The Preferred Ring-Tautomeric Form of a Bicyclic γ-Ketocarboxylic Acid: An Equilibrium Driven by Relief of Angular Hybridization Strain

(±)-2-exo-Carboxy-2-endo-methyl-7-oxobicyclo[2.2.1]heptane exists preferentially in its closed, ringtautomer form, the tricyclic lactol (C9H12O3), which aggregates catemerically by forming hydroxyl-to-carbonyl hydrogen bonds [O......O = 2.7667(16) , O-H......O = 170o] among molecules screw related in b. This ring-chain equilibrium is driven by relief of angular strain at the 7-ketone, whose origin is the ketone’s sp2 hybridization vs. the angle enforced by the bicyclic system. In the analogous compound with transposed functional groups, the equilibrium favors the keto acid, which is easily isolated.