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ISSN: 2056-9890

2,10-Dihydr­­oxy-13-methyl-13-aza­tetra­cyclo­[9.3.1.02,10.03,8]penta­deca-3(8),4,6-triene-9,15-dione

aDepartment of Physics, The Madura College, Madurai 625 011, India, bSchool of Chemistry, Madurai Kamaraj University, Madurai 625 021, India, and cDepartment of Food Science and Technology, Faculty of Agriculture, University of Ruhuna, Mapalana, Kamburupitiya 81100, Sri Lanka
*Correspondence e-mail: nilanthalakshman@yahoo.co.uk

(Received 22 April 2009; accepted 28 April 2009; online 7 May 2009)

In the title compound, C15H15NO4, the n-methyl­piperidone ring adopts a chair conformation and both five-membered rings adopt a twist conformation. An intra­molecular O—H⋯O hydrogen bond is observed. Inversion-related mol­ecules are linked into R22(10) dimers by pairs of O—H⋯O hydrogen bonds. The crystal structure is further stabilized by C—H⋯O hydrogen bonds.

Related literature

For the biological activity of piperidine compounds, see: Watson et al. (2000[Watson, P. S., Jiang, B. & Scott, B. (2000). Org. Lett. 2, 3679-3681.]). For ring conformation details, see: Cremer & Pople (1975[Cremer, D. & Pople, J. A. (1975). J. Am. Chem. Soc. 97, 1354-1358.]).

[Scheme 1]

Experimental

Crystal data
  • C15H15NO4

  • Mr = 273.28

  • Triclinic, [P \overline 1]

  • a = 7.5616 (7) Å

  • b = 8.9033 (8) Å

  • c = 10.8091 (11) Å

  • α = 72.764 (11)°

  • β = 80.486 (12)°

  • γ = 72.369 (11)°

  • V = 660.09 (11) Å3

  • Z = 2

  • Mo Kα radiation

  • μ = 0.10 mm−1

  • T = 293 K

  • 0.18 × 0.15 × 0.11 mm

Data collection
  • Nonius MACH-3 diffractometer

  • Absorption correction: ψ scan (North et al., 1968[North, A. C. T., Phillips, D. C. & Mathews, F. S. (1968). Acta Cryst. A24, 351-359.]) Tmin = 0.982, Tmax = 0.989

  • 2890 measured reflections

  • 2316 independent reflections

  • 2025 reflections with I > 2σ(I)

  • Rint = 0.010

  • 2 standard reflections frequency: 60 min intensity decay: none

Refinement
  • R[F2 > 2σ(F2)] = 0.037

  • wR(F2) = 0.097

  • S = 1.11

  • 2316 reflections

  • 184 parameters

  • H-atom parameters constrained

  • Δρmax = 0.22 e Å−3

  • Δρmin = −0.23 e Å−3

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A D—H H⋯A DA D—H⋯A
O3—H3⋯O2 0.82 2.11 2.6080 (15) 119
O2—H2⋯O1i 0.82 1.90 2.7168 (15) 175
C2—H2A⋯O1i 0.98 2.59 3.3778 (18) 137
C18—H18A⋯O1ii 0.96 2.57 3.432 (2) 149
C5—H5⋯O3iii 0.98 2.46 3.4381 (18) 173
Symmetry codes: (i) -x, -y+1, -z; (ii) -x+1, -y+1, -z; (iii) -x, -y, -z+1.

Data collection: CAD-4 EXPRESS (Enraf–Nonius, 1994[Enraf-Nonius (1994). CAD-4 EXPRESS. Enraf-Nonius, Delft, The Netherlands.]); cell refinement: CAD-4 EXPRESS; data reduction: XCAD4 (Harms & Wocadlo, 1996[Harms, K. & Wocadlo, S. (1996). XCAD4. University of Marburg, Germany.]); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]); molecular graphics: PLATON (Spek, 2009[Spek, A. L. (2009). Acta Cryst. D65, 148-155.]); software used to prepare material for publication: SHELXL97.

Supporting information


Comment top

The piperidine ring is a distinct structural feature of a variety of alkaloid natural products and drug candidates. Watson et al. (2000) observed that during the past decade there were thousands of piperidine compounds mentioned in clinical and preclinical studies. Piperidinones, though relatively less prominent, have also been regarded as precursors of a host of biologically active compounds and natural alkaloids, prior to their conversion to piperidines. Ninhydrin is a chemical used to detect ammonia or primary and secondary amines.

In the molecule of the title compound, (Fig. 1), the six-membered ring A (N1/C4/C5/C1/C2/C3), and the five membered rings B (C1/C2/C7/C6/C5) and C(C6-C10) are not planar. Rings B and C both adopt twist conformations, as indicated by Cremer & Pople (1975) puckering parameters Q = 0.455 (2) Å and Φ = 160.4 (2)° for ring B, and Q = 0.149 (2) Å and Φ = 21.6 (6)° for ring C. Ring A adopts a chair conformation.

In the crystal structure, the molecules are linked to form dimers by intermolecular O—H···O hydrogen bonds (Table 1), generating a graph set motif of R22(10) (Fig.2). In addition, the structure is stabilized by C—H···O and van der Waals interactions.

Related literature top

For the biological activity of piperidine compounds, see: Watson et al. (2000). For ring conformation details, see: Cremer & Pople (1975).

Experimental top

A mixture of 1-methyl-4-piperidinone (0.2 g, 0.002 mol), ninhydrin (0.315 g, 0.002 mol) and sarcosine (0.156 g, 0.002 mol) in methanol (30 ml) was refluxed in a water bath for 10 h. After completion of the reaction as monitored by TLC, the excess solvent was removed under vacuum and the residue was subjected to flash column chromatography using petroleum ether-ethyl acetate mixture (8:2 v/v) as eluent to obtain crystals of title compound in 8% yield along with a other product (yield 13%, m.p. 447–448 K).

Refinement top

The H atoms were placed in calculated positions and allowed to ride on their carrier atoms, with C-H = 0.93–0.98 Å, O-H = 0.82 Å and Uiso(H) = 1.2Ueq(C) and 1.5Ueq(Cmethyl,O).

Computing details top

Data collection: CAD-4 EXPRESS (Enraf–Nonius, 1994); cell refinement: CAD-4 EXPRESS (Enraf–Nonius, 1994); data reduction: XCAD4 (Harms & Wocadlo, 1996); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: PLATON (Spek, 2009); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures top
[Figure 1] Fig. 1. The molecular structure of the title compound, showing 30% probability displacement ellipsoids and the atom-numbering scheme. H atoms have been omitted for clarity.
[Figure 2] Fig. 2. Partial packing view down the b axis. Hydrogen bonds are shown as dashed lines.
2,10-Dihydroxy-13-methyl-13-azatetracyclo[9.3.1.02,10.03,8]pentadeca- 3(8),4,6-triene-9,15-dione top
Crystal data top
C15H15NO4Z = 2
Mr = 273.28F(000) = 288
Triclinic, P1Dx = 1.375 Mg m3
Hall symbol: -P 1Mo Kα radiation, λ = 0.71073 Å
a = 7.5616 (7) ÅCell parameters from 25 reflections
b = 8.9033 (8) Åθ = 2–25°
c = 10.8091 (11) ŵ = 0.10 mm1
α = 72.764 (11)°T = 293 K
β = 80.486 (12)°Block, colourless
γ = 72.369 (11)°0.18 × 0.15 × 0.11 mm
V = 660.09 (11) Å3
Data collection top
Nonius MACH-3
diffractometer
2025 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tubeRint = 0.010
Graphite monochromatorθmax = 25.0°, θmin = 2.5°
ω–2θ scansh = 18
Absorption correction: ψ scan
(North et al., 1968)
k = 1010
Tmin = 0.982, Tmax = 0.989l = 1212
2890 measured reflections2 standard reflections every 60 min
2316 independent reflections intensity decay: none
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullSecondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.037Hydrogen site location: inferred from neighbouring sites
wR(F2) = 0.097H-atom parameters constrained
S = 1.11 w = 1/[σ2(Fo2) + (0.0454P)2 + 0.1839P]
where P = (Fo2 + 2Fc2)/3
2316 reflections(Δ/σ)max = 0.001
184 parametersΔρmax = 0.22 e Å3
0 restraintsΔρmin = 0.23 e Å3
Crystal data top
C15H15NO4γ = 72.369 (11)°
Mr = 273.28V = 660.09 (11) Å3
Triclinic, P1Z = 2
a = 7.5616 (7) ÅMo Kα radiation
b = 8.9033 (8) ŵ = 0.10 mm1
c = 10.8091 (11) ÅT = 293 K
α = 72.764 (11)°0.18 × 0.15 × 0.11 mm
β = 80.486 (12)°
Data collection top
Nonius MACH-3
diffractometer
2025 reflections with I > 2σ(I)
Absorption correction: ψ scan
(North et al., 1968)
Rint = 0.010
Tmin = 0.982, Tmax = 0.9892 standard reflections every 60 min
2890 measured reflections intensity decay: none
2316 independent reflections
Refinement top
R[F2 > 2σ(F2)] = 0.0370 restraints
wR(F2) = 0.097H-atom parameters constrained
S = 1.11Δρmax = 0.22 e Å3
2316 reflectionsΔρmin = 0.23 e Å3
184 parameters
Special details top

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) top
xyzUiso*/Ueq
O10.29809 (15)0.41843 (13)0.00498 (10)0.0446 (3)
O20.04087 (14)0.29519 (13)0.08496 (10)0.0421 (3)
H20.11390.38460.05810.063*
O30.02584 (16)0.06248 (14)0.30007 (11)0.0499 (3)
H30.07240.08520.23130.075*
N10.29935 (17)0.38335 (15)0.28995 (12)0.0388 (3)
O40.19177 (16)0.40313 (16)0.45054 (11)0.0538 (3)
C80.2679 (2)0.30815 (17)0.09849 (13)0.0353 (3)
C40.2670 (2)0.26334 (19)0.41090 (14)0.0403 (3)
H4A0.24450.31210.48320.048*
H4B0.37680.17120.42520.048*
C100.3067 (2)0.04870 (17)0.24458 (14)0.0372 (3)
C70.07412 (19)0.30768 (16)0.17048 (13)0.0322 (3)
C50.0984 (2)0.20371 (18)0.40485 (13)0.0375 (3)
H50.07300.12290.48470.045*
C60.1144 (2)0.14491 (16)0.28236 (13)0.0348 (3)
C90.3957 (2)0.14516 (17)0.14346 (14)0.0372 (3)
C10.0622 (2)0.35556 (19)0.37706 (14)0.0383 (3)
C30.1375 (2)0.52059 (18)0.25385 (15)0.0406 (3)
H3A0.16610.59590.17280.049*
H3B0.09690.57880.32090.049*
C20.0144 (2)0.44610 (17)0.23864 (13)0.0362 (3)
H2A0.12200.52750.19700.043*
C140.5784 (2)0.0851 (2)0.09741 (16)0.0486 (4)
H140.63750.15030.03010.058*
C110.3986 (2)0.11196 (19)0.30063 (17)0.0496 (4)
H110.33940.17770.36740.059*
C120.5804 (3)0.1720 (2)0.25488 (18)0.0565 (5)
H120.64420.27940.29170.068*
C130.6699 (2)0.0751 (2)0.15496 (19)0.0572 (5)
H130.79270.11830.12630.069*
C180.4702 (3)0.4293 (3)0.2839 (2)0.0624 (5)
H18A0.48810.50360.20100.094*
H18B0.57370.33340.29490.094*
H18C0.46150.48110.35170.094*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
O10.0438 (6)0.0456 (6)0.0332 (5)0.0098 (5)0.0005 (4)0.0021 (5)
O20.0419 (6)0.0433 (6)0.0416 (6)0.0060 (5)0.0146 (5)0.0116 (5)
O30.0522 (7)0.0488 (6)0.0518 (7)0.0269 (5)0.0081 (5)0.0022 (5)
N10.0360 (7)0.0438 (7)0.0393 (7)0.0155 (5)0.0001 (5)0.0116 (5)
O40.0413 (6)0.0706 (8)0.0475 (7)0.0118 (6)0.0104 (5)0.0234 (6)
C80.0365 (8)0.0379 (7)0.0285 (7)0.0084 (6)0.0037 (6)0.0053 (6)
C40.0413 (8)0.0455 (8)0.0329 (7)0.0085 (6)0.0075 (6)0.0095 (6)
C100.0422 (8)0.0343 (7)0.0341 (7)0.0068 (6)0.0073 (6)0.0090 (6)
C70.0320 (7)0.0335 (7)0.0289 (7)0.0067 (6)0.0057 (5)0.0053 (6)
C50.0396 (8)0.0413 (8)0.0268 (7)0.0130 (6)0.0003 (6)0.0013 (6)
C60.0369 (8)0.0322 (7)0.0338 (7)0.0120 (6)0.0039 (6)0.0030 (6)
C90.0378 (8)0.0387 (8)0.0326 (7)0.0050 (6)0.0052 (6)0.0101 (6)
C10.0350 (8)0.0468 (8)0.0360 (8)0.0141 (6)0.0002 (6)0.0135 (6)
C30.0491 (9)0.0361 (8)0.0370 (8)0.0128 (7)0.0008 (6)0.0100 (6)
C20.0349 (7)0.0354 (7)0.0339 (7)0.0028 (6)0.0053 (6)0.0080 (6)
C140.0396 (8)0.0557 (10)0.0453 (9)0.0037 (7)0.0015 (7)0.0160 (8)
C110.0600 (10)0.0342 (8)0.0482 (9)0.0044 (7)0.0111 (8)0.0064 (7)
C120.0617 (11)0.0404 (9)0.0592 (11)0.0085 (8)0.0191 (9)0.0161 (8)
C130.0442 (9)0.0592 (11)0.0625 (11)0.0096 (8)0.0102 (8)0.0286 (9)
C180.0456 (10)0.0723 (12)0.0787 (13)0.0281 (9)0.0013 (9)0.0244 (10)
Geometric parameters (Å, º) top
O1—C81.2285 (17)C5—C11.509 (2)
O2—C71.4171 (16)C5—C61.537 (2)
O2—H20.82C5—H50.98
O3—C61.4219 (17)C9—C141.387 (2)
O3—H30.82C1—C21.517 (2)
N1—C31.453 (2)C3—C21.541 (2)
N1—C181.455 (2)C3—H3A0.97
N1—C41.4613 (19)C3—H3B0.97
O4—C11.2088 (18)C2—H2A0.98
C8—C91.471 (2)C14—C131.385 (3)
C8—C71.5419 (19)C14—H140.93
C4—C51.539 (2)C11—C121.381 (3)
C4—H4A0.97C11—H110.93
C4—H4B0.97C12—C131.388 (3)
C10—C111.386 (2)C12—H120.93
C10—C91.391 (2)C13—H130.93
C10—C61.507 (2)C18—H18A0.96
C7—C21.5443 (19)C18—H18B0.96
C7—C61.5720 (18)C18—H18C0.96
C7—O2—H2109.5C14—C9—C8128.62 (14)
C6—O3—H3109.5C10—C9—C8109.95 (13)
C3—N1—C18114.32 (13)O4—C1—C5128.54 (14)
C3—N1—C4113.97 (12)O4—C1—C2126.94 (14)
C18—N1—C4112.86 (13)C5—C1—C2104.23 (11)
O1—C8—C9126.53 (13)N1—C3—C2105.67 (11)
O1—C8—C7124.00 (12)N1—C3—H3A110.6
C9—C8—C7108.44 (12)C2—C3—H3A110.6
N1—C4—C5110.50 (11)N1—C3—H3B110.6
N1—C4—H4A109.6C2—C3—H3B110.6
C5—C4—H4A109.6H3A—C3—H3B108.7
N1—C4—H4B109.6C1—C2—C3103.05 (11)
C5—C4—H4B109.6C1—C2—C7103.33 (11)
H4A—C4—H4B108.1C3—C2—C7109.41 (11)
C11—C10—C9120.35 (15)C1—C2—H2A113.4
C11—C10—C6128.39 (14)C3—C2—H2A113.4
C9—C10—C6111.20 (12)C7—C2—H2A113.4
O2—C7—C8108.50 (11)C13—C14—C9117.82 (16)
O2—C7—C2114.39 (11)C13—C14—H14121.1
C8—C7—C2115.40 (12)C9—C14—H14121.1
O2—C7—C6108.69 (11)C12—C11—C10118.30 (16)
C8—C7—C6103.21 (11)C12—C11—H11120.8
C2—C7—C6105.84 (11)C10—C11—H11120.8
C1—C5—C6100.91 (11)C11—C12—C13121.32 (15)
C1—C5—C4105.49 (12)C11—C12—H12119.3
C6—C5—C4112.08 (11)C13—C12—H12119.3
C1—C5—H5112.5C14—C13—C12120.77 (16)
C6—C5—H5112.5C14—C13—H13119.6
C4—C5—H5112.5C12—C13—H13119.6
O3—C6—C10114.30 (12)N1—C18—H18A109.5
O3—C6—C5106.98 (11)N1—C18—H18B109.5
C10—C6—C5114.94 (12)H18A—C18—H18B109.5
O3—C6—C7111.58 (11)N1—C18—H18C109.5
C10—C6—C7104.97 (11)H18A—C18—H18C109.5
C5—C6—C7103.62 (11)H18B—C18—H18C109.5
C14—C9—C10121.43 (14)
C3—N1—C4—C554.45 (16)C6—C10—C9—C14176.26 (14)
C18—N1—C4—C5172.98 (13)C11—C10—C9—C8178.38 (14)
O1—C8—C7—O266.57 (17)C6—C10—C9—C84.31 (17)
C9—C8—C7—O2102.49 (12)O1—C8—C9—C144.9 (3)
O1—C8—C7—C263.28 (18)C7—C8—C9—C14173.60 (15)
C9—C8—C7—C2127.66 (12)O1—C8—C9—C10174.49 (14)
O1—C8—C7—C6178.24 (13)C7—C8—C9—C105.78 (16)
C9—C8—C7—C612.70 (14)C6—C5—C1—O4138.05 (16)
N1—C4—C5—C156.97 (15)C4—C5—C1—O4105.16 (17)
N1—C4—C5—C651.96 (16)C6—C5—C1—C247.81 (13)
C11—C10—C6—O348.1 (2)C4—C5—C1—C268.98 (13)
C9—C10—C6—O3134.84 (13)C18—N1—C3—C2168.31 (13)
C11—C10—C6—C576.15 (19)C4—N1—C3—C259.82 (15)
C9—C10—C6—C5100.88 (14)O4—C1—C2—C398.33 (17)
C11—C10—C6—C7170.70 (15)C5—C1—C2—C375.95 (13)
C9—C10—C6—C712.27 (15)O4—C1—C2—C7147.76 (15)
C1—C5—C6—O380.03 (13)C5—C1—C2—C737.97 (14)
C4—C5—C6—O3168.14 (11)N1—C3—C2—C169.03 (14)
C1—C5—C6—C10151.91 (12)N1—C3—C2—C740.39 (14)
C4—C5—C6—C1040.09 (16)O2—C7—C2—C1106.41 (13)
C1—C5—C6—C737.97 (13)C8—C7—C2—C1126.65 (12)
C4—C5—C6—C773.85 (14)C6—C7—C2—C113.22 (14)
O2—C7—C6—O323.90 (15)O2—C7—C2—C3144.35 (12)
C8—C7—C6—O3138.96 (12)C8—C7—C2—C317.42 (15)
C2—C7—C6—O399.41 (13)C6—C7—C2—C396.02 (13)
O2—C7—C6—C10100.42 (12)C10—C9—C14—C130.3 (2)
C8—C7—C6—C1014.64 (14)C8—C9—C14—C13178.97 (15)
C2—C7—C6—C10136.28 (11)C9—C10—C11—C121.0 (2)
O2—C7—C6—C5138.66 (11)C6—C10—C11—C12175.81 (15)
C8—C7—C6—C5106.28 (12)C10—C11—C12—C130.3 (3)
C2—C7—C6—C515.36 (14)C9—C14—C13—C120.4 (3)
C11—C10—C9—C141.1 (2)C11—C12—C13—C140.4 (3)
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
O3—H3···O20.822.112.6080 (15)119
O2—H2···O1i0.821.902.7168 (15)175
C2—H2A···O1i0.982.593.3778 (18)137
C18—H18A···O1ii0.962.573.432 (2)149
C5—H5···O3iii0.982.463.4381 (18)173
Symmetry codes: (i) x, y+1, z; (ii) x+1, y+1, z; (iii) x, y, z+1.

Experimental details

Crystal data
Chemical formulaC15H15NO4
Mr273.28
Crystal system, space groupTriclinic, P1
Temperature (K)293
a, b, c (Å)7.5616 (7), 8.9033 (8), 10.8091 (11)
α, β, γ (°)72.764 (11), 80.486 (12), 72.369 (11)
V3)660.09 (11)
Z2
Radiation typeMo Kα
µ (mm1)0.10
Crystal size (mm)0.18 × 0.15 × 0.11
Data collection
DiffractometerNonius MACH-3
diffractometer
Absorption correctionψ scan
(North et al., 1968)
Tmin, Tmax0.982, 0.989
No. of measured, independent and
observed [I > 2σ(I)] reflections
2890, 2316, 2025
Rint0.010
(sin θ/λ)max1)0.594
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.037, 0.097, 1.11
No. of reflections2316
No. of parameters184
H-atom treatmentH-atom parameters constrained
Δρmax, Δρmin (e Å3)0.22, 0.23

Computer programs: CAD-4 EXPRESS (Enraf–Nonius, 1994), XCAD4 (Harms & Wocadlo, 1996), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), PLATON (Spek, 2009).

Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
O3—H3···O20.822.112.6080 (15)119
O2—H2···O1i0.821.902.7168 (15)175
C2—H2A···O1i0.982.593.3778 (18)137
C18—H18A···O1ii0.962.573.432 (2)149
C5—H5···O3iii0.982.463.4381 (18)173
Symmetry codes: (i) x, y+1, z; (ii) x+1, y+1, z; (iii) x, y, z+1.
 

Acknowledgements

JS and UCN thank the Management of the Madura College, Madurai, for their constant support.

References

First citationCremer, D. & Pople, J. A. (1975). J. Am. Chem. Soc. 97, 1354–1358.  CrossRef CAS Web of Science Google Scholar
First citationEnraf–Nonius (1994). CAD-4 EXPRESS. Enraf–Nonius, Delft, The Netherlands.  Google Scholar
First citationHarms, K. & Wocadlo, S. (1996). XCAD4. University of Marburg, Germany.  Google Scholar
First citationNorth, A. C. T., Phillips, D. C. & Mathews, F. S. (1968). Acta Cryst. A24, 351–359.  CrossRef IUCr Journals Web of Science Google Scholar
First citationSheldrick, G. M. (2008). Acta Cryst. A64, 112–122.  Web of Science CrossRef CAS IUCr Journals Google Scholar
First citationSpek, A. L. (2009). Acta Cryst. D65, 148–155.  Web of Science CrossRef CAS IUCr Journals Google Scholar
First citationWatson, P. S., Jiang, B. & Scott, B. (2000). Org. Lett. 2, 3679–3681.  Web of Science CrossRef PubMed CAS Google Scholar

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