Ring Conformational Requirement for Biological Activity of Abscisic Acid Probed by the Cyclopropane Analogues.

Abstract For investigating ring conformational requirement for the biological activity of abscisic acid, the cyclopropane analogues 6, 7, 9 , and 10 were synthesized and their biological activities in four bioassays were tested. The activity of the achiral cyclohexadienone analogue 8 also was examined. Analogue 7 in which the 6′ β-substituent is constrained essentially to the axial-like orientation between axial and bisectional showed no activity, while 6 and 8 with no axial-like substituent at C-6′β independent of the conformational preference exhibited the equivalent activity to abscisic acid. This result suggested that the axial substituent at the β-side of the ring is fatal to the activity. The active conformation of abscisic acid would be a conformation where C-9′ is equatorial and the side chain is between pseudo-axial and bisectional, that is, close to the favored half-chair with the side chain pseudo-axial rather than the less favored half-chair with the side chain pseudo-equatorial.