The Open Crystallography Journal

2008, 1 : 56-58
Published online 2008 November 21. DOI: 10.2174/1874846500801010056
Publisher ID: TOCRYJ-1-56

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

Jesse K. Wong , Roger A. Lalancette and Hugh W. Thompson
Rutgers University, Department of Chemistry, Newark NJ 07102, USA.

ABSTRACT

(±)-2-exo-Carboxy-2-endo-methyl-7-oxobicyclo[2.2.1]heptane exists preferentially in its closed, ringtautomer form, the tricyclic lactol (C9H21O3), which aggregates catemerically by forming hydroxyl-to-carbonyl hydrogen bonds […O = 2.7667(16)Å, O-H…O = 170°] 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.

Keywords:

Hydrogen bonding, angle strain, keto carboxylic acids, ring-chain tautomerism, X-ray crystallography.