Molecular Structure and Gibberellin Activity. II.* Further Studies on Helminthosporic Acid Analogues

Abstract

The gibberellin-like activity of twelve synthetic helminthosporins has been examined in the barley endosperm bioassay. (±)-14-Norhelminthospor-7(12)-en-13-oic acid (5) was more potent than GA*9 and elicited similar responses in the assay measuring production of a-amylase, sugar and refractive compounds. Responses to acid (5) and GA*3 were retarded similary by abscisic acid. Log-doseresponse curves after 41 and 65 h incubation were of two types: linear with two C4-cyclohexyl helminthosporins and GA*3; and biphasic with two norhelminthosporins, showing initially a shallow curve then a steep slope at high concentrations. Structure-activity comparisons reveal that an alkyl group at C7 on a helminthosporin bridge contributes significantly to activity, but an olefinic bond has a minor effect. Three different orientations of the bridge carboxyl group did not materially alter activity. Oxygenated cyclohexyl helminthosporins had less biological activity than the deoxygenated analogue, perhaps reflecting a critical orientation of a hydroxyl group in the A ring of the more potent gibberellins.