Several compounds having the basic α-ionylideneacetic acid structure were tested in Cercospora rosicola resuspensions. At 100μM, all the compounds inhibited abscisic acid (ABA) biosynthesis. Time studies with unlabelled and deuterated (2Z, 4E)- and (2E, 4E)-α-ionylideneacetic acids showed rapid conversions into both (2Z, 4E)- and (2E, 4E)-4'-keto-α-ionylideneacetic acids as major products. Incorporation of the label into ABA was specific for the 2Z, 4E-isomer. Minor products, identified by GC-MS, were (2Z, 4E)- and (2E, 4E)-4'-hydroxy-α-ionylideneacetic acids and (2Z, 4E)-1'-hydroxy-α-ionylideneacetic acid. The conversion to (2Z, 4E)-1'-hydroxy-α-xionylideneacetic acid has not been previously reported and was specific for the 2Z, 4E-isomer. A time study for the conversion of methyl esters of [²H₃]-(2Z, 4E)- and [²H₃]-(2E, 4E)-4'-keto-α-ionylideneacetates showed a slow introduction of the 1'-hydroxyl group and specificity for 2Z, 4E-isomer. Conversion of the ethyl esters of (2Z, 4E)- and (ZE, 4E)-1'-hydroxy-α-ionylideneacetates into the ethyl esters of both ABA and (2E, 4E)-ABA demonstrated that ABA can be formed by oxidation of the 4'-position after the insertion of the 1'-hydroxy group. The ethyl 1'-hydroxy acids were also isomerized to the corresponding ethyl (2Z, 4E)- and ethyl (2E, 4E)-3'-hydroxy-β-ionylideneacetates. Ethyl (2Z, 4E)-1'-hydroxy acid also gave small amounts of ethyl 1', 4'-trans-diol of ABA. These results suggest that ABA may be formed through a (2Z, 4E)-1'-hydroxy-α-ionylidene-type intermediate in addition to the previously proposed route through (2Z, 4E)-4'-ketoa-α-ionylideneacetic acid.

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