OXIDATION OF THE 8'-POSITION OF A BIOLOGICALLY-ACTIVE ABSCISIC-ACID ANALOG

The metabolism of a biologically active abscisic acid (ABA) analogue, (+)-(1'S,2'S)-2',3'-dihydroabscisic acid tadienyl)-4-hydroxy-3,3,5-tri methylcyclohexanone], which cannot form phaseic acid was studied in a cell suspension culture of bromegrass (Bromus inermis). The analogue i nduced freezing tolerance in bromegrass cells similar to (S)-(+)-ABA a nd was oxidized to (1'S,2'S)-8'-hydroxy-2',3'-dihydroabscisic acid ydr oxy-3-hydroxymethyl-3,5-dimethylcyclohexanone]. The enantiomer (-)-(1' R,2'R)-2',3'-dihydroabscisic acid, which was inactive in inducing free zing tolerance, was converted to two metabolites, viz., the correspond ing enantiomeric (1'R,2'R)-8'-hydroxy-2',3'-dihydroabscisic acid hydro xy-3-hydroxymethyl-3,5-dimethylcyclohexanone] and reduction product ,3 -butadienyl)-2,2,6-trimethylcyclohexan-1,4-diol. Both enantiomers of t he analogue were transformed to the glucose esters. The structures of the metabolites were established by both spectroscopic analyses and ch emical syntheses. The metabolite 8'-hydroxy-2',3'-hydroxyabscisic acid is a cyclohexanone analogue of 8'-hydroxyabscisic acid, the transient oxidation product in the metabolism of ABA to phaseic acid. Like phas eic acid, the racemic form of 8'-hydroxy-2',3'-dihydroabscisic acid wa s inactive in inducing freezing tolerance in bromegrass cells, suggest ing that ABA and (+)-(1'S,2'S)-2',3'-dihydroabscisic acid were the act ive compounds.