Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270107011821/av3076sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108270107011821/av3076Vsup2.hkl |
CCDC reference: 649083
The synthesis of the enantiomerically pure α-methylene-δ-valerolactone, (V), was based on a highly stereoselective Michael reaction of the chiral enamine derived from (R)-1-phenylethylamine and (R)-dihydrocarvone 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. The enantiomeric purity of (V) was confirmed as higher than 0.99 by gas chromatographic analysis on a chiral column. Details of the procedure have been described elsewhere (Krawczyk & Śliwiński, 2003; Krawczyk, Śliwiński et al., 2004; Krawczyk et al., 2006). Colourless crystals of (V) (m.p. 397 K) were grown within 4 d by slow evaporation of a solution in a 1:1 mixture of methanol and ethyl acetate.
All H atoms, except those of the methyl groups, were located in a difference Fourier map calculated after three cycles of anisotropic refinement. Their positional and isotropic displacement parameters were allowed to refine freely [C—H = 0.945 (18)–1.029 (17) Å]. The methyl H atoms were placed in calculated positions [C—H = 0.96 (2) Å] and refined as riding. Refinement of the Flack (1983) parameter is in agreement with the absolute configuration as assigned from the mechanism of the highly stereoselective Michael reaction (Krawczyk & Śliwiński, 2003). An attempt to refine the inverted structure led to a Flack parameter of 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.
C14H20O2 | Dx = 1.161 Mg m−3 |
Mr = 220.30 | Cu Kα radiation, λ = 1.54178 Å |
Orthorhombic, P212121 | Cell parameters from 5207 reflections |
a = 6.4451 (1) Å | θ = 6.3–71.0° |
b = 13.9619 (2) Å | µ = 0.60 mm−1 |
c = 14.0081 (2) Å | T = 293 K |
V = 1260.53 (3) Å3 | Prism, colourless |
Z = 4 | 0.35 × 0.20 × 0.10 mm |
F(000) = 480 |
Bruker SMART APEX CCD area-detector diffractometer | 2404 independent reflections |
Radiation source: fine-focus sealed tube | 2362 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.019 |
ω scans | θmax = 71.0°, θmin = 4.5° |
Absorption correction: multi-scan (SHELXTL; Bruker, 2003) | h = −7→7 |
Tmin = 0.876, Tmax = 0.943 | k = −16→17 |
14510 measured reflections | l = −17→16 |
Refinement on F2 | Hydrogen site location: inferred from neighbouring sites |
Least-squares matrix: full | H atoms treated by a mixture of independent and constrained refinement |
R[F2 > 2σ(F2)] = 0.034 | w = 1/[σ2(Fo2) + (0.0627P)2 + 0.0879P] where P = (Fo2 + 2Fc2)/3 |
wR(F2) = 0.097 | (Δ/σ)max = 0.006 |
S = 1.05 | Δρmax = 0.14 e Å−3 |
2404 reflections | Δρmin = −0.14 e Å−3 |
211 parameters | Extinction correction: SHELXTL (Bruker, 2003), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4 |
0 restraints | Extinction coefficient: 0.0043 (9) |
Primary atom site location: structure-invariant direct methods | Absolute structure: Flack (1983), with 933 Friedel pairs |
Secondary atom site location: difference Fourier map | Absolute structure parameter: 0.0 (2) |
C14H20O2 | V = 1260.53 (3) Å3 |
Mr = 220.30 | Z = 4 |
Orthorhombic, P212121 | Cu Kα radiation |
a = 6.4451 (1) Å | µ = 0.60 mm−1 |
b = 13.9619 (2) Å | T = 293 K |
c = 14.0081 (2) Å | 0.35 × 0.20 × 0.10 mm |
Bruker SMART APEX CCD area-detector diffractometer | 2404 independent reflections |
Absorption correction: multi-scan (SHELXTL; Bruker, 2003) | 2362 reflections with I > 2σ(I) |
Tmin = 0.876, Tmax = 0.943 | Rint = 0.019 |
14510 measured reflections |
R[F2 > 2σ(F2)] = 0.034 | H atoms treated by a mixture of independent and constrained refinement |
wR(F2) = 0.097 | Δρmax = 0.14 e Å−3 |
S = 1.05 | Δρmin = −0.14 e Å−3 |
2404 reflections | Absolute structure: Flack (1983), with 933 Friedel pairs |
211 parameters | Absolute structure parameter: 0.0 (2) |
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 | 0.24221 (16) | 0.14905 (8) | 0.46270 (6) | 0.0616 (3) | |
O2 | 0.2594 (2) | 0.09988 (13) | 0.61025 (8) | 0.0934 (5) | |
C1 | 0.34711 (19) | 0.17901 (9) | 0.37539 (9) | 0.0463 (3) | |
C2 | 0.3514 (3) | 0.11511 (11) | 0.53709 (10) | 0.0607 (4) | |
C3 | 0.5783 (2) | 0.09699 (10) | 0.52438 (10) | 0.0558 (3) | |
C4 | 0.6806 (4) | 0.06331 (15) | 0.59859 (14) | 0.0776 (5) | |
C5 | 0.6777 (2) | 0.11617 (11) | 0.42942 (11) | 0.0575 (3) | |
C6 | 0.5202 (2) | 0.11055 (9) | 0.34789 (9) | 0.0488 (3) | |
C7 | 0.6136 (2) | 0.14868 (14) | 0.25465 (11) | 0.0612 (4) | |
C8 | 0.4542 (2) | 0.15914 (13) | 0.17503 (10) | 0.0623 (4) | |
C9 | 0.2776 (2) | 0.22594 (10) | 0.20560 (9) | 0.0512 (3) | |
C10 | 0.1818 (2) | 0.19015 (10) | 0.29952 (9) | 0.0494 (3) | |
C11 | 0.4454 (3) | 0.00745 (10) | 0.33590 (12) | 0.0658 (4) | |
C12 | 0.1109 (2) | 0.24099 (11) | 0.13121 (10) | 0.0570 (3) | |
C13 | −0.0006 (4) | 0.33299 (15) | 0.13465 (17) | 0.0894 (6) | |
C14 | 0.0569 (3) | 0.17350 (15) | 0.06828 (12) | 0.0717 (4) | |
H11 | 0.406 (2) | 0.2413 (10) | 0.3895 (9) | 0.045 (3)* | |
H41 | 0.609 (3) | 0.0522 (14) | 0.6575 (14) | 0.075 (5)* | |
H42 | 0.823 (5) | 0.047 (2) | 0.5893 (18) | 0.114 (8)* | |
H51 | 0.741 (3) | 0.1839 (12) | 0.4312 (11) | 0.058 (4)* | |
H52 | 0.791 (3) | 0.0723 (14) | 0.4181 (13) | 0.078 (5)* | |
H71 | 0.668 (3) | 0.2100 (13) | 0.2679 (12) | 0.060 (4)* | |
H72 | 0.732 (4) | 0.1077 (16) | 0.2325 (14) | 0.084 (6)* | |
H81 | 0.403 (3) | 0.0933 (14) | 0.1536 (13) | 0.076 (5)* | |
H82 | 0.524 (3) | 0.1857 (14) | 0.1191 (13) | 0.082 (6)* | |
H91 | 0.330 (3) | 0.2898 (12) | 0.2211 (11) | 0.057 (4)* | |
H101 | 0.113 (3) | 0.1295 (12) | 0.2878 (11) | 0.056 (4)* | |
H102 | 0.071 (3) | 0.2349 (11) | 0.3228 (10) | 0.052 (4)* | |
H111 | 0.3856 | −0.0145 | 0.3947 | 0.086 (6)* | |
H112 | 0.5607 | −0.0327 | 0.3192 | 0.126 (9)* | |
H113 | 0.3429 | 0.0048 | 0.2862 | 0.089 (6)* | |
H131 | 0.0925 | 0.3839 | 0.1178 | 0.133 (10)* | |
H132 | −0.0527 | 0.3434 | 0.1980 | 0.113 (9)* | |
H133 | −0.1142 | 0.3316 | 0.0904 | 0.111 (8)* | |
H141 | −0.062 (3) | 0.1831 (13) | 0.0286 (13) | 0.071 (5)* | |
H142 | 0.131 (4) | 0.1139 (19) | 0.0662 (17) | 0.106 (7)* |
U11 | U22 | U33 | U12 | U13 | U23 | |
O1 | 0.0517 (5) | 0.0867 (7) | 0.0466 (5) | 0.0080 (5) | 0.0059 (4) | 0.0064 (4) |
O2 | 0.0829 (8) | 0.1415 (12) | 0.0557 (6) | 0.0078 (9) | 0.0081 (7) | 0.0230 (7) |
C1 | 0.0435 (6) | 0.0502 (6) | 0.0451 (6) | −0.0004 (5) | 0.0019 (5) | −0.0008 (5) |
C2 | 0.0642 (9) | 0.0703 (8) | 0.0475 (7) | 0.0006 (7) | −0.0001 (6) | 0.0034 (6) |
C3 | 0.0609 (8) | 0.0489 (6) | 0.0575 (7) | −0.0017 (6) | −0.0104 (7) | −0.0013 (5) |
C4 | 0.0820 (12) | 0.0824 (11) | 0.0684 (10) | 0.0101 (10) | −0.0138 (10) | 0.0108 (8) |
C5 | 0.0461 (7) | 0.0645 (8) | 0.0620 (8) | 0.0015 (6) | −0.0060 (6) | 0.0011 (6) |
C6 | 0.0417 (6) | 0.0522 (7) | 0.0525 (7) | 0.0004 (5) | −0.0011 (5) | −0.0017 (5) |
C7 | 0.0405 (7) | 0.0840 (10) | 0.0591 (8) | 0.0042 (7) | 0.0087 (6) | 0.0032 (7) |
C8 | 0.0499 (7) | 0.0870 (11) | 0.0501 (7) | 0.0032 (7) | 0.0088 (6) | 0.0063 (7) |
C9 | 0.0474 (7) | 0.0548 (7) | 0.0514 (7) | −0.0040 (5) | 0.0013 (6) | 0.0057 (5) |
C10 | 0.0409 (6) | 0.0574 (7) | 0.0498 (7) | 0.0029 (5) | 0.0028 (5) | 0.0002 (6) |
C11 | 0.0769 (10) | 0.0513 (7) | 0.0691 (9) | 0.0033 (7) | −0.0119 (8) | −0.0068 (6) |
C12 | 0.0515 (7) | 0.0689 (8) | 0.0506 (7) | −0.0031 (6) | 0.0016 (6) | 0.0114 (6) |
C13 | 0.0918 (13) | 0.0818 (12) | 0.0947 (14) | 0.0205 (11) | −0.0313 (12) | 0.0038 (10) |
C14 | 0.0698 (10) | 0.0842 (11) | 0.0611 (8) | −0.0032 (8) | −0.0127 (8) | −0.0001 (8) |
O1—C2 | 1.3439 (17) | C5—H51 | 1.029 (17) |
O1—C1 | 1.4588 (14) | C5—H52 | 0.96 (2) |
O2—C2 | 1.203 (2) | C7—H71 | 0.945 (18) |
C3—C4 | 1.318 (2) | C7—H72 | 1.00 (2) |
C3—C5 | 1.501 (2) | C8—H81 | 1.02 (2) |
C2—C3 | 1.495 (2) | C8—H82 | 0.979 (19) |
C9—C12 | 1.5115 (19) | C9—H91 | 0.977 (17) |
C12—C14 | 1.337 (2) | C10—H101 | 0.969 (16) |
C12—C13 | 1.473 (2) | C10—H102 | 1.004 (16) |
C1—C10 | 1.5130 (18) | C11—H111 | 0.9600 |
C1—C6 | 1.5185 (18) | C11—H112 | 0.9600 |
C5—C6 | 1.5303 (18) | C11—H113 | 0.9600 |
C6—C11 | 1.5272 (19) | C13—H131 | 0.9600 |
C6—C7 | 1.5337 (19) | C13—H132 | 0.9600 |
C7—C8 | 1.524 (2) | C13—H133 | 0.9600 |
C8—C9 | 1.532 (2) | C14—H141 | 0.955 (19) |
C9—C10 | 1.5370 (17) | C14—H142 | 0.96 (3) |
C1—H11 | 0.970 (15) | ||
C2—O1—C1 | 120.56 (10) | C7—C8—H81 | 110.2 (11) |
O2—C2—O1 | 117.68 (14) | C9—C8—H81 | 113.0 (12) |
O2—C2—C3 | 123.64 (14) | C7—C8—H82 | 108.1 (12) |
O1—C2—C3 | 118.68 (13) | C9—C8—H82 | 109.7 (12) |
C4—C3—C2 | 117.12 (16) | H81—C8—H82 | 104.7 (15) |
C4—C3—C5 | 123.34 (16) | C12—C9—C8 | 114.82 (11) |
C2—C3—C5 | 119.54 (13) | C12—C9—C10 | 110.47 (10) |
O1—C1—C10 | 106.97 (10) | C8—C9—C10 | 109.86 (11) |
O1—C1—C6 | 111.87 (10) | C12—C9—H91 | 105.8 (10) |
C10—C1—C6 | 113.82 (10) | C8—C9—H91 | 111.2 (10) |
O1—C1—H11 | 105.6 (8) | C10—C9—H91 | 104.2 (9) |
C10—C1—H11 | 109.2 (8) | C1—C10—C9 | 110.58 (10) |
C6—C1—H11 | 109.1 (8) | C1—C10—H101 | 110.6 (9) |
C3—C4—H41 | 119.8 (13) | C9—C10—H101 | 108.8 (9) |
C3—C4—H42 | 117.3 (16) | C1—C10—H102 | 109.7 (8) |
H41—C4—H42 | 123 (2) | C9—C10—H102 | 111.2 (8) |
C3—C5—C6 | 111.66 (12) | H101—C10—H102 | 105.9 (13) |
C3—C5—H51 | 108.1 (9) | C6—C11—H111 | 109.5 |
C6—C5—H51 | 109.0 (9) | C6—C11—H112 | 109.5 |
C3—C5—H52 | 110.8 (11) | H111—C11—H112 | 109.5 |
C6—C5—H52 | 110.2 (12) | C6—C11—H113 | 109.5 |
H51—C5—H52 | 106.9 (15) | H111—C11—H113 | 109.5 |
C1—C6—C11 | 112.93 (12) | H112—C11—H113 | 109.5 |
C1—C6—C5 | 105.42 (11) | C14—C12—C13 | 120.63 (16) |
C11—C6—C5 | 109.86 (12) | C14—C12—C9 | 122.82 (15) |
C1—C6—C7 | 106.62 (12) | C13—C12—C9 | 116.46 (14) |
C11—C6—C7 | 110.94 (12) | C12—C13—H131 | 109.5 |
C5—C6—C7 | 110.92 (11) | C12—C13—H132 | 109.5 |
C8—C7—C6 | 113.06 (11) | H131—C13—H132 | 109.5 |
C8—C7—H71 | 108.0 (11) | C12—C13—H133 | 109.5 |
C6—C7—H71 | 107.1 (10) | H131—C13—H133 | 109.5 |
C8—C7—H72 | 109.9 (12) | H132—C13—H133 | 109.5 |
C6—C7—H72 | 111.4 (12) | C12—C14—H141 | 119.5 (11) |
H71—C7—H72 | 107.1 (17) | C12—C14—H142 | 120.0 (15) |
C7—C8—C9 | 110.77 (12) | H141—C14—H142 | 120.3 (19) |
O2—C2—C3—C4 | −0.7 (3) | O1—C1—C6—C7 | 178.78 (11) |
O1—C2—C3—C4 | −179.70 (15) | C10—C1—C6—C7 | 57.37 (14) |
C1—C6—C5—C3 | 54.19 (14) | C3—C5—C6—C11 | −67.77 (16) |
C6—C5—C3—C2 | −24.85 (18) | C3—C5—C6—C7 | 169.22 (13) |
C5—C3—C2—O1 | −0.5 (2) | C1—C6—C7—C8 | −56.85 (17) |
C3—C2—O1—C1 | −7.7 (2) | C11—C6—C7—C8 | 66.48 (18) |
C2—O1—C1—C6 | 41.39 (16) | C5—C6—C7—C8 | −171.13 (13) |
C8—C9—C12—C14 | 31.9 (2) | C6—C7—C8—C9 | 57.89 (18) |
C10—C9—C12—C14 | −93.00 (17) | C7—C8—C9—C12 | 180.00 (12) |
C2—O1—C1—C10 | 166.67 (12) | C7—C8—C9—C10 | −54.78 (16) |
C1—O1—C2—O2 | 173.23 (15) | O1—C1—C10—C9 | 177.58 (10) |
O2—C2—C3—C5 | 178.54 (16) | C6—C1—C10—C9 | −58.33 (14) |
C4—C3—C5—C6 | 154.29 (16) | C12—C9—C10—C1 | −177.61 (11) |
O1—C1—C6—C11 | 56.69 (14) | C8—C9—C10—C1 | 54.70 (15) |
C10—C1—C6—C11 | −64.71 (15) | C8—C9—C12—C13 | −151.69 (16) |
O1—C1—C6—C5 | −63.26 (13) | C10—C9—C12—C13 | 83.41 (18) |
C10—C1—C6—C5 | 175.33 (11) |
Experimental details
Crystal data | |
Chemical formula | C14H20O2 |
Mr | 220.30 |
Crystal system, space group | Orthorhombic, P212121 |
Temperature (K) | 293 |
a, b, c (Å) | 6.4451 (1), 13.9619 (2), 14.0081 (2) |
V (Å3) | 1260.53 (3) |
Z | 4 |
Radiation type | Cu Kα |
µ (mm−1) | 0.60 |
Crystal size (mm) | 0.35 × 0.20 × 0.10 |
Data collection | |
Diffractometer | Bruker SMART APEX CCD area-detector diffractometer |
Absorption correction | Multi-scan (SHELXTL; Bruker, 2003) |
Tmin, Tmax | 0.876, 0.943 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 14510, 2404, 2362 |
Rint | 0.019 |
(sin θ/λ)max (Å−1) | 0.613 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.034, 0.097, 1.05 |
No. of reflections | 2404 |
No. of parameters | 211 |
H-atom treatment | H atoms treated by a mixture of independent and constrained refinement |
Δρmax, Δρmin (e Å−3) | 0.14, −0.14 |
Absolute structure | Flack (1983), with 933 Friedel pairs |
Absolute structure parameter | 0.0 (2) |
Computer programs: SMART (Bruker, 2003), SMART, SAINT-Plus (Bruker, 2003), SHELXTL (Bruker, 2003), SHELXTL.
O1—C2 | 1.3439 (17) | C2—C3 | 1.495 (2) |
O1—C1 | 1.4588 (14) | C9—C12 | 1.5115 (19) |
O2—C2 | 1.203 (2) | C12—C14 | 1.337 (2) |
C3—C4 | 1.318 (2) | C12—C13 | 1.473 (2) |
C3—C5 | 1.501 (2) | ||
C2—O1—C1 | 120.56 (10) | C4—C3—C2 | 117.12 (16) |
O2—C2—O1 | 117.68 (14) | C4—C3—C5 | 123.34 (16) |
O2—C2—C3 | 123.64 (14) | C2—C3—C5 | 119.54 (13) |
O1—C2—C3 | 118.68 (13) | ||
O2—C2—C3—C4 | −0.7 (3) | C3—C2—O1—C1 | −7.7 (2) |
O1—C2—C3—C4 | −179.70 (15) | C2—O1—C1—C6 | 41.39 (16) |
C1—C6—C5—C3 | 54.19 (14) | C8—C9—C12—C14 | 31.9 (2) |
C6—C5—C3—C2 | −24.85 (18) | C10—C9—C12—C14 | −93.00 (17) |
C5—C3—C2—O1 | −0.5 (2) |
Type of interaction | Stabilization energy (kJ mol-1) | ||||
(I) | (II) | (III) | (IV) | (V) | |
σ(C3═C4)–σ*(C2—O1) | 7.8 | 8.6 | 8.9 | 9.2 | 9.8 |
σ(C2—O1)–σ*(C3═C4) | 5.3 | 4.7 | 4.8 | 5.0 | 4.6 |
π(C3═C4)–π*(C2═O2) | 57.8 | 57.2 | 56.8 | 62.0 | 60.2 |
π(C2═O2)–π*(C3═C4) | 13.3 | 13.1 | 13.1 | 14.0 | 13.3 |
nπ(O2)–σ*(C2—C3) | 66.3 | 66.9 | 66.1 | 64.8 | 67.3 |
Stabilization energies were calculated using GAUSSIAN03 (Frisch et al.. 2004) at the HF/6-311++G(d,p) level of theory for X-ray determined coordinates. The standard NBO deletion procedure (Glendening et al., 1992) was applied. |
The α-methylene-δ-valerolactone moiety is found in a wide range of natural products. Several of them, like vernolepin (Kupchan et al., 1968), teucriumlactone (Nangia et al., 1997) and artemisitene (Liao et al., 2001), have proven antibacterial and antitumour activities. Morever, the δ-valerolactones are also useful substrates for the preparation of versatile biodegradable polyesters with good mechanical properties (Lou et al., 2002), which may find biomedical and pharmaceutical applications (Albertsson & Varma, 2003). Enantiomerically pure α-methylene-δ-valerolactones are interesting chiral building blocks whose use in organic chemistry has been restricted by the limited availability of their synthesis (Suzuki et al., 1991; Krishna et al., 2004) The first synthesis by an asymmetric Michael reaction, leading to the enantio-enriched species, has been described recently by us (Krawczyk & Śliwiński, 2003; Krawczyk, Śliwiński et al., 2004; Krawczyk et al., 2006).
The present study is a continuation of our structural investigations of optically active bicyclic α-methylene-δ-valerolactones. Four crystal structures have been published previously, namely (4aS,8aS)-4a-methyl-3-methyleneperhydrochromen-2-one, (I) (Krawczyk, Śliwiński & Wolf, 2004), ethyl trans-(4aS,8aS)-3-methylene-2-oxohexahydrochromene-4a-carboxylate, (II) (Krawczyk, Śliwiński et al., 2004), trans-(4aR,8aR)-4a-methoxy-3-methyleneperhydrochromen-2-one, (III) (Wojciechowski et al., 2005), and (5R,6R)-methyl-9-methylene-2,7-dioxa-spiro[4.5]decane-1,8-dione, (IV) (Krawczyk et al., 2006). The title compound, (V), is fifth in the series. In compounds (I), (II), (III) and (V), the δ-valerolactone ring is condensed with the cyclohexane moiety along the individual Cδ—Cγ single bond. The molecule of (IV) adopts an unusual spiro arrangement, with the γ-lactone and δ-lactone rings sharing the pivotal C atom and strongly twisted with respect to one another.
A view of (V) with the atom-numbering scheme is shown in Fig. 1. The δ-valerolactone ring adopts a conformation close to a 5E envelope (Boeyens, 1978), with atoms O1, C1, C2, C3 and C5 almost coplanar (the average r.m.s. deviation from the mean plane is 0.04 Å) and atom C6 situated at the flap. The Cremer & Pople (1975) puckering parameters for the ring atom sequence O1/C2/C3/C5/C6/C1 are Q = 0.529 (1) Å, θ = 52.5 (2)° and ϕ = 251.8 (2)°. The conformation of unsaturated δ-valerolactones has been investigated by Brandänge et al. (2003). Their ab initio HF/6–31G* calculations on isolated molecules showed the high conformational mobility of the ring and indicated that the energy of the envelope conformer is almost 8.5 kJ mol-1 above the theoretically most stable half-chair arrangement.
The γ-methyl substituent occupies an axial position with respect to both the δ-valerolactone and the cyclohexane rings. The molecular conformation can be defined as extended with both rings almost coplanar to one another. A similar arrangement has been observed in compounds (II) and (III). In (I), both rings are roughly perpendicular to one another, leading to the folded conformation of the molecule. A superposition of (V) on the four structures, (I)–(IV), as presented in Fig. 2, clearly shows the high degree of similarity of the δ-valerolactone rings in all five compounds investigated to date.
Bond lengths in (V) are close to those observed in the related compounds (I)–(IV). In particular, two exocyclic double bonds, C2═O2 [1.203 (2) Å] and C3═C4 [1.318 (2) Å], are shorter than similar bonds observed in the O═ C—C═C moiety [1.222 and 1.340 Å, respectively; Allen et al., 2004]. These bonds are separated by a relatively long C2—C3 bond [1.495 (2) Å; standard value 1.465 Å] and are quite coplanar, as indicated by a close to zero value of the O2—C2—C3—C4 torsion angle [-0.7 (3)°].
The syn conformation of the O2═C2—C3═C4 fragment in all investigated δ-valerolactones (I)–(V) prompts electronic interactions involving the bonding σ and π orbitals and the antibonding σ* and π* orbitals. The most important values (Table 2, Fig. 3) were computed by the Weinhold natural bond orbitals deletion procedure (Glendening et al., 1992) for wavefunctions calculated with GAUSSIAN03 (Frisch et al., 2004) at the HF/6–311++G(d,p) level of theory for the X-ray determined coordinates. In particular, the exocyclic C3═C4 bond participates in electron-density transfer towards the carbonyl group in the π–π* fashion (Giuffreda et al., 2004), while the reverse back-donation is much weaker [60.2 and 13.3 kJ mol-1, respectively, for (V)]. In comparison with the above effect, the energies of mutual anti σ–σ* hyperconjugation (Weinhold, 2001) involving the endocyclic C2—O1 and vinyl C3═C4 bonds are smaller [9.8 and 4.6 kJ mol-1, respectively, for (V)]. The resulting surplus of electron density accumulated on the carbonyl atom O2 is back-donated towards atoms C2 and C3 through the nπ(O2)–σ*(C2—C3) stereoelectronic effect (Graczyk & Mikołajczyk, 1994).
Examination of the crystal packing of (V) indicates that the intermolecular distances are larger than the sums of the respective van der Waals radii (Bondi, 1964).