(2SR,8aSR)-1,2,3,4,6,7,8,8a-Octa­hydro-6-oxonaphthalene-2-acetic acid: hydrogen bonding in an unsaturated bicyclic keto acid

The title racemate, C₁₂H₁₆O₃, aggregates in the solid state as translational acid-to-ketone hydrogen-bonded catemers. The stereochemistry of the side chain relative to the ring junction arises spontaneously during the synthesis. The four hydrogen-bonding chains passing through the unit cell are of alternating handedness, and are aligned along the [101] direction. Starting at the origin, the order of the directional alignment of these four chains with respect to the a axis is ++−−. One inter­molecular C—H...O=C close contact exists to the carboxyl O atom.

Key indicators

• Single-crystal X-ray study
• T = 100 K
• R factor = 0.038
• wR factor = 0.104
• Data-to-parameter ratio = 13.6

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S1. Comment

Among the five hydrogen-bonding modes known for ketocarboxylic acids, the commonest involves centrosymmetric dimerization. However, we have shown that when centrosymmetry is thwarted or disfavored, the frequency of acid-to-ketone catemers increases. Thus, among chiral non-racemates this chain-mode actually predominates, and its occurrence also rises markedly among conformationally constrained molecules. The latter is the case for compound (I), whose structure and hydrogen-bonding pattern we report here.

Fig. 1 shows the asymmetric unit, whose only conformational options lie in the side-chain. The C2—C9 staggering requires that C10 be involved in a gauche interaction with an equatorial proton either at C1 or at C3. Compared to alkane systems in which all centers are tetrahedral, such gauche arrangements are less serious here because the sp² hybridization at the carboxyl diminishes the steric repulsions involved. Within the asymmetric unit, any energy advantage to the arrangement actually found [torsion angle C3—C2—C9—C10 = 64.16 (15)°] appears negligible and the observed choice is probably dictated by packing considerations. The remaining available rotation yields a C2—C9—C10—O2 torsion angle of 38.58 (19)° for the carboxyl group.

The disordering of bond lengths and angles often seen in carboxyl dimers is not possible in (I), which is not dimeric. Thus its C—O bond lengths [O2—C10 = 1.2054 (18) and O3—C10 = 1.3213 (17) Å] and C—C—O angles [O2—C10—C9 = 124.23 (12) and O3—C10—C9 = 112.41 (12) °] are typical of those in highly ordered dimeric carboxyls (Borthwick, 1980).

Fig. 2 shows the centrosymmetric packing of the cell, with extra molecules included to illustrate the acid-to-ketone hydrogen-bonding scheme. Each carboxylic acid is linked to the ketone in a molecule translationally related in both the a and c directions, so that the chains advance at an angle to the cell axes, along the [101] direction. The four hydrogen-bonding chains passing through the cell are of alternating handedness. Starting at the origin, the order of the directional alignment of these four chains with respect to the a axis is + + - -. The racemate aggregates in the solid as translational acid-to-ketone hydrogen-bonding catemers [O···O = 2.6793 (14)Å and O—H···O = 163°].

We characterize the geometry of H bonding to carbonyls using a combination of the H···O=C angle and the H···O=C—C torsion angle. These describe the approach of the H atom to the receptor O in terms of its deviation from, respectively, C=O axiality (ideal = 120°) and planarity with the carbonyl (ideal = 0°). In (I) these angles are 124.8 & 0.1°.

One intermolecular C—H···O close contact was found involving the carboxyl oxygen [H7B—O2 = 2.53; C7—O2 = 3.4758 (17) Å; C7—H7B—O2 = 161 °]. This distance lies within the 2.6-Å range we standardly survey for such packing interactions (Steiner, 1997).

Related literature

For related literature, see: Borthwick (1980); House et al. (1965); Steiner (1997); Stork et al. (1963).

S2. Experimental

Compound (I) has not previously been reported. The methyl ester of cyclohexanone-4-acetic acid (see Acknowledgments) was converted to its pyrrolidine enamine and then treated with methyl vinyl ketone as described by Stork et al. (1963). The usual workup and isolation yielded directly the crystalline methyl ester of (I), which was then saponified. Crystals of (I) suitable for X-ray were obtained from ether/CH₂Cl₂, mp 423 K. The stereochemistry obtained for C4a versus C6 arises during the synthesis, probably as the result of equilibrations occurring during saponification or earlier (House et al., 1965), as (I) is clearly the stabler of the two epimers possible.

The solid-state (KBr) infrared spectrum of (I) has C=O absorptions at 1724 & 1633 cm^-1, with a peak separation typical of the shifts seen in catemers, due, respectively, to removal of H bonding from the acid C=O and addition of H bonding to the ketone; an alkene shoulder appears at ca 1618 cm^-1. In CHCl₃ solution, where dimers predominate, these bands appear, respectively, at 1710, 1662 and 1621 cm^-1.

S3. Refinement

All H atoms for (I) were found in electron density difference maps. The O—H was constrained to an idealized position with distance fixed at 0.84 Å and U_iso(H) = 1.5U_eq(O). The methylene and methine Hs were placed in geometrically idealized positions and constrained to ride on their parent C atoms with C—H distances of 0.99 and 1.00 Å, respectively, and U_iso(H) = 1.2U_eq(C).

Computing details

Data collection: APEX2 (Bruker, 2006); cell refinement: APEX2; data reduction: SAINT (Bruker, 2005); program(s) used to solve structure: SHELXTL (Sheldrick, 2004); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL.

Figures

	Fig. 1. The asymmetric unit of (I), with its numbering. Displacement ellipsoids are shown at the 40% probability level.
	Fig. 2. A packing diagram with extracellular molecules, illustrating the four translational acid-to-ketone hydrogen-bonding chains of alternating handedness. The molecules are aligned along the [101] direction in the cell. All carbon-bound H atoms are removed for clarity. Displacement ellipsoids are shown at the 40% probability level.

(2SR,8aSR)-1,2,3,4,6,7,8,8a-Octahydro-6-oxonaphthalene-2-acetic acid

Crystal data

C12H16O3	F(000) = 448
Mr = 208.26	Dx = 1.292 Mg m^−3
Monoclinic, P21/c	Melting point: 423 K
Hall symbol: -P 2ybc	Cu Kα radiation, λ = 1.54178 Å
a = 7.5832 (2) Å	Cell parameters from 5950 reflections
b = 16.9532 (4) Å	θ = 5.9–68.1°
c = 8.3785 (2) Å	µ = 0.75 mm^−1
β = 96.323 (1)°	T = 100 K
V = 1070.58 (5) Å^3	Block, colourless
Z = 4	0.33 × 0.28 × 0.22 mm

Data collection

Bruker SMART APEXII CCD area-detector diffractometer	1879 independent reflections
Radiation source: fine-focus sealed tube	1818 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.017
φ and ω scans	θmax = 68.1°, θmin = 5.9°
Absorption correction: multi-scan (SADABS; Sheldrick, 2001)	h = −9→9
Tmin = 0.790, Tmax = 0.853	k = −18→19
5950 measured reflections	l = −10→9

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)] = 0.038	H-atom parameters constrained
wR(F^2) = 0.104	w = 1/[σ^2(Fo^2) + (0.0518P)^2 + 0.5351P] where P = (Fo^2 + 2Fc^2)/3
S = 1.05	(Δ/σ)max < 0.001
1879 reflections	Δρmax = 0.24 e Å^−3
138 parameters	Δρmin = −0.18 e Å^−3
0 restraints	Extinction correction: SHELXTL (Sheldrick, 2004), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.0015 (4)

Special details

Experimental. crystal mounted on cryoloop using Paratone-N

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
O1	0.33270 (13)	0.62327 (6)	1.12458 (11)	0.0253 (3)
C1	0.51750 (16)	0.66286 (8)	0.51501 (15)	0.0199 (3)
H1A	0.4298	0.6718	0.4202	0.024*
H1B	0.5558	0.7151	0.5586	0.024*
O2	0.90901 (13)	0.56090 (6)	0.22963 (14)	0.0340 (3)
C2	0.67798 (17)	0.62052 (8)	0.46056 (15)	0.0197 (3)
H2A	0.6371	0.5698	0.4079	0.024*
O3	1.04100 (14)	0.67826 (6)	0.24051 (14)	0.0346 (3)
H3	1.1187	0.6536	0.1965	0.052*
C3	0.81062 (17)	0.60176 (8)	0.60609 (16)	0.0224 (3)
H3A	0.8576	0.6517	0.6553	0.027*
H3B	0.9115	0.5718	0.5707	0.027*
C4A	0.55889 (17)	0.59049 (8)	0.77883 (16)	0.0199 (3)
C4	0.72564 (17)	0.55353 (9)	0.73159 (16)	0.0237 (3)
H4A	0.6977	0.5001	0.6882	0.028*
H4B	0.8120	0.5477	0.8284	0.028*
C5	0.52730 (17)	0.59475 (8)	0.93349 (16)	0.0212 (3)
H5A	0.6199	0.5814	1.0147	0.025*
C6	0.35638 (18)	0.61914 (8)	0.98126 (16)	0.0205 (3)
C7	0.20938 (17)	0.63442 (8)	0.84973 (16)	0.0230 (3)
H7A	0.1229	0.6715	0.8888	0.028*
H7B	0.1467	0.5844	0.8203	0.028*
C8A	0.42640 (16)	0.61828 (8)	0.64259 (15)	0.0194 (3)
H8AA	0.3684	0.5705	0.5902	0.023*
C8	0.28119 (17)	0.66910 (8)	0.70197 (16)	0.0216 (3)
H8A	0.1826	0.6747	0.6149	0.026*
H8B	0.3292	0.7224	0.7282	0.026*
C9	0.76216 (18)	0.67094 (8)	0.33847 (17)	0.0231 (3)
H9A	0.6693	0.6862	0.2515	0.028*
H9B	0.8092	0.7199	0.3918	0.028*
C10	0.91038 (17)	0.62978 (8)	0.26540 (15)	0.0205 (3)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1	0.0254 (5)	0.0284 (6)	0.0232 (5)	−0.0011 (4)	0.0073 (4)	−0.0009 (4)
C1	0.0182 (6)	0.0209 (7)	0.0206 (6)	0.0022 (5)	0.0016 (5)	0.0008 (5)
O2	0.0282 (6)	0.0243 (6)	0.0520 (7)	−0.0004 (4)	0.0152 (5)	−0.0066 (5)
C2	0.0183 (6)	0.0192 (7)	0.0221 (7)	0.0009 (5)	0.0037 (5)	0.0004 (5)
O3	0.0302 (6)	0.0287 (6)	0.0489 (7)	−0.0064 (4)	0.0218 (5)	−0.0073 (5)
C3	0.0166 (6)	0.0264 (7)	0.0248 (7)	0.0032 (5)	0.0040 (5)	0.0008 (5)
C4A	0.0181 (6)	0.0167 (7)	0.0248 (7)	−0.0002 (5)	0.0027 (5)	0.0019 (5)
C4	0.0200 (7)	0.0275 (8)	0.0238 (7)	0.0062 (5)	0.0031 (5)	0.0040 (6)
C5	0.0187 (6)	0.0207 (7)	0.0238 (7)	0.0008 (5)	0.0011 (5)	0.0024 (5)
C6	0.0221 (7)	0.0155 (7)	0.0244 (7)	−0.0028 (5)	0.0053 (5)	−0.0003 (5)
C7	0.0166 (6)	0.0259 (7)	0.0272 (7)	0.0010 (5)	0.0053 (5)	−0.0007 (5)
C8A	0.0167 (6)	0.0196 (7)	0.0220 (7)	0.0003 (5)	0.0022 (5)	−0.0014 (5)
C8	0.0156 (6)	0.0245 (7)	0.0247 (7)	0.0027 (5)	0.0017 (5)	0.0007 (5)
C9	0.0224 (7)	0.0217 (7)	0.0260 (7)	0.0018 (5)	0.0061 (5)	0.0009 (5)
C10	0.0197 (7)	0.0221 (8)	0.0195 (6)	0.0004 (5)	0.0016 (5)	0.0023 (5)

Geometric parameters (Å, º)

O1—C6	1.2356 (17)	C4A—C8A	1.5105 (18)
C1—C2	1.5253 (17)	C4—H4A	0.9900
C1—C8A	1.5341 (18)	C4—H4B	0.9900
C1—H1A	0.9900	C5—C6	1.4578 (18)
C1—H1B	0.9900	C5—H5A	0.9500
O2—C10	1.2054 (18)	C6—C7	1.5012 (19)
C2—C9	1.5260 (18)	C7—C8	1.5243 (18)
C2—C3	1.5263 (18)	C7—H7A	0.9900
C2—H2A	1.0000	C7—H7B	0.9900
O3—C10	1.3213 (17)	C8A—C8	1.5241 (17)
O3—H3	0.8400	C8A—H8AA	1.0000
C3—C4	1.5293 (18)	C8—H8A	0.9900
C3—H3A	0.9900	C8—H8B	0.9900
C3—H3B	0.9900	C9—C10	1.5088 (18)
C4A—C5	1.3454 (19)	C9—H9A	0.9900
C4A—C4	1.5029 (18)	C9—H9B	0.9900
C2—C1—C8A	114.33 (11)	O1—C6—C5	120.78 (12)
C2—C1—H1A	108.7	O1—C6—C7	121.87 (12)
C8A—C1—H1A	108.7	C5—C6—C7	117.27 (11)
C2—C1—H1B	108.7	C6—C7—C8	111.10 (11)
C8A—C1—H1B	108.7	C6—C7—H7A	109.4
H1A—C1—H1B	107.6	C8—C7—H7A	109.4
C1—C2—C9	110.08 (11)	C6—C7—H7B	109.4
C1—C2—C3	109.66 (11)	C8—C7—H7B	109.4
C9—C2—C3	111.39 (11)	H7A—C7—H7B	108.0
C1—C2—H2A	108.5	C4A—C8A—C8	111.96 (11)
C9—C2—H2A	108.5	C4A—C8A—C1	111.54 (10)
C3—C2—H2A	108.5	C8—C8A—C1	110.24 (11)
C10—O3—H3	109.5	C4A—C8A—H8AA	107.6
C2—C3—C4	111.73 (11)	C8—C8A—H8AA	107.6
C2—C3—H3A	109.3	C1—C8A—H8AA	107.6
C4—C3—H3A	109.3	C8A—C8—C7	112.50 (11)
C2—C3—H3B	109.3	C8A—C8—H8A	109.1
C4—C3—H3B	109.3	C7—C8—H8A	109.1
H3A—C3—H3B	107.9	C8A—C8—H8B	109.1
C5—C4A—C4	121.35 (12)	C7—C8—H8B	109.1
C5—C4A—C8A	122.47 (12)	H8A—C8—H8B	107.8
C4—C4A—C8A	116.10 (11)	C10—C9—C2	113.47 (11)
C4A—C4—C3	112.87 (11)	C10—C9—H9A	108.9
C4A—C4—H4A	109.0	C2—C9—H9A	108.9
C3—C4—H4A	109.0	C10—C9—H9B	108.9
C4A—C4—H4B	109.0	C2—C9—H9B	108.9
C3—C4—H4B	109.0	H9A—C9—H9B	107.7
H4A—C4—H4B	107.8	O2—C10—O3	123.33 (12)
C4A—C5—C6	122.52 (12)	O2—C10—C9	124.23 (12)
C4A—C5—H5A	118.7	O3—C10—C9	112.41 (12)
C6—C5—H5A	118.7

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O3—H3···O1^i	0.84	1.86	2.6793 (14)	163
C7—H7B···O2^ii	0.99	2.53	3.4758 (17)	161

Symmetry codes: (i) x+1, y, z−1; (ii) −x+1, −y+1, −z+1.