The stereochemistry of cyclization in abscisic acid

The hydroxylation of the pro-6′-(R)-methyl of (+)-abscisic acid, which then cyclises to phaseic acid, was used to define the origin in mevalonate of the 6′-methyl groups. Abscisic acid (ABA), biosynthesised from 2-¹⁴C, 2-³H₂]-mevalonate, was metabolized to phaseic acid by tomato shoots. The slight loss of ³H] from the phaseate, and to a lesser extent from the ABA, suggested that the unlabelled 6′-methyl was hydroxylated. This was confirmed by Kuhn-Roth oxidation of methyl phaseate to give ¹⁴C, ³H]-acetate. The data also suggest that ABA is converted to dihydrophaseate via free phaseate, the conjugates being formed from each free acid.