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

摘要

(±)-2-exo-Carboxy-2-endo-methyl-7-oxobicyclo[2.2.1]heptane exists preferentially in its closed, ringtautomerform, the tricyclic lactol (C9H21O3), which aggregates catemerically by forming hydroxyl-to-carbonyl hydrogenbonds […O = 2.7667(16)?, O-H…O = 170°] among molecules screw related in b. This ring-chain equilibrium is drivenby relief of angular strain at the 7-ketone, whose origin is the ketone’s sp2 hybridization vs. the angle enforced by the bicyclicsystem. In the analogous compound with transposed functional groups, the equilibrium favors the keto acid, whichis easily isolated.