(±)-trans-3-Benzoylbicyclo[2.2.2]octane-2-carboxylic acid

The title keto acid, C16H18O3, displays significant twisting of all three ethylene bridges in its bicyclo[2.2.2]octane structure owing to steric interactions; the bridgehead-to-bridgehead torsion angles are 13.14 (12), 13.14 (13) and 9.37 (13)°. The compound crystallizes as centrosymmetric carboxyl dimers [O⋯O = 2.6513 (12) Å and O—H⋯O = 178°], which have two orientations within the cell and contain no significant carboxyl disorder.

The benzoyl group has component parts that are only approximately coplanar [the dihedral angle for C3-C10-C11-O1 versus the aromatic ring = 24.60 (7)°], and is oriented so that the ketone C=O is aimed toward C2.
Although carboxyl dimers frequently display complete or partial averaging of C-O bond lengths and C-C-O angles due to disorder, no significant averaging is observed in (I), where these lengths and angles are similar to those in other highly ordered dimeric carboxyls. Fig. 2 shows the packing for (I), typical for racemic keto acids that are dimeric. Centrosymmetric dimers are centered at 1/2,1/2,1/2 in the chosen cell, with a second screw-related set centered on the a cell edge. No close intermolecular contacts were found within the 2.6 Å range we routinely survey for non-bonded C-H···O packing interactions.
Experimental endo-Bicyclo[2.2.2]oct-5-ene-2,3-dicarboxylic anhydride, purchased from Aldrich Chemical Co., Milwaukee, Wisconsin, USA, was hydrogenated under typical conditions (atmospheric pressure, room temperature, 5%Pd/C, EtOAc) and the isolated product used directly in a Friedel-Crafts acylation of benzene (AlCl 3 ). The cis keto acid initially obtained (mp 446 K) was epimerized by refluxing in excess aqueous KOH (Scribner & Miller, 1965). The isolated trans product (I) was vacuum-distilled and crystallized from acetonitrile to give the crystal used, mp 444 K.
supplementary materials sup-2 The solid-state IR spectrum (KBr) of (I) has C=O absorptions at 1692 (acid) and 1677 cm -1 (ketone), normal for dimerized COOH and for a benzoyl group without H bonding but with significant coplanarity. In CHCl 3 solution these peaks appear at 1702 & 1679 cm -1 .

Refinement
All H atoms for (I) were found in electron density difference maps. The O-H was constrained to an idealized position with its distance fixed at 0.84 Å and U iso (H) = 1.5U eq (O). The methylene, methine and aromatic Hs were placed in geometrically idealized positions and constrained to ride on their parent C atoms with C-H distances of 0.99, 1.00 and 0.95 Å, respectively, and U iso (H) = 1.2U eq (C). Fig. 1. The asymmetric unit with its numbering. Displacement ellipsoids are drawn at the 40% probability level. Fig. 2. A partial packing diagram, with an extracellular molecule included to show the dimers centered at 1/2,1/2,1/2 and 1/2,0,0. For clarity, all C-bound H atoms are omitted. Displacement ellipsoids are drawn at the 40% probability level.