(2SR,4aSR,8aSR)-6-Oxoperhydronaphthalene-2-carboxylic acid

In the title racemic compound, C11H16O3, the molecule adopts a conformation that places its carboxyl group in an equatorial position. Molecules aggregate by hydrogen-bond pairing of carboxyl groups, yielding centrosymmetric dimers that are arranged into layers in the (020) planes.

H atoms treated by a mixture of independent and constrained refinement Á max = 0.24 e Å À3 Á min = À0.18 e Å À3 Table 1 Hydrogen-bond geometry (Å , ). Our study of H-bonding modes in crystalline ketocarboxylic acids includes a variety of examples based on the naphthalene skeleton. Many of these are accessible from cyclohexanones via annulation reactions yielding enones, from which subsequent alkene reduction may then provide additional isoskeletal keto acids. The title racemate is the reduction product of an unsaturated keto acid whose structure we have previously reported (Efthimiopoulos et al., 2008), and we have also reported the structure of the isoskeletal trans isomer (Lalancette et al., 2007). Fig. 1 shows the molecular structure and conformation. The unfavorable alternative conformer would place the carboxyl group at C2 not only on an axial bond, but inside the molecule's C-shaped curvature, while the observed conformer orients the carboxyl equatorially, with far less strain due to hindrance. This leaves as the only conformational option the rotational orientation of the carboxyl, which is turned so that the O2-C9-C2-C1 torsion angle is 31.57 (17) Å, presumably minimizing steric interactions with nearby H atoms at C1, C2 and C3. Although disorder-averaging of C-O bond lengths and C-C-O angles is common in carboxyl dimers, this is not observed in the current structure, where these values conform to ones typical for highly ordered cases (Borthwick, 1980). dimers are centered at 1/2,1/2,1/2 and, in a second orientation, at 0,0,0. No intermolecular close contacts were found within the 2.6 Å range we standardly survey for C-H···O packing interactions (Steiner, 1997).

Experimental
The title compound was synthesized from the isoskeletal unsaturated keto acid we have previously reported (Efthimiopoulos et al., 2008), by room-temperature catalytic hydrogenation over 5% Pd/C in absolute EtOH at atmospheric pressure. This yielded the expected cis stereochemistry, and recrystallization from methyl pivalate gave material suitable for X-ray diffraction analysis (mp 399 K). The solid-state (KBr) infrared spectrum features a single broad peak at 1705 cm -1 for both C=O functions, typical of unstrained carboxyl-paired keto acids. In CHCl 3 solution this combined absorption is seen at 1706 cm -1 .

Refinement
All H atoms were visible in Fourier difference maps. The position of the acid H was allowed to refine with its displacement parameter fixed at U iso (H) = 1.5U eq (O). The methylene and methine H atoms 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).
supplementary materials sup-2 Figures   Fig. 1. Molecular structure showing displacement ellipsoids at the 40% probability level for non-H atoms. Fig. 2. Partial packing diagram illustrating the pairing of molecules into centrosymmetric dimers centered at 1/2,1/2,1/2 and 0,0,0. All carbon-bound H atoms are omitted and displacement ellipsoids are drawn at the 40% probability level for non-H atoms.
(2SR,4aSR,8aSR)-6-Oxoperhydronaphthalene-2-carboxylic acid Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.