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Mikanolide [systematic names: 1,10:2,3-diepoxy-6,8-dihydroxy-11-vinylgermacr-4-ene 12,14-di-γ-lactone and 7,10a-dimethyl-1a,1b,2a,6a,7,9a,10,10a-octahydro-4
H-6,3-methenofuro[3,2-
c]bisoxireno[
f,
h]oxacycloundecin-4,8(6
H)-dione], C
15H
14O
6, derived from a variety of
Mikania micrantha growing in Portland, Jamaica, contains a methylcyclodecane ring fused to an unsaturated planar α,γ-lactone, an envelope-type near-planar vinyl-β,γ-lactone and two epoxide moieties. The crystal packing shows stacks of mikanolide molecules interlocked
via a network of non-classical C—H
O hydrogen bonds between the lactone units.
Supporting information
CCDC reference: 257014
Mikanolide extracted from Mikania micrantha Kunth (from Portland, Jamaica) was isolated as described by Facey et al. (1999). Crystals were obtained when an extract in hexane–ethyl acetate (1:3) was allowed to stand at room temperature for several days.
##AUTHOR: ## Was the hand (enantiomer) chosen at random, because the ## absolute structure is not known, or was there evidence from other ## sources as to which hand is adopted by (1)?
All H atoms were assigned by assuming idealized geometry, C—H distances of 0.98, 0.96, 0.97 and 0.93 Å for tertiary CH, methyl CH3, secondary CH2 and terminal ═CH2 atoms, respectively, and with Uiso(H) values of 1.5Ueq(C) for methyl H atoms and 1.2Ueq(C) for all other H atoms. ##AUTHOR: Please check the change above.
Data collection: XSCANS (Bruker, 1997); cell refinement: XSCANS; data reduction: XSCANS; program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL (Bruker, 1997) and PLATON (Spek, 2003); software used to prepare material for publication: SHELXTL.
7,10
a-dimethyl-1a,1 b,2a,6a,7,9a,10,10
a-octahydro-4
H-6,3- methenofuro[3,2-
c]bisoxireno[f,h]oxacycloundecin-4,8(6
H)-dione or 1,10:2,3-diepoxy-6,8-dihydroxygermacr-4-ene-11-vinyl-12,14- di-
γ-lactone
top
Crystal data top
C15H14O6 | F(000) = 304 |
Mr = 290.26 | Dx = 1.452 Mg m−3 |
Monoclinic, P21 | Mo Kα radiation, λ = 0.71073 Å |
a = 8.9090 (12) Å | Cell parameters from 43 reflections |
b = 7.1840 (8) Å | θ = 9.5–24.3° |
c = 10.4940 (11) Å | µ = 0.11 mm−1 |
β = 98.830 (9)° | T = 293 K |
V = 663.60 (14) Å3 | Rectangular prism, colourless |
Z = 2 | 0.4 × 0.3 × 0.2 mm |
Data collection top
Bruker P4 diffractometer | Rint = 0.028 |
Radiation source: fine-focus sealed tube | θmax = 25.0°, θmin = 2.0° |
Graphite monochromator | h = −10→1 |
2θ/ω scans | k = −8→1 |
1782 measured reflections | l = −12→12 |
1483 independent reflections | 3 standard reflections every 97 reflections |
1269 reflections with I > 2σ(I) | intensity decay: none |
Refinement top
Refinement on F2 | Hydrogen site location: inferred from neighbouring sites |
Least-squares matrix: full | H-atom parameters constrained |
R[F2 > 2σ(F2)] = 0.037 | w = 1/[σ2(Fo2) + (0.049P)2 + 0.069P] where P = (Fo2 + 2Fc2)/3 |
wR(F2) = 0.098 | (Δ/σ)max < 0.001 |
S = 1.04 | Δρmax = 0.13 e Å−3 |
1483 reflections | Δρmin = −0.11 e Å−3 |
191 parameters | Extinction correction: SHELXL97, Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4 |
1 restraint | Extinction coefficient: 0.028 (6) |
Primary atom site location: structure-invariant direct methods | Absolute structure: Flack H D (1983), Acta Cryst. A39, 876-881 |
Secondary atom site location: difference Fourier map | Absolute structure parameter: not reliably determined |
Crystal data top
C15H14O6 | V = 663.60 (14) Å3 |
Mr = 290.26 | Z = 2 |
Monoclinic, P21 | Mo Kα radiation |
a = 8.9090 (12) Å | µ = 0.11 mm−1 |
b = 7.1840 (8) Å | T = 293 K |
c = 10.4940 (11) Å | 0.4 × 0.3 × 0.2 mm |
β = 98.830 (9)° | |
Data collection top
Bruker P4 diffractometer | Rint = 0.028 |
1782 measured reflections | 3 standard reflections every 97 reflections |
1483 independent reflections | intensity decay: none |
1269 reflections with I > 2σ(I) | |
Refinement top
R[F2 > 2σ(F2)] = 0.037 | H-atom parameters constrained |
wR(F2) = 0.098 | Δρmax = 0.13 e Å−3 |
S = 1.04 | Δρmin = −0.11 e Å−3 |
1483 reflections | Absolute structure: Flack H D (1983), Acta Cryst. A39, 876-881 |
191 parameters | Absolute structure parameter: not reliably determined |
1 restraint | |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | x | y | z | Uiso*/Ueq | |
O1 | 0.5920 (3) | 0.0333 (4) | 0.4209 (2) | 0.0852 (10) | |
O2 | 0.3885 (3) | 0.5396 (5) | 0.2236 (2) | 0.0851 (10) | |
O3 | 0.5364 (2) | 0.4654 (4) | 0.07723 (18) | 0.0692 (9) | |
O4 | 1.1290 (3) | 0.3778 (4) | −0.0908 (3) | 0.0934 (11) | |
O5 | 1.0709 (2) | 0.3466 (4) | 0.1070 (2) | 0.0720 (9) | |
O6 | 0.9159 (3) | 0.3606 (5) | 0.51889 (19) | 0.0823 (9) | |
C1 | 0.8165 (4) | 0.2399 (6) | 0.4363 (3) | 0.0636 (11) | |
C2 | 0.6632 (4) | 0.2007 (6) | 0.4739 (3) | 0.0686 (13) | |
C3 | 0.5248 (4) | 0.2058 (5) | 0.3769 (3) | 0.0624 (11) | |
C4 | 0.5427 (3) | 0.2613 (5) | 0.2457 (3) | 0.0512 (10) | |
C5 | 0.6328 (3) | 0.1907 (5) | 0.1694 (3) | 0.0534 (9) | |
C6 | 0.6465 (3) | 0.3220 (5) | 0.0634 (3) | 0.0583 (10) | |
C7 | 0.8046 (3) | 0.4124 (5) | 0.0706 (2) | 0.0494 (9) | |
C8 | 0.9358 (3) | 0.3340 (5) | 0.1703 (3) | 0.0526 (10) | |
C9 | 0.9779 (3) | 0.4304 (6) | 0.2996 (3) | 0.0628 (10) | |
C10 | 0.8584 (3) | 0.4230 (5) | 0.3877 (2) | 0.0551 (10) | |
C11 | 0.8706 (4) | 0.4031 (5) | −0.0531 (3) | 0.0592 (11) | |
C12 | 1.0341 (4) | 0.3768 (5) | −0.0215 (4) | 0.0681 (11) | |
C13 | 0.8000 (5) | 0.4149 (7) | −0.1742 (3) | 0.0855 (15) | |
C14 | 0.4773 (3) | 0.4340 (6) | 0.1874 (3) | 0.0602 (12) | |
C15 | 0.7570 (4) | 0.5868 (5) | 0.3851 (3) | 0.0647 (11) | |
H1 | 0.86590 | 0.13260 | 0.40260 | 0.0930* | |
H2 | 0.65090 | 0.23090 | 0.56280 | 0.1090* | |
H3 | 0.42960 | 0.24020 | 0.40690 | 0.0920* | |
H5 | 0.68070 | 0.07550 | 0.18060 | 0.0810* | |
H6 | 0.61910 | 0.25910 | −0.01980 | 0.0850* | |
H7 | 0.79340 | 0.54440 | 0.09090 | 0.0810* | |
H8 | 0.91530 | 0.20250 | 0.18560 | 0.0750* | |
H9A | 0.99990 | 0.55990 | 0.28400 | 0.0730* | |
H9B | 1.07030 | 0.37440 | 0.34410 | 0.0730* | |
H13A | 0.85550 | 0.40630 | −0.24220 | 0.1350* | |
H13B | 0.69540 | 0.43170 | −0.19090 | 0.1350* | |
H15A | 0.68610 | 0.56760 | 0.44410 | 0.0930* | |
H15B | 0.70260 | 0.60310 | 0.29950 | 0.0930* | |
H15C | 0.81660 | 0.69590 | 0.40990 | 0.0930* | |
Atomic displacement parameters (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
O1 | 0.115 (2) | 0.0651 (18) | 0.0830 (16) | −0.0083 (17) | 0.0395 (15) | 0.0212 (16) |
O2 | 0.0606 (14) | 0.106 (2) | 0.0894 (17) | 0.0246 (17) | 0.0137 (13) | 0.0044 (19) |
O3 | 0.0562 (12) | 0.098 (2) | 0.0520 (11) | 0.0065 (14) | 0.0041 (9) | 0.0165 (13) |
O4 | 0.1121 (19) | 0.0646 (18) | 0.125 (2) | 0.0031 (16) | 0.0869 (18) | 0.0127 (16) |
O5 | 0.0621 (13) | 0.0731 (17) | 0.0885 (16) | 0.0009 (13) | 0.0363 (11) | 0.0103 (14) |
O6 | 0.0795 (14) | 0.116 (2) | 0.0458 (11) | −0.0012 (16) | −0.0082 (10) | 0.0142 (15) |
C1 | 0.073 (2) | 0.069 (2) | 0.0475 (16) | 0.0109 (19) | 0.0049 (14) | 0.0140 (17) |
C2 | 0.089 (2) | 0.070 (3) | 0.0524 (17) | 0.003 (2) | 0.0283 (16) | 0.0133 (18) |
C3 | 0.0711 (19) | 0.065 (2) | 0.0565 (17) | −0.0052 (18) | 0.0272 (15) | 0.0043 (17) |
C4 | 0.0476 (15) | 0.062 (2) | 0.0459 (14) | −0.0080 (16) | 0.0133 (12) | −0.0036 (15) |
C5 | 0.0615 (16) | 0.0476 (17) | 0.0549 (15) | −0.0149 (15) | 0.0211 (13) | −0.0087 (15) |
C6 | 0.0659 (18) | 0.070 (2) | 0.0416 (14) | −0.0126 (18) | 0.0163 (13) | −0.0134 (16) |
C7 | 0.0605 (15) | 0.0438 (16) | 0.0471 (14) | −0.0062 (15) | 0.0189 (12) | −0.0037 (14) |
C8 | 0.0541 (16) | 0.0481 (17) | 0.0610 (17) | −0.0019 (15) | 0.0262 (13) | 0.0015 (15) |
C9 | 0.0528 (15) | 0.076 (2) | 0.0583 (16) | −0.0081 (19) | 0.0048 (13) | 0.0008 (18) |
C10 | 0.0567 (16) | 0.068 (2) | 0.0376 (13) | 0.0000 (17) | −0.0024 (12) | −0.0034 (15) |
C11 | 0.083 (2) | 0.0457 (18) | 0.0569 (17) | −0.0101 (18) | 0.0358 (15) | −0.0060 (16) |
C12 | 0.092 (2) | 0.0382 (18) | 0.086 (2) | −0.0039 (17) | 0.0514 (19) | 0.0021 (17) |
C13 | 0.118 (3) | 0.091 (3) | 0.0557 (18) | −0.017 (3) | 0.0398 (19) | −0.001 (2) |
C14 | 0.0450 (14) | 0.083 (3) | 0.0518 (15) | −0.0008 (19) | 0.0052 (12) | −0.0036 (18) |
C15 | 0.073 (2) | 0.061 (2) | 0.0589 (18) | −0.0047 (18) | 0.0063 (16) | −0.0175 (17) |
Geometric parameters (Å, º) top
O1—C2 | 1.432 (5) | C8—C9 | 1.518 (5) |
O1—C3 | 1.422 (5) | C9—C10 | 1.514 (4) |
O2—C14 | 1.199 (5) | C10—C15 | 1.481 (5) |
O3—C6 | 1.445 (4) | C11—C12 | 1.456 (5) |
O3—C14 | 1.361 (4) | C11—C13 | 1.331 (5) |
O4—C12 | 1.197 (5) | C1—H1 | 0.9802 |
O5—C8 | 1.464 (3) | C2—H2 | 0.9802 |
O5—C12 | 1.355 (5) | C3—H3 | 0.9806 |
O6—C1 | 1.432 (5) | C5—H5 | 0.9303 |
O6—C10 | 1.464 (3) | C6—H6 | 0.9801 |
C1—C2 | 1.505 (5) | C7—H7 | 0.9804 |
C1—C10 | 1.479 (5) | C8—H8 | 0.9801 |
C2—C3 | 1.474 (5) | C9—H9A | 0.9698 |
C3—C4 | 1.465 (4) | C9—H9B | 0.9693 |
C4—C5 | 1.319 (4) | C13—H13A | 0.9305 |
C4—C14 | 1.464 (5) | C13—H13B | 0.9293 |
C5—C6 | 1.478 (5) | C15—H15A | 0.9599 |
C6—C7 | 1.542 (4) | C15—H15B | 0.9601 |
C7—C8 | 1.550 (4) | C15—H15C | 0.9600 |
C7—C11 | 1.507 (4) | | |
| | | |
C2—O1—C3 | 62.2 (2) | O5—C12—C11 | 109.2 (3) |
C6—O3—C14 | 108.9 (3) | O2—C14—O3 | 121.6 (4) |
C8—O5—C12 | 111.8 (2) | O2—C14—C4 | 130.0 (3) |
C1—O6—C10 | 61.4 (2) | O3—C14—C4 | 108.3 (3) |
O6—C1—C2 | 117.1 (3) | O6—C1—H1 | 115.10 |
O6—C1—C10 | 60.3 (2) | C2—C1—H1 | 115.03 |
C2—C1—C10 | 123.0 (3) | C10—C1—H1 | 115.08 |
O1—C2—C1 | 114.9 (3) | O1—C2—H2 | 116.62 |
O1—C2—C3 | 58.6 (2) | C1—C2—H2 | 116.52 |
C1—C2—C3 | 120.7 (3) | C3—C2—H2 | 116.61 |
O1—C3—C2 | 59.2 (2) | O1—C3—H3 | 117.20 |
O1—C3—C4 | 116.2 (3) | C2—C3—H3 | 117.23 |
C2—C3—C4 | 117.1 (3) | C4—C3—H3 | 117.21 |
C3—C4—C5 | 129.0 (3) | C4—C5—H5 | 125.13 |
C3—C4—C14 | 122.0 (3) | C6—C5—H5 | 125.01 |
C5—C4—C14 | 108.2 (3) | O3—C6—H6 | 109.76 |
C4—C5—C6 | 109.9 (3) | C5—C6—H6 | 109.77 |
O3—C6—C5 | 104.1 (2) | C7—C6—H6 | 109.78 |
O3—C6—C7 | 108.9 (3) | C6—C7—H7 | 107.18 |
C5—C6—C7 | 114.4 (2) | C8—C7—H7 | 107.23 |
C6—C7—C8 | 117.9 (3) | C11—C7—H7 | 107.24 |
C6—C7—C11 | 114.4 (2) | O5—C8—H8 | 108.71 |
C8—C7—C11 | 102.2 (2) | C7—C8—H8 | 108.70 |
O5—C8—C7 | 105.0 (2) | C9—C8—H8 | 108.66 |
O5—C8—C9 | 105.7 (2) | C8—C9—H9A | 108.38 |
C7—C8—C9 | 119.7 (3) | C8—C9—H9B | 108.44 |
C8—C9—C10 | 115.4 (2) | C10—C9—H9A | 108.42 |
O6—C10—C1 | 58.3 (2) | C10—C9—H9B | 108.42 |
O6—C10—C9 | 114.1 (2) | H9A—C9—H9B | 107.50 |
O6—C10—C15 | 112.7 (2) | C11—C13—H13A | 119.93 |
C1—C10—C9 | 118.7 (3) | C11—C13—H13B | 120.10 |
C1—C10—C15 | 121.8 (3) | H13A—C13—H13B | 119.97 |
C9—C10—C15 | 116.4 (3) | C10—C15—H15A | 109.50 |
C7—C11—C12 | 108.6 (3) | C10—C15—H15B | 109.45 |
C7—C11—C13 | 129.1 (3) | C10—C15—H15C | 109.47 |
C12—C11—C13 | 122.4 (3) | H15A—C15—H15B | 109.47 |
O4—C12—O5 | 121.3 (3) | H15A—C15—H15C | 109.50 |
O4—C12—C11 | 129.5 (4) | H15B—C15—H15C | 109.44 |
| | | |
C3—O1—C2—C1 | −112.1 (3) | C3—C4—C14—O3 | 168.4 (3) |
C2—O1—C3—C4 | 107.3 (3) | C3—C4—C5—C6 | −163.5 (3) |
C14—O3—C6—C5 | 6.3 (3) | C5—C4—C14—O2 | 178.7 (4) |
C14—O3—C6—C7 | −116.0 (3) | C5—C4—C14—O3 | −2.2 (4) |
C6—O3—C14—O2 | 176.3 (3) | C14—C4—C5—C6 | 6.3 (4) |
C6—O3—C14—C4 | −2.9 (3) | C3—C4—C14—O2 | −10.7 (6) |
C12—O5—C8—C7 | −14.7 (4) | C4—C5—C6—O3 | −7.9 (3) |
C12—O5—C8—C9 | −142.0 (3) | C4—C5—C6—C7 | 110.8 (3) |
C8—O5—C12—O4 | −174.6 (3) | O3—C6—C7—C8 | 126.1 (3) |
C8—O5—C12—C11 | 4.9 (4) | O3—C6—C7—C11 | −113.6 (3) |
C10—O6—C1—C2 | −114.3 (4) | C5—C6—C7—C11 | 130.4 (3) |
C1—O6—C10—C9 | −110.0 (3) | C5—C6—C7—C8 | 10.2 (4) |
C1—O6—C10—C15 | 114.4 (3) | C6—C7—C8—C9 | −97.7 (4) |
O6—C1—C10—C15 | −98.6 (3) | C6—C7—C8—O5 | 144.1 (3) |
C2—C1—C10—O6 | 104.9 (3) | C8—C7—C11—C13 | 164.3 (4) |
O6—C1—C10—C9 | 102.0 (3) | C11—C7—C8—C9 | 135.9 (3) |
C2—C1—C10—C15 | 6.3 (4) | C6—C7—C11—C12 | −144.1 (3) |
O6—C1—C2—C3 | 135.3 (4) | C11—C7—C8—O5 | 17.6 (3) |
C10—C1—C2—O1 | 131.5 (3) | C8—C7—C11—C12 | −15.4 (4) |
C10—C1—C2—C3 | 64.6 (5) | C6—C7—C11—C13 | 35.7 (6) |
O6—C1—C2—O1 | −157.8 (3) | C7—C8—C9—C10 | 66.3 (4) |
C2—C1—C10—C9 | −153.1 (3) | O5—C8—C9—C10 | −175.8 (3) |
O1—C2—C3—C4 | −105.8 (3) | C8—C9—C10—C1 | 64.3 (4) |
C1—C2—C3—C4 | −3.6 (5) | C8—C9—C10—C15 | −96.3 (4) |
C1—C2—C3—O1 | 102.2 (4) | C8—C9—C10—O6 | 129.9 (3) |
O1—C3—C4—C14 | 179.0 (3) | C13—C11—C12—O4 | 7.1 (7) |
O1—C3—C4—C5 | −12.4 (5) | C13—C11—C12—O5 | −172.4 (4) |
C2—C3—C4—C5 | 54.7 (5) | C7—C11—C12—O4 | −173.2 (4) |
C2—C3—C4—C14 | −113.9 (4) | C7—C11—C12—O5 | 7.4 (4) |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
C5—H5···O4i | 0.93 | 2.50 | 3.282 (4) | 141 |
C7—H7···O4ii | 0.98 | 2.49 | 3.396 (4) | 153 |
C8—H8···O4i | 0.98 | 2.54 | 3.409 (4) | 147 |
Symmetry codes: (i) −x+2, y−1/2, −z; (ii) −x+2, y+1/2, −z. |
Experimental details
Crystal data |
Chemical formula | C15H14O6 |
Mr | 290.26 |
Crystal system, space group | Monoclinic, P21 |
Temperature (K) | 293 |
a, b, c (Å) | 8.9090 (12), 7.1840 (8), 10.4940 (11) |
β (°) | 98.830 (9) |
V (Å3) | 663.60 (14) |
Z | 2 |
Radiation type | Mo Kα |
µ (mm−1) | 0.11 |
Crystal size (mm) | 0.4 × 0.3 × 0.2 |
|
Data collection |
Diffractometer | Bruker P4 diffractometer |
Absorption correction | – |
No. of measured, independent and observed [I > 2σ(I)] reflections | 1782, 1483, 1269 |
Rint | 0.028 |
(sin θ/λ)max (Å−1) | 0.595 |
|
Refinement |
R[F2 > 2σ(F2)], wR(F2), S | 0.037, 0.098, 1.04 |
No. of reflections | 1483 |
No. of parameters | 191 |
No. of restraints | 1 |
H-atom treatment | H-atom parameters constrained |
Δρmax, Δρmin (e Å−3) | 0.13, −0.11 |
Absolute structure | Flack H D (1983), Acta Cryst. A39, 876-881 |
Absolute structure parameter | not reliably determined |
Selected torsion angles (º) topC14—O3—C6—C5 | 6.3 (3) | C6—C7—C11—C13 | 35.7 (6) |
C3—C4—C14—O2 | −10.7 (6) | C13—C11—C12—O4 | 7.1 (7) |
C5—C6—C7—C8 | 10.2 (4) | C13—C11—C12—O5 | −172.4 (4) |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
C5—H5···O4i | 0.93 | 2.50 | 3.282 (4) | 141 |
C7—H7···O4ii | 0.98 | 2.49 | 3.396 (4) | 153 |
C8—H8···O4i | 0.98 | 2.54 | 3.409 (4) | 147 |
Symmetry codes: (i) −x+2, y−1/2, −z; (ii) −x+2, y+1/2, −z. |
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Mikanolide, (I), is a sequiterpene dilactone isolated from a variety of Mikania species abundant in many regions. It is used locally as a folk medicine and has attracted the attention of natural products chemists because of its [AUTHOR: can these properties be tied to individual references?] anti-bacterial, antitumor, antimicrobial, cytotoxic and phytotoxic activities (Ahmed et al., 2001; Aguinaldo et al., 1995; Bohlmann et al., 1984; Facey et al., 1999; Gutierrez et al., 1985; Pickman, 1986; Valdes, 1998). Although structural studies of (I) and dihydromikanolide, (II), [1,10:2,3-diepoxy-6,8-dihydroxygermacr-4-ene-12,14- di-γ-lactone, C15H16O6] from Mikania scandens (L.) Willd have been published by Herz et al. (1970) and the crystal structure of (II) has been reported (Cox & Sim 1974), to our knowledge the solid-state structure of (I) has never been investigated. As part of an effort to explore structure–activity relationships of natural products isolated from medicinal plants that are used as alternative medicine in Jamaican society, we report here the structure of mikanolide isolated from Mikania micrantha from Port Antonio, [AUTHOR: It says "Portland" in the Abstract: please clarify.] Jamaica, and compare the results with those reported for dihydromikanolide from Mikania scandens (L.) Willd.
A displacement ellipsoid plot of (I) (Fig. 1) shows a methyl-substituted cyclodecane ring fused to a planar unsaturated α,γ-lactone, a nearly planar envelope-type vinyl β,γ-lactone and two epoxides. The conformations of the cyclodecane ring and epoxide units in (I) are very similar to those reported in the dihydromikanolide isolated from Mikania scandens (L.) Willd (Cox & Sim, 1974), although subtle differences were noted. These are evident in the appearance on the same face of the macrocycle of a methyl group (C15) attached to atoms C10 and C14 of the α,γ-lactone, and in the bond distances, valence angles and endocyclic torsion angles of the cyclododecane ring, which are comparable to the corresponding angles in dihydromikanolide (see Table 1). The same is true of both epoxides at C1—C10 and C2—C3 and the near planar α-γ-lactone at C4—C5—C6. The dihedral angle between the epoxide rings is nearly identical for the two compounds, its value being 36.1 (4)° in (I) and 34.4 (7)° in (II). An exception is the endocyclic C5—C6—C7—C8 torsion angle, which has a value of 10.2 (4)° in the case of (I) and −2.0 (2)° in (II). Although the C4═C5 double bond of the α,γ-lactone in (I) shows the same strain as that in (II), slight variations in the conformations of the α,γ-lactones in (I) and (II) were noted, as apparent from the torsion angles around the α,γ-lactone rings. For example, in (I), the C14—O3—C6—C5 and C3–C4—C14—O2 torsion angles take values of 6.3 (3) and −10.7 (6)°, respectively, while in (II), these values are 2.0 (7) and −17.0 (7)°. The major structural and conformational differences between (I) and (II) occur in the β,γ-trans-fused γ-lactone at C7—C8, which has a more planar five-membered lactone ring in (I), with endocyclic torsion angles approximately half the values of those in the saturated γ-lactone of dihydromikanolide. In (I), torsion angles of 35.7 (6), −172.4 (4) and 7.1 (7)° were observed for C6—C7—C11—C13, C13—C11—C12—O5 and C13—C11—C12—O4, respectively, while corresponding values of 77.0 (7), 148.0 (7) and −34.0 (7)° have been reported for dihydromikanolide.
Molecules of (I) are stacked along the b axis; they are linked by a network of C—H···O hydrogen bonds between the lactone units of neighbouring molecules related by a 21 screw axis (see Table 2 and Fig. 2). The geometric parameters for these non-classical hydrogen bonds are normal and similar to those reported for a variety of compounds containing such bonds, for example, in diospyrin [1',5-dihydroxy-3',7-dimethyl-2,2'-binaphthalene-1,4,5',8'-tetrone C22H14O6, see diagram 3] (Harrison & Musgrave, 2004). ##AUTHOR: Please check the changes in wording abve.
In conclusion, the structural determination of (I) is the first for an unsaturated mikanolide and reveals a near-planar β-lactone unit that may allow for intercalation of mikanolide into the minor groove of DNA, a feature that provides a possible explanation for its biological activity. In view of the use of folk medicinal plants as a guide to the development of new pharmaceuticals, studies are currently in progress in our laboratories to explore the structure–activity relationship of natural products isolated from a variety of common native Jamaican plants.