4ii | 0.96 | 2.52 | 3.3295 (16) | 142 |
C16—H16c···O2iii | 0.96 | 2.55 | 3.4863 (15) | 165 |
Symmetry codes: (i) −x, −y+1, −z; (ii) −x, −y+2, −z; (iii) −x+1, −y+1, −z+1. |
D—H···A | D—H | H···A | D···A | D—H···A |
C12—H12b···O5i | 0.97 | 2.47 | 3.4390 (16) | 174 |
C14—H14a···O4ii | 0.96 | 2.52 | 3.3295 (16) | 142 |
C16—H16c···O2iii | 0.96 | 2.55 | 3.4863 (15) | 165 |
Symmetry codes: (i) −x, −y+1, −z; (ii) −x, −y+2, −z; (iii) −x+1, −y+1, −z+1. |
Acknowledgements
We thank the Brazilian agencies Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq: 306532/2009–3,to JZ-S), Coordenação de Aperfeicoamento de Pessoal de Nível Superior (CAPES: 808/09 to JZ-S and scholarship to CLH) and FAPESP for financial support. We also thank the Ministry of Higher Education (Malaysia) for funding structural studies through the High-Impact Research scheme (UM.C/HIR-MOHE/SC/03).
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Zukerman-Schpector, J., Abd Salim, S. N., Moran, P. J. S., Paula, B. R. S. de, Rodrigues, J. A. R. & Tiekink, E. R. T. (2011). Acta Cryst. E67, o1044. Web of Science CSD CrossRef IUCr Journals Google Scholar
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As part of the continuing interest in the bio-reduction of α-haloketones and enones (Rodrigues et al., 2004; Zukerman-Schpector et al., 2011), the title compound, (I), was synthesized by means of a Knoevenagel condensation reaction between ethylacetoacetate and 3,4,5-trimethoxybenzaldehyde to afford a mixture of E and Z isomers that were separated by column chromatography (hexane/ethyl acetate, gradient from pure hexane to 95% hexane/5% ethyl acetate). The crystallized isomer, obtained by slow evaporation from a dichloromethane/hexane mixture, was shown by X-ray crystallography to be the Z isomer.
In (I), Fig. 1, the conformation about the C7═C8 bond [1.3444 (15) Å] is Z. The O1–O3 methoxy groups are approximately planar, perpendicular and co-planar, respectively, with the benzene ring to which they are attached as seen in the C2—C3—O1—C14, C3—C4—O2—C15 and C4—C5—O3—C16 torsion angles of 3.54 (15), 80.08 (12) and 177.70 (9)°, respectively. With respect to the ethylene bond, the ketone group is co-planar [C7—C8—C9—O4 = -179.60 (10)°] but the ester is almost perpendicular [C7—C8—C11—O6 = 78.42 (13)°]. With the exception of the ester-carbonyl-O5 atom, the two perpendicularly orientated groups lie to the same side of the central plane.
In the crystal packing, C—H···O, Table 1, and π—π [inter-centroid distance = 3.6283 (6) Å for symmetry operation: 1 - x, 2 - y, -z] combine to link molecules into a three-dimensional architecture. Globally, molecule pack in layers in an ···ABA···fashion running parallel to the (2,0,-1) lattice planes, Fig. 2.