(+)- and (+/-)-ketopinic acid: hydrogen-bonding patterns in a beta-keto acid in its enantiomeric and racemic forms and enantiomeric disordering in the racemate

Crystal structures have been determined for both (+)- and (+/-)-7,7-dimethy l-2-oxobicyclo[2.2.1]heptane-1-carboxylic acid, C10H14O3. The asymmetric un it of the (+)-form, (I), contains two molecules paired by mutual hydrogen b onding of their carboxyl groups, without ketone involvement. Both carboxyl groups are disordered and the conformation of each relative to its own mole cule differs by 46.3(4)degrees for the two halves of the dimer. In the race mate, (II), the asymmetric unit also contains two molecules, each of which dimerizes with a centrosymmetric counterpart by mutual hydrogen bonding of carboxyl groups across the a or b cell edge, also without ketone involvemen t. Besides disorder of the carboxyl C-O lengths and angles, each half of th e asymmetric unit of (II) displays extensive enantiomeric disorder, corresp onding to transposition of the ethylene and methylene bridges, with the car boxyl and ketone C atoms plus the anti-methyl C atom exactly coinciding. Ea ch contributor to the structure of (II) has a different rotational conforma tion for its carboxyl group, none corresponding to those found in (I).