supplementary materials


rk2403 scheme

Acta Cryst. (2013). E69, o1055    [ doi:10.1107/S1600536813014967 ]

Methyl 4-anilino-2',5-dioxo-1',2'-dihydro-5H-spiro[furan-2,3'-indole]-3-carboxylate

R. Gangadharan, K. Sethusankar, S. E. Kiruthika and P. T. Perumal

Abstract top

In the title compound, C19H14N2O5, the spiro junction links an oxindole moeity and a furan ring, which subtend a dihedral angle of 83.49 (6)°. The molecular structure features an N-H...O hydrogen bond, which generates an S(6) ring motif. The crystal packing is governed by two N-H...O interactions, one of which generates a centrosymmetric R22(14) dimer. The other N-H...O interaction along with a C-H...O hydrogen bond contributes to the formation of a C22[R22(9)] dimeric chain running along the b-axis direction.

Comment top

Spiro compounds represent an important class of naturally occurring substances characterized by highly pronounced biological properties (Kobayashi et al., 1991). Among them the spiro oxindole is an important structural motif which finds wide applications as antimicrobial and antitumour agents and as inhibitors of the human NK1 receptor (Kornet & Thio, 1976). The construction of polyfunctionalized furans, spiro furans and furan fused cycloalkanes are important from the standpoint of synthesis of biologically active natural products such as aflatoxin, asteltoxin, monensin, panacene etc., (Schoop et al., 2000).

The bond lengths and angles in the title compound are within normal ranges except those at the spiro junction which reflects the presence of bulky subsituents. The dihedral angle between the five (C5/C6/N1/C7/C8) and six membered (C1–C6) rings in the indole group is 4.15 (8)°. The indole moeity is orthogonal to the furan ring as indicated by the dihedral angle of 83.49 (6)° between them.

The furan ring in the structure adopts a twisted conformation with a psuedo–twofold axis passing through the C8 atom and C9–C10 bond. The puckering parameters (Cremer & Pople, 1975) for the furan ring are q2 = 0.0810 (14)Å, φ2 = 310.9 (10)°. The benzene ring is bisectionally oriented to the furan ring, the dihedral angle between them being 52.80 (8)°. The title compound exihibits structural similarities with an already reported related structure (Gayathri et al., 2006.)

The molecular structure is stabilized by intramolecular N2—H2A···O5 bond which generates S(6) ring motif. Atom N2 acts as a donor to O1ii generating a centrosymmetric dimer with graph set descriptor of R22(14) (Bernstein et al., 1995). The bifurcated H bond at O1 facilitates the C13—H13···O1iii bond, which together with a N1—H1(A)···O5i interaction forms a non–centrosymmetric R22(9) dimer. These non–centrosymmetric dimers aggregate to form C22[R22(9)] supramolecular chains running along the b axis. The symmetry codes for the interactions are: (i) 1-x, -1/2+y, 3/2-z; (ii) 1-x, 1-y, 2-z; (iii) 1-x, 1/2+y, 3/2-z.

Related literature top

For applications of spiro oxindoles, see: Kornet & Thio (1976); Kobayashi et al. (1991). For applications of furans, see: Schoop et al. (2000). For puckering and asymmetry parameters, see: Cremer & Pople (1975). For a related structure, see: Gayathri et al. (2006). For graph-set notation, see: Bernstein et al. (1995).

Experimental top

Isatin (1 mmol), aromatic amine (1 mmol), and dimethyl acetylene dicarboxylate (1 mmol) were stirred at room temperature in methanol in the presence of triethylamine (20 mol%) for 4 hrs to give the spirolactones which was filtered out and recrystallized from methanol to afford the pure product (85% yield) as yellow solid.

Refinement top

Positions of the H atoms were localized from the difference electron density maps and their distances were geometrically constrained. The H atoms of amine groups were refined freely with Uiso(H) = 1.2Ueq(N). The atoms bound to the C atoms were treated as riding atoms with d(C—H) = 0.93Å and Uiso(H) = 1.2Ueq(C) for aromatic H; d(C—H) = 0.96Å and Uiso(H) = 1.5Ueq(C) for methyl H. The rotation angle methyl group was optimized by least squares.

Computing details top

Data collection: APEX2 (Bruker, 2008); cell refinement: SAINT (Bruker, 2008); data reduction: SAINT (Bruker, 2008); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008) and PLATON (Spek, 2009).

Figures top
[Figure 1] Fig. 1. The molecular structure of the title compound with the atom numbering scheme. Displacement ellipsoids are drawn at 30% probability level. H atoms are present as small spheres of arbitary radius.
[Figure 2] Fig. 2. Part of crystal structure of the title compound, showing the formation the intramolecular S(6) ring motif, R22(14) centrosymmetric and the R22(9) non–centrosymmetric dimers as viewed along the b–axis.
Methyl 4-anilino-2',5-dioxo-1',2'-dihydro-5H-spiro[furan-2,3'-indole]-3-carboxylate top
Crystal data top
C19H14N2O5F(000) = 728
Mr = 350.32Dx = 1.411 Mg m3
Monoclinic, P21/cMo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybcCell parameters from 3502 reflections
a = 12.1713 (6) Åθ = 2.4–31.0°
b = 13.6144 (7) ŵ = 0.10 mm1
c = 10.9602 (6) ÅT = 296 K
β = 114.813 (2)°Block, yellow
V = 1648.50 (15) Å30.30 × 0.25 × 0.20 mm
Z = 4
Data collection top
Bruker SMART APEXII CCD
diffractometer
5167 independent reflections
Radiation source: fine–focus sealed tube3502 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.026
ω– and φ–scansθmax = 31.0°, θmin = 2.4°
Absorption correction: multi-scan
(SADABS; Bruker, 2008)
h = 1717
Tmin = 0.969, Tmax = 0.979k = 1919
20901 measured reflectionsl = 1515
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)] = 0.044Hydrogen site location: inferred from neighbouring sites
wR(F2) = 0.132H atoms treated by a mixture of independent and constrained refinement
S = 1.00 w = 1/[σ2(Fo2) + (0.0665P)2 + 0.2325P]
where P = (Fo2 + 2Fc2)/3
5167 reflections(Δ/σ)max = 0.001
242 parametersΔρmax = 0.26 e Å3
2 restraintsΔρmin = 0.16 e Å3
Crystal data top
C19H14N2O5V = 1648.50 (15) Å3
Mr = 350.32Z = 4
Monoclinic, P21/cMo Kα radiation
a = 12.1713 (6) ŵ = 0.10 mm1
b = 13.6144 (7) ÅT = 296 K
c = 10.9602 (6) Å0.30 × 0.25 × 0.20 mm
β = 114.813 (2)°
Data collection top
Bruker SMART APEXII CCD
diffractometer
5167 independent reflections
Absorption correction: multi-scan
(SADABS; Bruker, 2008)
3502 reflections with I > 2σ(I)
Tmin = 0.969, Tmax = 0.979Rint = 0.026
20901 measured reflectionsθmax = 31.0°
Refinement top
R[F2 > 2σ(F2)] = 0.044H atoms treated by a mixture of independent and constrained refinement
wR(F2) = 0.132Δρmax = 0.26 e Å3
S = 1.00Δρmin = 0.16 e Å3
5167 reflectionsAbsolute structure: ?
242 parametersAbsolute structure parameter: ?
2 restraintsRogers parameter: ?
Special details top

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 > 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
C10.20517 (13)0.35795 (13)0.37822 (16)0.0565 (4)
H10.22330.30540.33550.068*
C20.13329 (14)0.43504 (14)0.30526 (17)0.0602 (4)
H20.10410.43470.21210.072*
C30.10387 (14)0.51262 (13)0.36769 (17)0.0573 (4)
H30.05550.56350.31640.069*
C40.14643 (12)0.51452 (11)0.50668 (16)0.0494 (3)
H40.12650.56600.54940.059*
C50.21853 (11)0.43880 (9)0.57979 (14)0.0401 (3)
C60.24886 (11)0.36182 (10)0.51613 (15)0.0444 (3)
C70.35751 (11)0.32910 (9)0.73933 (15)0.0440 (3)
C80.28153 (11)0.42334 (9)0.72866 (14)0.0383 (3)
C90.21736 (12)0.45201 (9)0.89615 (14)0.0416 (3)
C100.30798 (11)0.53001 (8)0.90518 (13)0.0355 (3)
C110.35077 (10)0.50798 (8)0.81357 (13)0.0346 (3)
C120.45130 (11)0.55592 (8)0.79841 (13)0.0347 (3)
C130.57815 (14)0.55042 (11)0.68384 (18)0.0541 (4)
H13A0.55930.61500.64550.081*
H13B0.59510.50790.62400.081*
H13C0.64770.55370.76860.081*
C140.26345 (11)0.64409 (9)1.05174 (13)0.0383 (3)
C150.30989 (14)0.68008 (11)1.18109 (15)0.0512 (3)
H150.39240.67581.23550.061*
C160.23301 (17)0.72265 (13)1.22943 (18)0.0649 (5)
H160.26430.74721.31670.078*
C170.11072 (16)0.72924 (13)1.1501 (2)0.0646 (5)
H170.05940.75651.18440.077*
C180.06501 (14)0.69548 (12)1.02087 (18)0.0592 (4)
H180.01740.70070.96650.071*
C190.14097 (12)0.65364 (11)0.97084 (15)0.0482 (3)
H190.10970.63180.88230.058*
N10.32908 (11)0.29741 (8)0.61328 (14)0.0520 (3)
H1A0.3634 (15)0.2457 (10)0.5943 (17)0.062*
N20.34264 (10)0.60019 (8)1.00166 (11)0.0393 (2)
H2A0.4110 (10)0.6321 (10)1.0165 (15)0.047*
O10.42614 (9)0.29188 (7)0.84405 (12)0.0569 (3)
O20.19725 (8)0.39643 (6)0.78561 (10)0.0445 (2)
O30.17129 (11)0.43426 (8)0.96977 (12)0.0624 (3)
O40.47595 (8)0.51218 (7)0.70423 (10)0.0448 (2)
O50.50692 (8)0.62507 (6)0.86606 (9)0.0437 (2)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
C10.0449 (8)0.0651 (9)0.0649 (10)0.0106 (7)0.0283 (8)0.0261 (8)
C20.0438 (8)0.0854 (12)0.0511 (8)0.0129 (8)0.0197 (7)0.0146 (8)
C30.0410 (7)0.0686 (10)0.0575 (9)0.0008 (7)0.0160 (7)0.0021 (8)
C40.0405 (7)0.0483 (7)0.0580 (9)0.0048 (6)0.0193 (7)0.0054 (6)
C50.0299 (6)0.0394 (6)0.0516 (7)0.0037 (5)0.0176 (5)0.0109 (5)
C60.0326 (6)0.0419 (6)0.0606 (8)0.0062 (5)0.0214 (6)0.0156 (6)
C70.0325 (6)0.0304 (6)0.0668 (9)0.0038 (5)0.0186 (6)0.0062 (6)
C80.0319 (6)0.0315 (5)0.0542 (8)0.0026 (4)0.0206 (6)0.0058 (5)
C90.0381 (6)0.0368 (6)0.0533 (8)0.0040 (5)0.0224 (6)0.0004 (5)
C100.0325 (5)0.0308 (5)0.0442 (7)0.0012 (4)0.0172 (5)0.0022 (5)
C110.0321 (5)0.0280 (5)0.0444 (7)0.0020 (4)0.0167 (5)0.0012 (5)
C120.0330 (5)0.0303 (5)0.0428 (6)0.0001 (4)0.0180 (5)0.0019 (5)
C130.0546 (8)0.0540 (8)0.0706 (10)0.0117 (7)0.0429 (8)0.0084 (7)
C140.0401 (6)0.0340 (6)0.0454 (7)0.0037 (5)0.0224 (6)0.0024 (5)
C150.0463 (7)0.0529 (8)0.0534 (8)0.0050 (6)0.0199 (7)0.0145 (7)
C160.0698 (11)0.0678 (10)0.0635 (10)0.0082 (8)0.0344 (9)0.0273 (8)
C170.0623 (10)0.0629 (10)0.0865 (12)0.0056 (8)0.0489 (10)0.0207 (9)
C180.0431 (8)0.0626 (9)0.0763 (11)0.0004 (7)0.0293 (8)0.0091 (8)
C190.0426 (7)0.0546 (8)0.0484 (8)0.0004 (6)0.0200 (6)0.0041 (6)
N10.0444 (6)0.0364 (6)0.0737 (8)0.0024 (5)0.0234 (6)0.0171 (5)
N20.0356 (5)0.0399 (5)0.0455 (6)0.0056 (4)0.0200 (5)0.0062 (4)
O10.0459 (5)0.0375 (5)0.0743 (7)0.0017 (4)0.0126 (5)0.0019 (5)
O20.0383 (5)0.0376 (4)0.0634 (6)0.0096 (4)0.0268 (5)0.0070 (4)
O30.0679 (7)0.0621 (7)0.0739 (7)0.0214 (5)0.0460 (6)0.0051 (6)
O40.0443 (5)0.0417 (5)0.0575 (6)0.0095 (4)0.0304 (5)0.0116 (4)
O50.0456 (5)0.0371 (4)0.0523 (5)0.0115 (4)0.0245 (4)0.0072 (4)
Geometric parameters (Å, º) top
C1—C61.377 (2)C11—C121.4564 (16)
C1—C21.385 (2)C12—O51.2137 (14)
C1—H10.9300C12—O41.3304 (15)
C2—C31.385 (2)C13—O41.4498 (16)
C2—H20.9300C13—H13A0.9600
C3—C41.388 (2)C13—H13B0.9600
C3—H30.9300C13—H13C0.9600
C4—C51.3729 (19)C14—C151.3773 (19)
C4—H40.9300C14—C191.3842 (19)
C5—C61.3920 (17)C14—N21.4253 (16)
C5—C81.4981 (19)C15—C161.381 (2)
C6—N11.4083 (19)C15—H150.9300
C7—O11.2112 (17)C16—C171.376 (3)
C7—N11.3466 (19)C16—H160.9300
C7—C81.5571 (17)C17—C181.366 (2)
C8—O21.4537 (15)C17—H170.9300
C8—C111.4986 (16)C18—C191.380 (2)
C9—O31.1847 (16)C18—H180.9300
C9—O21.3607 (17)C19—H190.9300
C9—C101.5041 (17)N1—H1A0.887 (9)
C10—C111.3448 (17)N2—H2A0.890 (9)
C10—N21.3545 (16)
C6—C1—C2117.66 (14)O5—C12—O4124.86 (11)
C6—C1—H1121.2O5—C12—C11123.91 (11)
C2—C1—H1121.2O4—C12—C11111.20 (10)
C3—C2—C1121.61 (15)O4—C13—H13A109.5
C3—C2—H2119.2O4—C13—H13B109.5
C1—C2—H2119.2H13A—C13—H13B109.5
C2—C3—C4120.11 (16)O4—C13—H13C109.5
C2—C3—H3119.9H13A—C13—H13C109.5
C4—C3—H3119.9H13B—C13—H13C109.5
C5—C4—C3118.64 (14)C15—C14—C19119.50 (12)
C5—C4—H4120.7C15—C14—N2119.60 (12)
C3—C4—H4120.7C19—C14—N2120.88 (12)
C4—C5—C6120.78 (13)C14—C15—C16119.51 (14)
C4—C5—C8130.60 (12)C14—C15—H15120.2
C6—C5—C8108.54 (11)C16—C15—H15120.2
C1—C6—C5121.17 (14)C17—C16—C15120.83 (15)
C1—C6—N1129.20 (13)C17—C16—H16119.6
C5—C6—N1109.60 (12)C15—C16—H16119.6
O1—C7—N1128.04 (12)C18—C17—C16119.66 (14)
O1—C7—C8124.58 (13)C18—C17—H17120.2
N1—C7—C8107.35 (12)C16—C17—H17120.2
O2—C8—C11103.98 (10)C17—C18—C19120.11 (15)
O2—C8—C5111.85 (10)C17—C18—H18119.9
C11—C8—C5117.76 (11)C19—C18—H18119.9
O2—C8—C7105.30 (10)C18—C19—C14120.34 (14)
C11—C8—C7115.18 (10)C18—C19—H19119.8
C5—C8—C7102.35 (10)C14—C19—H19119.8
O3—C9—O2122.23 (12)C7—N1—C6111.98 (11)
O3—C9—C10130.00 (13)C7—N1—H1A123.5 (11)
O2—C9—C10107.70 (11)C6—N1—H1A124.2 (11)
C11—C10—N2130.46 (11)C10—N2—C14123.85 (10)
C11—C10—C9107.25 (11)C10—N2—H2A116.6 (10)
N2—C10—C9121.93 (11)C14—N2—H2A117.1 (10)
C10—C11—C12126.32 (11)C9—O2—C8110.27 (9)
C10—C11—C8110.00 (10)C12—O4—C13116.51 (10)
C12—C11—C8123.66 (11)
C6—C1—C2—C31.3 (2)C5—C8—C11—C10122.03 (12)
C1—C2—C3—C40.1 (2)C7—C8—C11—C10117.01 (12)
C2—C3—C4—C50.6 (2)O2—C8—C11—C12176.23 (10)
C3—C4—C5—C60.3 (2)C5—C8—C11—C1259.41 (15)
C3—C4—C5—C8176.78 (13)C7—C8—C11—C1261.55 (16)
C2—C1—C6—C52.1 (2)C10—C11—C12—O52.4 (2)
C2—C1—C6—N1175.49 (13)C8—C11—C12—O5179.27 (12)
C4—C5—C6—C11.7 (2)C10—C11—C12—O4175.70 (12)
C8—C5—C6—C1178.88 (12)C8—C11—C12—O42.62 (16)
C4—C5—C6—N1176.37 (12)C19—C14—C15—C161.8 (2)
C8—C5—C6—N10.85 (14)N2—C14—C15—C16179.97 (14)
C4—C5—C8—O273.88 (17)C14—C15—C16—C170.2 (3)
C6—C5—C8—O2109.27 (11)C15—C16—C17—C181.6 (3)
C4—C5—C8—C1146.46 (18)C16—C17—C18—C191.0 (3)
C6—C5—C8—C11130.39 (11)C17—C18—C19—C141.1 (2)
C4—C5—C8—C7173.86 (13)C15—C14—C19—C182.5 (2)
C6—C5—C8—C72.99 (12)N2—C14—C19—C18179.41 (13)
O1—C7—C8—O265.30 (15)O1—C7—N1—C6177.92 (13)
N1—C7—C8—O2112.83 (12)C8—C7—N1—C64.03 (15)
O1—C7—C8—C1148.61 (18)C1—C6—N1—C7175.69 (14)
N1—C7—C8—C11133.26 (12)C5—C6—N1—C72.14 (15)
O1—C7—C8—C5177.64 (12)C11—C10—N2—C14151.63 (13)
N1—C7—C8—C54.23 (13)C9—C10—N2—C1436.17 (18)
O3—C9—C10—C11167.68 (15)C15—C14—N2—C10152.11 (13)
O2—C9—C10—C119.33 (14)C19—C14—N2—C1029.79 (18)
O3—C9—C10—N26.1 (2)O3—C9—O2—C8169.38 (13)
O2—C9—C10—N2176.87 (11)C10—C9—O2—C87.91 (14)
N2—C10—C11—C121.5 (2)C11—C8—O2—C93.79 (13)
C9—C10—C11—C12171.60 (11)C5—C8—O2—C9131.88 (11)
N2—C10—C11—C8179.99 (12)C7—C8—O2—C9117.71 (11)
C9—C10—C11—C86.91 (14)O5—C12—O4—C130.81 (19)
O2—C8—C11—C102.33 (13)C11—C12—O4—C13177.27 (11)
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
N1—H1A···O5i0.89 (2)2.19 (2)3.0268 (16)157 (2)
N2—H2A···O1ii0.89 (1)2.19 (1)2.9907 (17)149 (1)
N2—H2A···O50.89 (1)2.39 (2)2.9691 (16)123 (1)
C13—H13A···O1iii0.962.413.2999 (18)153
Symmetry codes: (i) x+1, y1/2, z+3/2; (ii) x+1, y+1, z+2; (iii) x+1, y+1/2, z+3/2.

Experimental details

Crystal data
Chemical formulaC19H14N2O5
Mr350.32
Crystal system, space groupMonoclinic, P21/c
Temperature (K)296
a, b, c (Å)12.1713 (6), 13.6144 (7), 10.9602 (6)
β (°) 114.813 (2)
V3)1648.50 (15)
Z4
Radiation typeMo Kα
µ (mm1)0.10
Crystal size (mm)0.30 × 0.25 × 0.20
Data collection
DiffractometerBruker SMART APEXII CCD
diffractometer
Absorption correctionMulti-scan
(SADABS; Bruker, 2008)
Tmin, Tmax0.969, 0.979
No. of measured, independent and
observed [I > 2σ(I)] reflections
20901, 5167, 3502
Rint0.026
(sin θ/λ)max1)0.725
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.044, 0.132, 1.00
No. of reflections5167
No. of parameters242
No. of restraints2
H-atom treatmentH atoms treated by a mixture of independent and constrained refinement
Δρmax, Δρmin (e Å3)0.26, 0.16

Computer programs: APEX2 (Bruker, 2008), SAINT (Bruker, 2008), SHELXS97 (Sheldrick, 2008), ORTEP-3 for Windows (Farrugia, 1997), SHELXL97 (Sheldrick, 2008) and PLATON (Spek, 2009).

Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
N1—H1A···O5i0.887 (16)2.191 (17)3.0268 (16)156.9 (15)
N2—H2A···O1ii0.891 (14)2.189 (14)2.9907 (17)149.4 (13)
N2—H2A···O50.891 (14)2.389 (15)2.9691 (16)122.95 (14)
C13—H13A···O1iii0.962.413.2999 (18)153
Symmetry codes: (i) x+1, y1/2, z+3/2; (ii) x+1, y+1, z+2; (iii) x+1, y+1/2, z+3/2.
Acknowledgements top

The authors thank Dr Babu Varghese, SAIF, IIT, Chennai, India, for the data collection.

references
References top

Bernstein, J., Davis, R. E., Shimoni, L. & Chang, N.-L. (1995). Angew. Chem. Int. Ed. Engl. 34, 1555–1573.

Bruker (2008). APEX2, SAINT and SADABS. Bruker AXS Inc., Madison, Wisconsin, U.S.A.

Cremer, D. & Pople, J. A. (1975). J. Am. Chem. Soc. 97, 1354–1358.

Farrugia, L. J. (1997). J. Appl. Cryst. 30, 565.

Gayathri, D., Velmurugan, D., Ravikumar, K., Savitha, G. & Perumal, P. T. (2006). Acta Cryst. E62, o5947–o5949.

Kobayashi, J., Tsuda, M., Agemi, K., Shigemori, H., Ishibashi, M., Sasaki, T. & Mikami, Y. (1991). Tetrahedron, 47, 6617–6622.

Kornet, M. J. & Thio, A. P. (1976). J. Med. Chem. 19, 892–898.

Schoop, A., Grieving, H. & Gohrt, A. (2000). Tetrahedron Lett. 41, 1913–1916.

Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122.

Spek, A. L. (2009). Acta Cryst. D65, 148–155.