Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270104027027/sx1146sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108270104027027/sx1146IIsup2.hkl |
CCDC reference: 259049
Synthesis of enantiomerically pure compound (II) was achieved by employing a highly stereoselective Michael reaction of chiral imine derived from (R)-1-phenylethylamine and 2-methylcyclohexanone with dicyclohexylammonium 2-(diethoxyphosphoryl)acrylate. Subsequent reduction of the carbonyl group in the adduct with KBH4 was followed by lactonization of the resulting 2-(diethoxyphosphoryl)-5-hydroxyalkanoic acid. The final step in the synthesis pathway was the Horner–Wadsworth–Emmons olefination of the obtained α-phosphono-δ-valerolactone with formaldehyde. Details of the procedure have been described elsewhere (Krawczyk & Śliwiński, 2003; Krawczyk et al., 2004). Colourless crystals (m.p. 344 K) were grown in 8 d by slow evaporation from a 1:1 mixture of methanol and ethyl acetate.
All H atoms were located on a difference Fourier map calculated after three cycles of anisotropic refinement. The H-atom positional and isotropic displacement parameters were allowed to refine freely [C—H =0.94 (3)–1.04 (2) Å]. Refinement of the Flack (1983) parameter is in agreement with the absolute configuration as assigned from the mechanism of the highly stereoselective Michael (Krawczyk & Śliwiński, 2003) reaction. An attempt to refine the inverted structure led to a Flack parameter equal to 1.0 (2).
Data collection: SMART (Bruker, 2003); cell refinement: SMART; data reduction: SAINT-Plus (Bruker, 2003); program(s) used to solve structure: SHELXTL (Bruker, 2003); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL.
C11H16O2 | Dx = 1.197 Mg m−3 |
Mr = 180.24 | Melting point: 344 K |
Orthorhombic, P212121 | Cu Kα radiation, λ = 1.54178 Å |
a = 9.248 (1) Å | Cell parameters from 9898 reflections |
b = 9.317 (1) Å | θ = 5–70° |
c = 11.612 (1) Å | µ = 0.64 mm−1 |
V = 1000.53 (18) Å3 | T = 293 K |
Z = 4 | Prism, colourless |
F(000) = 392 | 0.18 × 0.15 × 0.12 mm |
Bruker SMART APEX diffractometer | 1911 independent reflections |
Radiation source: fine-focus sealed tube | 1886 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.020 |
ω scans | θmax = 71.0°, θmin = 6.1° |
Absorption correction: numerical (SHELXTL; Bruker, 2003) | h = −11→10 |
Tmin = 0.881, Tmax = 0.942 | k = −11→11 |
11840 measured reflections | l = −14→14 |
Refinement on F2 | Hydrogen site location: difference Fourier map |
Least-squares matrix: full | All H-atom parameters refined |
R[F2 > 2σ(F2)] = 0.029 | w = 1/[σ2(Fo2) + (0.0704P)2 + 0.0293P] where P = (Fo2 + 2Fc2)/3 |
wR(F2) = 0.088 | (Δ/σ)max < 0.001 |
S = 1.02 | Δρmax = 0.11 e Å−3 |
1911 reflections | Δρmin = −0.12 e Å−3 |
184 parameters | Extinction correction: SHELXL97, Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4 |
0 restraints | Extinction coefficient: 0.035 (3) |
Primary atom site location: structure-invariant direct methods | Absolute structure: Absolute structure Flack (1983), 773 Friedel pairs |
Secondary atom site location: difference Fourier map | Absolute structure parameter: 0.06 (20) |
C11H16O2 | V = 1000.53 (18) Å3 |
Mr = 180.24 | Z = 4 |
Orthorhombic, P212121 | Cu Kα radiation |
a = 9.248 (1) Å | µ = 0.64 mm−1 |
b = 9.317 (1) Å | T = 293 K |
c = 11.612 (1) Å | 0.18 × 0.15 × 0.12 mm |
Bruker SMART APEX diffractometer | 1911 independent reflections |
Absorption correction: numerical (SHELXTL; Bruker, 2003) | 1886 reflections with I > 2σ(I) |
Tmin = 0.881, Tmax = 0.942 | Rint = 0.020 |
11840 measured reflections |
R[F2 > 2σ(F2)] = 0.029 | All H-atom parameters refined |
wR(F2) = 0.088 | Δρmax = 0.11 e Å−3 |
S = 1.02 | Δρmin = −0.12 e Å−3 |
1911 reflections | Absolute structure: Absolute structure Flack (1983), 773 Friedel pairs |
184 parameters | Absolute structure parameter: 0.06 (20) |
0 restraints |
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. |
x | y | z | Uiso*/Ueq | ||
O1 | 0.78686 (10) | 0.34595 (9) | 0.97412 (6) | 0.0583 (2) | |
O2 | 0.72242 (13) | 0.17661 (13) | 0.85586 (8) | 0.0821 (3) | |
C1 | 0.84320 (11) | 0.40040 (11) | 1.08412 (9) | 0.0471 (3) | |
H11 | 0.9468 (13) | 0.3995 (12) | 1.0752 (10) | 0.043 (3)* | |
C2 | 0.79834 (11) | 0.30787 (10) | 1.18609 (8) | 0.0455 (2) | |
C3 | 0.84338 (13) | 0.15383 (12) | 1.15805 (11) | 0.0558 (3) | |
H31 | 0.9490 (18) | 0.1520 (18) | 1.1523 (14) | 0.072 (4)* | |
H32 | 0.8172 (16) | 0.0883 (17) | 1.2221 (13) | 0.062 (4)* | |
C4 | 0.77973 (13) | 0.10227 (11) | 1.04729 (11) | 0.0564 (3) | |
C5 | 0.7349 (2) | −0.03012 (16) | 1.03080 (19) | 0.0892 (5) | |
H51 | 0.693 (3) | −0.060 (3) | 0.962 (2) | 0.113 (7)* | |
H52 | 0.740 (3) | −0.106 (2) | 1.0946 (17) | 0.102 (6)* | |
C6 | 0.76205 (11) | 0.20792 (14) | 0.95178 (10) | 0.0564 (3) | |
C7 | 0.78920 (15) | 0.55378 (12) | 1.09147 (11) | 0.0576 (3) | |
H71 | 0.8384 (17) | 0.6001 (15) | 1.1567 (14) | 0.068 (4)* | |
H72 | 0.8193 (15) | 0.6063 (15) | 1.0216 (13) | 0.063 (4)* | |
C8 | 0.62606 (15) | 0.55967 (14) | 1.10839 (13) | 0.0645 (3) | |
H81 | 0.5762 (18) | 0.5135 (17) | 1.0382 (13) | 0.067 (4)* | |
H82 | 0.588 (2) | 0.663 (2) | 1.1177 (16) | 0.092 (5)* | |
C9 | 0.58022 (14) | 0.47390 (14) | 1.21300 (12) | 0.0622 (3) | |
H92 | 0.475 (2) | 0.4706 (18) | 1.2172 (14) | 0.073 (4)* | |
H91 | 0.6149 (17) | 0.5178 (17) | 1.2867 (13) | 0.064 (4)* | |
C10 | 0.63444 (12) | 0.31887 (13) | 1.20564 (11) | 0.0545 (3) | |
H101 | 0.5839 (16) | 0.2712 (14) | 1.1433 (12) | 0.054 (3)* | |
H102 | 0.6113 (19) | 0.2663 (17) | 1.2771 (15) | 0.070 (4)* | |
C11 | 0.88261 (16) | 0.35604 (15) | 1.29293 (11) | 0.0633 (3) | |
H111 | 0.855 (2) | 0.292 (2) | 1.3636 (17) | 0.088 (5)* | |
H112 | 0.851 (2) | 0.454 (2) | 1.3159 (16) | 0.086 (5)* | |
H113 | 0.985 (2) | 0.3544 (19) | 1.2754 (15) | 0.082 (5)* |
U11 | U22 | U33 | U12 | U13 | U23 | |
O1 | 0.0713 (5) | 0.0589 (4) | 0.0446 (4) | 0.0027 (4) | 0.0032 (4) | 0.0021 (3) |
O2 | 0.0815 (6) | 0.1049 (8) | 0.0598 (5) | 0.0042 (6) | −0.0098 (5) | −0.0251 (5) |
C1 | 0.0434 (5) | 0.0473 (5) | 0.0506 (5) | −0.0045 (4) | 0.0030 (4) | 0.0008 (4) |
C2 | 0.0507 (5) | 0.0414 (5) | 0.0443 (5) | −0.0045 (4) | 0.0004 (4) | 0.0003 (4) |
C3 | 0.0621 (6) | 0.0445 (5) | 0.0608 (6) | 0.0027 (5) | −0.0031 (5) | 0.0028 (5) |
C4 | 0.0518 (5) | 0.0484 (5) | 0.0691 (7) | 0.0044 (4) | 0.0036 (5) | −0.0116 (5) |
C5 | 0.1013 (11) | 0.0559 (8) | 0.1105 (12) | −0.0035 (8) | −0.0096 (10) | −0.0209 (9) |
C6 | 0.0481 (5) | 0.0668 (6) | 0.0544 (5) | 0.0057 (4) | 0.0021 (4) | −0.0128 (5) |
C7 | 0.0624 (6) | 0.0428 (5) | 0.0675 (6) | −0.0055 (5) | 0.0031 (6) | 0.0059 (5) |
C8 | 0.0639 (7) | 0.0527 (6) | 0.0769 (8) | 0.0109 (5) | 0.0015 (6) | −0.0016 (6) |
C9 | 0.0556 (6) | 0.0623 (7) | 0.0686 (7) | 0.0021 (5) | 0.0111 (5) | −0.0156 (6) |
C10 | 0.0529 (5) | 0.0537 (6) | 0.0568 (6) | −0.0104 (5) | 0.0119 (5) | −0.0054 (5) |
C11 | 0.0738 (7) | 0.0627 (7) | 0.0535 (6) | −0.0049 (6) | −0.0113 (6) | −0.0025 (5) |
O1—C6 | 1.3319 (15) | C3—H31 | 0.979 (16) |
O1—C1 | 1.4698 (13) | C3—H32 | 0.992 (16) |
O2—C6 | 1.2083 (15) | C5—H51 | 0.94 (3) |
C1—C7 | 1.5162 (15) | C5—H52 | 1.03 (2) |
C1—C2 | 1.5222 (13) | C7—H71 | 0.983 (16) |
C2—C3 | 1.5294 (15) | C7—H72 | 0.988 (15) |
C2—C11 | 1.5324 (15) | C8—H81 | 1.031 (16) |
C2—C10 | 1.5361 (14) | C8—H82 | 1.030 (19) |
C3—C4 | 1.4939 (16) | C9—H92 | 0.976 (18) |
C4—C5 | 1.3153 (19) | C9—H91 | 1.001 (16) |
C4—C6 | 1.4919 (18) | C10—H101 | 0.969 (14) |
C7—C8 | 1.5225 (18) | C10—H102 | 0.987 (18) |
C8—C9 | 1.515 (2) | C11—H111 | 1.04 (2) |
C9—C10 | 1.5314 (17) | C11—H112 | 0.99 (2) |
C1—H11 | 0.963 (12) | C11—H113 | 0.97 (2) |
C5—C4—C6 | 118.43 (14) | C4—C5—H51 | 122.5 (15) |
C5—C4—C3 | 123.44 (14) | C4—C5—H52 | 122.0 (12) |
C6—C4—C3 | 118.11 (9) | H51—C5—H52 | 115.4 (18) |
O2—C6—O1 | 117.70 (12) | C1—C7—H71 | 107.8 (9) |
O2—C6—C4 | 124.01 (12) | C8—C7—H71 | 110.1 (9) |
O1—C6—C4 | 118.26 (9) | C1—C7—H72 | 109.2 (8) |
C6—O1—C1 | 124.30 (9) | C8—C7—H72 | 111.6 (8) |
O1—C1—C7 | 104.92 (9) | H71—C7—H72 | 106.5 (11) |
O1—C1—C2 | 112.58 (8) | C9—C8—H81 | 106.8 (9) |
C7—C1—C2 | 113.59 (9) | C7—C8—H81 | 109.0 (9) |
C1—C2—C3 | 106.96 (9) | C9—C8—H82 | 108.2 (10) |
C1—C2—C11 | 108.98 (9) | C7—C8—H82 | 112.7 (11) |
C3—C2—C11 | 107.98 (10) | H81—C8—H82 | 108.7 (14) |
C1—C2—C10 | 110.25 (9) | C8—C9—H92 | 109.6 (10) |
C3—C2—C10 | 111.27 (9) | C10—C9—H92 | 107.4 (10) |
C11—C2—C10 | 111.26 (9) | C8—C9—H91 | 112.3 (9) |
C4—C3—C2 | 112.19 (9) | C10—C9—H91 | 109.2 (9) |
C1—C7—C8 | 111.57 (9) | H92—C9—H91 | 106.8 (14) |
C9—C8—C7 | 111.21 (11) | C9—C10—H101 | 108.4 (8) |
C9—C10—C2 | 113.22 (9) | C2—C10—H101 | 109.6 (8) |
C8—C9—C10 | 111.18 (10) | C9—C10—H102 | 110.5 (9) |
O1—C1—H11 | 104.8 (7) | C2—C10—H102 | 107.8 (10) |
C7—C1—H11 | 110.0 (7) | H101—C10—H102 | 107.2 (12) |
C2—C1—H11 | 110.5 (7) | C2—C11—H111 | 110.2 (11) |
C4—C3—H31 | 109.2 (10) | C2—C11—H112 | 109.8 (11) |
C2—C3—H31 | 107.6 (10) | H111—C11—H112 | 104.0 (15) |
C4—C3—H32 | 110.6 (9) | C2—C11—H113 | 108.8 (11) |
C2—C3—H32 | 110.6 (9) | H111—C11—H113 | 113.2 (15) |
H31—C3—H32 | 106.5 (13) | H112—C11—H113 | 110.7 (15) |
O1—C1—C2—C3 | 53.59 (11) | C7—C1—C2—C11 | −70.85 (12) |
C1—C2—C3—C4 | −56.38 (12) | C6—O1—C1—C7 | −154.30 (10) |
C2—C1—O1—C6 | −30.29 (14) | C7—C1—C2—C3 | 172.66 (9) |
C3—C4—C6—O1 | −8.15 (15) | O1—C1—C2—C11 | 170.08 (9) |
C4—C6—O1—C1 | 5.71 (15) | O1—C1—C2—C10 | −67.52 (11) |
C6—C4—C3—C2 | 35.04 (14) | C11—C2—C3—C4 | −173.54 (10) |
C1—C2—C10—C9 | −51.39 (13) | C10—C2—C3—C4 | 64.08 (13) |
C2—C10—C9—C8 | 54.14 (14) | C2—C3—C4—C5 | −143.00 (14) |
C10—C9—C8—C7 | −55.17 (15) | C1—O1—C6—O2 | −176.39 (10) |
C9—C8—C7—C1 | 55.34 (15) | C3—C4—C6—O2 | 174.09 (11) |
C8—C7—C1—C2 | −54.31 (14) | C5—C4—C6—O1 | 169.99 (13) |
C7—C1—C2—C10 | 51.54 (12) | O1—C1—C7—C8 | 69.05 (12) |
O2—C6—C4—C5 | −7.8 (2) | C3—C2—C10—C9 | −169.90 (10) |
C9—C10—C2—C11 | 69.64 (13) |
D—H···A | D—H | H···A | D···A | D—H···A |
C11—H113···O2i | 0.97 (2) | 2.69 (2) | 3.599 (2) | 157 (2) |
C9—H91···O1ii | 1.00 (2) | 2.68 (2) | 3.677 (2) | 175 (2) |
Symmetry codes: (i) x+1/2, −y+1/2, −z+2; (ii) −x+3/2, −y+1, z+1/2. |
Experimental details
Crystal data | |
Chemical formula | C11H16O2 |
Mr | 180.24 |
Crystal system, space group | Orthorhombic, P212121 |
Temperature (K) | 293 |
a, b, c (Å) | 9.248 (1), 9.317 (1), 11.612 (1) |
V (Å3) | 1000.53 (18) |
Z | 4 |
Radiation type | Cu Kα |
µ (mm−1) | 0.64 |
Crystal size (mm) | 0.18 × 0.15 × 0.12 |
Data collection | |
Diffractometer | Bruker SMART APEX diffractometer |
Absorption correction | Numerical (SHELXTL; Bruker, 2003) |
Tmin, Tmax | 0.881, 0.942 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 11840, 1911, 1886 |
Rint | 0.020 |
(sin θ/λ)max (Å−1) | 0.613 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.029, 0.088, 1.02 |
No. of reflections | 1911 |
No. of parameters | 184 |
H-atom treatment | All H-atom parameters refined |
Δρmax, Δρmin (e Å−3) | 0.11, −0.12 |
Absolute structure | Absolute structure Flack (1983), 773 Friedel pairs |
Absolute structure parameter | 0.06 (20) |
Computer programs: SMART (Bruker, 2003), SMART, SAINT-Plus (Bruker, 2003), SHELXTL (Bruker, 2003), SHELXTL.
O1—C6 | 1.3319 (15) | C2—C10 | 1.5361 (14) |
O1—C1 | 1.4698 (13) | C3—C4 | 1.4939 (16) |
O2—C6 | 1.2083 (15) | C4—C5 | 1.3153 (19) |
C1—C7 | 1.5162 (15) | C4—C6 | 1.4919 (18) |
C1—C2 | 1.5222 (13) | C7—C8 | 1.5225 (18) |
C2—C3 | 1.5294 (15) | C8—C9 | 1.515 (2) |
C2—C11 | 1.5324 (15) | C9—C10 | 1.5314 (17) |
C5—C4—C6 | 118.43 (14) | O2—C6—O1 | 117.70 (12) |
C5—C4—C3 | 123.44 (14) | O2—C6—C4 | 124.01 (12) |
C6—C4—C3 | 118.11 (9) | O1—C6—C4 | 118.26 (9) |
O1—C1—C2—C3 | 53.59 (11) | C10—C9—C8—C7 | −55.17 (15) |
C1—C2—C3—C4 | −56.38 (12) | C9—C8—C7—C1 | 55.34 (15) |
C2—C1—O1—C6 | −30.29 (14) | C8—C7—C1—C2 | −54.31 (14) |
C3—C4—C6—O1 | −8.15 (15) | C7—C1—C2—C10 | 51.54 (12) |
C4—C6—O1—C1 | 5.71 (15) | O2—C6—C4—C5 | −7.8 (2) |
C6—C4—C3—C2 | 35.04 (14) | C9—C10—C2—C11 | 69.64 (13) |
C1—C2—C10—C9 | −51.39 (13) | C7—C1—C2—C11 | −70.85 (12) |
C2—C10—C9—C8 | 54.14 (14) |
D—H···A | D—H | H···A | D···A | D—H···A |
C11—H113···O2i | 0.97 (2) | 2.69 (2) | 3.599 (2) | 157 (2) |
C9—H91···O1ii | 1.00 (2) | 2.68 (2) | 3.677 (2) | 175 (2) |
Symmetry codes: (i) x+1/2, −y+1/2, −z+2; (ii) −x+3/2, −y+1, z+1/2. |
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Recently, we have employed a highly stereoselective Michael (Krawczyk & Śliwiński, 2003) reaction for a synthesis of the optically active α-methylene-δ-valerolactones and described the crystal structure of the first compound in the series: 3-methylene-2-oxo-hexahydro-chromene-4a-carboxylic acid ethyl ester, (I) (Krawczyk et al., 2004). In the present paper, we report the following structure of the S,S enantiomorph of 4a-methyl-3-methylene-octahydro-chromen-2-one, (II).
The α-methylene-δ-valerolactone moiety is present in various biologically active natural compounds. Several, such as vernolepin and vernomenin (Kupchan et al., 1968), pentalenolactone E (Cane & Rossi, 1979), teucriumlactone (Nangia et al., 1997), artemisitene (Liao et al., 2001), and crassin (Weinheimer et al., 1979; McMurry & Dushin, 1990), have been isolated and identified. These compounds have been reported to exhibit anibacterial, antifungal and in some cases antitumor activities (Ekthawatchai et al., 2001; Avery et al., 2002). However, work on isolation and synthesis of new α-methylene-δ-valerolactones has not led to a significant number of crystal structure investigations. A search of the Cambridge Structural Database (Version 5.25, March 2004 update; Allen, 2002) shows that high-quality single-crystal X-ray structure determinations are restricted to six naturally occurring compounds. In only three structures is the cyclic δ-valerolactone moiety fused with the cycloxexane ring, i.e. juniperine (III) (Maldonado et al., 1985), zaluzanin B (IV) (Toscano et al., 1997) and aciculatalactone (V) (Takaoka et al., 1993). However, the system in which the δ-valerolactone ring is condensed with the cyclohexane moiety along the Cδ—Cγ single bond [as in (I) and (II)] is unique among crystal structures examined to date.
A view of (II), with the atom-numbering scheme, is shown in Fig. 1. The overall molecular conformation can be defined as semi-folded, with the δ-lactone and cyclohexane rings almost perpendicular to one another. The β-methyl substituent occupies an axial position with respect to the cyclohexane ring. The δ-lactone ring adopts a slightly distorted half-chair arrangement, while the cyclohexane ring exists in an almost ideal chair conformation. The upper flap of the latter ring points away from the δ-lactone ring. The resulting molecular conformation is stabilized by two weak intramolecular interactions (Desiraju & Steiner, 1999) between axial H atoms at atoms C8 and C10, and atoms O1 and C4, which bear substantial negative charges (−0.64 and −0.32 e); the respective interatomic distances are 2.60 (2) and 2.65 (2) Å. Atomic charges derived from electrostatic potentials were calculated using GAUSSIAN03 (Frisch et al., 2003) at the MP2/6–31+G(d,p) level for the X-ray determined coordinates. Grid points were selected accordingly to the CHELPG procedure of Breneman & Wiberg (1990).
The X-ray determined structures, in which the α-methylene-δ-valerolactone moiety is condensed with the cyclohexane ring as in (III), (IV) and (V), adopt a fully folded conformation, with the cyclohexane flap aiming towards the δ-lactone ring. The only compound existing in the extended conformation is (I). The superposition of title structure, (II), on to the structure of (I), as presented in Fig. 2, clearly shows the similarity of their δ-lactone rings and the substantial difference between the positions of the cyclohexane rings. The least-squares fit was based on all common non-H atoms of the α-methylene-δ-valerolactone fragment. The r.m.s. deviation was 0.87 Å.
The bond lengths in (II) are close to those observed in the related compounds (I) and (III)–(V). In particular, the two exocyclic double bonds [C6═O2 = 1.2083 (15) Å and C4═C5 = 1.3153 (19) Å] are shorter than similar bonds observed in the C═O—C═C moiety [1.222 and 1.340 Å, respectively; Allen et al., 1992]. Those bonds are separated by a relatively long C4—C6 bond [1.4919 (18) Å; the standard value is 1.465 Å] and are not strictly coplanar, as indicated by the non-zero value of the O2—C6—C4—C5 torsion angle [−7.8 (2)°]. These results suggest that the highly polar character of the carbonyl group hinders π electron density delocalization within the O2═C6—C4═C5 moiety.
In the crystal, the endocyclic O1 and exocyclic O2 atoms are involved in close contacts with the surrounding H atoms of the cyclohexane ring and the methyl group, respectively. According to the definition of Desiraju and Steiner (Steiner, 1997; Desiraju & Steiner, 1998), these contacts could be classified as weak C—H···O hydrogen bonds. Details are summarized in Table 2.