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Acta Crystallogr Sect E Struct Rep Online. 2010 November 1; 66(Pt 11): o2822.
Published online 2010 October 20. doi:  10.1107/S1600536810040456
PMCID: PMC3009344

Ethyl 4-(furan-2-yl)-6-methyl-2-oxo-1,2,3,4-tetra­hydro­pyrimidine-5-carboxyl­ate

Abstract

The asymmetric unit of the title compound, C12H14N2O4, contains two independent mol­ecules. In one independent mol­ecule, the furanyl fragment is rotationally disordered between two orientations in a 0.625 (6):0.375 (6) ratio. In the crystal, inter­molecular pyrimidine–pyrimidinone N—H(...)O hydrogen bonds link the mol­ecules into centrosymmetric tetra­mers, which are further associated into ribbons extending in [010] via weak inter­molecular pyrimidine–carboxyl N—H(...)O hydrogen bonds.

Related literature

The Biginelli reaction is the most important procedure in the synthesis of 3,4-dihydro­pyrimidin-2-(1H)-ones, see: Biginelli (1893 [triangle]). For related structures, see: Nizam Mohideen et al. (2008 [triangle]); Qing-Fang et al. (2007 [triangle]).

An external file that holds a picture, illustration, etc.
Object name is e-66-o2822-scheme1.jpg

Experimental

Crystal data

  • C12H14N2O4
  • M r = 250.25
  • Monoclinic, An external file that holds a picture, illustration, etc.
Object name is e-66-o2822-efi1.jpg
  • a = 12.1720 (14) Å
  • b = 13.3180 (15) Å
  • c = 17.116 (2) Å
  • α = 90°
  • β = 118.300 (2)°
  • γ = 90°
  • V = 2443.0 (5) Å3
  • Z = 8
  • Mo Kα radiation
  • μ = 0.10 mm−1
  • T = 298 K
  • 0.48 × 0.45 × 0.17 mm

Data collection

  • Bruker SMART APEX CCD area-detector diffractometer
  • Absorption correction: multi-scan (SADABS; Sheldrick, 1996 [triangle]) T min = 0.952, T max = 0.983
  • 11604 measured reflections
  • 4298 independent reflections
  • 1943 reflections with I > 2σ(I)
  • R int = 0.069

Refinement

  • R[F 2 > 2σ(F 2)] = 0.071
  • wR(F 2) = 0.237
  • S = 0.94
  • 4298 reflections
  • 338 parameters
  • H-atom parameters constrained
  • Δρmax = 0.30 e Å−3
  • Δρmin = −0.23 e Å−3

Data collection: SMART (Bruker, 2007 [triangle]); cell refinement: SAINT (Bruker, 2007 [triangle]); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 [triangle]); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008 [triangle]); molecular graphics: SHELXTL (Sheldrick, 2008 [triangle]); software used to prepare material for publication: SHELXTL.

Table 1
Hydrogen-bond geometry (Å, °)

Supplementary Material

Crystal structure: contains datablocks I, global. DOI: 10.1107/S1600536810040456/cv2771sup1.cif

Structure factors: contains datablocks I. DOI: 10.1107/S1600536810040456/cv2771Isup2.hkl

Additional supplementary materials: crystallographic information; 3D view; checkCIF report

Acknowledgments

The authors acknowledge financial support by the Foundation of Binzhou University (grant No. BZXYQNLG­2005015).

supplementary crystallographic information

Comment

Biginelli reaction is a well known multicomponent reaction involving a one-pot cyclocondensation of an aldehyde, β-ketoester and urea/thiourea. It is the most important procedure in the synthesis of 3,4-dihydropyrimidin-2-(1H)-ones (Biginelli, 1893). Herewith we report the crystal structure of the title compound, (I), obtained by the three-component reaction of furfuraldehyde, acetoacetate and urea.

In (I) (Fig. 1), the dihydropyrimidinone rings adopt flattened boat conformation. The asymmetric unit contains two independent molecules. In one independent molecule, the furanyl fragment is rotationally disordered in a ratio 0.625 (6):0.375 (6). The bond lengths an angles are normal and comparable to the values observed in similar compounds (Nizam Mohideen et al., 2008; Qing-Fang et al., 2007) The dihedral angles between the furan rings (C3—C6/O2, C15—C18/O6) and the mean planes of the dihydropyrimidinone rings (N1/C1/N2/C9/C8, N3/C13/N4/C21/C20) unit in two independent molecules are 88.79 (4) ° and 86.73 (2)°, respectively, indicating that the furan rings and the dihydropyrimidinone rings are nearly perpendicular.

In the crystal structure, intermolecular NH···Opyrimidinone hydrogen bonds (Table 1) link the molecules into centrosymmetric tetramers. Tetramers are further associated into ribbons extended in direction [010] via the weak intermolecular N—H···Ocarboxyl hydrogen bonds (Table 1).

Experimental

A mixture of ethylacetoacetate (0.5 mol), furfural (0.5 mol) and urea (0.6 mol) was refluxed in 50.0 ml of ethanol for 2.0 hrs. The reaction completion was monitored through thin layer chromatography and the reaction mixture was quenched in ice cold water. The precipitate obtained was filtered, dried and crystallized from methanol to obtain the title compound.

Refinement

All H atoms were placed in geometrically idealized positions (N—H 0.86 and C—H = 0.93–0.97 Å) and treated as riding on their parent atoms, with Uiso(H) = 1.2U-1.5eq(C, N). Atoms C4, C5, C6, O2 were treated as disordered between two positions, with refined occupancies of 0.375 (6) and 0.625 (6).

Figures

Fig. 1.
The content of asymmetric unit of the title compound showing the atomic numbering scheme and 30% probability displacement ellipsoids. Only major components of the disordered atoms are shown.

Crystal data

C12H14N2O4F(000) = 1056
Mr = 250.25Dx = 1.361 Mg m3
Monoclinic, P21/cMo Kα radiation, λ = 0.71073 Å
a = 12.1720 (14) ÅCell parameters from 1664 reflections
b = 13.3180 (15) Åθ = 2.3–22.3°
c = 17.116 (2) ŵ = 0.10 mm1
β = 118.300 (2)°T = 298 K
V = 2443.0 (5) Å3Block, yellow
Z = 80.48 × 0.45 × 0.17 mm

Data collection

Bruker SMART APEX CCD area-detector diffractometer4298 independent reflections
Radiation source: fine-focus sealed tube1943 reflections with I > 2σ(I)
graphiteRint = 0.069
[var phi] and ω scansθmax = 25.0°, θmin = 1.9°
Absorption correction: multi-scan (SADABS; Sheldrick, 1996)h = −14→14
Tmin = 0.952, Tmax = 0.983k = −14→15
11604 measured reflectionsl = −20→19

Refinement

Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullSecondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.071Hydrogen site location: inferred from neighbouring sites
wR(F2) = 0.237H-atom parameters constrained
S = 0.94w = 1/[σ2(Fo2) + (0.1244P)2] where P = (Fo2 + 2Fc2)/3
4298 reflections(Δ/σ)max = 0.001
338 parametersΔρmax = 0.30 e Å3
0 restraintsΔρmin = −0.23 e Å3

Special details

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

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

xyzUiso*/UeqOcc. (<1)
N10.0677 (3)0.8975 (2)0.4632 (2)0.0478 (9)
H10.03510.95550.44370.057*
N20.0574 (3)0.7467 (2)0.5219 (2)0.0479 (8)
H20.04680.71350.56090.057*
N30.8736 (3)0.6124 (2)0.7072 (2)0.0499 (9)
H30.89880.55110.71820.060*
N40.8981 (3)0.7698 (2)0.6635 (2)0.0497 (8)
H40.91960.80750.63210.060*
O1−0.0199 (2)0.88634 (18)0.55364 (19)0.0574 (8)
O20.3239 (6)0.8613 (5)0.5879 (5)0.086 (2)0.625 (6)
C4'0.3192 (18)0.9228 (13)0.5941 (13)0.086 (2)0.375 (6)
H4'0.26720.94250.61730.103*0.375 (6)
O30.2203 (2)0.7669 (2)0.32562 (17)0.0565 (8)
O40.1728 (3)0.6108 (2)0.3447 (2)0.0686 (9)
O50.9657 (3)0.63333 (18)0.6200 (2)0.0600 (8)
O60.6054 (4)0.6411 (3)0.5976 (3)0.1107 (13)
O70.7327 (3)0.7358 (2)0.85737 (19)0.0690 (9)
O80.7471 (3)0.8936 (2)0.8223 (2)0.0780 (10)
C10.0329 (3)0.8474 (3)0.5145 (3)0.0456 (10)
C20.1585 (3)0.8589 (3)0.4384 (3)0.0429 (10)
H2A0.14380.89070.38260.052*
C30.2873 (4)0.8830 (3)0.5074 (3)0.0559 (11)
C40.3859 (8)0.9166 (7)0.4951 (7)0.079 (2)0.625 (6)
H4A0.38520.93490.44240.094*0.625 (6)
C50.4870 (15)0.9164 (9)0.5815 (12)0.086 (4)0.625 (6)
H50.56800.93590.59660.103*0.625 (6)
C60.448 (2)0.8828 (11)0.6403 (17)0.091 (4)0.625 (6)
H60.49430.87640.70160.109*0.625 (6)
O2'0.3875 (9)0.8450 (8)0.5162 (7)0.079 (2)0.375 (6)
C5'0.459 (4)0.924 (2)0.636 (3)0.091 (4)0.375 (6)
H5'0.51160.96190.68550.109*0.375 (6)
C6'0.492 (3)0.8685 (16)0.598 (2)0.086 (4)0.375 (6)
H6'0.57250.84450.61840.103*0.375 (6)
C70.1783 (3)0.6992 (3)0.3629 (2)0.0461 (9)
C80.1410 (3)0.7470 (2)0.4236 (2)0.0408 (9)
C90.0980 (3)0.6952 (3)0.4705 (2)0.0425 (9)
C100.0877 (4)0.5845 (3)0.4755 (3)0.0621 (12)
H10A0.00240.56460.43950.093*
H10B0.11490.56520.53590.093*
H10C0.13900.55230.45420.093*
C110.2614 (4)0.7310 (3)0.2650 (3)0.0666 (12)
H11A0.19380.69690.21550.080*
H11B0.32990.68420.29460.080*
C120.3030 (4)0.8203 (4)0.2328 (3)0.0828 (15)
H12A0.23440.86600.20370.124*
H12B0.33120.79880.19180.124*
H12C0.37000.85330.28230.124*
C130.9139 (3)0.6678 (3)0.6603 (3)0.0473 (10)
C140.7910 (3)0.6476 (3)0.7409 (3)0.0477 (10)
H140.81830.61610.79890.057*
C150.6605 (4)0.6159 (3)0.6829 (3)0.0588 (12)
C160.5810 (5)0.5651 (4)0.7005 (4)0.0925 (17)
H160.59690.53870.75520.111*
C170.4691 (6)0.5587 (5)0.6215 (6)0.113 (2)
H170.39610.52810.61410.136*
C180.4851 (6)0.6031 (6)0.5604 (6)0.126 (3)
H180.42540.60870.50110.151*
C190.7596 (4)0.8057 (3)0.8129 (3)0.0563 (11)
C200.8035 (3)0.7603 (3)0.7555 (2)0.0452 (9)
C210.8509 (3)0.8162 (3)0.7128 (2)0.0444 (9)
C220.8605 (4)0.9280 (3)0.7135 (3)0.0591 (12)
H22A0.77990.95620.67560.089*
H22B0.91780.94740.69240.089*
H22C0.89010.95240.77290.089*
C230.6852 (5)0.7708 (4)0.9158 (3)0.0831 (15)
H23A0.61480.81540.88390.100*
H23B0.74960.80740.96540.100*
C240.6464 (5)0.6833 (5)0.9477 (4)0.110 (2)
H24A0.57800.65080.89870.165*
H24B0.62060.70410.99030.165*
H24C0.71500.63730.97530.165*

Atomic displacement parameters (Å2)

U11U22U33U12U13U23
N10.047 (2)0.0349 (18)0.072 (2)0.0098 (14)0.037 (2)0.0113 (15)
N20.058 (2)0.0331 (19)0.065 (2)0.0014 (15)0.0384 (19)0.0075 (14)
N30.053 (2)0.0363 (18)0.074 (2)0.0055 (14)0.041 (2)0.0085 (15)
N40.056 (2)0.0355 (19)0.068 (2)0.0017 (15)0.038 (2)0.0059 (15)
O10.0673 (18)0.0414 (16)0.088 (2)0.0067 (13)0.0563 (18)0.0067 (14)
O20.068 (3)0.082 (5)0.086 (4)−0.010 (4)0.019 (3)0.002 (5)
C4'0.068 (3)0.082 (5)0.086 (4)−0.010 (4)0.019 (3)0.002 (5)
O30.0712 (19)0.0502 (17)0.0654 (18)−0.0097 (13)0.0466 (17)−0.0051 (13)
O40.089 (2)0.0406 (18)0.097 (2)−0.0088 (15)0.061 (2)−0.0133 (15)
O50.0728 (19)0.0429 (17)0.091 (2)0.0006 (14)0.0604 (19)0.0001 (14)
O60.077 (3)0.116 (3)0.105 (3)−0.018 (2)0.015 (3)0.006 (2)
O70.083 (2)0.070 (2)0.076 (2)−0.0077 (16)0.055 (2)−0.0078 (16)
O80.100 (3)0.052 (2)0.105 (3)−0.0020 (17)0.066 (2)−0.0134 (17)
C10.041 (2)0.041 (2)0.064 (3)0.0000 (17)0.032 (2)0.0031 (19)
C20.044 (2)0.037 (2)0.055 (2)−0.0022 (17)0.030 (2)0.0033 (17)
C30.049 (3)0.041 (2)0.074 (3)−0.001 (2)0.027 (3)−0.004 (2)
C40.058 (3)0.084 (6)0.090 (5)−0.003 (5)0.031 (4)−0.011 (5)
C50.052 (4)0.090 (11)0.100 (9)−0.006 (8)0.024 (6)−0.015 (9)
C60.068 (6)0.081 (14)0.095 (6)−0.007 (10)0.016 (5)−0.005 (11)
O2'0.058 (3)0.084 (6)0.090 (5)−0.003 (5)0.031 (4)−0.011 (5)
C5'0.068 (6)0.081 (14)0.095 (6)−0.007 (10)0.016 (5)−0.005 (11)
C6'0.052 (4)0.090 (11)0.100 (9)−0.006 (8)0.024 (6)−0.015 (9)
C70.043 (2)0.045 (3)0.053 (2)−0.0040 (18)0.025 (2)−0.0023 (19)
C80.040 (2)0.033 (2)0.053 (2)−0.0022 (16)0.025 (2)−0.0002 (16)
C90.041 (2)0.037 (2)0.057 (2)−0.0012 (16)0.029 (2)−0.0004 (17)
C100.079 (3)0.035 (2)0.091 (3)−0.009 (2)0.055 (3)−0.005 (2)
C110.075 (3)0.070 (3)0.071 (3)−0.005 (2)0.048 (3)−0.004 (2)
C120.086 (4)0.094 (4)0.088 (4)−0.023 (3)0.057 (3)−0.005 (3)
C130.044 (2)0.039 (2)0.064 (3)0.0000 (17)0.029 (2)0.0050 (18)
C140.049 (2)0.044 (2)0.059 (3)0.0016 (18)0.033 (2)0.0051 (18)
C150.054 (3)0.049 (3)0.081 (4)−0.005 (2)0.038 (3)−0.007 (2)
C160.077 (4)0.106 (5)0.107 (5)−0.027 (3)0.054 (4)−0.013 (3)
C170.068 (4)0.110 (6)0.161 (7)−0.021 (4)0.054 (5)−0.043 (5)
C180.069 (5)0.127 (6)0.123 (7)−0.007 (4)−0.002 (5)−0.019 (5)
C190.046 (3)0.059 (3)0.066 (3)−0.005 (2)0.028 (2)−0.003 (2)
C200.038 (2)0.041 (2)0.057 (2)0.0002 (17)0.022 (2)−0.0021 (18)
C210.038 (2)0.037 (2)0.058 (3)−0.0007 (16)0.022 (2)−0.0011 (17)
C220.063 (3)0.039 (2)0.080 (3)−0.0017 (19)0.037 (3)0.000 (2)
C230.103 (4)0.090 (4)0.082 (3)−0.009 (3)0.065 (3)−0.013 (3)
C240.122 (5)0.140 (6)0.106 (4)−0.030 (4)0.084 (4)−0.017 (4)

Geometric parameters (Å, °)

N1—C11.322 (4)O2'—C6'1.41 (3)
N1—C21.454 (4)C5'—C6'1.17 (6)
N1—H10.8600C5'—H5'0.9300
N2—C11.365 (4)C6'—H6'0.9300
N2—C91.379 (4)C7—C81.462 (5)
N2—H20.8600C8—C91.339 (5)
N3—C131.343 (5)C9—C101.485 (5)
N3—C141.454 (4)C10—H10A0.9600
N3—H30.8600C10—H10B0.9600
N4—C211.373 (4)C10—H10C0.9600
N4—C131.377 (4)C11—C121.496 (6)
N4—H40.8600C11—H11A0.9700
O1—C11.241 (4)C11—H11B0.9700
O2—C31.263 (8)C12—H12A0.9600
O2—C61.37 (2)C12—H12B0.9600
C4'—C31.44 (2)C12—H12C0.9600
C4'—C5'1.50 (5)C14—C151.480 (5)
C4'—H4'0.9300C14—C201.518 (5)
O3—C71.338 (4)C14—H140.9800
O3—C111.430 (4)C15—C161.327 (6)
O4—C71.211 (4)C16—C171.395 (8)
O5—C131.224 (4)C16—H160.9300
O6—C151.330 (6)C17—C181.294 (9)
O6—C181.387 (7)C17—H170.9300
O7—C191.337 (5)C18—H180.9300
O7—C231.449 (5)C19—C201.455 (5)
O8—C191.201 (4)C20—C211.349 (5)
C2—C31.484 (5)C21—C221.494 (5)
C2—C81.509 (5)C22—H22A0.9600
C2—H2A0.9800C22—H22B0.9600
C3—O2'1.262 (10)C22—H22C0.9600
C3—C41.387 (10)C23—C241.458 (7)
C4—C51.405 (18)C23—H23A0.9700
C4—H4A0.9300C23—H23B0.9700
C5—C61.38 (3)C24—H24A0.9600
C5—H50.9300C24—H24B0.9600
C6—H60.9300C24—H24C0.9600
C1—N1—C2122.9 (3)H10A—C10—H10B109.5
C1—N1—H1118.6C9—C10—H10C109.5
C2—N1—H1118.6H10A—C10—H10C109.5
C1—N2—C9123.9 (3)H10B—C10—H10C109.5
C1—N2—H2118.0O3—C11—C12107.3 (4)
C9—N2—H2118.0O3—C11—H11A110.3
C13—N3—C14125.2 (3)C12—C11—H11A110.3
C13—N3—H3117.4O3—C11—H11B110.3
C14—N3—H3117.4C12—C11—H11B110.3
C21—N4—C13125.1 (3)H11A—C11—H11B108.5
C21—N4—H4117.5C11—C12—H12A109.5
C13—N4—H4117.5C11—C12—H12B109.5
C3—O2—C6112.1 (12)H12A—C12—H12B109.5
C3—C4'—C5'101 (2)C11—C12—H12C109.5
C3—C4'—H4'129.5H12A—C12—H12C109.5
C5'—C4'—H4'129.5H12B—C12—H12C109.5
C7—O3—C11117.6 (3)O5—C13—N3124.2 (4)
C15—O6—C18106.5 (5)O5—C13—N4120.8 (3)
C19—O7—C23117.0 (4)N3—C13—N4115.0 (4)
O1—C1—N1123.9 (4)N3—C14—C15111.5 (3)
O1—C1—N2120.4 (3)N3—C14—C20110.5 (3)
N1—C1—N2115.6 (3)C15—C14—C20112.6 (3)
N1—C2—C3110.7 (3)N3—C14—H14107.3
N1—C2—C8109.4 (3)C15—C14—H14107.3
C3—C2—C8111.0 (3)C20—C14—H14107.3
N1—C2—H2A108.5C16—C15—O6109.7 (5)
C3—C2—H2A108.5C16—C15—C14131.2 (5)
C8—C2—H2A108.5O6—C15—C14119.1 (4)
O2'—C3—O287.6 (7)C15—C16—C17107.0 (6)
O2'—C3—C444.8 (5)C15—C16—H16126.5
O2—C3—C4110.9 (6)C17—C16—H16126.5
O2'—C3—C4'104.6 (10)C18—C17—C16107.7 (6)
O2—C3—C4'35.0 (6)C18—C17—H17126.2
C4—C3—C4'101.9 (9)C16—C17—H17126.2
O2'—C3—C2127.2 (6)C17—C18—O6109.1 (7)
O2—C3—C2120.8 (5)C17—C18—H18125.4
C4—C3—C2127.8 (6)O6—C18—H18125.4
C4'—C3—C2124.4 (9)O8—C19—O7121.5 (4)
C3—C4—C5103.2 (10)O8—C19—C20127.3 (4)
C3—C4—H4A128.4O7—C19—C20111.3 (4)
C5—C4—H4A128.4C21—C20—C19121.7 (4)
C6—C5—C4109.5 (15)C21—C20—C14119.5 (3)
C6—C5—H5125.2C19—C20—C14118.8 (3)
C4—C5—H5125.2C20—C21—N4119.7 (3)
O2—C6—C5104.1 (18)C20—C21—C22126.7 (3)
O2—C6—H6127.9N4—C21—C22113.5 (3)
C5—C6—H6127.9C21—C22—H22A109.5
C3—O2'—C6'113.1 (15)C21—C22—H22B109.5
C6'—C5'—C4'110 (3)H22A—C22—H22B109.5
C6'—C5'—H5'125.2C21—C22—H22C109.5
C4'—C5'—H5'125.2H22A—C22—H22C109.5
C5'—C6'—O2'108 (3)H22B—C22—H22C109.5
C5'—C6'—H6'125.9O7—C23—C24107.9 (4)
O2'—C6'—H6'125.9O7—C23—H23A110.1
O4—C7—O3121.3 (3)C24—C23—H23A110.1
O4—C7—C8127.4 (3)O7—C23—H23B110.1
O3—C7—C8111.2 (3)C24—C23—H23B110.1
C9—C8—C7122.9 (3)H23A—C23—H23B108.4
C9—C8—C2118.2 (3)C23—C24—H24A109.5
C7—C8—C2118.9 (3)C23—C24—H24B109.5
C8—C9—N2119.1 (3)H24A—C24—H24B109.5
C8—C9—C10127.9 (3)C23—C24—H24C109.5
N2—C9—C10113.0 (3)H24A—C24—H24C109.5
C9—C10—H10A109.5H24B—C24—H24C109.5
C9—C10—H10B109.5

Hydrogen-bond geometry (Å, °)

D—H···AD—HH···AD···AD—H···A
N1—H1···O1i0.862.122.924 (4)156
N2—H2···O5ii0.862.022.851 (4)162
N3—H3···O4iii0.862.383.077 (4)138
N4—H4···O1iv0.862.102.952 (4)174

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

Footnotes

Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: CV2771).

References

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  • Qing-Fang, C., Xu, X.-Y., Bao, J.-Y. & Zhang, C.-F. (2007). Acta Cryst. E63, o2391–o2392.
  • Sheldrick, G. M. (1996). SADABS University of Göttingen, Germany.
  • Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122. [PubMed]

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