For the elucidation of the role of methyl substituents on cyclohexene ring of abscisic acid (ABA, I) on the physiological activity, 2'-demethyl-ABA (II), the two stereoisomers (III and IV) of 6'-demethyl-ABA and methyl 5-(1, ' 6'-epoxy-2', 2'-dimethylcyclohexyl)-3-methyl-(2Z, 4E)-2, 4-pentadienoate (VI) were synthesized. From the result of rice seedling assay, it was concluded that 2'-methyl group of ABA was indispensable for strong growth inhibitory activities and that at least one of the 6'-gem-dimethyl groups was unnecessary for growth inhibition.

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