organic compounds\(\def\hfill{\hskip 5em}\def\hfil{\hskip 3em}\def\eqno#1{\hfil {#1}}\)

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(+)-(1S,5R,6R)-6-[(S)-1-Hy­dr­oxy-2-(meth­­oxy­meth­yl­oxy)eth­yl]-1-methyl-3-tri­chloro­methyl-2-aza-4,7-dioxa­bi­cyclo­[3.3.0]oct-2-en-8-one

aSchool of Medicine, Keio University, Hiyoshi 4-1-1, Kohoku-ku, Yokohama 223-8521, Japan, and bDepartment of Applied Chemistry, Faculty of Science and Technology, Keio University, Hiyoshi 3-14-1, Kohoku-ku, Yokohama 223-8522, Japan
*Correspondence e-mail: oec@a6.keio.jp

(Received 9 October 2012; accepted 15 October 2012; online 20 October 2012)

In the title compound, C11H14Cl3NO6, the fused five-membered oxazoline and tetra­hydro­furan rings are essentially planar with maximum deviations of 0.069 (1) and 0.031 (1) Å, respectively, and make a dihedral angle of 64.23 (11)° with each other. In the crystal, mol­ecules are linked by O—H⋯O and C—H⋯O hydrogen bonds, forming chains along the b-axis direction. Further C—H⋯O hydrogen bonds are observed between the chains.

Related literature

For the synthesis, see: Oishi et al. (2012[Oishi, H., Tsuzaki, S., Sato, T. & Chida, N. (2012). In preparation.]). For the isolation of sphingofungins, see: VanMiddlesworth, Giacobbe et al. (1992[VanMiddlesworth, F., Giacobbe, R. A., Lopez, M., Garrity, G., Bland, J. A., Bartizal, K., Fromtling, R. A., Polishook, J., Zweerink, M., Edison, A. M., Rozdilsky, W., Wilson, K. E. & Monaghan, R. L. (1992). J. Antibiot. 45, 861-867.]); VanMiddlesworth, Dufresne et al. (1992[VanMiddlesworth, F., Dufresne, C., Wincott, F. E., Mosley, R. T. & Wilson, K. E. (1992). Tetrahedron Lett. 33, 297-300.]); Horn et al. (1992[Horn, W. S., Smith, J. L., Bills, G. F., Raghoobar, S. L., Helms, G. L., Kurts, M. B., Marrinan, J. A., Frommer, B. R., Thornton, R. A. & Mandala, S. M. (1992). J. Antibiot. 45, 1692-1696.]).

[Scheme 1]

Experimental

Crystal data
  • C11H14Cl3NO6

  • Mr = 362.58

  • Monoclinic, P 21

  • a = 8.9311 (7) Å

  • b = 6.0283 (4) Å

  • c = 13.8694 (10) Å

  • β = 99.699 (2)°

  • V = 736.05 (9) Å3

  • Z = 2

  • Mo Kα radiation

  • μ = 0.65 mm−1

  • T = 90 K

  • 0.50 × 0.25 × 0.16 mm

Data collection
  • Bruker D8 goniometer diffractometer

  • Absorption correction: multi-scan (SADABS; Bruker, 2012[Bruker (2012). APEX2, SAINT and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA.]) Tmin = 0.738, Tmax = 0.904

  • 6662 measured reflections

  • 2364 independent reflections

  • 2291 reflections with I > 2σ(I)

  • Rint = 0.028

Refinement
  • R[F2 > 2σ(F2)] = 0.027

  • wR(F2) = 0.069

  • S = 1.34

  • 2364 reflections

  • 193 parameters

  • 1 restraint

  • H-atom parameters constrained

  • Δρmax = 0.37 e Å−3

  • Δρmin = −0.22 e Å−3

  • Absolute structure: Flack (1983[Flack, H. D. (1983). Acta Cryst. A39, 876-881.]), 947 Friedel pairs

  • Flack parameter: 0.01 (5)

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A D—H H⋯A DA D—H⋯A
O12—H12⋯O15i 0.84 1.90 2.695 (2) 157
C6—H6⋯O9ii 1.00 2.43 3.402 (3) 164
C17—H17C⋯O9iii 0.98 2.53 3.320 (3) 137
Symmetry codes: (i) x, y-1, z; (ii) x, y+1, z; (iii) [-x+1, y+{\script{1\over 2}}, -z+1].

Data collection: APEX2 (Bruker, 2012[Bruker (2012). APEX2, SAINT and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA.]); cell refinement: SAINT (Bruker, 2012[Bruker (2012). APEX2, SAINT and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA.]); data reduction: SAINT; 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: Mercury (Macrae et al., 2006[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.]); software used to prepare material for publication: SHELXL97.

Supporting information


Comment top

Sphingofungins are natural antifungal agents isolated from Aspergillus and reported to be potent inhibitors of the biosynthesis of sphingolipids (VanMiddlesworth, Giacobbe et al., 1992; VanMiddlesworth, Dufresne et al., 1992; Horn et al., 1992). The title compound (I), C11H14Cl3NO6, which has four contiguous stereogenic center including a tetra-substituted carbon with nitrogen (Fig. 1), was provided in a synthetic study on the natural products sphingofungins from D-ribose (Oishi et al., 2012). The absolute configurations were confirmed by the X-ray analysis as C1S, C5R, C6R and C10S. The crystal packing was stabilized by an intermolecular O—H···O hydrogen bond, forming molecular a chain along the b axis (Fig. 2). There are also C—H···O hydrogen bonds and intermolecular Cl···O short contacts, Cl19···O4 (x, y - 1, z) and Cl20···O7 (x + 1, y, z) being 3.070 (2) and 3.142 (2) Å, respectively.

Related literature top

For the synthesis, see: Oishi et al. (2012). For the isolation of sphingofungins, see: VanMiddlesworth, Giacobbe et al. (1992); VanMiddlesworth, Dufresne et al. (1992); Horn et al. (1992).

Experimental top

The title compound was obtained in a synthetic study of sphingofungins from D-ribose (Oishi et al., 2012), and recrystallized from ethyl acetate solution. [α]27D +83.1 (c 0.345, CHCl3); m.p. 409.7–411.2 K.

Refinement top

C-bound H atoms were positioned geometrically with C—H = 0.98–1.00 Å, and constrained to ride on their parent atoms with Uiso(H) = 1.2Ueq(C) or 1.5Ueq(methyl C). The H atom of hydroxyl group (O12) was placed guided by difference maps, with O—H = 0.84 Å and with Uiso(H) = 1.5Ueq(O). Three reflections (6 3 6, 6 3 7, 0 1 16) have been omitted in the final refinement.

Computing details top

Data collection: APEX2 (Bruker, 2012); cell refinement: SAINT (Bruker, 2012); data reduction: SAINT (Bruker, 2012); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: Mercury (Macrae et al., 2006); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures top
[Figure 1] Fig. 1. The molecular structure of the title compound, with atom labels and 50% probability displacement ellipsoids for non-H atoms.
[Figure 2] Fig. 2. The crystal packing of the title compound, viewed down the a axis. The dashed lines indicate O—H···O hydrogen bonds.
(+)-(1S,5R,6R)-6-[(S)-1-Hydroxy-2-(methoxymethyloxy)ethyl]-1-methyl-3-trichloromethyl-2-aza-4,7-dioxabicyclo[3.3.0]oct-2-en-8-one top
Crystal data top
C11H14Cl3NO6Dx = 1.636 Mg m3
Mr = 362.58Melting point: 409.7 K
Monoclinic, P21Mo Kα radiation, λ = 0.71073 Å
a = 8.9311 (7) ÅCell parameters from 5778 reflections
b = 6.0283 (4) Åθ = 2.3–25.1°
c = 13.8694 (10) ŵ = 0.65 mm1
β = 99.699 (2)°T = 90 K
V = 736.05 (9) Å3Prism, colourless
Z = 20.50 × 0.25 × 0.16 mm
F(000) = 372
Data collection top
Bruker D8 goniometer
diffractometer
2364 independent reflections
Radiation source: fine-focus sealed tube2291 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.028
Detector resolution: 10.4167 pixels mm-1θmax = 25.1°, θmin = 2.3°
ω scansh = 1010
Absorption correction: multi-scan
(SADABS; Bruker, 2012)
k = 76
Tmin = 0.738, Tmax = 0.904l = 1616
6662 measured reflections
Refinement top
Refinement on F2Secondary atom site location: difference Fourier map
Least-squares matrix: fullHydrogen site location: inferred from neighbouring sites
R[F2 > 2σ(F2)] = 0.027H-atom parameters constrained
wR(F2) = 0.069 w = 1/[σ2(Fo2) + (0.0338P)2]
where P = (Fo2 + 2Fc2)/3
S = 1.34(Δ/σ)max = 0.001
2364 reflectionsΔρmax = 0.37 e Å3
193 parametersΔρmin = 0.22 e Å3
1 restraintAbsolute structure: Flack (1983), 947 Friedel pairs
Primary atom site location: structure-invariant direct methodsAbsolute structure parameter: 0.01 (5)
Crystal data top
C11H14Cl3NO6V = 736.05 (9) Å3
Mr = 362.58Z = 2
Monoclinic, P21Mo Kα radiation
a = 8.9311 (7) ŵ = 0.65 mm1
b = 6.0283 (4) ÅT = 90 K
c = 13.8694 (10) Å0.50 × 0.25 × 0.16 mm
β = 99.699 (2)°
Data collection top
Bruker D8 goniometer
diffractometer
2364 independent reflections
Absorption correction: multi-scan
(SADABS; Bruker, 2012)
2291 reflections with I > 2σ(I)
Tmin = 0.738, Tmax = 0.904Rint = 0.028
6662 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.027H-atom parameters constrained
wR(F2) = 0.069Δρmax = 0.37 e Å3
S = 1.34Δρmin = 0.22 e Å3
2364 reflectionsAbsolute structure: Flack (1983), 947 Friedel pairs
193 parametersAbsolute structure parameter: 0.01 (5)
1 restraint
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
C10.7195 (2)0.5236 (4)0.43062 (15)0.0152 (5)
N20.7869 (2)0.3750 (3)0.36335 (12)0.0139 (4)
C30.8554 (2)0.5035 (4)0.31378 (15)0.0134 (5)
O40.85179 (17)0.7255 (3)0.32714 (10)0.0156 (3)
C50.7464 (2)0.7590 (4)0.39563 (15)0.0148 (5)
H50.79280.85500.45160.018*
C60.5893 (2)0.8481 (4)0.34831 (15)0.0142 (5)
H60.56520.98250.38510.017*
O70.48036 (16)0.6720 (3)0.36046 (10)0.0162 (4)
C80.5464 (2)0.4965 (4)0.40903 (15)0.0142 (5)
O90.47385 (18)0.3412 (3)0.42994 (10)0.0181 (4)
C100.5688 (3)0.9041 (4)0.24039 (15)0.0161 (5)
H100.64641.01850.23160.019*
C110.4125 (3)1.0049 (4)0.20566 (15)0.0165 (5)
H11A0.33350.90270.22160.020*
H11B0.40391.14620.24080.020*
O120.59596 (18)0.7153 (3)0.18539 (11)0.0208 (4)
H120.51320.65200.16420.031*
O130.38660 (19)1.0460 (3)0.10212 (11)0.0205 (4)
C140.4558 (3)1.2396 (4)0.07599 (16)0.0192 (5)
H14A0.45771.23650.00490.023*
H14B0.56221.24370.11050.023*
O150.38097 (18)1.4326 (3)0.09848 (11)0.0195 (4)
C160.2416 (3)1.4735 (5)0.03432 (17)0.0256 (6)
H16A0.16631.36250.04560.038*
H16B0.25821.46410.03360.038*
H16C0.20451.62200.04680.038*
C170.7852 (3)0.4676 (4)0.53576 (15)0.0194 (5)
H17A0.89620.47930.54540.029*
H17B0.75640.31590.55030.029*
H17C0.74550.57140.57960.029*
C180.9561 (2)0.4317 (4)0.24145 (15)0.0155 (5)
Cl190.95353 (6)0.14171 (10)0.22820 (4)0.02283 (16)
Cl201.14459 (6)0.51691 (10)0.28730 (4)0.02415 (16)
Cl210.89393 (7)0.55359 (12)0.12613 (4)0.03251 (19)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
C10.0208 (12)0.0143 (12)0.0113 (10)0.0026 (11)0.0052 (9)0.0013 (10)
N20.0157 (10)0.0130 (10)0.0135 (8)0.0008 (8)0.0040 (8)0.0008 (8)
C30.0127 (11)0.0159 (12)0.0107 (10)0.0001 (10)0.0005 (8)0.0026 (10)
O40.0157 (8)0.0130 (8)0.0195 (8)0.0007 (7)0.0071 (6)0.0001 (7)
C50.0158 (11)0.0172 (13)0.0122 (10)0.0006 (10)0.0050 (8)0.0015 (11)
C60.0191 (12)0.0100 (11)0.0147 (10)0.0041 (10)0.0063 (9)0.0026 (10)
O70.0162 (8)0.0161 (9)0.0174 (8)0.0006 (7)0.0061 (6)0.0032 (7)
C80.0219 (12)0.0125 (12)0.0094 (10)0.0023 (11)0.0065 (9)0.0024 (10)
O90.0228 (9)0.0154 (9)0.0174 (7)0.0033 (7)0.0074 (7)0.0001 (7)
C100.0195 (12)0.0159 (12)0.0143 (10)0.0009 (10)0.0064 (9)0.0022 (10)
C110.0216 (11)0.0175 (13)0.0109 (10)0.0012 (10)0.0041 (9)0.0026 (10)
O120.0208 (9)0.0239 (9)0.0179 (8)0.0014 (8)0.0035 (7)0.0072 (8)
O130.0294 (9)0.0177 (9)0.0135 (8)0.0011 (8)0.0010 (7)0.0023 (7)
C140.0231 (13)0.0195 (12)0.0164 (11)0.0038 (11)0.0071 (9)0.0027 (11)
O150.0219 (9)0.0171 (8)0.0194 (8)0.0012 (7)0.0038 (7)0.0004 (8)
C160.0234 (13)0.0262 (15)0.0257 (12)0.0023 (11)0.0005 (10)0.0014 (12)
C170.0190 (12)0.0231 (14)0.0168 (11)0.0052 (10)0.0048 (9)0.0037 (11)
C180.0158 (12)0.0159 (12)0.0151 (11)0.0009 (10)0.0032 (9)0.0003 (10)
Cl190.0254 (3)0.0168 (3)0.0290 (3)0.0034 (3)0.0126 (3)0.0074 (3)
Cl200.0162 (3)0.0259 (3)0.0317 (3)0.0044 (3)0.0079 (2)0.0094 (3)
Cl210.0345 (4)0.0461 (5)0.0202 (3)0.0192 (3)0.0139 (3)0.0130 (3)
Geometric parameters (Å, º) top
C1—N21.491 (3)C11—H11A0.9900
C1—C171.515 (3)C11—H11B0.9900
C1—C51.532 (3)O12—H120.8400
C1—C81.533 (3)O13—C141.396 (3)
N2—C31.260 (3)C14—O151.403 (3)
C3—O41.352 (3)C14—H14A0.9900
C3—C181.519 (3)C14—H14B0.9900
O4—C51.459 (3)O15—C161.424 (3)
C5—C61.542 (3)C16—H16A0.9800
C5—H51.0000C16—H16B0.9800
C6—O71.468 (3)C16—H16C0.9800
C6—C101.515 (3)C17—H17A0.9800
C6—H61.0000C17—H17B0.9800
O7—C81.337 (3)C17—H17C0.9800
C8—O91.202 (3)C18—Cl191.757 (3)
C10—O121.414 (3)C18—Cl211.763 (2)
C10—C111.524 (3)C18—Cl201.773 (2)
C10—H101.0000Cl19—O4i3.0697 (17)
C11—O131.437 (2)Cl20—O7ii3.1419 (16)
N2—C1—C17109.67 (18)O13—C11—H11A109.3
N2—C1—C5104.89 (16)C10—C11—H11A109.3
C17—C1—C5117.1 (2)O13—C11—H11B109.3
N2—C1—C8108.29 (17)C10—C11—H11B109.3
C17—C1—C8112.22 (18)H11A—C11—H11B107.9
C5—C1—C8104.10 (18)C10—O12—H12109.5
C3—N2—C1104.83 (19)C14—O13—C11113.55 (17)
N2—C3—O4120.6 (2)O13—C14—O15112.79 (17)
N2—C3—C18125.5 (2)O13—C14—H14A109.0
O4—C3—C18113.8 (2)O15—C14—H14A109.0
C3—O4—C5105.01 (17)O13—C14—H14B109.0
O4—C5—C1103.34 (17)O15—C14—H14B109.0
O4—C5—C6114.30 (16)H14A—C14—H14B107.8
C1—C5—C6106.11 (18)C14—O15—C16113.79 (18)
O4—C5—H5110.9O15—C16—H16A109.5
C1—C5—H5110.9O15—C16—H16B109.5
C6—C5—H5110.9H16A—C16—H16B109.5
O7—C6—C10107.59 (17)O15—C16—H16C109.5
O7—C6—C5105.81 (18)H16A—C16—H16C109.5
C10—C6—C5116.62 (18)H16B—C16—H16C109.5
O7—C6—H6108.9C1—C17—H17A109.5
C10—C6—H6108.9C1—C17—H17B109.5
C5—C6—H6108.9H17A—C17—H17B109.5
C8—O7—C6112.65 (16)C1—C17—H17C109.5
O9—C8—O7121.9 (2)H17A—C17—H17C109.5
O9—C8—C1127.1 (2)H17B—C17—H17C109.5
O7—C8—C1111.04 (19)C3—C18—Cl19110.75 (17)
O12—C10—C11112.46 (18)C3—C18—Cl21110.46 (16)
O12—C10—C6110.51 (19)Cl19—C18—Cl21108.87 (12)
C11—C10—C6110.68 (17)C3—C18—Cl20108.21 (15)
O12—C10—H10107.7Cl19—C18—Cl20108.69 (13)
C11—C10—H10107.7Cl21—C18—Cl20109.84 (12)
C6—C10—H10107.7C18—Cl19—O4i139.97 (8)
O13—C11—C10111.67 (17)C18—Cl20—O7ii177.84 (8)
C17—C1—N2—C3118.3 (2)C17—C1—C8—O947.7 (3)
C5—C1—N2—C38.3 (2)C5—C1—C8—O9175.3 (2)
C8—C1—N2—C3119.0 (2)N2—C1—C8—O7105.6 (2)
C1—N2—C3—O41.7 (3)C17—C1—C8—O7133.2 (2)
C1—N2—C3—C18173.61 (19)C5—C1—C8—O75.6 (2)
N2—C3—O4—C56.0 (3)O7—C6—C10—O1259.9 (2)
C18—C3—O4—C5178.18 (16)C5—C6—C10—O1258.7 (3)
C3—O4—C5—C110.3 (2)O7—C6—C10—C1165.4 (2)
C3—O4—C5—C6104.5 (2)C5—C6—C10—C11176.0 (2)
N2—C1—C5—O411.3 (2)O12—C10—C11—O1352.1 (3)
C17—C1—C5—O4110.49 (19)C6—C10—C11—O13176.20 (19)
C8—C1—C5—O4125.02 (17)C10—C11—O13—C1479.6 (2)
N2—C1—C5—C6109.24 (18)C11—O13—C14—O1574.1 (2)
C17—C1—C5—C6128.9 (2)O13—C14—O15—C1674.3 (2)
C8—C1—C5—C64.4 (2)N2—C3—C18—Cl194.6 (3)
O4—C5—C6—O7115.31 (19)O4—C3—C18—Cl19179.78 (15)
C1—C5—C6—O72.1 (2)N2—C3—C18—Cl21125.3 (2)
O4—C5—C6—C104.2 (3)O4—C3—C18—Cl2159.1 (2)
C1—C5—C6—C10117.4 (2)N2—C3—C18—Cl20114.4 (2)
C10—C6—O7—C8126.83 (19)O4—C3—C18—Cl2061.2 (2)
C5—C6—O7—C81.5 (2)C3—C18—Cl19—O4i12.9 (2)
C6—O7—C8—O9176.28 (19)Cl21—C18—Cl19—O4i134.58 (9)
C6—O7—C8—C14.6 (2)Cl20—C18—Cl19—O4i105.81 (13)
N2—C1—C8—O973.5 (3)
Symmetry codes: (i) x, y1, z; (ii) x+1, y, z.
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
O12—H12···O15i0.841.902.695 (2)157
C6—H6···O9iii1.002.433.402 (3)164
C17—H17C···O9iv0.982.533.320 (3)137
Symmetry codes: (i) x, y1, z; (iii) x, y+1, z; (iv) x+1, y+1/2, z+1.

Experimental details

Crystal data
Chemical formulaC11H14Cl3NO6
Mr362.58
Crystal system, space groupMonoclinic, P21
Temperature (K)90
a, b, c (Å)8.9311 (7), 6.0283 (4), 13.8694 (10)
β (°) 99.699 (2)
V3)736.05 (9)
Z2
Radiation typeMo Kα
µ (mm1)0.65
Crystal size (mm)0.50 × 0.25 × 0.16
Data collection
DiffractometerBruker D8 goniometer
diffractometer
Absorption correctionMulti-scan
(SADABS; Bruker, 2012)
Tmin, Tmax0.738, 0.904
No. of measured, independent and
observed [I > 2σ(I)] reflections
6662, 2364, 2291
Rint0.028
(sin θ/λ)max1)0.597
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.027, 0.069, 1.34
No. of reflections2364
No. of parameters193
No. of restraints1
H-atom treatmentH-atom parameters constrained
Δρmax, Δρmin (e Å3)0.37, 0.22
Absolute structureFlack (1983), 947 Friedel pairs
Absolute structure parameter0.01 (5)

Computer programs: APEX2 (Bruker, 2012), SAINT (Bruker, 2012), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), Mercury (Macrae et al., 2006).

Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
O12—H12···O15i0.841.902.695 (2)157
C6—H6···O9ii1.002.433.402 (3)164
C17—H17C···O9iii0.982.533.320 (3)137
Symmetry codes: (i) x, y1, z; (ii) x, y+1, z; (iii) x+1, y+1/2, z+1.
 

Acknowledgements

We thank Professor S. Ohba, Professor N. Yoshioka and Dr C. Maeda (Keio University, Japan) for providing valuable advice.

References

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