Synthesis and biological activity of 3 '-chloro, -bromo, and -iodoabscisicacids, and biological activity of 3 '-fluoro-8 '-hydroxyabscisic acid

(+)-3'-Chloro, -bromo and -iodoabscisic acids were synthesized, and their b iological 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'-chlor o acid bromoabscisic acids were similar to, or higher than, those of(+)-abs cisic acid in four bioassays, while (+)-3'-iodoabscisic acid was more poten t in two bioassays and less potent than (+)-abscisic acid in the other two bioassays. The biological activities of 3'-haloabscisic acids seem to be de pendent on the 3'-halogens, but not on the electron density at C-2'. The me tabolism 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'-f luoro-8'- hydroxyabscisic acid showed biological activities lower than (+)- 3'-fluoroabscisic acid, but higher than (-)-3'alpha-Fluorophascsic acid. Th ese findings suggested that the biological activity of (+)-8'-hydroxyabscis ic acid is intermediate between those of (+)abscisic acid and (-)-phaseic a cid. Abscisic acid may be inactivated in a stepwise manner by metabolism in plants. (C) 1999 Elsevier Science Ltd. All rights reserved.