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Acta Crystallogr Sect E Struct Rep Online. 2009 April 1; 65(Pt 4): o935.
Published online 2009 March 31. doi:  10.1107/S1600536809010770
PMCID: PMC2968801

11β,17aα-Dihydr­oxy-17aβ-methyl-d-homoandrosta-1,4-diene-3,17-dione monohydrate

Abstract

In the title compound, C21H28O4·H2O, the cyclo­hexa­dienone ring is planar (r.m.s. deviation 0.0186 Å), whereas the two cyclo­hexane rings and the cyclo­hexa­none ring adopt chair conformations. The crystal structure is stabilized by O—H(...)O and C—H(...)O hydrogen bonds.

Related literature

For general background, see: Conrow (1999 [triangle]). For details of the synthesis, see: Huo (2003 [triangle]).

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Object name is e-65-0o935-scheme1.jpg

Experimental

Crystal data

  • C21H28O4·H2O
  • M r = 362.45
  • Monoclinic, An external file that holds a picture, illustration, etc.
Object name is e-65-0o935-efi1.jpg
  • a = 6.641 (2) Å
  • b = 18.642 (6) Å
  • c = 8.017 (3) Å
  • β = 103.797 (4)°
  • V = 963.9 (6) Å3
  • Z = 2
  • Mo Kα radiation
  • μ = 0.09 mm−1
  • T = 293 K
  • 0.25 × 0.20 × 0.15 mm

Data collection

  • Bruker SMART APEX CCD area-detector diffractometer
  • Absorption correction: multi-scan (SADABS; Bruker, 2000 [triangle]) T min = 0.978, T max = 0.987
  • 4866 measured reflections
  • 2182 independent reflections
  • 1778 reflections with I > 2σ(I)
  • R int = 0.034

Refinement

  • R[F 2 > 2σ(F 2)] = 0.042
  • wR(F 2) = 0.104
  • S = 0.97
  • 2182 reflections
  • 254 parameters
  • 5 restraints
  • H atoms treated by a mixture of independent and constrained refinement
  • Δρmax = 0.24 e Å−3
  • Δρmin = −0.15 e Å−3

Data collection: SMART (Bruker, 2000 [triangle]); cell refinement: SAINT (Bruker, 2000 [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, 2008b [triangle]); software used to prepare material for publication: SHELXTL.

Table 1
Hydrogen-bond geometry (Å, °)

Supplementary Material

Crystal structure: contains datablocks global, I. DOI: 10.1107/S1600536809010770/gk2193sup1.cif

Structure factors: contains datablocks I. DOI: 10.1107/S1600536809010770/gk2193Isup2.hkl

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

supplementary crystallographic information

Comment

21-Methyl-11β,17α-dihydroxy-1,4-pregnadien-3,20-dione derivatives are intermediates in the synthesis of steroid agents (Conrow, 1999). We tried to prepare 21-methyl-11β,17α-dihydroxy-1,4-pregnadien-3,20-dione by 21-methylation of 11β,17α-dihydroxy-21-chloro-1,4-pregnadien-3,20-dione with organic metal reagent. However, the reaction of 11β,17α-dihydroxy-21-chloro-1,4-pregnadien-3,20-dione with a mixture of iodomethane, zinc dust, I2(cat.) and Cl2Ni(PPh3)2(cat.) in the solvent DMA offered the title compound as one of the main products. The crystal structure determination of the title compound was carried out in order to determine the exact molecular structure.

The molecular structure of the title compound is shown in Fig.1. The cyclohexadienone ring is planar. The two cyclohexane rings and the cyclohexanone ring adopt chair conformations. Intra- and intermolecular C—H···O and O—H···O hydrogen bonding are observed in the crystal structure.

Experimental

A mixture of CH3I (568 mg, 4 mmol) (Huo,2003), zinc dust (387 mg, 6 mmol) and I2 (50 mg, 0.2 mmol) in dry DMA was stirred under N2 at 5°C until the red color of I2 disappeared. After stirring further at 30°C for 3 h, the11β,17α-dihydroxy-21-chloro-1,4-pregnadien-3,20-dione (750 mg, 1.98 mmol) and Cl2Ni(PPh3)2 (35 mg, 0.053 mmol) were added successively. The mixture was stirred at 90°C for 4 h. The TLC showed that the starting material was consumed completely. The mixture was filtered and the filtrate was poured into ice water(150 ml) and extracted with ethyl acetate. The organic layer was washed with brine, dried over Na2SO4, filtered and concentrated under reduced pressure. The residue was isolated by chromatography on silicagel column with petroleum ether/EtOAc(1:1) as eluent to afford the pure title compound, which was recrystallized from ethyl acetate to give colorless crystals for the single-crystal X-ray diffraction analysis. Yield: 26.5%.

Refinement

In the absence of significant anomalous scattering, Friedel pairs were merged. All H atoms from C-H groups were positioned geometrically and refined using a riding model with C—H equal 0.93 - 0.98Å and Uiso(H) = 1.2Ueq(C), except methyl groups where Uiso(H) = 1.5Ueq(C). The H atoms of the OH groups were fully refined.

Figures

Fig. 1.
The molecular structure of the title compound showing displacement ellipsoids at the 30% probability level.
Fig. 2.
Crystal packing of the title compound - projection along the b axis.

Crystal data

C21H28O4·H2OF(000) = 392
Mr = 362.45Dx = 1.249 Mg m3
Monoclinic, P21Mo Kα radiation, λ = 0.71073 Å
Hall symbol: P 2ybCell parameters from 1024 reflections
a = 6.641 (2) Åθ = 2.8–27.1°
b = 18.642 (6) ŵ = 0.09 mm1
c = 8.017 (3) ÅT = 293 K
β = 103.797 (4)°Plate, colorless
V = 963.9 (6) Å30.25 × 0.20 × 0.15 mm
Z = 2

Data collection

Bruker SMART APEX CCD area-detector diffractometer2182 independent reflections
Radiation source: fine-focus sealed tube1778 reflections with I > 2σ(I)
graphiteRint = 0.034
[var phi] and ω scansθmax = 27.1°, θmin = 2.8°
Absorption correction: multi-scan (SADABS; Sheldrick, 2008a)h = −8→8
Tmin = 0.978, Tmax = 0.987k = −23→23
4866 measured reflectionsl = −10→6

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)] = 0.042Hydrogen site location: inferred from neighbouring sites
wR(F2) = 0.104H atoms treated by a mixture of independent and constrained refinement
S = 0.97w = 1/[σ2(Fo2) + (0.0626P)2] where P = (Fo2 + 2Fc2)/3
2182 reflections(Δ/σ)max < 0.001
254 parametersΔρmax = 0.24 e Å3
5 restraintsΔρmin = −0.15 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*/Ueq
O10.1926 (4)0.41488 (11)0.9326 (3)0.0555 (5)
O20.6229 (3)0.61548 (11)0.4180 (3)0.0509 (5)
H2X0.691 (4)0.6524 (13)0.440 (4)0.055 (10)*
O30.1121 (4)0.77632 (12)−0.2435 (3)0.0583 (6)
O4−0.0063 (3)0.78258 (11)0.1395 (3)0.0499 (5)
H4X−0.050 (4)0.8214 (11)0.102 (4)0.039 (8)*
O50.8509 (4)0.74238 (15)0.4291 (4)0.0686 (7)
H5Y0.936 (5)0.756 (3)0.523 (4)0.089 (15)*
H5X0.894 (6)0.750 (2)0.340 (4)0.086 (14)*
C10.4634 (4)0.52002 (13)0.6909 (4)0.0427 (6)
H10.57530.55140.71110.051*
C20.4114 (4)0.49166 (14)0.8260 (4)0.0433 (6)
H20.48940.50280.93550.052*
C30.2351 (4)0.44350 (13)0.8073 (4)0.0409 (6)
C40.1154 (4)0.43065 (13)0.6331 (4)0.0403 (6)
H4−0.00300.40230.61690.048*
C50.1682 (4)0.45781 (13)0.4951 (3)0.0371 (6)
C60.0458 (4)0.44352 (13)0.3178 (4)0.0437 (6)
H6A−0.07880.41730.32300.052*
H6B0.12650.41360.25910.052*
C7−0.0148 (4)0.51213 (14)0.2155 (4)0.0418 (6)
H7A−0.12140.53640.25850.050*
H7B−0.07380.49970.09640.050*
C80.1668 (4)0.56397 (12)0.2240 (3)0.0328 (5)
H80.26640.54170.16730.039*
C90.2742 (4)0.57722 (12)0.4142 (3)0.0332 (5)
H90.16320.59320.46710.040*
C100.3559 (4)0.50534 (13)0.5089 (3)0.0366 (6)
C110.4311 (4)0.63875 (13)0.4440 (4)0.0404 (6)
H110.45510.65190.56550.048*
C120.3482 (4)0.70560 (12)0.3407 (3)0.0392 (6)
H12A0.45960.74040.35420.047*
H12B0.24060.72650.38860.047*
C130.2592 (4)0.69250 (12)0.1481 (3)0.0337 (5)
C140.0878 (4)0.63452 (12)0.1298 (3)0.0342 (5)
H14−0.01770.65380.18470.041*
C15−0.0196 (5)0.62098 (16)−0.0586 (4)0.0485 (7)
H15A0.07800.5987−0.11530.058*
H15B−0.13370.5879−0.06440.058*
C16−0.1023 (5)0.68968 (17)−0.1538 (4)0.0535 (7)
H16A−0.14900.6796−0.27560.064*
H16B−0.22030.7067−0.11350.064*
C170.0599 (4)0.74649 (14)−0.1266 (4)0.0415 (6)
C17A0.1594 (4)0.76373 (13)0.0615 (3)0.0396 (6)
C180.4297 (4)0.67001 (16)0.0598 (4)0.0453 (6)
H18A0.47660.62260.09630.068*
H18B0.54370.70300.09010.068*
H18C0.37580.6704−0.06250.068*
C190.5131 (4)0.46446 (16)0.4290 (4)0.0526 (7)
H19A0.53830.41780.48050.079*
H19B0.64080.49080.44930.079*
H19C0.45750.45940.30750.079*
C200.3116 (5)0.82621 (15)0.0757 (4)0.0559 (8)
H20A0.37110.83640.19460.084*
H20B0.23950.86780.02130.084*
H20C0.41960.81360.02010.084*

Atomic displacement parameters (Å2)

U11U22U33U12U13U23
O10.0786 (14)0.0499 (11)0.0436 (12)−0.0146 (10)0.0255 (10)0.0001 (9)
O20.0356 (9)0.0403 (11)0.0753 (15)−0.0050 (8)0.0104 (10)0.0063 (10)
O30.0817 (14)0.0535 (12)0.0414 (12)−0.0120 (11)0.0181 (10)0.0069 (10)
O40.0664 (12)0.0407 (10)0.0477 (12)0.0179 (9)0.0239 (10)0.0110 (9)
O50.0723 (15)0.0808 (17)0.0542 (15)−0.0300 (13)0.0177 (13)0.0031 (13)
C10.0380 (13)0.0372 (13)0.0484 (16)−0.0063 (11)0.0014 (12)0.0077 (12)
C20.0526 (15)0.0356 (13)0.0384 (15)−0.0026 (11)0.0040 (12)0.0039 (11)
C30.0528 (14)0.0294 (12)0.0445 (16)0.0016 (11)0.0195 (12)0.0001 (11)
C40.0419 (13)0.0328 (12)0.0482 (16)−0.0065 (10)0.0147 (12)0.0030 (11)
C50.0410 (12)0.0267 (11)0.0430 (15)−0.0010 (10)0.0091 (11)−0.0008 (10)
C60.0512 (14)0.0335 (13)0.0460 (15)−0.0118 (11)0.0111 (13)−0.0026 (12)
C70.0439 (14)0.0387 (13)0.0404 (15)−0.0111 (11)0.0053 (12)−0.0011 (12)
C80.0346 (12)0.0295 (11)0.0329 (13)−0.0045 (9)0.0055 (10)−0.0011 (9)
C90.0339 (11)0.0309 (11)0.0345 (13)−0.0027 (9)0.0079 (10)0.0011 (9)
C100.0339 (12)0.0348 (13)0.0414 (14)−0.0007 (10)0.0098 (10)0.0072 (11)
C110.0410 (13)0.0378 (13)0.0385 (15)−0.0083 (10)0.0017 (11)0.0030 (11)
C120.0460 (13)0.0309 (12)0.0385 (14)−0.0086 (10)0.0056 (11)−0.0009 (10)
C130.0377 (12)0.0283 (11)0.0344 (13)−0.0044 (9)0.0073 (10)−0.0003 (9)
C140.0350 (12)0.0332 (12)0.0337 (14)−0.0032 (9)0.0069 (10)−0.0012 (10)
C150.0567 (16)0.0434 (14)0.0386 (15)−0.0154 (13)−0.0024 (13)0.0006 (12)
C160.0551 (15)0.0609 (18)0.0382 (16)−0.0120 (14)−0.0014 (13)0.0072 (13)
C170.0504 (14)0.0373 (13)0.0357 (15)0.0037 (11)0.0082 (12)0.0060 (11)
C17A0.0497 (14)0.0338 (13)0.0365 (14)0.0013 (11)0.0129 (12)0.0019 (10)
C180.0452 (13)0.0444 (14)0.0498 (17)−0.0029 (12)0.0180 (12)−0.0005 (12)
C190.0520 (16)0.0423 (15)0.068 (2)0.0121 (13)0.0239 (15)0.0125 (14)
C200.075 (2)0.0386 (15)0.0492 (18)−0.0124 (14)0.0058 (15)0.0027 (13)

Geometric parameters (Å, °)

O1—C31.228 (3)C9—H90.9800
O2—C111.407 (3)C10—C191.549 (4)
O2—H2X0.821 (19)C11—C121.525 (3)
O3—C171.209 (4)C11—H110.9800
O4—C17A1.433 (3)C12—C131.535 (4)
O4—H4X0.810 (18)C12—H12A0.9700
O5—H5Y0.86 (2)C12—H12B0.9700
O5—H5X0.85 (2)C13—C181.531 (4)
C1—C21.323 (4)C13—C141.551 (3)
C1—C101.489 (4)C13—C17A1.570 (3)
C1—H10.9300C14—C151.530 (4)
C2—C31.454 (4)C14—H140.9800
C2—H20.9300C15—C161.524 (4)
C3—C41.452 (4)C15—H15A0.9700
C4—C51.337 (4)C15—H15B0.9700
C4—H40.9300C16—C171.489 (4)
C5—C61.483 (4)C16—H16A0.9700
C5—C101.512 (3)C16—H16B0.9700
C6—C71.521 (4)C17—C17A1.529 (4)
C6—H6A0.9700C17A—C201.528 (4)
C6—H6B0.9700C18—H18A0.9599
C7—C81.535 (3)C18—H18B0.9599
C7—H7A0.9700C18—H18C0.9599
C7—H7B0.9700C19—H19A0.9599
C8—C91.541 (3)C19—H19B0.9599
C8—C141.545 (3)C19—H19C0.9599
C8—H80.9800C20—H20A0.9599
C9—C111.530 (3)C20—H20B0.9599
C9—C101.571 (3)C20—H20C0.9599
C11—O2—H2X101 (2)C13—C12—H12A108.6
C17A—O4—H4X108 (2)C11—C12—H12B108.6
H5Y—O5—H5X114 (4)C13—C12—H12B108.6
C2—C1—C10124.8 (2)H12A—C12—H12B107.6
C2—C1—H1117.6C18—C13—C12111.0 (2)
C10—C1—H1117.6C18—C13—C14111.9 (2)
C1—C2—C3121.5 (3)C12—C13—C14107.69 (19)
C1—C2—H2119.2C18—C13—C17A107.9 (2)
C3—C2—H2119.2C12—C13—C17A109.33 (19)
O1—C3—C4122.3 (2)C14—C13—C17A108.94 (19)
O1—C3—C2121.3 (3)C15—C14—C8111.5 (2)
C4—C3—C2116.3 (2)C15—C14—C13111.5 (2)
C5—C4—C3122.8 (2)C8—C14—C13113.00 (19)
C5—C4—H4118.6C15—C14—H14106.8
C3—C4—H4118.6C8—C14—H14106.8
C4—C5—C6122.2 (2)C13—C14—H14106.8
C4—C5—C10122.4 (2)C16—C15—C14112.5 (2)
C6—C5—C10115.4 (2)C16—C15—H15A109.1
C5—C6—C7112.3 (2)C14—C15—H15A109.1
C5—C6—H6A109.1C16—C15—H15B109.1
C7—C6—H6A109.1C14—C15—H15B109.1
C5—C6—H6B109.1H15A—C15—H15B107.8
C7—C6—H6B109.1C17—C16—C15111.3 (2)
H6A—C6—H6B107.9C17—C16—H16A109.4
C6—C7—C8113.7 (2)C15—C16—H16A109.4
C6—C7—H7A108.8C17—C16—H16B109.4
C8—C7—H7A108.8C15—C16—H16B109.4
C6—C7—H7B108.8H16A—C16—H16B108.0
C8—C7—H7B108.8O3—C17—C16123.0 (3)
H7A—C7—H7B107.7O3—C17—C17A122.1 (3)
C7—C8—C9108.5 (2)C16—C17—C17A114.9 (2)
C7—C8—C14110.02 (19)O4—C17A—C20110.1 (2)
C9—C8—C14111.72 (19)O4—C17A—C17106.6 (2)
C7—C8—H8108.8C20—C17A—C17110.9 (2)
C9—C8—H8108.8O4—C17A—C13107.5 (2)
C14—C8—H8108.8C20—C17A—C13114.2 (2)
C11—C9—C8114.3 (2)C17—C17A—C13107.2 (2)
C11—C9—C10114.83 (19)C13—C18—H18A109.5
C8—C9—C10111.49 (19)C13—C18—H18B109.5
C11—C9—H9105.0H18A—C18—H18B109.5
C8—C9—H9105.0C13—C18—H18C109.5
C10—C9—H9105.0H18A—C18—H18C109.5
C1—C10—C5111.9 (2)H18B—C18—H18C109.5
C1—C10—C19106.4 (2)C10—C19—H19A109.5
C5—C10—C19107.8 (2)C10—C19—H19B109.5
C1—C10—C9110.3 (2)H19A—C19—H19B109.5
C5—C10—C9106.31 (19)C10—C19—H19C109.5
C19—C10—C9114.1 (2)H19A—C19—H19C109.5
O2—C11—C12113.2 (2)H19B—C19—H19C109.5
O2—C11—C9110.6 (2)C17A—C20—H20A109.5
C12—C11—C9112.6 (2)C17A—C20—H20B109.5
O2—C11—H11106.7H20A—C20—H20B109.5
C12—C11—H11106.7C17A—C20—H20C109.5
C9—C11—H11106.7H20A—C20—H20C109.5
C11—C12—C13114.8 (2)H20B—C20—H20C109.5
C11—C12—H12A108.6
C10—C1—C2—C3−1.7 (4)O2—C11—C12—C1374.9 (3)
C1—C2—C3—O1176.9 (3)C9—C11—C12—C13−51.5 (3)
C1—C2—C3—C4−2.3 (4)C11—C12—C13—C18−66.8 (3)
O1—C3—C4—C5−175.5 (3)C11—C12—C13—C1456.0 (3)
C2—C3—C4—C53.7 (4)C11—C12—C13—C17A174.3 (2)
C3—C4—C5—C6179.3 (2)C7—C8—C14—C15−59.1 (3)
C3—C4—C5—C10−1.0 (4)C9—C8—C14—C15−179.7 (2)
C4—C5—C6—C7127.3 (3)C7—C8—C14—C13174.4 (2)
C10—C5—C6—C7−52.5 (3)C9—C8—C14—C1353.8 (3)
C5—C6—C7—C849.2 (3)C18—C13—C14—C15−61.1 (3)
C6—C7—C8—C9−52.6 (3)C12—C13—C14—C15176.6 (2)
C6—C7—C8—C14−175.2 (2)C17A—C13—C14—C1558.1 (3)
C7—C8—C9—C11−168.7 (2)C18—C13—C14—C865.4 (3)
C14—C8—C9—C11−47.2 (3)C12—C13—C14—C8−56.9 (3)
C7—C8—C9—C1059.0 (2)C17A—C13—C14—C8−175.36 (19)
C14—C8—C9—C10−179.54 (19)C8—C14—C15—C16179.1 (2)
C2—C1—C10—C54.1 (4)C13—C14—C15—C16−53.6 (3)
C2—C1—C10—C19−113.5 (3)C14—C15—C16—C1750.2 (4)
C2—C1—C10—C9122.3 (3)C15—C16—C17—O3124.4 (3)
C4—C5—C10—C1−2.7 (3)C15—C16—C17—C17A−54.6 (4)
C6—C5—C10—C1177.0 (2)O3—C17—C17A—O4125.0 (3)
C4—C5—C10—C19114.0 (3)C16—C17—C17A—O4−56.0 (3)
C6—C5—C10—C19−66.2 (3)O3—C17—C17A—C205.2 (4)
C4—C5—C10—C9−123.2 (3)C16—C17—C17A—C20−175.8 (3)
C6—C5—C10—C956.6 (3)O3—C17—C17A—C13−120.1 (3)
C11—C9—C10—C146.5 (3)C16—C17—C17A—C1358.9 (3)
C8—C9—C10—C1178.55 (19)C18—C13—C17A—O4177.4 (2)
C11—C9—C10—C5168.0 (2)C12—C13—C17A—O4−61.7 (2)
C8—C9—C10—C5−59.9 (3)C14—C13—C17A—O455.7 (3)
C11—C9—C10—C19−73.2 (3)C18—C13—C17A—C20−60.2 (3)
C8—C9—C10—C1958.8 (3)C12—C13—C17A—C2060.7 (3)
C8—C9—C11—O2−81.9 (3)C14—C13—C17A—C20178.2 (2)
C10—C9—C11—O248.8 (3)C18—C13—C17A—C1763.1 (3)
C8—C9—C11—C1245.8 (3)C12—C13—C17A—C17−176.0 (2)
C10—C9—C11—C12176.5 (2)C14—C13—C17A—C17−58.6 (3)

Hydrogen-bond geometry (Å, °)

D—H···AD—HH···AD···AD—H···A
O2—H2X···O50.82 (2)2.00 (2)2.799 (3)166 (3)
C1—H1···O20.932.723.191 (3)112
C12—H12A···O50.972.523.315 (4)139
C18—H18A···O20.962.533.028 (4)112
C19—H19B···O20.962.342.915 (4)118
O4—H4X···O1i0.81 (2)1.97 (2)2.758 (3)164 (3)
O5—H5Y···O3ii0.86 (2)1.99 (2)2.851 (4)171 (4)
O5—H5X···O4iii0.85 (2)1.97 (2)2.811 (3)172 (4)
C16—H16A···O5iv0.972.643.427 (4)138
C6—H6A···O3v0.972.703.297 (3)120

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

Footnotes

Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: GK2193).

References

  • Bruker (2000). SMART, SAINT and SADABS Bruker AXS Inc., Madison, Wisconsin, USA.
  • Conrow, E. (1999). J. Org. Chem.3, 1042–1044. [PubMed]
  • Huo, S. Q. (2003). Org. Lett.5, 423–425. [PubMed]
  • Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122. [PubMed]

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