Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270113013991/lg3110sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108270113013991/lg3110Isup2.hkl | |
Chemical Markup Language (CML) file https://doi.org/10.1107/S0108270113013991/lg3110Isup3.cml |
CCDC reference: 957013
Aristotelia chilensis (Maqui) was collected in Concepción, VIII Region of Chile (36°50'00" S and 73°01'54" W) in February 2012.
Leaves (2 kg) were dried at 313 K, powdered and extracted with EtOAc three times (6 l). The solvent was concentrated in vacuum obtaining 40 g of a dark gum which was purified by silica gel column eluted with increasing polarity solvents from hexane to ethyl acetate. The breaking up was followed by thin-layer chromatography and similar fractions were concentrated together giving fivr fractions. From fraction 30% EtOAc–70% hexane was obtained a white solid, which was recrystallized from ethyl acetate producing large colorless crystals which were suitable for X-ray diffraction analysis.
All H atoms were found in a difference map, but C-bound H atoms were repositioned at their ideal values, with methylene C—H = 0.97 Å, aromatic C—H = 0.93 Å and methyl C—H = 0.96 Å, and allowed to ride, with Uiso(H) = kUeq(C), where k = 1.2 for methylene and aromatic H atoms, and k = 1.5 for methyl H atoms. Hydroxy H atoms were freely refined.
Data collection: CrysAlis PRO (Oxford Diffraction, 2009); cell refinement: CrysAlis PRO (Oxford Diffraction, 2009); data reduction: CrysAlis PRO (Oxford Diffraction, 2009); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008) and PLATON (Spek, 2009).
C11H12O4 | F(000) = 440 |
Mr = 208.21 | Dx = 1.315 Mg m−3 |
Monoclinic, P21/c | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: -P 2ybc | Cell parameters from 1254 reflections |
a = 7.7326 (9) Å | θ = 3.7–29.2° |
b = 10.9427 (10) Å | µ = 0.10 mm−1 |
c = 12.6997 (13) Å | T = 291 K |
β = 101.948 (11)° | Block, colourless |
V = 1051.3 (2) Å3 | 0.42 × 0.34 × 0.28 mm |
Z = 4 |
Oxford Diffraction Gemini CCD S Ultra diffractometer | 2416 independent reflections |
Radiation source: fine-focus sealed tube | 1699 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.017 |
ω scans, thick slices (1°) | θmax = 29.2°, θmin = 3.7° |
Absorption correction: multi-scan (CrysAlis PRO; Oxford Diffraction, 2009) | h = −6→9 |
Tmin = 0.95, Tmax = 0.98 | k = −13→14 |
4801 measured reflections | l = −17→17 |
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)] = 0.044 | H atoms treated by a mixture of independent and constrained refinement |
wR(F2) = 0.119 | w = 1/[σ2(Fo2) + (0.049P)2 + 0.1986P] where P = (Fo2 + 2Fc2)/3 |
S = 1.03 | (Δ/σ)max < 0.001 |
2416 reflections | Δρmax = 0.18 e Å−3 |
146 parameters | Δρmin = −0.18 e Å−3 |
0 restraints | Extinction correction: SHELXL97 (Sheldrick, 2008), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4 |
Primary atom site location: structure-invariant direct methods | Extinction coefficient: 0.014 (3) |
C11H12O4 | V = 1051.3 (2) Å3 |
Mr = 208.21 | Z = 4 |
Monoclinic, P21/c | Mo Kα radiation |
a = 7.7326 (9) Å | µ = 0.10 mm−1 |
b = 10.9427 (10) Å | T = 291 K |
c = 12.6997 (13) Å | 0.42 × 0.34 × 0.28 mm |
β = 101.948 (11)° |
Oxford Diffraction Gemini CCD S Ultra diffractometer | 2416 independent reflections |
Absorption correction: multi-scan (CrysAlis PRO; Oxford Diffraction, 2009) | 1699 reflections with I > 2σ(I) |
Tmin = 0.95, Tmax = 0.98 | Rint = 0.017 |
4801 measured reflections |
R[F2 > 2σ(F2)] = 0.044 | 0 restraints |
wR(F2) = 0.119 | H atoms treated by a mixture of independent and constrained refinement |
S = 1.03 | Δρmax = 0.18 e Å−3 |
2416 reflections | Δρmin = −0.18 e Å−3 |
146 parameters |
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. |
x | y | z | Uiso*/Ueq | ||
O1 | −0.00655 (17) | 0.86442 (10) | 0.06502 (10) | 0.0485 (3) | |
H1 | −0.071 (3) | 0.8341 (18) | 0.1062 (16) | 0.063 (6)* | |
O2 | 0.10857 (18) | 0.90070 (11) | −0.12009 (10) | 0.0520 (4) | |
H2 | 0.054 (3) | 0.952 (2) | −0.0881 (19) | 0.092 (9)* | |
O3 | 0.21251 (16) | 0.26008 (10) | 0.31465 (10) | 0.0531 (4) | |
O4 | 0.32630 (16) | 0.21932 (10) | 0.17089 (9) | 0.0505 (3) | |
C1 | 0.11952 (19) | 0.66131 (13) | 0.08159 (12) | 0.0354 (3) | |
H1A | 0.0810 | 0.6479 | 0.1453 | 0.042* | |
C2 | 0.08573 (19) | 0.77133 (12) | 0.02941 (12) | 0.0344 (3) | |
C3 | 0.1441 (2) | 0.79284 (13) | −0.06617 (12) | 0.0383 (4) | |
C4 | 0.2359 (2) | 0.70332 (15) | −0.10765 (14) | 0.0468 (4) | |
H4 | 0.2757 | 0.7175 | −0.1708 | 0.056* | |
C5 | 0.2686 (2) | 0.59272 (15) | −0.05538 (13) | 0.0447 (4) | |
H5 | 0.3301 | 0.5326 | −0.0842 | 0.054* | |
C6 | 0.21149 (19) | 0.56917 (13) | 0.03981 (12) | 0.0351 (4) | |
C7 | 0.24649 (19) | 0.44970 (13) | 0.09088 (12) | 0.0366 (4) | |
H7 | 0.3013 | 0.3924 | 0.0547 | 0.044* | |
C8 | 0.2086 (2) | 0.41411 (13) | 0.18329 (13) | 0.0395 (4) | |
H8 | 0.1537 | 0.4700 | 0.2208 | 0.047* | |
C9 | 0.2475 (2) | 0.29288 (14) | 0.22975 (13) | 0.0394 (4) | |
C10 | 0.3621 (3) | 0.09494 (15) | 0.20958 (16) | 0.0573 (5) | |
H10A | 0.2563 | 0.0586 | 0.2261 | 0.069* | |
H10B | 0.4541 | 0.0945 | 0.2744 | 0.069* | |
C11 | 0.4198 (3) | 0.02527 (18) | 0.12296 (18) | 0.0673 (6) | |
H11A | 0.3307 | 0.0307 | 0.0581 | 0.101* | |
H11B | 0.4373 | −0.0588 | 0.1440 | 0.101* | |
H11C | 0.5286 | 0.0588 | 0.1106 | 0.101* |
U11 | U22 | U33 | U12 | U13 | U23 | |
O1 | 0.0682 (8) | 0.0298 (6) | 0.0568 (7) | 0.0073 (5) | 0.0345 (6) | 0.0050 (5) |
O2 | 0.0750 (9) | 0.0359 (6) | 0.0516 (7) | 0.0062 (6) | 0.0280 (6) | 0.0122 (6) |
O3 | 0.0727 (8) | 0.0459 (7) | 0.0485 (7) | 0.0054 (6) | 0.0304 (6) | 0.0081 (6) |
O4 | 0.0697 (8) | 0.0386 (6) | 0.0498 (7) | 0.0166 (5) | 0.0278 (6) | 0.0089 (5) |
C1 | 0.0434 (8) | 0.0320 (7) | 0.0333 (8) | −0.0017 (6) | 0.0139 (6) | 0.0006 (7) |
C2 | 0.0397 (8) | 0.0270 (7) | 0.0383 (8) | −0.0015 (6) | 0.0125 (6) | −0.0027 (6) |
C3 | 0.0456 (9) | 0.0323 (8) | 0.0388 (8) | −0.0039 (6) | 0.0126 (7) | 0.0044 (7) |
C4 | 0.0603 (10) | 0.0438 (9) | 0.0430 (9) | 0.0028 (8) | 0.0259 (8) | 0.0051 (8) |
C5 | 0.0550 (10) | 0.0393 (9) | 0.0448 (9) | 0.0072 (7) | 0.0221 (8) | −0.0008 (8) |
C6 | 0.0379 (8) | 0.0322 (8) | 0.0363 (8) | 0.0007 (6) | 0.0099 (6) | −0.0007 (6) |
C7 | 0.0404 (8) | 0.0308 (7) | 0.0400 (8) | 0.0029 (6) | 0.0116 (6) | −0.0019 (7) |
C8 | 0.0468 (9) | 0.0321 (8) | 0.0416 (9) | 0.0029 (6) | 0.0139 (7) | −0.0025 (7) |
C9 | 0.0428 (8) | 0.0375 (8) | 0.0403 (8) | 0.0000 (6) | 0.0140 (7) | 0.0006 (7) |
C10 | 0.0714 (13) | 0.0425 (9) | 0.0603 (11) | 0.0181 (9) | 0.0189 (10) | 0.0138 (9) |
C11 | 0.0697 (13) | 0.0522 (11) | 0.0771 (14) | 0.0152 (9) | 0.0085 (11) | −0.0103 (10) |
O1—C2 | 1.3730 (17) | C5—C6 | 1.394 (2) |
O1—H1 | 0.86 (2) | C5—H5 | 0.9300 |
O2—C3 | 1.3637 (18) | C6—C7 | 1.460 (2) |
O2—H2 | 0.85 (2) | C7—C8 | 1.326 (2) |
O3—C9 | 1.2189 (18) | C7—H7 | 0.9300 |
O4—C9 | 1.3289 (18) | C8—C9 | 1.458 (2) |
O4—C10 | 1.4540 (19) | C8—H8 | 0.9300 |
C1—C2 | 1.373 (2) | C10—C11 | 1.481 (3) |
C1—C6 | 1.400 (2) | C10—H10A | 0.9700 |
C1—H1A | 0.9300 | C10—H10B | 0.9700 |
C2—C3 | 1.400 (2) | C11—H11A | 0.9600 |
C3—C4 | 1.377 (2) | C11—H11B | 0.9600 |
C4—C5 | 1.378 (2) | C11—H11C | 0.9600 |
C4—H4 | 0.9300 | ||
C2—O1—H1 | 108.7 (13) | C8—C7—C6 | 127.01 (14) |
C3—O2—H2 | 113.5 (16) | C8—C7—H7 | 116.5 |
C9—O4—C10 | 117.02 (13) | C6—C7—H7 | 116.5 |
C2—C1—C6 | 120.62 (14) | C7—C8—C9 | 124.11 (15) |
C2—C1—H1A | 119.7 | C7—C8—H8 | 117.9 |
C6—C1—H1A | 119.7 | C9—C8—H8 | 117.9 |
O1—C2—C1 | 123.47 (13) | O3—C9—O4 | 122.06 (14) |
O1—C2—C3 | 116.34 (13) | O3—C9—C8 | 124.12 (14) |
C1—C2—C3 | 120.18 (13) | O4—C9—C8 | 113.82 (13) |
O2—C3—C4 | 119.33 (14) | O4—C10—C11 | 107.24 (15) |
O2—C3—C2 | 120.90 (13) | O4—C10—H10A | 110.3 |
C4—C3—C2 | 119.76 (14) | C11—C10—H10A | 110.3 |
C3—C4—C5 | 119.89 (14) | O4—C10—H10B | 110.3 |
C3—C4—H4 | 120.1 | C11—C10—H10B | 110.3 |
C5—C4—H4 | 120.1 | H10A—C10—H10B | 108.5 |
C4—C5—C6 | 121.35 (14) | C10—C11—H11A | 109.5 |
C4—C5—H5 | 119.3 | C10—C11—H11B | 109.5 |
C6—C5—H5 | 119.3 | H11A—C11—H11B | 109.5 |
C5—C6—C1 | 118.19 (13) | C10—C11—H11C | 109.5 |
C5—C6—C7 | 119.20 (13) | H11A—C11—H11C | 109.5 |
C1—C6—C7 | 122.59 (13) | H11B—C11—H11C | 109.5 |
C6—C1—C2—O1 | 178.75 (14) | C2—C1—C6—C5 | 0.7 (2) |
C6—C1—C2—C3 | −0.6 (2) | C2—C1—C6—C7 | −178.33 (14) |
C1—C2—C3—O2 | 178.73 (15) | C5—C6—C7—C8 | 176.87 (17) |
O1—C2—C3—O2 | −0.7 (2) | C1—C6—C7—C8 | −4.1 (3) |
C1—C2—C3—C4 | 0.1 (2) | C6—C7—C8—C9 | −179.98 (15) |
O1—C2—C3—C4 | −179.30 (15) | C10—O4—C9—O3 | 2.7 (2) |
O2—C3—C4—C5 | −178.36 (16) | C10—O4—C9—C8 | −177.25 (15) |
C2—C3—C4—C5 | 0.3 (3) | C7—C8—C9—O3 | −179.93 (17) |
C3—C4—C5—C6 | −0.2 (3) | C7—C8—C9—O4 | 0.1 (2) |
C4—C5—C6—C1 | −0.3 (3) | C9—O4—C10—C11 | 170.22 (16) |
C4—C5—C6—C7 | 178.76 (16) |
D—H···A | D—H | H···A | D···A | D—H···A |
O1—H1···O3i | 0.86 (2) | 1.82 (2) | 2.6801 (18) | 175 (2) |
O2—H2···O1ii | 0.85 (2) | 2.07 (2) | 2.8189 (17) | 145 (2) |
Symmetry codes: (i) −x, y+1/2, −z+1/2; (ii) −x, y+5/2, −z+1/2. |
Experimental details
Crystal data | |
Chemical formula | C11H12O4 |
Mr | 208.21 |
Crystal system, space group | Monoclinic, P21/c |
Temperature (K) | 291 |
a, b, c (Å) | 7.7326 (9), 10.9427 (10), 12.6997 (13) |
β (°) | 101.948 (11) |
V (Å3) | 1051.3 (2) |
Z | 4 |
Radiation type | Mo Kα |
µ (mm−1) | 0.10 |
Crystal size (mm) | 0.42 × 0.34 × 0.28 |
Data collection | |
Diffractometer | Oxford Diffraction Gemini CCD S Ultra diffractometer |
Absorption correction | Multi-scan (CrysAlis PRO; Oxford Diffraction, 2009) |
Tmin, Tmax | 0.95, 0.98 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 4801, 2416, 1699 |
Rint | 0.017 |
(sin θ/λ)max (Å−1) | 0.687 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.044, 0.119, 1.03 |
No. of reflections | 2416 |
No. of parameters | 146 |
H-atom treatment | H atoms treated by a mixture of independent and constrained refinement |
Δρmax, Δρmin (e Å−3) | 0.18, −0.18 |
Computer programs: CrysAlis PRO (Oxford Diffraction, 2009), SHELXS97 (Sheldrick, 2008), SHELXTL (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008) and PLATON (Spek, 2009).
D—H···A | D—H | H···A | D···A | D—H···A |
O1—H1···O3i | 0.86 (2) | 1.82 (2) | 2.6801 (18) | 175 (2) |
O2—H2···O1ii | 0.85 (2) | 2.07 (2) | 2.8189 (17) | 145 (2) |
Symmetry codes: (i) −x, y+1/2, −z+1/2; (ii) −x, y+5/2, −z+1/2. |
Group 1···Group 2 | ccd | ipd | sa |
Cg1···Cg2iii | 3.435 | 3.4201 | 5.3 |
Symmetry code: (iii) -x, -y+1, -z Cg1 is the centroid of the C1–C6 ring and Cg2 is the mid-point of the C7═C8 bond. Notes: ccd is the center-to-center distance (distance between centroids), ipd is the interplanar distance (distance from one plane to the neighbouring centroid) and sa is the mean slippage angle (angle subtended by the intercentroid vector to the plane normal). For details, see Janiak (2000). |
D—H···A | D—H | H···A | D···A | D—H···A |
O1—H1···O3i | 0.82 | 1.92 | 2.7308 (17) | 170 |
O2—H2···O2ii | 0.82 | 2.30 | 3.0364 (18) | 150 |
O2—H2···O1ii | 0.82 | 2.22 | 2.8699 (17) | 135 |
Symmetry codes: (i) -x-2, y+1/2, -z+1/2; (ii) x+1, y-1, z. |
n | CSD refcode (reference) | Space group | a (Å) | b (Å) | c (Å) | α (°) | β (°) | γ(°) | Dc (Mg Mm-3) | Conformational isomer |
1 | MHPOAT (Chen et al., 1979) | P21/n | 11.377 (9) | 11.561 (5) | 7.146 (2) | 90.00 | 104.50 (5) | 90.00 | 1.417 | Z |
1 | MHPOAT01 (Wang, Meng et al., 2011) | P1 | 5.129 (5) | 9.969 (8) | 10.586 (9) | 117.63 (2) | 97.92 (2) | 94.32 (2) | 1.375 | E |
2 | (I) (this work) | P21/c | 7.7326 (9) | 10.9427 (10) | 12.6997 (13) | 90.00 | 101.948 (11) | 90.00 | 1.315 | Z |
2 | (II) (Xia et al., 2004) | P21/c | 6.659 (2) | 5.811 (2) | 25.992 (7) | 90.00 | 91.51 (2) | 90.00 | 1.376 | E |
3 | CICLIK (Xia et al., 2007) | C2/c | 18.883 (3) | 10.965 (2) | 12.478 (2) | 90.00 | 116.95 (2) | 90.00 | 1.282 | E |
4 | YEFMAY (Xia et al., 2006a) | P1 | 5.282 (5) | 10.490 (5) | 11.558(7 | 83.95 (6) | 84.31 (7) | 81.14 (6) | 1.251 | E |
5 | YAGCIU (Wang, Gu et al., 2011) | P1 | 5.307 (2) | 10.567 (2) | 11.816 (2) | 90.96 (3) | 91.84 (3) | 98.60 (3) | 1.270 | E |
6 | SABGAF (Wang et al., 2012) | P1 | 5.292 (2)) | 10.689 (2) | 12.732 (3) | 95.45 (3) | 92.76 (3) | 96.84 (3) | 1.235 | E |
7 | XENQEN01 (Xia et al., 2008) | P1 | 5.296 (3) | 10.711 (13) | 13.870 (4) | 98.84 (7) | 90.97 (4) | 96.77 (7) | 1.198 | E |
8 | MENJAR (Xia et al., 2006b) | P1 | 5.454 (2) | 12.152 (5) | 12.584 (5) | 87.76 (2) | 80.56 (2) | 81.39 (2) | 1.194 | Z |
Aristotelia chilensis (Mol.) Stuntz (Elaeocaepaceae) is an evergreen tree distributed in central and southern Chile and in southwestern Argentina. Folk medicine attributes properties to this tree such as anti-inflammatory, analgesic and antihaemorrhagic activities (Bhakuni, 1976). Crushed fresh leaves are used as a poultice for treating burns and counteracting fever (Muñoz et al., 2001). The fruit from A. chilensis (commonly known as Maqui) is an edible wild blackberry, recently reported as one of the healthiest exotic berries due to its displaying one of the highest ORAC (Oxygen Radical Absorbance Capacity) antioxidant capacities (Céspedes et al., 2010; Morariou, 2007). The berry presents high concentrations of bioactive agents, such as anthocyanins (viz. delphinidin 3-sambubioside-5-glucoside, delphinidin 3,5-diglucoside, delphinidin 3-sambubioside, delphinidin 3-glucoside, cyanidin 3-sambubioside-5-glucoside, cyanidin 3,5-diglucoside, cyanidin 3-sambubioside and cyanidin 3-glucoside; Escribano-Bailón et al., 2006) and polyphenols (viz. quercetin, rutin and derivates from coumaric acid; Céspedes et al., 2010).
On the other hand, caffeic acid and its esters have displayed antioxidant, anti-inflammatory, antiproliferative, cytostatic and most improtantly antineoplastic properties (Ozturk et al., 2012).
We present here the crystal and molecular structure of a natural derivative from caffeic acid, extracted from the chilean tree A. chilensis, namely (Z)-ethyl 3-(3,4-dihydroxyphenyl)prop-2-enoate, (I), a compound of which a synthetic E polymorph, (II), obtained from a Knoevenagel condensation reaction, had been described elsewhere (Xia et al., 2004). In addition, an exhaustive Raman analysis on potentially useful properties of this synthetic compound as a nonlinear optical (NLO) device has been reported (Bena Jothy et al., 2009).
Fig. 1(a) shows the atom-numbering scheme for (I) and Fig. 1(b) shows a superposition of molecules (I) and (II), where only atoms O1/O2/C1–C7 have been used for the least-squares overlap. The only detectable differences at a molecular level derive from their being rotational conformers in the solid state [Z–syn in (I) and E–anti in (II)], as seen in the C5—C6—C7—C8 and C4—C3—O2—H2 torsion angles (see the circled zones in Fig. 1b) with extreme values of ~180° in (I) and ~0° in (II) [176.83 (16)/-178.3 (17) and -0.4 (2)/-12°, respectively].
Even if in both cases the main 3,4-dihydroxyphenyl)prop-2-enoate core is basically planar [maximum deviations = 0.076 (2)Å for atom O4 in (I) and 0.039 (2) Å for C7 in (II)], the pendant ethyl group deviates from this plane, more significantly in (I) [by 12.1 (2)°, and only slightly in (II), by 1.2 (2)°].
Table 1 presents the hydrogen bonds in (I) while Fig. 2(a) shows the bimolecular arrangement generated by the centrosymmetric R22(10) motif built up by the second entry in Table 1. This dimeric entity is the elemental packing `brick' in (I), as we shall see below. Since the dihydroxyphenyl plane in (I) leaves the nearby centre of symmetry 0.272 (2) Å away, the generated dimer presents a 0.54 Å interplanar shift between both parallel aromatic planes.
The packing sequence can be viewed as a two-step process: (i) an initial concatenation of dimers via the π–π interaction between the benzene ring and the central double bond of the propenyl group (Table 2 and Fig. 2b) generating chains along [010]; (ii) the strong lateral interaction between neighbouring chains related by a 21 axis running parallel to, but external from, the chains themselves. Figs. 3(a) and 3(b) show lateral views of two neighbouring chains (z ~0, 1/2, respectively); dimers in vicinal chains are rotated by 180°, subtending angles of 40.12 (2)° to each other. Figs. 3(c) and 3(d) show two overlapping views when both types of chains are considered (Fig. 3c sideways and Fig 3d normal to the plane). The linkage between chains is achieved through the strong O1—H1···O3i hydrogen bond [symmetry code: (i) -x, y+1/2, -z+1/2; Table 1, first entry), giving rise to a C(9) transversal chain (A···A' in Fig. 3d).
Fig. 3(c) shows that the central core is composed of the hydrophilic nuclei, shielded from significant interplanar interactionswhile by an outer hydrophobic cover of methyl groups. The plane width, as defined by the outermost C11 methyl atoms, is ~6.35 Å, which with a = 7.7326 (9) Å leaves ~1.40 Å for the interplanar region.
Table 3 presents the hydrogen-bonding interactions for (II). There are some analogies with those in (I) but, in spite of these formal similarities, the final packing arrays are entirely different. The hydrogen bonds involving O2—H2 (entries 2 and 3) generate an R21(5) ring and define, as in (I), the elemental packing `brick' in the crystal structure, but here it consists of a broad strip running along [010], halved by a 21 axis acting as its "backbone" (Fig. 4a). These roughly planar structures (~19 Å wide and~1 Å thick) are almost parallel to the (601) plane. The hydrogen-bond involving atom H1 (first entry in Table 3) serves via a C(9) chain to connect strips into the final packing array, a very broad two-dimensional structure (~13.68 Å thick) parallel to (001) (Fig. 4b). Considering that there are two of these structures (2 × 13.68 = 27.36 Å) per unit-cell c translation (25.99 Å), it is apparent that in this case the particular planes interpenetrate, with no interpanar spacing left.
This suggests a better packing efficiency for (II), as shown by the larger calculated densities [1.315 versus 1.375 Mg m-3 for (I)], as well as packing indices [67.6 for (II) versus 70.8 for (I); calculated with PLATON (Spek, 2009)].
A check with the Cambridge Structural Database (CSD, Version 5.33; Allen, 2002) in order to find out structures closely related to (I) and (II) uncovered a whole family of 3-(3,4-dihydroxyphenyl)propeno-2-ate derivatives, in particular, with pendant alkyl groups (CnH2n+1), a family in which (I) and (II) would be the n = 2 (ethyl) members. Fig. 5 shows schematic views of the complete set, all of them synthetic [which makes (I) the first of natural occurrence]. Inspection of Fig. 5 shows that there are, in addition to the n = 2 case herein discussed, other reported cases of polymorphism associated with conformational isomerism [viz. the n = 1 (methyl) case]. Surprisingly, these particular situations seem to appear only for the shortest chains (n = 1 and 2), as if the existence of longer `tails' would reduce the number of favourable spatial arrangements into a single more feasible one. This latter assumption seems to be sustained by the fact that structures with n = 4–7 are isostructural (Table 4), with identical conformations (Fig. 5). Regarding the main difference between structures (I) and (II), viz. the relative orientation of the O—H groups, there seems to be a marked preference for the conformation present in (I); that in (II) appears to be highly unusual.