Synthesis and biological activity of 3′-chloro, -bromo, and -iodoabscisic acids, and biological activity of 3′-fluoro-8′-hydroxyabscisic acid

(+)-3'-Chloro, -bromo and -iodoabscisic acids were synthesized, and their biological activities and metabolism were examined, as well as that of (+)-3'-fluoroabscisic acid. This was in order to estimate the effects of altered electron density at the C-2' position of abscisic acid on both biological activity and metabolic stability. The biological activities of (+)-3'-chloro acid bromoabscisic acids were similar to, or higher than, those of(+)-abscisic acid in four bioassays, while (+)-3'-iodoabscisic acid was more potent in two bioassays and less potent than (+)-abscisic acid in the other two bioassays. The biological activities of 3'-haloabscisic acids seem to be dependent on the 3'-halogens, but not on the electron density at C-2'. The metabolism of(+)3'-haloabscisic acids to ca. 50% of their original levels in the media of rice cell suspension culture was similar to that of(+)abscisic acid. Metabolites from (+)-3'-chloro, -bromo and -iodoabscisic acids were not found in the culture media and cells, while (+)-3'-fluoroabscisic acid gave (+)-3'-fluoro-8'-hydroxyabscisic acid as a stable metabolite. (+)-3'-fluoro-8'- hydroxyabscisic acid showed biological activities lower than (+)-3'-fluoroabscisic acid, but higher than (-)-3'alpha-Fluorophascsic acid. These findings suggested that the biological activity of (+)-8'-hydroxyabscisic acid is intermediate between those of (+)abscisic acid and (-)-phaseic acid. Abscisic acid may be inactivated in a stepwise manner by metabolism in plants. (C) 1999 Elsevier Science Ltd. All rights reserved.