Conformational Analysis of Abscisic Acid Analogs Produced by Cercospora cruenta.

The conformation of hydroxy-γ-ionylideneacetic acids produced by Cercospora cruenta was examined by 'H-NMR analysis. (1)H-(1)H COSY and NOE data indicated that (2Z,4E,1'R,4'#)-4'-hydroxy-(γ)- ionylideneacetic acid has a cyclohexane ring of chair form furnishing a hydroxyl group and a 2,4-pentadienoic acid moiety, each with equatorial orientation, while (2Z,4E,1'S,3'S)-3'-hydroxy-γ-ionylideneacetic acid has a flexible twisted-boat form of cyclohexane ring. It is suggested that the structure of another (2Z,4E,1'S,4'R)-isomer of 4'-hydroxy-γ-ionylideneacetic acid with an equatorial hydrogen at the l'-position is the key for further oxygenation to abscisic acid.