Conformational analysis of the cyclohexenone ring in abscisic acid and itsanalogs 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 l ow-temperature NMR analysis and computer-aided calculations of the model co mpounds. The H-1 signals of 1 separated into two sets below 250 K, which wa s a coalescence temperature, in a 99.4:0.6 ratio at 185 K, although the sma ll set of signals was not complete. The large and small sets of signals wer e 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-e nergy 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 conform ers was ca. 1.4 kcal/mol. The H-1 signals of 2-5 never broadened even at 20 0 K, suggesting that these analogs are more flexible than 1. These experime ntal results were consistent with those obtained by calculations using mode ls 6-10. Introducing a cyclopropyl group into the ring of 1 lowered the ene rgy barrier for ring inversion and varied the conformational ratio at equil ibrium between the minimum-energy conformers. (C) 2000 Elsevier Science Ltd . All rights reserved.