(5S)-3-Oxo-4-oxa-endo-tri­cyclo[5.2.1.0^2,6]­dec-8-en-5-yl acetate

The molecular structure of C₁₁H₁₂O₄, based on a norbornene core, was established to confirm the configuration of an acetoxy side-chain group in addition to the formation of the endo product. The acetoxy side chain lies in an axial position relative to the five-membered fused ring. Bond distances and angles show no unusual features, with all geometric parameters lying within their expected ranges. The overall stereochemistry of the mol­ecule was ascertained from the chiral furan­one starting material.

S1. Comment

Strigol derivatives and members of the related sesquiterpene lactone family are known to induce germination of seeds from parasitic weeds. Early synthetic routes to the formation of endo-tricyclo exo-hydroxy lactones consisted of laborious processes which included the separation of diastereomers and selective recrystallization. A simpler synthetic procedure was achieved by reaction of the racemic alcohol mixture with an acyl donor in the presence of lipase, which acts as a catalyst (Thuring et al., 1996). The synthesis of the title compound, (I), encompasses the optically pure 5-acetoxy-2(5H)furanon (van der Deen et al., 1994), the acetyl functionality acting as temporary protection for the hydroxy group. \sch

The molecular structure of (I) is shown in Fig. 1, with selected geometric parameters provided in Table 1. The conformation of (I) is based on the standard bicyclo[2.2.1]heptene (norbornene) core, comprising two five-membered rings, with a third fused system in an endo configuration. Under the reaction conditions, there has been retention of configuration at the carbon centre (C4) and the molecular structure was based on having an S configuration here. Thus the configuration at atoms C1, C5, C6 and C9 are R, S, R and S.

All three five-membered rings in (I) are in envelope conformations. The C6, C7, C8, C9, C10 ring has 97% in the cis form (Evans & Boeyens, 1989) with puckering parameters of Q = 0.546 (2) Å, ϕ = 323.5 (2)°, (calculations from PLATON; Spek, 2000), whilst the second ring comprising C1, C5, C6, C10, C9, has 85% in the cis form [Q = 0.642 (2) Å, ϕ = 105.34 (19)°]. Both rings have C10 as the `envelope' atom. The third ring, with atoms O3, C2, C1, C5, C4, is not as puckered as the other two rings, being more `twisted' at C4 by 64% [Q = 0.150 (2) Å, ϕ = 150.6 (7)°].

Comparison of the bond distances and angles in (I) with those determined for norbornane carboxylic anhydride derivatives (Garbauskas & Buese, 1992; Schonk et al., 1992; Shnulin et al., 1982) show no significant variations associated with acetylation at O12. The three types of C—O bonds within each O═C—O—C unit of (I) are consistent: C2—O11 = 1.194 (2) cf. C13—O15 = 1.195 (2)°; C2—O3 = 1.360 (2) cf. C13—O12 = 1.369 (2)°; and C4—O3 = 1.425 (2) cf. C4—O12 = 1.425 (2)°, and all other bond types lie in their expected ranges. The inter bridgehead angles C10—C6—C5 and C10—C9—C1 angles of 98.44 (15) and 98.99 (15)°, respectively, are contracted with respect to the tetrahedral value, as is the C6—C10—C9 angle of 93.82 (14)°. The angles at the non-substituted end of the norbornene fragment are 4° wider on average than those at the fused end [the C—C—C bond angles being in the range 102.70 (15)–108.53 (16)°].

The acetoxy side-chain (O12, C13, C14, O15) at C4 is axial and almost perpendicular to the carboxylate group which forms part of the fused ring (O11, C2, O3, C4); the angle between the two planes is 85.62 (16)°. The twisting of the acetoxy functionalities is indicated by the torsion angle values of −169.71 (17)° for O11—C2—O3—C4, and C4—O12—C13—O15 being 10.9 (3)°, each deviating from the trans conformation.

The crystal structure is not stabilized by inter- (or intra-) molecular hydrogen bonds - all contacts to potential donor atoms are quite long: O3···H10Bⁱ of 2.60 Å, O11···H14Cⁱⁱ (2.55 Å), O12···H9ⁱⁱⁱ (2.81 Å) and O15···H6ⁱ of 2.69 Å [symmetry codes: (i) 1 + x, y, z, (ii) x, −1 + y, z, (iii) −x, 1/2 + y, 1/2 − z].

S2. Experimental

C₁₁H₁₂O₄ was synthesized from a Diels-Alder reaction of (+)5-acetoxy-2(5H)-furanon (van der Deen et al., 1994), with cyclopentadiene. The compound was recrystallized from a diethyl ether/hexane mixture.

S3. Refinement

The data set included some Friedel-related reflections, but due to the absence of anomalous scatterers, the absolute configuration could not be established reliably (Flack & Bernardinelli, 1999). A final value of 0.4 (13) was obtained for the Flack x parameter (Flack, 1983). The absolute configuration was therefore set in accordance with the known configuration of the optically pure precursor (+)5-acetoxy-2(5H)furanon. The estimated number of Friedel pairs measured was 935 (70%), and these reflections were merged in the final refinement cycles. The presence of only one hydrogen on atoms C7 and C8, was confirmed by inspection of the difference Fourier map. All hydrogen atoms except for those on methyl group C14 were placed in idealized positions and constrained to ride on their carbon atoms with U_iso(H) = 1.2U_eq(C). The methyl H atoms on C14 were constrained to an ideal geometry with U_iso(H) = 1.5U_eq(C), and allowed to rotate freely about the C—C bonds.

Computing details

Data collection: locally modified CAD4-Version 5 Software (Enraf-Nonius, 1989); cell refinement: SET4 (de Boer & Duisenberg, 1984); data reduction: HELENA (Spek, 1997); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: PLATON (Spek, 2000); software used to prepare material for publication: PLATON.

Figures

Fig. 1. Displacement ellipsoid plot (PLATON; Spek, 2000) of (I) drawn at 50% probability level.

(I)

Crystal data

C11H12O4	Dx = 1.392 Mg m^−3
Mr = 208.21	Mo Kα radiation, λ = 0.71073 Å
Orthorhombic, P212121	Cell parameters from 25 reflections
a = 7.4538 (8) Å	θ = 9.7–14.0°
b = 7.5568 (7) Å	µ = 0.11 mm^−1
c = 17.6385 (13) Å	T = 150 K
V = 993.52 (16) Å^3	Plates, colourless
Z = 4	0.50 × 0.50 × 0.50 mm
F(000) = 440

Data collection

Enraf-Nonius CAD4T diffractometer	Rint = 0.023
Radiation source: rotating anode	θmax = 27.4°, θmin = 2.3°
Graphite monochromator	h = −9→9
w/2θ scans	k = −9→7
3710 measured reflections	l = 0→22
1334 independent reflections	3 standard reflections every 60 min
1213 reflections with I > 2σ(I)	intensity decay: 2.2%

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.034	H-atom parameters constrained
wR(F^2) = 0.083	w = 1/[σ^2(Fo^2) + (0.0449P)^2 + 0.1472P] where P = (Fo^2 + 2Fc^2)/3
S = 1.08	(Δ/σ)max = 0.001
1334 reflections	Δρmax = 0.17 e Å^−3
149 parameters	Δρmin = −0.24 e Å^−3
0 restraints	Absolute structure: see experimental
Primary atom site location: structure-invariant direct methods

Special details

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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
C1	−0.0254 (2)	0.3942 (3)	0.19351 (10)	0.0204 (4)
H1	−0.0448	0.4530	0.2437	0.028 (6)*
C2	0.1636 (2)	0.3264 (2)	0.18536 (10)	0.0218 (4)
O3	0.24164 (16)	0.39637 (18)	0.12243 (7)	0.0238 (3)
C4	0.1383 (2)	0.5390 (2)	0.09274 (10)	0.0215 (4)
H4	0.1340	0.5360	0.0361	0.021 (5)*
C5	−0.0487 (2)	0.5251 (3)	0.12713 (10)	0.0223 (4)
H5	−0.0901	0.6431	0.1460	0.025 (6)*
C6	−0.1972 (3)	0.4352 (3)	0.07829 (11)	0.0296 (5)
H6	−0.2570	0.5127	0.0399	0.032 (6)*
C7	−0.1257 (3)	0.2615 (3)	0.04904 (11)	0.0327 (5)
H7	−0.0962	0.2358	−0.0022	0.043 (6)*
C8	−0.1111 (3)	0.1514 (3)	0.10681 (12)	0.0298 (5)
H8	−0.0701	0.0325	0.1046	0.051 (8)*
C9	−0.1715 (2)	0.2492 (3)	0.17728 (10)	0.0239 (4)
H9	−0.2078	0.1741	0.2214	0.029 (6)*
C10	−0.3181 (2)	0.3680 (3)	0.14336 (11)	0.0286 (5)
H10A	−0.3565	0.4635	0.1782	0.028 (5)*
H10B	−0.4234	0.3005	0.1251	0.031 (6)*
O11	0.24175 (19)	0.2223 (2)	0.22411 (9)	0.0344 (4)
O12	0.21915 (17)	0.69783 (17)	0.11977 (8)	0.0246 (3)
C13	0.3743 (3)	0.7491 (3)	0.08444 (10)	0.0256 (4)
C14	0.4628 (3)	0.8931 (3)	0.12751 (14)	0.0427 (6)
H14A	0.5640	0.9393	0.0981	0.072 (10)*
H14B	0.5066	0.8467	0.1760	0.095 (12)*
H14C	0.3765	0.9883	0.1369	0.084 (11)*
O15	0.42704 (19)	0.68259 (19)	0.02715 (8)	0.0327 (4)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C1	0.0190 (8)	0.0225 (9)	0.0196 (8)	−0.0002 (8)	0.0002 (7)	−0.0019 (8)
C2	0.0199 (9)	0.0205 (9)	0.0250 (9)	−0.0034 (8)	−0.0017 (7)	−0.0010 (8)
O3	0.0183 (6)	0.0246 (6)	0.0283 (7)	0.0013 (6)	0.0036 (6)	0.0015 (6)
C4	0.0194 (8)	0.0226 (9)	0.0223 (8)	−0.0008 (8)	−0.0023 (7)	−0.0005 (8)
C5	0.0174 (8)	0.0228 (9)	0.0266 (9)	0.0014 (7)	0.0000 (7)	0.0006 (8)
C6	0.0192 (8)	0.0407 (12)	0.0290 (9)	−0.0030 (9)	−0.0051 (8)	0.0063 (9)
C7	0.0240 (9)	0.0486 (13)	0.0255 (9)	−0.0120 (11)	0.0001 (8)	−0.0130 (9)
C8	0.0227 (9)	0.0279 (10)	0.0388 (11)	−0.0070 (9)	0.0028 (9)	−0.0112 (9)
C9	0.0206 (9)	0.0249 (9)	0.0263 (9)	−0.0032 (8)	0.0015 (7)	0.0016 (8)
C10	0.0185 (9)	0.0345 (11)	0.0328 (10)	−0.0021 (8)	0.0000 (8)	−0.0008 (9)
O11	0.0248 (7)	0.0361 (8)	0.0423 (8)	0.0018 (7)	−0.0040 (6)	0.0139 (7)
O12	0.0237 (6)	0.0242 (7)	0.0258 (6)	−0.0053 (6)	0.0034 (6)	−0.0016 (6)
C13	0.0245 (9)	0.0226 (9)	0.0295 (9)	−0.0020 (9)	−0.0003 (8)	0.0053 (8)
C14	0.0412 (12)	0.0395 (12)	0.0474 (14)	−0.0191 (12)	0.0071 (11)	−0.0049 (11)
O15	0.0317 (7)	0.0349 (8)	0.0316 (7)	−0.0070 (7)	0.0097 (6)	−0.0016 (7)

Geometric parameters (Å, º)

C1—C2	1.506 (3)	C7—C8	1.320 (3)
C1—C5	1.543 (2)	C7—H7	0.9500
C1—C9	1.572 (3)	C8—C9	1.515 (3)
C1—H1	1.0000	C8—H8	0.9500
C2—O11	1.194 (2)	C9—C10	1.536 (3)
C2—O3	1.360 (2)	C9—H9	1.0000
O3—C4	1.425 (2)	C10—H10A	0.9900
C4—O12	1.425 (2)	C10—H10B	0.9900
C4—C5	1.524 (3)	O12—C13	1.369 (2)
C4—H4	1.0000	C13—O15	1.195 (2)
C5—C6	1.559 (3)	C13—C14	1.482 (3)
C5—H5	1.0000	C14—H14A	0.9800
C6—C7	1.507 (3)	C14—H14B	0.9800
C6—C10	1.545 (3)	C14—H14C	0.9800
C6—H6	1.0000
C2—C1—C5	104.56 (14)	C8—C7—C6	108.30 (16)
C2—C1—C9	113.17 (15)	C8—C7—H7	125.9
C5—C1—C9	103.35 (14)	C6—C7—H7	125.9
C2—C1—H1	111.7	C7—C8—C9	107.53 (17)
C5—C1—H1	111.7	C7—C8—H8	126.2
C9—C1—H1	111.7	C9—C8—H8	126.2
O11—C2—O3	120.96 (17)	C8—C9—C10	100.20 (15)
O11—C2—C1	128.77 (17)	C8—C9—C1	106.48 (14)
O3—C2—C1	110.23 (15)	C10—C9—C1	98.99 (15)
C2—O3—C4	111.26 (14)	C8—C9—H9	116.2
O3—C4—O12	106.58 (13)	C10—C9—H9	116.2
O3—C4—C5	107.23 (14)	C1—C9—H9	116.2
O12—C4—C5	108.17 (15)	C9—C10—C6	93.82 (14)
O3—C4—H4	111.5	C9—C10—H10A	113.0
O12—C4—H4	111.5	C6—C10—H10A	113.0
C5—C4—H4	111.5	C9—C10—H10B	113.0
C4—C5—C1	104.07 (14)	C6—C10—H10B	113.0
C4—C5—C6	117.36 (15)	H10A—C10—H10B	110.4
C1—C5—C6	102.70 (15)	C13—O12—C4	116.33 (15)
C4—C5—H5	110.7	O15—C13—O12	122.91 (18)
C1—C5—H5	110.7	O15—C13—C14	126.55 (19)
C6—C5—H5	110.7	O12—C13—C14	110.53 (16)
C7—C6—C10	100.04 (17)	C13—C14—H14A	109.5
C7—C6—C5	108.53 (16)	C13—C14—H14B	109.5
C10—C6—C5	98.44 (15)	H14A—C14—H14B	109.5
C7—C6—H6	115.8	C13—C14—H14C	109.5
C10—C6—H6	115.8	H14A—C14—H14C	109.5
C5—C6—H6	115.8	H14B—C14—H14C	109.5
C5—C1—C2—O11	179.82 (19)	C1—C5—C6—C10	39.82 (18)
C9—C1—C2—O11	−68.4 (2)	C10—C6—C7—C8	−32.7 (2)
C5—C1—C2—O3	−2.61 (19)	C5—C6—C7—C8	69.9 (2)
C9—C1—C2—O3	109.14 (16)	C6—C7—C8—C9	−0.4 (2)
O11—C2—O3—C4	−169.71 (17)	C7—C8—C9—C10	33.6 (2)
C1—C2—O3—C4	12.50 (19)	C7—C8—C9—C1	−69.0 (2)
C2—O3—C4—O12	98.48 (16)	C2—C1—C9—C8	−44.0 (2)
C2—O3—C4—C5	−17.17 (19)	C5—C1—C9—C8	68.50 (18)
O3—C4—C5—C1	14.44 (18)	C2—C1—C9—C10	−147.52 (16)
O12—C4—C5—C1	−100.16 (16)	C5—C1—C9—C10	−35.02 (17)
O3—C4—C5—C6	−98.20 (18)	C8—C9—C10—C6	−49.74 (17)
O12—C4—C5—C6	147.20 (16)	C1—C9—C10—C6	58.94 (16)
C2—C1—C5—C4	−7.24 (18)	C7—C6—C10—C9	49.44 (17)
C9—C1—C5—C4	−125.89 (15)	C5—C6—C10—C9	−61.21 (17)
C2—C1—C5—C6	115.59 (16)	O3—C4—O12—C13	76.42 (17)
C9—C1—C5—C6	−3.06 (17)	C5—C4—O12—C13	−168.55 (15)
C4—C5—C6—C7	49.6 (2)	C4—O12—C13—O15	10.9 (3)
C1—C5—C6—C7	−63.81 (18)	C4—O12—C13—C14	−168.50 (17)
C4—C5—C6—C10	153.23 (17)