supplementary materials


zp2007 scheme

Acta Cryst. (2013). E69, o1494-o1495    [ doi:10.1107/S160053681302391X ]

(3S,3aS,6R,6aR)-2-Oxohexahydrofuro[3,2-b]furan-3,6-diyl dibenzoate

V. Piccialli, G. Oliviero, S. Zaccaria, R. Centore and A. Tuzi

Abstract top

The title compound, C20H16O7, contains a cis-fused [gamma]-lactone tetrahydrofuran ring system functionalized with two benzoyloxy groups. Both rings adopt an envelope conformation. The molecule assumes an elongated shape and exibits non-crystallographic C2 symmetry. The benzoyloxy groups are almost planar [maximum deviations of 0.0491 (15) and 0.0336 (17) Å for the O atoms] and their mean planes are inclined to one another by 16.51 (4)°. The crystal packing features weak C-H...O interactions. The aryl groups of adjacent molecules are parallel shifted with face-to-face contacts and a shortest intermolecular C...C distance of 3.482 (4) Å.

Comment top

D-mannitol plays an important role in organic synthesis as a readily available chiral building block (Hanessian, 1993). In addition, mannitol and its derivatives are widely used as chiral reagents and chiral auxiliaries (Masaki et al., 1999) and can be transformed into biologically active and pharmaceutically important compounds (Lohray et al., 1999). As a part of our ongoing interest in oxidative processes mediated by transition metals oxo-species (De Champdorè et al.. 1998; Piccialli, Oliviero, Borbone et al., 2013; Piccialli, Tuzi, Oliviero et al., 2013; Piccialli, 2007), and in particular in the synthesis of new THF-containing compounds, we recently focused on the catalytic use of chlorochromatoperiodate (CCP), a powerful oxidizing reagent generated by the condensation of pyridinium chlorochromate (PCC) and periodic acid (Piccialli, D'Errico, Borbone et al., 2013; Piccialli, Zaccaria, Oliviero et al., 2012).

The title compound, C20H18O7, contains a functionalized cis–fused γ-lactone-tetrahydrofuran ring system, substituted at C2 and C5 positions with benzoyloxy groups. Both rings adopt an envelope conformation with O2 and C3 at the flap. The molecule assumes an elongated shape and exibits a C2 symmetry of the benzoyloxy groups (Fig.2). The benzoyloxy groups are almost planar (maximum deviation from least square plane (O3/O4/C7—C13) is -0.0491 (15) Å for O3; maximum deviation from least square plane (O5/O6/C14—C20) is 0.0336 (17) Å for O6) and their mean planes are inclined with respect to each other by 16.51 (4)°. No strong H–bonding donor groups are present in the molecule. The crystal packing (Fig.3) is stabilized by weak intermolecular CH···O interactions. Aryl groups of adjacent molecules are parallel shifted with face-to-face contacts (shortest intermolecular distance is C12···C17i = 3.482 (4) Å, i = 1 + x, -1 + y, z).

Related literature top

For the use of carbohydrate in the synthesis of complex natural chiral substances, see: Hanessian (1993). For mannitol as chiral reagent and as a precursor of biologically active derivatives, see: Masaki et al. (1999); Lohray et al. (1999). For oxidative processes mediated by transition metal oxo-species, see: De Champdorè et al.. (1998); Piccialli (2007); Piccialli, Oliviero, Borbone et al. (2013); Piccialli, Tuzi, Oliviero et al. (2013). For the catalytic use of chlorochromatoperiodate, see: Piccialli, D'Errico, Borbone et al. (2013); Piccialli, Zaccaria, Oliviero et al. (2012). For the synthesis of the precursor, see: Hockett et al. (1946).

Experimental top

The title compound 2 has been synthesized by oxidation of bis-tetrahydrofuran 1 with chlorochromatoperiodate (CCP) according to the scheme in Fig. 1. Compound 1 has been obtained from mannitol according to literature (Hockett et al., 1946). Since the oxidative process does not affect any of the chiral centres of the molecule, the absolute configuration of 2 matches that of mannitol. Crystals suitable for X-ray analysis were obtained by recrystallization of 2 from methanol.

Compound 2: 1H-NMR (200 MHz, CDCl3) 8.23–8.03 (4H, m), 7.62 (2H, t, J= 7.6 Hz), 7.48 (4H, t, J= 7.5 Hz), 5.63 (1H, d, J= 5.6 Hz, H-2), 5.58–5.34 (2H, overlapped multiplets, H-4 and H-5), 5.06 (1H, dd, J= 5.6, 4.0 Hz, H-3), 4.33 (1H, dd, J= 9.3, 6.9 Hz, Ha-6), 4.03 (1H, dd, J= 9.3, 7.8 Hz, Hb-6); 13C-NMR (50 MHz, CDCl3) 170.4, 165.8, 165.3, 133.9, 133.7, 130.2, 130.0, 129.8, 128.65, 128.56, 128.4, 128.2, 78.6, 76.1, 72.9, 69.4, 69.2.

Refinement top

All H atoms were generated stereochemically and refined by the riding model with Uiso=1.2×Ueq of the carrier atom. In the absence of strong anomalous scatterer the Flack parameter is not meaningful. Data were merged using MERG 3 instruction and the absolute configuration was assigned on the basis of the configuration of its percursor.

Computing details top

Data collection: COLLECT (Nonius, 1999); cell refinement: DIRAX/LSQ (Duisenberg et al., 2000); data reduction: EVALCCD (Duisenberg et al., 2003); program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012) and Mercury (Macrae et al., 2006); software used to prepare material for publication: WinGX (Farrugia, 2012).

Figures top
[Figure 1] Fig. 1. Synthesis of the title compound by chlorochromatoperiodate (CCP) catalyzed oxidation of 1.
[Figure 2] Fig. 2. ORTEP view of the the title compound. Thermal ellipsoids are drawn at 30% probability level.
[Figure 3] Fig. 3. Crystal packing viewed along b axis. Shortest aryl intermolecular C···C distance is shown as dashed line (i = 1 + x, -1 + y, z).
(3S,3aS,6R,6aR)-2-Oxohexahydrofuro[3,2-b]furan-3,6-diyl dibenzoate top
Crystal data top
C20H16O7F(000) = 768
Mr = 368.33Dx = 1.440 Mg m3
Orthorhombic, P212121Mo Kα radiation, λ = 0.71073 Å
Hall symbol: P 2ac 2abCell parameters from 119 reflections
a = 7.4870 (7) Åθ = 3.8–22.3°
b = 10.2050 (14) ŵ = 0.11 mm1
c = 22.232 (2) ÅT = 173 K
V = 1698.6 (3) Å3Block, white
Z = 40.50 × 0.40 × 0.08 mm
Data collection top
Bruker–Nonius KappaCCD
diffractometer
2108 independent reflections
Radiation source: normal-focus sealed tube1797 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.032
Detector resolution: 9 pixels mm-1θmax = 27.5°, θmin = 3.3°
CCD rotation images, thick slices scansh = 95
Absorption correction: multi-scan
(SADABS; Bruker, 2001)
k = 1213
Tmin = 0.947, Tmax = 0.991l = 2828
8078 measured reflections
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)] = 0.034Hydrogen site location: inferred from neighbouring sites
wR(F2) = 0.078H-atom parameters constrained
S = 1.07 w = 1/[σ2(Fo2) + (0.0383P)2 + 0.2376P]
where P = (Fo2 + 2Fc2)/3
2108 reflections(Δ/σ)max < 0.001
244 parametersΔρmax = 0.16 e Å3
0 restraintsΔρmin = 0.19 e Å3
Crystal data top
C20H16O7V = 1698.6 (3) Å3
Mr = 368.33Z = 4
Orthorhombic, P212121Mo Kα radiation
a = 7.4870 (7) ŵ = 0.11 mm1
b = 10.2050 (14) ÅT = 173 K
c = 22.232 (2) Å0.50 × 0.40 × 0.08 mm
Data collection top
Bruker–Nonius KappaCCD
diffractometer
2108 independent reflections
Absorption correction: multi-scan
(SADABS; Bruker, 2001)
1797 reflections with I > 2σ(I)
Tmin = 0.947, Tmax = 0.991Rint = 0.032
8078 measured reflectionsθmax = 27.5°
Refinement top
R[F2 > 2σ(F2)] = 0.034H-atom parameters constrained
wR(F2) = 0.078Δρmax = 0.16 e Å3
S = 1.07Δρmin = 0.19 e Å3
2108 reflectionsAbsolute structure: ?
244 parametersAbsolute structure parameter: ?
0 restraintsRogers parameter: ?
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
C10.7473 (3)0.5893 (2)0.57766 (9)0.0274 (5)
C20.7006 (3)0.45523 (19)0.55289 (9)0.0256 (5)
H20.76050.44160.51320.031*
C30.5016 (3)0.46156 (19)0.54459 (9)0.0248 (5)
H30.45870.40850.50960.030*
C40.4649 (3)0.60851 (19)0.53746 (9)0.0260 (5)
H40.45460.63450.49420.031*
C50.2879 (3)0.6297 (2)0.57190 (9)0.0276 (5)
H50.19040.65740.54390.033*
C60.2490 (3)0.4957 (2)0.59859 (11)0.0324 (5)
H6A0.20040.50450.63980.039*
H6B0.16120.44800.57350.039*
C70.7436 (3)0.2341 (2)0.57841 (9)0.0277 (5)
C80.8031 (3)0.1437 (2)0.62616 (9)0.0267 (5)
C90.7809 (3)0.0097 (2)0.61663 (11)0.0350 (6)
H90.73480.02150.57950.042*
C100.8264 (4)0.0772 (2)0.66150 (12)0.0425 (7)
H100.81310.16870.65510.051*
C110.8912 (4)0.0320 (2)0.71559 (12)0.0425 (6)
H110.92040.09250.74650.051*
C120.9142 (4)0.1007 (2)0.72531 (11)0.0369 (6)
H120.95910.13140.76270.044*
C130.8714 (3)0.1880 (2)0.68034 (10)0.0320 (5)
H130.88890.27930.68650.038*
C140.3130 (3)0.8490 (2)0.60421 (9)0.0271 (5)
C150.3326 (3)0.9379 (2)0.65625 (9)0.0256 (5)
C160.3399 (3)1.0720 (2)0.64556 (10)0.0330 (5)
H160.32901.10490.60580.040*
C170.3629 (4)1.1568 (2)0.69311 (11)0.0381 (6)
H170.36791.24850.68600.046*
C180.3788 (4)1.1098 (2)0.75109 (10)0.0382 (6)
H180.39431.16890.78370.046*
C190.3722 (4)0.9768 (2)0.76157 (10)0.0349 (6)
H190.38430.94430.80140.042*
C200.3479 (3)0.8907 (2)0.71458 (9)0.0282 (5)
H200.34180.79920.72210.034*
O10.6118 (2)0.67337 (13)0.56690 (6)0.0276 (4)
O20.4158 (2)0.42772 (14)0.59961 (6)0.0294 (4)
O30.7629 (2)0.36076 (13)0.59524 (6)0.0272 (4)
O40.6838 (3)0.20348 (14)0.53016 (7)0.0386 (4)
O50.3071 (2)0.72131 (13)0.62053 (6)0.0268 (4)
O60.3053 (3)0.88274 (15)0.55255 (6)0.0438 (5)
O70.8798 (2)0.62237 (15)0.60320 (8)0.0403 (4)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
C10.0279 (12)0.0252 (11)0.0292 (11)0.0009 (10)0.0049 (10)0.0007 (8)
C20.0339 (14)0.0222 (10)0.0206 (9)0.0002 (10)0.0014 (9)0.0017 (8)
C30.0318 (13)0.0251 (11)0.0177 (9)0.0011 (10)0.0012 (9)0.0012 (8)
C40.0293 (13)0.0264 (11)0.0221 (10)0.0009 (10)0.0022 (9)0.0007 (8)
C50.0300 (13)0.0289 (11)0.0239 (10)0.0003 (10)0.0025 (10)0.0022 (9)
C60.0274 (13)0.0300 (12)0.0397 (12)0.0024 (10)0.0035 (11)0.0020 (9)
C70.0276 (12)0.0250 (11)0.0304 (11)0.0039 (10)0.0073 (10)0.0055 (8)
C80.0243 (12)0.0254 (11)0.0304 (10)0.0043 (9)0.0051 (10)0.0006 (9)
C90.0356 (15)0.0260 (12)0.0434 (13)0.0029 (11)0.0049 (12)0.0042 (9)
C100.0460 (17)0.0242 (12)0.0572 (16)0.0052 (12)0.0121 (14)0.0036 (11)
C110.0402 (15)0.0370 (13)0.0504 (14)0.0098 (12)0.0072 (14)0.0157 (12)
C120.0339 (15)0.0401 (13)0.0368 (12)0.0060 (12)0.0029 (11)0.0037 (10)
C130.0300 (13)0.0286 (12)0.0373 (12)0.0045 (10)0.0009 (11)0.0006 (9)
C140.0267 (13)0.0269 (11)0.0278 (10)0.0039 (10)0.0028 (10)0.0047 (9)
C150.0216 (12)0.0292 (11)0.0260 (10)0.0011 (9)0.0022 (9)0.0012 (8)
C160.0365 (15)0.0301 (11)0.0323 (11)0.0007 (11)0.0047 (10)0.0066 (9)
C170.0419 (16)0.0269 (11)0.0455 (13)0.0077 (11)0.0102 (12)0.0020 (10)
C180.0389 (15)0.0370 (13)0.0388 (12)0.0091 (12)0.0047 (12)0.0110 (10)
C190.0364 (15)0.0432 (13)0.0252 (10)0.0033 (12)0.0007 (10)0.0004 (10)
C200.0284 (13)0.0292 (11)0.0270 (10)0.0002 (10)0.0008 (9)0.0040 (9)
O10.0274 (9)0.0222 (7)0.0332 (8)0.0002 (7)0.0008 (7)0.0023 (6)
O20.0310 (9)0.0285 (7)0.0289 (7)0.0023 (7)0.0029 (7)0.0048 (6)
O30.0341 (9)0.0211 (7)0.0263 (7)0.0024 (6)0.0015 (7)0.0005 (6)
O40.0540 (12)0.0322 (8)0.0297 (8)0.0067 (8)0.0043 (8)0.0092 (6)
O50.0332 (9)0.0238 (7)0.0233 (7)0.0034 (7)0.0001 (7)0.0001 (6)
O60.0734 (14)0.0335 (8)0.0245 (7)0.0041 (9)0.0101 (9)0.0054 (6)
O70.0302 (10)0.0341 (9)0.0568 (10)0.0025 (8)0.0076 (9)0.0079 (8)
Geometric parameters (Å, º) top
C1—O71.191 (3)C9—C101.377 (3)
C1—O11.350 (3)C9—H90.9500
C1—C21.516 (3)C10—C111.376 (4)
C2—O31.426 (2)C10—H100.9500
C2—C31.502 (3)C11—C121.381 (4)
C2—H21.0000C11—H110.9500
C3—O21.424 (2)C12—C131.377 (3)
C3—C41.533 (3)C12—H120.9500
C3—H31.0000C13—H130.9500
C4—O11.441 (3)C14—O61.200 (2)
C4—C51.545 (3)C14—O51.353 (2)
C4—H41.0000C14—C151.477 (3)
C5—O51.436 (2)C15—C201.388 (3)
C5—C61.519 (3)C15—C161.390 (3)
C5—H51.0000C16—C171.377 (3)
C6—O21.428 (3)C16—H160.9500
C6—H6A0.9900C17—C181.381 (3)
C6—H6B0.9900C17—H170.9500
C7—O41.204 (3)C18—C191.378 (3)
C7—O31.354 (2)C18—H180.9500
C7—C81.475 (3)C19—C201.377 (3)
C8—C131.385 (3)C19—H190.9500
C8—C91.394 (3)C20—H200.9500
O7—C1—O1121.98 (19)C10—C9—H9120.3
O7—C1—C2128.4 (2)C8—C9—H9120.3
O1—C1—C2109.61 (19)C11—C10—C9120.3 (2)
O3—C2—C3115.75 (18)C11—C10—H10119.9
O3—C2—C1107.13 (16)C9—C10—H10119.9
C3—C2—C1103.58 (18)C10—C11—C12120.6 (2)
O3—C2—H2110.0C10—C11—H11119.7
C3—C2—H2110.0C12—C11—H11119.7
C1—C2—H2110.0C13—C12—C11119.5 (2)
O2—C3—C2109.37 (17)C13—C12—H12120.3
O2—C3—C4104.18 (16)C11—C12—H12120.3
C2—C3—C4103.46 (18)C12—C13—C8120.4 (2)
O2—C3—H3113.0C12—C13—H13119.8
C2—C3—H3113.0C8—C13—H13119.8
C4—C3—H3113.0O6—C14—O5122.07 (19)
O1—C4—C3105.38 (17)O6—C14—C15125.32 (19)
O1—C4—C5111.41 (15)O5—C14—C15112.60 (16)
C3—C4—C5103.87 (17)C20—C15—C16119.9 (2)
O1—C4—H4111.9C20—C15—C14121.81 (18)
C3—C4—H4111.9C16—C15—C14118.32 (19)
C5—C4—H4111.9C17—C16—C15119.5 (2)
O5—C5—C6108.12 (16)C17—C16—H16120.2
O5—C5—C4112.23 (18)C15—C16—H16120.2
C6—C5—C4103.39 (18)C16—C17—C18120.6 (2)
O5—C5—H5110.9C16—C17—H17119.7
C6—C5—H5110.9C18—C17—H17119.7
C4—C5—H5110.9C19—C18—C17119.8 (2)
O2—C6—C5106.02 (19)C19—C18—H18120.1
O2—C6—H6A110.5C17—C18—H18120.1
C5—C6—H6A110.5C20—C19—C18120.3 (2)
O2—C6—H6B110.5C20—C19—H19119.8
C5—C6—H6B110.5C18—C19—H19119.8
H6A—C6—H6B108.7C19—C20—C15119.9 (2)
O4—C7—O3122.27 (19)C19—C20—H20120.0
O4—C7—C8126.27 (19)C15—C20—H20120.0
O3—C7—C8111.46 (17)C1—O1—C4111.24 (15)
C13—C8—C9119.8 (2)C3—O2—C6105.25 (16)
C13—C8—C7122.24 (19)C7—O3—C2115.35 (16)
C9—C8—C7117.9 (2)C14—O5—C5115.34 (15)
C10—C9—C8119.5 (2)
O7—C1—C2—O339.4 (3)C7—C8—C13—C12175.6 (2)
O1—C1—C2—O3140.40 (18)O6—C14—C15—C20177.3 (3)
O7—C1—C2—C3162.3 (2)O5—C14—C15—C201.7 (3)
O1—C1—C2—C317.6 (2)O6—C14—C15—C161.2 (4)
O3—C2—C3—O231.1 (2)O5—C14—C15—C16179.9 (2)
C1—C2—C3—O285.81 (19)C20—C15—C16—C170.2 (4)
O3—C2—C3—C4141.68 (16)C14—C15—C16—C17178.3 (2)
C1—C2—C3—C424.7 (2)C15—C16—C17—C180.0 (4)
O2—C3—C4—O190.05 (19)C16—C17—C18—C190.2 (4)
C2—C3—C4—O124.3 (2)C17—C18—C19—C200.6 (4)
O2—C3—C4—C527.2 (2)C18—C19—C20—C150.9 (4)
C2—C3—C4—C5141.55 (16)C16—C15—C20—C190.6 (4)
O1—C4—C5—O57.1 (2)C14—C15—C20—C19177.8 (2)
C3—C4—C5—O5120.05 (18)O7—C1—O1—C4177.88 (19)
O1—C4—C5—C6109.20 (19)C2—C1—O1—C42.0 (2)
C3—C4—C5—C63.8 (2)C3—C4—O1—C114.2 (2)
O5—C5—C6—O298.2 (2)C5—C4—O1—C1126.26 (18)
C4—C5—C6—O220.9 (2)C2—C3—O2—C6151.80 (17)
O4—C7—C8—C13179.6 (3)C4—C3—O2—C641.7 (2)
O3—C7—C8—C130.8 (3)C5—C6—O2—C339.8 (2)
O4—C7—C8—C93.5 (4)O4—C7—O3—C22.5 (3)
O3—C7—C8—C9176.2 (2)C8—C7—O3—C2177.11 (18)
C13—C8—C9—C100.4 (4)C3—C2—O3—C769.5 (2)
C7—C8—C9—C10176.7 (2)C1—C2—O3—C7175.54 (18)
C8—C9—C10—C110.8 (4)O6—C14—O5—C51.1 (3)
C9—C10—C11—C121.1 (4)C15—C14—O5—C5179.92 (18)
C10—C11—C12—C130.1 (4)C6—C5—O5—C14169.6 (2)
C11—C12—C13—C81.1 (4)C4—C5—O5—C1477.1 (2)
C9—C8—C13—C121.3 (4)
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
C2—H2···O6i1.002.342.974 (2)121
C3—H3···O4ii1.002.513.356 (3)142
C10—H10···O7iii0.952.483.353 (3)154
Symmetry codes: (i) x+1/2, y+3/2, z+1; (ii) x1/2, y+1/2, z+1; (iii) x, y1, z.
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
C2—H2···O6i1.002.342.974 (2)120.7
C3—H3···O4ii1.002.513.356 (3)141.5
C10—H10···O7iii0.952.483.353 (3)153.6
Symmetry codes: (i) x+1/2, y+3/2, z+1; (ii) x1/2, y+1/2, z+1; (iii) x, y1, z.
Acknowledgements top

The authors thank the Centro Interdipartimentale di Metodologie Chimico–Fisiche, Università degli Studi di Napoli "Federico II" for X-ray and NMR facilities.

references
References top

Altomare, A., Burla, M. C., Camalli, M., Cascarano, G. L., Giacovazzo, C., Guagliardi, A., Moliterni, A. G. G., Polidori, G. & Spagna, R. (1999). J. Appl. Cryst. 32, 115–119.

Bruker . (2001). SADABS. Bruker AXS Inc., Madison, Wisconsin, USA.

De Champdorè, M., Lasalvia, M. & Piccialli, V. (1998). Tetrahedron Lett. 39, 9781–9784.

Duisenberg, A. J. M., Hooft, R. W. W., Schreurs, A. M. M. & Kroon, J. (2000). J. Appl. Cryst. 33, 893–898.

Duisenberg, A. J. M., Kroon-Batenburg, L. M. J. & Schreurs, A. M. M. (2003). J. Appl. Cryst. 36, 220–229.

Farrugia, L. J. (2012). J. Appl. Cryst. 45, 849–854.

Hanessian, S. (1993). Total synthesis of natural products `Chiron approach'. Organic Chemistry Series, edited by J. E. Baldwin. New York: Pergamon Press.

Hockett, R. C., Fletcher, H. G. Jr, Sheffield, E., Goepp, R. M. Jr & Soltzberg, S. (1946). J. Am. Chem. Soc. pp. 930–935.

Lohray, B. B., Baskaran, S., Rao, B. S., Reddy, B. Y. & Rao, I. N. (1999). Tetrahedron Lett. 40, 4855–4856.

Macrae, C. F., Edgington, P. R., McCabe, P., Pidcock, E., Shields, G. P., Taylor, R., Towler, M. & van de Streek, J. (2006). J. Appl. Cryst. 39, 453–457.

Masaki, Y., Arasaki, H. & Itoh, A. (1999). Tetrahedron Lett. 40, 4829–4832.

Nonius, B. V. (1999). COLLECT. Nonius BV, Delft, The Netherlands.

Piccialli, V. (2007). Synthesis, pp. 2585–2607.

Piccialli, V., D'Errico, S., Borbone, N., Oliviero, G., Centore, R. & Zaccaria, S. (2013). Eur. J. Org. Chem. pp. 1781–1789.

Piccialli, V., Oliviero, G., Borbone, N., Centore, R. & Tuzi, A. (2013). Acta Cryst. E69, o879–o880.

Piccialli, V., Tuzi, A., Oliviero, G., Borbone, N. & Centore, R. (2013). Acta Cryst. E69, o1109–o1110.

Piccialli, V., Zaccaria, S., Oliviero, G., D'Errico, S., D'Atri, V. & Borbone, N. (2012). Eur. J. Org. Chem. pp. 4293–4305.

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