The assignment of the correct structures and conformational analysis of the isomeric t-5- and t-4-phenyl-t(c)-2-benzoyl-r-1-cyclohexanecarboxylic acids by NMR and FT-IR spectroscopy

Abstract

The AlCl₃ catalysed addition of benzene to cis-2-benzoylcyclohex-4-enecarboxylic acid results in the product, t-5-phenyl-c-2-benzoyl-r-1-cyclohexanecarboxylic acid (2), but not in its 2-trans epimer (5) as has been reported previously in the literature. The latter was obtained instead by treating 2 with NaOH in EtOH–H₂O. The structures and conformational assignment of 2, its 4-trans phenyl positional isomer (4), and both their 2-trans benzoyl epimers (5 and 6, respectively, prepared by base-induced epimerisation) were elucidated by means of NMR spectroscopy. The postulated trans→cis transformation of the 2-trans isomer 5 to the product 2 does not in fact occur; indeed it is the reverse transformation that is viable. All of the compounds showed substantial intramolecular hydrogen-bonding in solution (e.g. in CDCl₃) by FT-IR measurements and, furthermore, a dynamic equilibrium between hydrogen-bonded and non hydrogen-bonded forms was observed by NMR for compound 2.