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In the crystal structure of the title compound, C
18H
20O
5, all geometric parameters fall within experimental error of the expected values. Analysis of the molecular-packing plots reveals an infinite one-dimensional linear array running parallel to the
c axis, formed by an O-H
O intermolecular hydrogen-bonding interaction. The stilbene framework and most of the substituents are approximately coplanar.
Supporting information
CCDC reference: 217146
To a dimethylformamide (7 ml) solution containing an isomeric mixture of the silyl-protected precursor to (III) and (IV) [3:1 Z:E-1-(2-tert-butyldimethylsiloxy-4- methoxyphenyl)-2-(3,4,5-trimethoxyphenyl)ethane] (1.12 g, 2.6 mmol) were added KF (150 mg, 2.6 mmol) and HBr (462 mg, 0.3 ml, 2.5 mmol). Additional HBr (0.3 ml) was added after 12 h, and the reaction mixture was stirred for total of a two days. Water (15 ml) was then added to the solution, and the resulting suspension was extracted with ethyl acetate (3 × 15 ml). The combined organic extracts were washed with water, dried over sodium sulfate, and evaporated to dryness. The residue was applied to a silica-gel column and eluted with hexane/ethyl acetate (75:25). Compound (III) was obtained as a white powder, which was crystallized from methanol to afford pure (III) as white crystals (210 mg, 0.49 mmol; yield 18%).
Systematic conditions suggested the unambiguous space group P2(1)/n. Atom H1 was located from a difference map, and H atoms were treated as riding. The space group was confirmed by successful convergence of the full-matrix least-squares refinement on F2 (Sheldrick, 1993). The highest peaks in the final difference Fourier map were in the vicinity of atom C16, and the final map had no other significant features.
Program(s) used to solve structure: SHELXS86 (Sheldrick, 1990); program(s) used to refine structure: SHELXL93 (Sheldrick, 1993).
(
E)-1-(2-hydroxy-4-methoxyphenyl)-2-(3,4,5-trimethoxyphenyl)ethene
top
Crystal data top
C18H20O5 | F(000) = 672 |
Mr = 316.34 | Dx = 1.284 Mg m−3 |
Monoclinic, P21/n | Mo Kα radiation, λ = 0.71073 Å |
a = 11.6693 (15) Å | Cell parameters from 25 reflections |
b = 7.9790 (8) Å | θ = 12.5–21.3° |
c = 17.5818 (13) Å | µ = 0.09 mm−1 |
β = 90.673 (9)° | T = 293 K |
V = 1636.9 (3) Å3 | Block, colorless |
Z = 4 | 0.54 × 0.47 × 0.41 mm |
Data collection top
Enraf-NoniusCAD 4 diffractometer | Rint = 0.016 |
Radiation source: fine-focus sealed tube | θmax = 24.9°, θmin = 2.1° |
Graphite monochromator | h = −1→13 |
ω scan | k = 0→9 |
3369 measured reflections | l = −20→20 |
2867 independent reflections | 3 standard reflections every 600 reflections |
2420 reflections with I > 2σ(I) | intensity decay: 2.1% |
Refinement top
Refinement on F2 | Secondary atom site location: difference Fourier map |
Least-squares matrix: full | Hydrogen site location: inferred from neighbouring sites |
R[F2 > 2σ(F2)] = 0.050 | H-atom parameters constrained |
wR(F2) = 0.128 | w = 1/[σ2(Fo2) + (0.0813P)2 + 0.3079P] where P = (Fo2 + 2Fc2)/3 |
S = 1.10 | (Δ/σ)max < 0.001 |
2867 reflections | Δρmax = 0.74 e Å−3 |
209 parameters | Δρmin = −0.20 e Å−3 |
0 restraints | Extinction correction: SHELXL, Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4 |
Primary atom site location: structure-invariant direct methods | Extinction coefficient: 0.164 (8) |
Crystal data top
C18H20O5 | V = 1636.9 (3) Å3 |
Mr = 316.34 | Z = 4 |
Monoclinic, P21/n | Mo Kα radiation |
a = 11.6693 (15) Å | µ = 0.09 mm−1 |
b = 7.9790 (8) Å | T = 293 K |
c = 17.5818 (13) Å | 0.54 × 0.47 × 0.41 mm |
β = 90.673 (9)° | |
Data collection top
Enraf-NoniusCAD 4 diffractometer | Rint = 0.016 |
3369 measured reflections | 3 standard reflections every 600 reflections |
2867 independent reflections | intensity decay: 2.1% |
2420 reflections with I > 2σ(I) | |
Refinement top
R[F2 > 2σ(F2)] = 0.050 | 0 restraints |
wR(F2) = 0.128 | H-atom parameters constrained |
S = 1.10 | Δρmax = 0.74 e Å−3 |
2867 reflections | Δρmin = −0.20 e Å−3 |
209 parameters | |
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 on F2 for ALL reflections except for 0 with very negative F2 or flagged by the user for potential systematic errors. Weighted R-factors wR and all goodnesses of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The observed criterion of F2 > σ(F2) is used only for calculating R factor_obs 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 | x | y | z | Uiso*/Ueq | |
O1 | −0.01721 (11) | −0.27129 (17) | −0.20934 (7) | 0.0470 (4) | |
H1 | −0.076 (2) | −0.312 (3) | −0.2367 (13) | 0.071* | |
O2 | 0.10768 (11) | −0.07871 (16) | −0.44871 (7) | 0.0426 (3) | |
O3 | 0.08719 (10) | −0.26337 (16) | 0.15941 (7) | 0.0458 (4) | |
O4 | 0.27702 (9) | −0.14811 (14) | 0.22835 (6) | 0.0351 (3) | |
O5 | 0.41793 (9) | 0.05986 (14) | 0.16041 (6) | 0.0365 (3) | |
C1 | 0.13402 (13) | −0.0753 (2) | −0.21153 (9) | 0.0330 (4) | |
C2 | 0.05354 (13) | −0.1737 (2) | −0.25163 (9) | 0.0326 (4) | |
C3 | 0.04788 (13) | −0.1716 (2) | −0.33008 (9) | 0.0341 (4) | |
H3 | −0.0062 | −0.2371 | −0.3555 | 0.041* | |
C4 | 0.12232 (13) | −0.0723 (2) | −0.37109 (9) | 0.0333 (4) | |
C5 | 0.20393 (14) | 0.0245 (2) | −0.33347 (10) | 0.0383 (4) | |
H5 | 0.2550 | 0.0901 | −0.3607 | 0.046* | |
C6 | 0.20797 (14) | 0.0217 (2) | −0.25498 (10) | 0.0384 (4) | |
H6 | 0.2624 | 0.0874 | −0.2300 | 0.046* | |
C7 | 0.13840 (14) | −0.0842 (2) | −0.12881 (10) | 0.0352 (4) | |
H7 | 0.0869 | −0.1576 | −0.1064 | 0.042* | |
C8 | 0.20736 (13) | 0.0001 (2) | −0.08117 (9) | 0.0352 (4) | |
H8 | 0.2541 | 0.0819 | −0.1021 | 0.042* | |
C9 | 0.21574 (13) | −0.0253 (2) | 0.00132 (9) | 0.0337 (4) | |
C10 | 0.13741 (14) | −0.1251 (2) | 0.04066 (10) | 0.0368 (4) | |
H10 | 0.0726 | −0.1659 | 0.0156 | 0.044* | |
C11 | 0.15574 (13) | −0.1633 (2) | 0.11647 (10) | 0.0349 (4) | |
C12 | 0.25160 (13) | −0.0991 (2) | 0.15493 (9) | 0.0321 (4) | |
C13 | 0.32740 (13) | 0.00532 (19) | 0.11699 (9) | 0.0310 (4) | |
C14 | 0.30978 (13) | 0.0426 (2) | 0.04074 (9) | 0.0327 (4) | |
H14 | 0.3605 | 0.1128 | 0.0157 | 0.039* | |
C15 | 0.17861 (18) | 0.0273 (3) | −0.49368 (11) | 0.0510 (5) | |
H15A | 0.1602 | 0.0113 | −0.5466 | 0.077* | |
H15B | 0.2576 | −0.0005 | −0.4846 | 0.077* | |
H15C | 0.1656 | 0.1422 | −0.4801 | 0.077* | |
C16 | −0.00427 (16) | −0.3492 (2) | 0.12219 (12) | 0.0515 (5) | |
H16A | −0.0455 | −0.4146 | 0.1587 | 0.077* | |
H16B | 0.0261 | −0.4218 | 0.0838 | 0.077* | |
H16C | −0.0552 | −0.2693 | 0.0989 | 0.077* | |
C17 | 0.20878 (16) | −0.0721 (2) | 0.28674 (10) | 0.0420 (4) | |
H17A | 0.2327 | −0.1139 | 0.3356 | 0.063* | |
H17B | 0.1295 | −0.0990 | 0.2781 | 0.063* | |
H17C | 0.2186 | 0.0473 | 0.2854 | 0.063* | |
C18 | 0.50324 (14) | 0.1574 (2) | 0.12366 (11) | 0.0441 (5) | |
H18A | 0.5618 | 0.1879 | 0.1599 | 0.066* | |
H18B | 0.4689 | 0.2570 | 0.1029 | 0.066* | |
H18C | 0.5366 | 0.0931 | 0.0834 | 0.066* | |
Atomic displacement parameters (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
O1 | 0.0465 (7) | 0.0571 (8) | 0.0372 (7) | −0.0226 (6) | −0.0084 (6) | 0.0115 (6) |
O2 | 0.0495 (7) | 0.0458 (7) | 0.0326 (6) | −0.0051 (6) | 0.0037 (5) | 0.0000 (5) |
O3 | 0.0374 (7) | 0.0501 (8) | 0.0499 (7) | −0.0114 (6) | −0.0019 (6) | 0.0152 (6) |
O4 | 0.0369 (6) | 0.0375 (7) | 0.0309 (6) | 0.0070 (5) | 0.0006 (5) | 0.0017 (5) |
O5 | 0.0337 (6) | 0.0395 (7) | 0.0362 (6) | −0.0055 (5) | −0.0042 (5) | −0.0033 (5) |
C1 | 0.0287 (8) | 0.0314 (8) | 0.0387 (9) | 0.0032 (6) | −0.0029 (7) | 0.0015 (7) |
C2 | 0.0284 (8) | 0.0311 (8) | 0.0381 (9) | 0.0001 (6) | −0.0027 (6) | 0.0063 (7) |
C3 | 0.0318 (8) | 0.0318 (8) | 0.0385 (9) | −0.0006 (7) | −0.0049 (7) | 0.0008 (7) |
C4 | 0.0326 (8) | 0.0327 (8) | 0.0345 (9) | 0.0065 (6) | 0.0015 (7) | −0.0002 (7) |
C5 | 0.0320 (8) | 0.0407 (9) | 0.0425 (9) | −0.0029 (7) | 0.0043 (7) | 0.0040 (8) |
C6 | 0.0324 (8) | 0.0396 (9) | 0.0431 (9) | −0.0060 (7) | −0.0048 (7) | 0.0020 (8) |
C7 | 0.0318 (8) | 0.0354 (9) | 0.0383 (9) | −0.0009 (7) | −0.0026 (7) | 0.0051 (7) |
C8 | 0.0311 (8) | 0.0343 (9) | 0.0401 (9) | −0.0013 (7) | −0.0016 (7) | 0.0035 (7) |
C9 | 0.0315 (8) | 0.0318 (8) | 0.0378 (9) | 0.0021 (7) | −0.0024 (7) | 0.0019 (7) |
C10 | 0.0287 (8) | 0.0386 (9) | 0.0429 (10) | −0.0006 (7) | −0.0057 (7) | 0.0023 (7) |
C11 | 0.0298 (8) | 0.0322 (9) | 0.0427 (9) | 0.0004 (7) | 0.0016 (7) | 0.0056 (7) |
C12 | 0.0309 (8) | 0.0319 (8) | 0.0334 (8) | 0.0056 (6) | 0.0003 (6) | 0.0027 (7) |
C13 | 0.0285 (8) | 0.0286 (8) | 0.0360 (9) | 0.0035 (6) | −0.0018 (6) | −0.0040 (7) |
C14 | 0.0312 (8) | 0.0307 (8) | 0.0363 (9) | −0.0009 (6) | 0.0022 (7) | −0.0003 (7) |
C15 | 0.0578 (12) | 0.0560 (12) | 0.0396 (10) | −0.0080 (10) | 0.0124 (9) | 0.0016 (9) |
C16 | 0.0371 (10) | 0.0450 (11) | 0.0722 (13) | −0.0093 (8) | −0.0037 (9) | 0.0144 (10) |
C17 | 0.0473 (10) | 0.0391 (9) | 0.0400 (10) | 0.0033 (8) | 0.0107 (8) | −0.0007 (7) |
C18 | 0.0329 (9) | 0.0501 (11) | 0.0491 (10) | −0.0093 (8) | −0.0012 (8) | −0.0054 (8) |
Geometric parameters (Å, º) top
O1—C2 | 1.3623 (19) | C8—C9 | 1.467 (2) |
O1—H1 | 0.89 (3) | C8—H8 | 0.93 |
O2—C4 | 1.374 (2) | C9—C14 | 1.400 (2) |
O2—C15 | 1.428 (2) | C9—C10 | 1.401 (2) |
O3—C11 | 1.365 (2) | C10—C11 | 1.381 (2) |
O3—C16 | 1.421 (2) | C10—H10 | 0.93 |
O4—C12 | 1.3778 (19) | C11—C12 | 1.397 (2) |
O4—C17 | 1.4405 (19) | C12—C13 | 1.391 (2) |
O5—C13 | 1.3671 (19) | C13—C14 | 1.386 (2) |
O5—C18 | 1.425 (2) | C14—H14 | 0.93 |
C1—C6 | 1.394 (2) | C15—H15A | 0.96 |
C1—C2 | 1.407 (2) | C15—H15B | 0.96 |
C1—C7 | 1.456 (2) | C15—H15C | 0.96 |
C2—C3 | 1.380 (2) | C16—H16A | 0.96 |
C3—C4 | 1.384 (2) | C16—H16B | 0.96 |
C3—H3 | 0.93 | C16—H16C | 0.96 |
C4—C5 | 1.388 (2) | C17—H17A | 0.96 |
C5—C6 | 1.380 (2) | C17—H17B | 0.96 |
C5—H5 | 0.93 | C17—H17C | 0.96 |
C6—H6 | 0.93 | C18—H18A | 0.96 |
C7—C8 | 1.336 (2) | C18—H18B | 0.96 |
C7—H7 | 0.93 | C18—H18C | 0.96 |
| | | |
C2—O1—H1 | 112.4 (15) | O3—C11—C12 | 114.69 (15) |
C4—O2—C15 | 117.49 (13) | C10—C11—C12 | 120.00 (15) |
C11—O3—C16 | 117.98 (14) | O4—C12—C13 | 119.21 (14) |
C12—O4—C17 | 115.69 (12) | O4—C12—C11 | 120.78 (14) |
C13—O5—C18 | 117.40 (12) | C13—C12—C11 | 119.82 (14) |
C6—C1—C2 | 116.68 (15) | O5—C13—C14 | 125.21 (14) |
C6—C1—C7 | 124.05 (15) | O5—C13—C12 | 114.49 (13) |
C2—C1—C7 | 119.23 (15) | C14—C13—C12 | 120.28 (14) |
O1—C2—C3 | 122.02 (14) | C13—C14—C9 | 120.15 (15) |
O1—C2—C1 | 116.79 (14) | C13—C14—H14 | 119.9 |
C3—C2—C1 | 121.19 (15) | C9—C14—H14 | 119.9 |
C2—C3—C4 | 120.30 (15) | O2—C15—H15A | 109.5 |
C2—C3—H3 | 119.8 | O2—C15—H15B | 109.5 |
C4—C3—H3 | 119.8 | H15A—C15—H15B | 109.5 |
O2—C4—C3 | 115.13 (14) | O2—C15—H15C | 109.5 |
O2—C4—C5 | 124.79 (15) | H15A—C15—H15C | 109.5 |
C3—C4—C5 | 120.09 (15) | H15B—C15—H15C | 109.5 |
C6—C5—C4 | 118.86 (15) | O3—C16—H16A | 109.5 |
C6—C5—H5 | 120.6 | O3—C16—H16B | 109.5 |
C4—C5—H5 | 120.6 | H16A—C16—H16B | 109.5 |
C5—C6—C1 | 122.87 (15) | O3—C16—H16C | 109.5 |
C5—C6—H6 | 118.6 | H16A—C16—H16C | 109.5 |
C1—C6—H6 | 118.6 | H16B—C16—H16C | 109.5 |
C8—C7—C1 | 127.95 (16) | O4—C17—H17A | 109.5 |
C8—C7—H7 | 116.0 | O4—C17—H17B | 109.5 |
C1—C7—H7 | 116.0 | H17A—C17—H17B | 109.5 |
C7—C8—C9 | 125.67 (15) | O4—C17—H17C | 109.5 |
C7—C8—H8 | 117.2 | H17A—C17—H17C | 109.5 |
C9—C8—H8 | 117.2 | H17B—C17—H17C | 109.5 |
C14—C9—C10 | 119.19 (15) | O5—C18—H18A | 109.5 |
C14—C9—C8 | 118.61 (15) | O5—C18—H18B | 109.5 |
C10—C9—C8 | 122.03 (14) | H18A—C18—H18B | 109.5 |
C11—C10—C9 | 120.47 (15) | O5—C18—H18C | 109.5 |
C11—C10—H10 | 119.8 | H18A—C18—H18C | 109.5 |
C9—C10—H10 | 119.8 | H18B—C18—H18C | 109.5 |
O3—C11—C10 | 125.30 (15) | | |
| | | |
C18—O5—C13—C14 | −2.6 (2) | C17—O4—C12—C11 | −77.41 (19) |
O5—C13—C14—C9 | 177.77 (14) | O4—C12—C11—O3 | 5.4 (2) |
C13—C14—C9—C8 | −172.71 (14) | C12—C11—O3—C16 | −172.68 (15) |
C14—C9—C8—C7 | 164.89 (16) | C16—O3—C11—C10 | 6.6 (2) |
C9—C8—C7—C1 | −173.83 (15) | C2—C1—C7—C8 | −179.23 (17) |
C8—C7—C1—C2 | −179.23 (17) | C15—O2—C4—C5 | −2.7 (2) |
C17—O4—C12—C13 | 107.59 (16) | C15—O2—C4—C3 | 177.12 (15) |
C18—O5—C13—C12 | 175.59 (14) | C6—C1—C7—C8 | 3.0 (3) |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
O1—H1···O4i | 0.89 (3) | 1.85 (3) | 2.7052 (16) | 161 (2) |
Symmetry code: (i) x−1/2, −y−1/2, z−1/2. |
Experimental details
Crystal data |
Chemical formula | C18H20O5 |
Mr | 316.34 |
Crystal system, space group | Monoclinic, P21/n |
Temperature (K) | 293 |
a, b, c (Å) | 11.6693 (15), 7.9790 (8), 17.5818 (13) |
β (°) | 90.673 (9) |
V (Å3) | 1636.9 (3) |
Z | 4 |
Radiation type | Mo Kα |
µ (mm−1) | 0.09 |
Crystal size (mm) | 0.54 × 0.47 × 0.41 |
|
Data collection |
Diffractometer | Enraf-NoniusCAD 4 diffractometer |
Absorption correction | – |
No. of measured, independent and observed [I > 2σ(I)] reflections | 3369, 2867, 2420 |
Rint | 0.016 |
(sin θ/λ)max (Å−1) | 0.593 |
|
Refinement |
R[F2 > 2σ(F2)], wR(F2), S | 0.050, 0.128, 1.10 |
No. of reflections | 2867 |
No. of parameters | 209 |
H-atom treatment | H-atom parameters constrained |
Δρmax, Δρmin (e Å−3) | 0.74, −0.20 |
Selected geometric parameters (Å, º) topO1—C2 | 1.3623 (19) | O4—C12 | 1.3778 (19) |
O2—C4 | 1.374 (2) | O4—C17 | 1.4405 (19) |
O2—C15 | 1.428 (2) | O5—C13 | 1.3671 (19) |
O3—C11 | 1.365 (2) | O5—C18 | 1.425 (2) |
O3—C16 | 1.421 (2) | | |
| | | |
C4—O2—C15 | 117.49 (13) | O2—C4—C5 | 124.79 (15) |
C11—O3—C16 | 117.98 (14) | O3—C11—C10 | 125.30 (15) |
C12—O4—C17 | 115.69 (12) | O3—C11—C12 | 114.69 (15) |
C13—O5—C18 | 117.40 (12) | O4—C12—C13 | 119.21 (14) |
O1—C2—C3 | 122.02 (14) | O4—C12—C11 | 120.78 (14) |
O1—C2—C1 | 116.79 (14) | O5—C13—C14 | 125.21 (14) |
O2—C4—C3 | 115.13 (14) | O5—C13—C12 | 114.49 (13) |
| | | |
C18—O5—C13—C14 | −2.6 (2) | O4—C12—C11—O3 | 5.4 (2) |
O5—C13—C14—C9 | 177.77 (14) | C12—C11—O3—C16 | −172.68 (15) |
C17—O4—C12—C13 | 107.59 (16) | C16—O3—C11—C10 | 6.6 (2) |
C18—O5—C13—C12 | 175.59 (14) | C15—O2—C4—C5 | −2.7 (2) |
C17—O4—C12—C11 | −77.41 (19) | C15—O2—C4—C3 | 177.12 (15) |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
O1—H1···O4i | 0.89 (3) | 1.85 (3) | 2.7052 (16) | 161 (2) |
Symmetry code: (i) x−1/2, −y−1/2, z−1/2. |
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The microtubule system of eukaryotic cells is an important target for the development of anticancer agents. Novel drugs have been developed that bind to tubulin, disrupting cellular microtuble structure and function, thus resulting in mitotic arrest. The combretastatins are tubulin polymerization inhibitors isolated from the bark of the South African tree Combretum caffrum (Pettit et al., 1982, 1989). The most potent of these is combretastatin A-4 (CA-4), (I), which is a potent cytotoxic agent and which strongly inhibits the polymerization of tubulin by binding to the colchicine site (Hamel, et al., 1983).
CA-4 shows potent cytotoxicity against a wide variety of human cancers, including multidrug resistant cell lines (El-Zayat et al., 1993), and is thus attractive as a lead compound for development of anticancer drugs. However, CA-4 has demonstrated lack of efficacy in vivo, presumably due to poor pharmacokinetics which arise from its high lipophilicity and limited aqueous solubility. A phosphate prodrug of CA-4 (CA-4P), (II) (Pettit et al., 1998), has been synthesized in order to improve aqueous solubility and pharmacokinetics.
To obtain compounds with pharmaceutically acceptable properties and improved antitumor activities, we have designed and synthesized a number of CA-4 analogs and their corresponding sodium phosphate prodrugs. Compounds (III) and (IV) are two isomeric derivatives that we have recently prepared. In order to unequivocally confirm the molecular structure, and to gather information for our molecular recognition studies, we have determined the crystal structure of (III). Fig. 1 shows the crystallographically determined molecular structure of (III), while selected parameters are presented in Table 1. A l l internuclear distances and angles fall within the range of expected values. The stilbene system exhibits only minor deviations from ideal planarity, as evidenced by the torsion angles (Table 1.). Each methoxy substituent is approximately coplanar with the aromatic rings, with the exception of the O4 methoxy group, which is twisted by approximately 75° relative to the plane of the aromatic ring. This twist minimizes steric interactions with the neighboring O3 and O5 methoxy groups.
The crystal packing of (III) (Fig. 2) consists of corrugated ribbons formed by an infinite linear array of molecules linked by an intermolecular O1—H···O4 hydrogen-bonding interaction that has an O–O distance of 2.705 (2) Å.