Conformational Analysis of the Cyclohexenone Ring in Abscisic Acid and its Analogs with a Fused Cyclopropyl Ring

The conformational behavior of the cyclohexenone ring in abscisic acid (1) and its analogs (2–5) with a fused cyclopropyl ring was investigated by a low-temperature NMR analysis and computer-aided calculations of the model compounds. The 1H signals of 1 separated into two sets below 250 K, which was a coalescence temperature, in a 99.4:0.6 ratio at 185 K, although the small set of signals was not complete. The large and small sets of signals were considered to correspond to an envelope 1-i with the axial side-chain and another envelope 1-ii with the equatorial side-chain. At 185 K, the free-energy barrier for the ring inversion between 1-i and 1-ii was determined to be ca. 11 kcal/mol, and the free-energy difference between the two conformers was ca. 1.4 kcal/mol. The 1H signals of 2–5 never broadened even at 200 K, suggesting that these analogs are more flexible than 1. These experimental results were consistent with those obtained by calculations using models 6–10. Introducing a cyclopropyl group into the ring of 1 lowered the energy barrier for ring inversion and varied the conformational ratio at equilibrium between the minimum-energy conformers.