Insight into the chemistry of cycloaddition between α-ketol oxylipin and epinephrine: isolation and structure elucidation of a new reaction product

The α-ketol oxylipin (12Z,15Z)-9-hydroxy-10-oxooctadeca-12,15-dienoic acid (1) reacts with epinephrine (2) to afford unique cycloadducts, KE1 (3) and KE2 (4), both of which strongly induce flowering in Lemna paucicostata. Here we isolate compound 5 from the reaction mixture and identify it as a Michael-type adduct of 1 with adrenochrome (6), an oxidized form of 2. Compound 5 was gradually converted into KEs in aqueous solution, suggesting that the compound is an intermediate of KEs. From these results, we newly propose the reaction mechanism of the cycloaddition between 1 and 2.