Metabolism of chiral ionylideneacetic acids on the abscisic acid biosynthetic pathway in Cercospora.

A Chiralcel OJ column was used to determine the absolute configuration of naturally occurring alpha-ionylideneacetic acid from Cercospora rosicola and gamma-ionylideneacetic acid from C. cruenta as (R) enantiomers in accordance with their biosynthetic product, (S)-ABA. Both enantiomers of [1, 2-(13)C2]-alpha and gamma-ionylideneacetic acids were prepared and fed to C. rosicola and C. cruenta. Six combinations of feeding experiments comparatively and unequivocally demonstrated stereoselectivity in the biosynthetic conversions, including stepwise hydroxylation at C-1' and 4'. Enzymatic isomerization from the gamma to alpha-intermediate was suggested not to be involved in ABA biosynthesis in C. rosicola.