Crystallographic and spectroscopic study of the keto-acid–hydroxy-lactone isomerization path

Abstract

I.r. and n.m.r. spectroscopic evidence shows that 3-hydroxy-trans-bicyclo[4.4.0]decane-1,3-carbolactone (2a) and its keto-acid isomer coexist in solution as a rapidly interconverting equilibrium mixture. Results of crystal structure analyses of this carbolactone and of three related compounds, 3-methoxy-trans-bicyclo[4.4.0]decane-1,3-carbolactone (2b), methyl 3-oxo-trans-bicyclo[4.4.0]decane-1-carboxylate (3b), and 3-oxo-cis-bicyclo[4.4.0]decane-1-carboxylic acid (4), are described. The hydroxy- and methoxy-lactones display a pattern of bond-length and -angle deformations corresponding to an initial stage of the ring-opening reaction. On the ring-opened side, the keto-methyl ester and the keto-acid show structural features (short O ⋯ CO distance, slight pyramidalization of the keto-group) corresponding to displacement along the reverse, ring-closure reaction pathway. The hydrogen-bonding patterns found in the hydroxy-lactone and the keto-acid correspond to incipient stages of the proton-transfer processes that accompany ring-opening and -closure.