Electrolytic reduction of abscisic acid methyl ester and its free acid

Abstract Abscisic acid (ABA, 1 ), a plant hormone, has electrophilicity derived almost entirely from the side-chain, 3-methylpenta-2,4-dienoic acid. The electrochemical property of ABA was investigated by analysis of its cathodic reaction. ABA methyl ester ( 1 - Me ) was reduced at a peak potential of −1.6 V to give a unique and unstable bicyclic compound ( 5 - Me ) as a major product at pH 3 and 7. This finding showed that an electron was absorbed in the conjugated dienecarboxyl group, and that C-5 with a high electron density attacked C-2′ through an intramolecular nucleophilic addition. At pH 10, in addition to 5 - Me , a compound 4 - Me was formed by isomerization of 5 - Me under alkaline conditions. For a cathodic reaction of ABA at pH 3 and 7, compound 5 was a major product as well as in the case of ABA methyl ester. However, at pH 10, a dimer ( 6 ) with an epoxy group, 1′-deoxy-ABA ( 7 ) and other compounds were formed instead of compounds 4 and 5 . Compounds 4 and 5 were biologically inactive, suggesting the importance of the electrophilic side-chain of ABA for biological activity.