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Acta Cryst. (2008). E64, o619    [ doi:10.1107/S1600536808004364 ]

(1R,2R)-4-Benzoyl-2-benzoyloxy-1-phenylbutyl imidazole-1-carboxylate

D. J. Fox, S. Parris, D. S. Pedersen and S. Warren

Abstract top

The title compound, C28H24N2O5, was prepared from (E)-2-cinnamyl-1,3-diphenylpropane-1,3-dione using standard Sharpless asymmetric dihydroxylation conditions, followed by treatment with 1,1'-carbonyl diimidazole. In the crystal structure, the phenyl rings form intermolecular face-to-face [pi]-[pi] contacts, with an interplanar angle of 15.5 (2)° and a centroid-centroid distance of 4.73 (1) Å. One phenyl ring also forms a C-H...[pi] contact to an adjacent imidazole ring, with an H...centroid distance of 3.18 Å.

Comment top

We recently reported a method for the preparation of optically active dihydrofurans starting from β-keto-diphenylphosphine oxides by intramolecular ring opening of cyclic carbonates (Fox et al., 2006). Currently we are investigating this synthetic concept in more detail to include other anion-stabilizing groups. Seeking to extend the methodology to diketones we performed the Sharpless asymmetric dihydroxylation (Kolb et al., 1994) on (E)-2-cinnamyl-1,3-diphenylpropane-1,3-dione, followed by treatment with 1,1'-carbonyl diimidazole. Surprisingly, this produced the title compound and the regioisomer (4R,5R)-5-benzoyloxy-1,5-diphenyl-4-imidazoyloxy-pentanone in a combined 50% yield. Presumably these products are formed by intramolecular acyl transfer during the asymmetric dihydroxylation step.

Related literature top

For related literature, see: Fox et al. (2006); Kolb et al. (1994).

Experimental top

The synthetic procedure is summarized in Fig. 2. By the method of Sharpless and co-workers (Kolb et al., 1994), olefin 1 (0.14 g, 0.41 mmol), was partially dissolved in t-BuOH (5 ml) with heating, and water (5 ml) was added. A freshly made mixture of OsCl3.xH2O (1 mol%), K3Fe(CN)6 (3 equiv.), K2CO3 (3 equiv.), MeSO2NH2 (1 equiv.) and hydroquinidine 1,4-phthalazinediyl diether (denoted (DHQD)2PHAL, 2 mol%) was added to the cooled solution (282 K) in one portion and it was stirred vigorously for 26 d. Sodium sulfite (ca 10 equiv.) was added and the reaction allowed to warm to room temperature with vigorous stirring. The reaction mixture was transferred to a separatory funnel and the organic layer separated, and concentrated in vacuo. The concentrated organic layer was partitioned between dichloromethane (20 ml) and water (20 ml), and the aqueous phase extracted with more dichloromethane (2 × 20 ml). The organic extracts were combined and washed with brine (20 ml), dried (Na2SO4), filtered and concentrated in vacuo. The residue was dissolved in anhydrous dichloromethane (5 ml) and 1,1'-carbonyldiimidazole (97 mg, 0.60 mmol) was added. After 7 h, the reaction was quenched with 1M aqueous hydrochloric acid (20 ml) and extracted with dichloromethane (3 × 25 ml). The combined organic phases were washed with saturated aqueous NaHCO3 (10 ml), dried (Na2SO4), filtered and concentrated in vacuo. The residue was purified by flash column chromatography (25–100% EtOAc in hexanes, v/v) to give a mixture of the title compound and the regioisomer 2 as a yellow oil (92 mg, 50%).

Refinement top

H atoms were placed geometrically and allowed to ride during refinement with C—H = 0.95–0.99 Å and with Uiso(H) = 1.2Ueq(C). Data were collected to θ = 20°, equivalent to a resolution of 1.04 Å. The resulting structure is therefore of relatively low precision. In the absence of significant anomalous scattering effects, 961 Friedel pairs were merged as equivalent data. The absolute structure is based on the known stereochemical outcome of the asymmetric dihydroxylation.

Computing details top

Data collection: COLLECT (Nonius, 1998); cell refinement: SCALEPACK (Otwinowski & Minor, 1997); data reduction: DENZO and SCALEPACK (Otwinowski & Minor, 1997); program(s) used to solve structure: SIR92 (Altomare et al., 1994); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

Figures top
[Figure 1] Fig. 1. Molecular structure with displacement parameters drawn at the 30% probability level for non-H atoms.
[Figure 2] Fig. 2. Summary of the synthetic procedure. CDI = 1,1'-carbonyldiimidazole, (DHQD)2PHAL = Hydroquinidine 1,4-phthalazinediyl diether.
(1R,2R)-4-Benzoyl-2-benzoyloxy-1-phenylbutyl imidazole-1-carboxylate top
Crystal data top
C28H24N2O5F000 = 984
Mr = 468.49Dx = 1.284 Mg m−3
Orthorhombic, P212121Mo Kα radiation
λ = 0.71073 Å
Hall symbol: P 2ac 2abCell parameters from 7747 reflections
a = 5.9512 (2) Åθ = 1.0–20.4º
b = 18.1330 (7) ŵ = 0.09 mm−1
c = 22.4612 (12) ÅT = 180 (2) K
V = 2423.86 (18) Å3Block, colourless
Z = 40.37 × 0.05 × 0.02 mm
Data collection top
Nonius KappaCCD
diffractometer		1230 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tubeRint = 0.076
T = 180(2) Kθmax = 20.4º
ω and φ scansθmin = 3.5º
Absorption correction: multi-scan
(SORTAV; Blessing, 1995)		h = −5→5
Tmin = 0.767, Tmax = 0.998k = −17→17
7293 measured reflectionsl = −21→22
1416 independent reflections
Refinement top
Refinement on F2Hydrogen site location: inferred from neighbouring sites
Least-squares matrix: fullH-atom parameters constrained
R[F2 > 2σ(F2)] = 0.036  w = 1/[σ2(Fo2) + (0.0295P)2 + 0.6591P]
where P = (Fo2 + 2Fc2)/3
wR(F2) = 0.080(Δ/σ)max < 0.001
S = 1.12Δρmax = 0.20 e Å−3
1416 reflectionsΔρmin = −0.18 e Å−3
317 parametersExtinction correction: SHELXL97 (Sheldrick, 2008), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
Primary atom site location: structure-invariant direct methodsExtinction coefficient: 0.0071 (12)
Secondary atom site location: difference Fourier mapAbsolute structure: In the absence of significant anomalous scattering effects, 961 Friedel pairs have been merged as equivalent data.
Crystal data top
C28H24N2O5V = 2423.86 (18) Å3
Mr = 468.49Z = 4
Orthorhombic, P212121Mo Kα
a = 5.9512 (2) ŵ = 0.09 mm−1
b = 18.1330 (7) ÅT = 180 (2) K
c = 22.4612 (12) Å0.37 × 0.05 × 0.02 mm
Data collection top
Nonius KappaCCD
diffractometer		1416 independent reflections
Absorption correction: multi-scan
(SORTAV; Blessing, 1995)		1230 reflections with I > 2σ(I)
Tmin = 0.767, Tmax = 0.998Rint = 0.076
7293 measured reflectionsθmax = 20.4º
Refinement top
R[F2 > 2σ(F2)] = 0.036Δρmax = 0.20 e Å−3
wR(F2) = 0.080Δρmin = −0.18 e Å−3
S = 1.12Absolute structure: In the absence of significant anomalous scattering effects, 961 Friedel pairs have been merged as equivalent data.
1416 reflectionsFlack parameter: ?
317 parametersRogers parameter: ?
H-atom parameters constrained
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 > 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
O10.4514 (6)−0.04408 (18)0.45568 (15)0.0606 (10)
O20.2125 (4)0.04215 (14)0.29114 (12)0.0370 (7)
O30.5837 (5)0.06215 (19)0.29436 (15)0.0627 (10)
O40.1450 (4)0.17552 (15)0.33765 (13)0.0397 (8)
O5−0.1887 (6)0.23422 (17)0.33492 (17)0.0663 (10)
N10.1272 (6)0.28828 (19)0.29753 (17)0.0397 (10)
N20.4070 (6)0.3576 (2)0.26476 (18)0.0477 (11)
C10.4404 (8)−0.0913 (3)0.4173 (2)0.0412 (12)
C20.2591 (7)−0.0889 (2)0.3705 (2)0.0411 (12)
H2B0.1717−0.13530.37230.049*
H2C0.3303−0.08610.33080.049*
C30.0990 (7)−0.0239 (2)0.3779 (2)0.0408 (12)
H3A−0.0419−0.03470.35620.049*
H3B0.0611−0.01860.42060.049*
C40.1935 (7)0.0484 (2)0.35549 (18)0.0338 (11)
H40.34490.05740.37340.041*
C50.0401 (7)0.1133 (2)0.3679 (2)0.0367 (11)
H5−0.11160.10370.35030.044*
C60.0092 (8)0.2308 (3)0.3247 (2)0.0441 (12)
C70.3534 (7)0.2942 (3)0.2878 (2)0.0424 (12)
H70.45880.25650.29680.051*
C80.2059 (8)0.3948 (3)0.2583 (2)0.0491 (13)
H80.19190.44290.24190.059*
C90.0327 (8)0.3542 (3)0.2781 (2)0.0509 (13)
H9−0.12150.36770.27870.061*
C100.4159 (8)0.0525 (2)0.2659 (2)0.0393 (11)
C110.4072 (7)0.0490 (2)0.20012 (19)0.0348 (11)
C120.5986 (8)0.0679 (3)0.1689 (2)0.0578 (14)
H120.72820.08430.18980.069*
C130.6024 (9)0.0632 (3)0.1082 (3)0.0734 (17)
H130.73520.07570.08700.088*
C140.4156 (10)0.0405 (3)0.0778 (2)0.0636 (15)
H140.41790.03850.03560.076*
C150.2263 (9)0.0207 (2)0.1080 (2)0.0547 (14)
H150.09790.00380.08690.066*
C160.2217 (7)0.0254 (2)0.1696 (2)0.0441 (12)
H160.08930.01210.19060.053*
C170.6070 (8)−0.1529 (2)0.41677 (18)0.0374 (11)
C180.7758 (8)−0.1535 (3)0.4594 (2)0.0466 (12)
H180.7844−0.11450.48750.056*
C190.9307 (8)−0.2095 (3)0.4615 (3)0.0601 (14)
H191.0444−0.20940.49120.072*
C200.9211 (10)−0.2657 (3)0.4207 (3)0.0641 (15)
H201.0288−0.30430.42200.077*
C210.7573 (10)−0.2660 (3)0.3785 (2)0.0646 (15)
H210.7515−0.30520.35040.077*
C220.5981 (8)−0.2100 (2)0.3758 (2)0.0513 (13)
H220.4843−0.21090.34620.062*
C230.0174 (7)0.1293 (2)0.4332 (2)0.0373 (12)
C24−0.1753 (8)0.1095 (2)0.4643 (3)0.0546 (13)
H24−0.29680.08680.44390.066*
C25−0.1905 (11)0.1227 (3)0.5249 (3)0.0722 (17)
H25−0.32170.10850.54610.087*
C26−0.0174 (14)0.1563 (4)0.5544 (3)0.080 (2)
H26−0.02930.16530.59590.097*
C270.1732 (11)0.1770 (3)0.5245 (3)0.0692 (16)
H270.29240.20060.54510.083*
C280.1910 (8)0.1634 (2)0.4642 (2)0.0498 (13)
H280.32370.17750.44360.060*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
O10.077 (2)0.052 (2)0.052 (2)0.0064 (19)−0.0150 (19)−0.015 (2)
O20.0302 (18)0.0434 (17)0.0374 (19)−0.0031 (14)−0.0014 (15)−0.0026 (15)
O30.0305 (18)0.104 (3)0.053 (2)−0.0117 (19)−0.0072 (19)−0.005 (2)
O40.0301 (16)0.0336 (17)0.055 (2)0.0057 (15)−0.0016 (15)0.0084 (16)
O50.034 (2)0.058 (2)0.107 (3)0.0085 (17)0.001 (2)0.021 (2)
N10.034 (2)0.036 (2)0.049 (2)0.003 (2)−0.0054 (19)0.006 (2)
N20.043 (3)0.047 (3)0.053 (3)−0.002 (2)0.001 (2)0.004 (2)
C10.051 (3)0.033 (3)0.039 (3)−0.005 (3)0.005 (3)−0.001 (3)
C20.046 (3)0.035 (2)0.042 (3)−0.004 (2)0.000 (3)0.002 (2)
C30.042 (3)0.039 (3)0.041 (3)−0.002 (2)−0.003 (2)0.003 (2)
C40.031 (2)0.039 (3)0.031 (3)−0.001 (2)−0.002 (2)−0.002 (2)
C50.027 (2)0.036 (3)0.046 (3)−0.002 (2)−0.004 (2)0.006 (2)
C60.034 (3)0.042 (3)0.056 (3)0.002 (3)−0.005 (2)0.002 (3)
C70.030 (3)0.052 (3)0.046 (3)0.008 (2)−0.004 (2)−0.001 (3)
C80.044 (3)0.040 (3)0.064 (4)0.006 (3)−0.002 (3)0.008 (3)
C90.044 (3)0.043 (3)0.066 (4)0.008 (3)0.000 (3)0.011 (3)
C100.028 (3)0.034 (3)0.056 (3)−0.002 (2)0.000 (3)0.003 (2)
C110.035 (3)0.035 (2)0.034 (3)0.001 (2)0.003 (3)0.004 (2)
C120.043 (3)0.085 (4)0.045 (4)−0.013 (3)0.001 (3)0.010 (3)
C130.049 (4)0.113 (5)0.059 (4)−0.003 (3)0.006 (3)0.020 (4)
C140.066 (4)0.079 (4)0.047 (4)0.007 (3)0.014 (4)0.007 (3)
C150.060 (4)0.054 (3)0.050 (4)−0.008 (3)0.000 (3)−0.008 (3)
C160.040 (3)0.047 (3)0.046 (3)−0.011 (2)0.003 (3)−0.004 (2)
C170.050 (3)0.035 (3)0.028 (3)−0.004 (2)0.002 (2)0.006 (2)
C180.051 (3)0.046 (3)0.043 (3)−0.011 (3)−0.002 (3)0.010 (2)
C190.051 (3)0.062 (4)0.067 (4)−0.001 (3)−0.008 (3)0.018 (3)
C200.062 (4)0.055 (4)0.075 (4)0.009 (3)−0.002 (4)0.011 (3)
C210.085 (4)0.043 (3)0.066 (4)0.009 (3)−0.004 (4)−0.006 (3)
C220.063 (3)0.042 (3)0.049 (3)0.005 (3)−0.006 (3)0.005 (3)
C230.035 (3)0.035 (3)0.042 (3)0.005 (2)−0.001 (2)0.001 (2)
C240.050 (3)0.052 (3)0.062 (4)0.008 (3)0.010 (3)−0.001 (3)
C250.091 (5)0.064 (4)0.062 (5)0.017 (4)0.034 (4)0.007 (3)
C260.122 (6)0.072 (4)0.047 (4)0.030 (4)−0.001 (5)−0.012 (4)
C270.083 (4)0.064 (4)0.061 (5)0.012 (3)−0.017 (4)−0.021 (3)
C280.053 (3)0.048 (3)0.049 (4)0.004 (3)−0.008 (3)−0.006 (3)
Geometric parameters (Å, °) top
O1—C11.217 (5)C12—C131.368 (7)
O2—C101.350 (5)C12—H120.950
O2—C41.454 (5)C13—C141.367 (7)
O3—C101.199 (5)C13—H130.950
O4—C61.321 (5)C14—C151.363 (7)
O4—C51.458 (5)C14—H140.950
O5—C61.201 (5)C15—C161.386 (6)
N1—C71.368 (5)C15—H150.950
N1—C91.392 (5)C16—H160.950
N1—C61.397 (5)C17—C221.386 (6)
N2—C71.300 (5)C17—C181.387 (6)
N2—C81.382 (5)C18—C191.373 (6)
C1—C171.494 (6)C18—H180.950
C1—C21.506 (6)C19—C201.370 (7)
C2—C31.526 (5)C19—H190.950
C2—H2B0.990C20—C211.360 (7)
C2—H2C0.990C20—H200.950
C3—C41.512 (5)C21—C221.390 (6)
C3—H3A0.990C21—H210.950
C3—H3B0.990C22—H220.950
C4—C51.515 (5)C23—C241.390 (6)
C4—H41.000C23—C281.391 (6)
C5—C231.501 (6)C24—C251.386 (8)
C5—H51.000C24—H240.950
C7—H70.950C25—C261.367 (8)
C8—C91.342 (6)C25—H250.950
C8—H80.950C26—C271.371 (8)
C9—H90.950C26—H260.950
C10—C111.479 (6)C27—C281.379 (7)
C11—C161.368 (6)C27—H270.950
C11—C121.381 (6)C28—H280.950
C10—O2—C4118.5 (3)C13—C12—C11120.3 (5)
C6—O4—C5115.4 (3)C13—C12—H12119.9
C7—N1—C9106.2 (4)C11—C12—H12119.9
C7—N1—C6128.6 (4)C14—C13—C12120.2 (5)
C9—N1—C6125.1 (4)C14—C13—H13119.9
C7—N2—C8105.1 (4)C12—C13—H13119.9
O1—C1—C17119.7 (4)C15—C14—C13120.2 (5)
O1—C1—C2120.8 (4)C15—C14—H14119.9
C17—C1—C2119.4 (4)C13—C14—H14119.9
C1—C2—C3113.2 (3)C14—C15—C16119.8 (5)
C1—C2—H2B108.9C14—C15—H15120.1
C3—C2—H2B108.9C16—C15—H15120.1
C1—C2—H2C108.9C11—C16—C15120.3 (4)
C3—C2—H2C108.9C11—C16—H16119.9
H2B—C2—H2C107.7C15—C16—H16119.9
C4—C3—C2113.7 (3)C22—C17—C18118.6 (4)
C4—C3—H3A108.8C22—C17—C1122.6 (4)
C2—C3—H3A108.8C18—C17—C1118.8 (4)
C4—C3—H3B108.8C19—C18—C17121.1 (5)
C2—C3—H3B108.8C19—C18—H18119.5
H3A—C3—H3B107.7C17—C18—H18119.5
O2—C4—C3107.0 (3)C20—C19—C18119.9 (5)
O2—C4—C5106.9 (3)C20—C19—H19120.1
C3—C4—C5112.8 (3)C18—C19—H19120.1
O2—C4—H4110.0C21—C20—C19120.0 (5)
C3—C4—H4110.0C21—C20—H20120.0
C5—C4—H4110.0C19—C20—H20120.0
O4—C5—C23110.1 (3)C20—C21—C22121.0 (5)
O4—C5—C4104.9 (3)C20—C21—H21119.5
C23—C5—C4112.6 (3)C22—C21—H21119.5
O4—C5—H5109.7C17—C22—C21119.4 (4)
C23—C5—H5109.7C17—C22—H22120.3
C4—C5—H5109.7C21—C22—H22120.3
O5—C6—O4126.6 (4)C24—C23—C28118.4 (4)
O5—C6—N1122.6 (4)C24—C23—C5121.0 (4)
O4—C6—N1110.8 (4)C28—C23—C5120.6 (4)
N2—C7—N1112.0 (4)C25—C24—C23120.2 (5)
N2—C7—H7124.0C25—C24—H24119.9
N1—C7—H7124.0C23—C24—H24119.9
C9—C8—N2111.3 (4)C26—C25—C24120.2 (5)
C9—C8—H8124.4C26—C25—H25119.9
N2—C8—H8124.4C24—C25—H25119.9
C8—C9—N1105.4 (4)C25—C26—C27120.5 (5)
C8—C9—H9127.3C25—C26—H26119.7
N1—C9—H9127.3C27—C26—H26119.7
O3—C10—O2122.9 (4)C26—C27—C28119.7 (5)
O3—C10—C11124.7 (4)C26—C27—H27120.1
O2—C10—C11112.5 (4)C28—C27—H27120.1
C16—C11—C12119.3 (4)C27—C28—C23120.9 (5)
C16—C11—C10122.9 (4)C27—C28—H28119.5
C12—C11—C10117.8 (4)C23—C28—H28119.5
O1—C1—C2—C3−0.8 (6)C16—C11—C12—C130.1 (7)
C17—C1—C2—C3178.6 (4)C10—C11—C12—C13177.7 (5)
C1—C2—C3—C478.5 (4)C11—C12—C13—C140.8 (8)
C10—O2—C4—C3−122.7 (4)C12—C13—C14—C15−1.6 (9)
C10—O2—C4—C5116.2 (4)C13—C14—C15—C161.5 (8)
C2—C3—C4—O267.6 (4)C12—C11—C16—C15−0.1 (7)
C2—C3—C4—C5−175.2 (4)C10—C11—C16—C15−177.6 (4)
C6—O4—C5—C23−80.9 (4)C14—C15—C16—C11−0.7 (7)
C6—O4—C5—C4157.7 (3)O1—C1—C17—C22177.3 (4)
O2—C4—C5—O4−56.5 (4)C2—C1—C17—C22−2.1 (6)
C3—C4—C5—O4−173.8 (3)O1—C1—C17—C18−2.0 (6)
O2—C4—C5—C23−176.2 (3)C2—C1—C17—C18178.6 (4)
C3—C4—C5—C2366.5 (4)C22—C17—C18—C19−0.5 (6)
C5—O4—C6—O5−0.5 (7)C1—C17—C18—C19178.8 (4)
C5—O4—C6—N1178.2 (3)C17—C18—C19—C200.7 (7)
C7—N1—C6—O5174.1 (5)C18—C19—C20—C21−0.5 (8)
C9—N1—C6—O5−1.6 (8)C19—C20—C21—C220.1 (8)
C7—N1—C6—O4−4.7 (7)C18—C17—C22—C210.1 (6)
C9—N1—C6—O4179.6 (4)C1—C17—C22—C21−179.2 (4)
C8—N2—C7—N1−0.9 (5)C20—C21—C22—C170.1 (7)
C9—N1—C7—N20.6 (6)O4—C5—C23—C24139.4 (4)
C6—N1—C7—N2−175.8 (4)C4—C5—C23—C24−103.9 (4)
C7—N2—C8—C90.9 (5)O4—C5—C23—C28−41.8 (5)
N2—C8—C9—N1−0.5 (5)C4—C5—C23—C2874.9 (5)
C7—N1—C9—C80.0 (5)C28—C23—C24—C25−0.8 (7)
C6—N1—C9—C8176.5 (4)C5—C23—C24—C25178.0 (4)
C4—O2—C10—O34.6 (6)C23—C24—C25—C260.8 (7)
C4—O2—C10—C11−176.5 (3)C24—C25—C26—C27−0.2 (8)
O3—C10—C11—C16168.4 (4)C25—C26—C27—C28−0.5 (8)
O2—C10—C11—C16−10.6 (6)C26—C27—C28—C230.5 (7)
O3—C10—C11—C12−9.1 (7)C24—C23—C28—C270.2 (7)
O2—C10—C11—C12172.0 (4)C5—C23—C28—C27−178.7 (4)
Hydrogen-bond geometry (Å, °) top
D—H···AD—HH···AD···AD—H···A
C26—H26A···Cg10.953.184.07 (1)155
Table 1
Hydrogen-bond geometry (Å, °) top
D—H···AD—HH···AD···AD—H···A
C26—H26A···Cg10.953.184.07 (1)155
Acknowledgements top

We are grateful to Dr John E. Davies (University of Cambridge) for collecting the X-ray data.

references
References top

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