Probing the role of the hydroxyl group of ABA: Analogues with a methyl ether at C-1'

(+)-(S)- and (-)-(R)-C-1'-O-methyl abscisic acids and their methylesters, as well as the methyl ethers of the acetylenic analogue of methyl ABA, were synthesized through an enantioselective route, giving a series of optically active, new C-1' substituted analogues with known stereochemistry. In a wheat embryo germination inhibition assay, (-)-C-1'-O-methyl ABA shows high activity, comparable with(+)- and (-)-ABA, whereas(+)-C-1'-O-methyl ABA is less active. In a wheat seedling transpiration assay, both analogues exhibit weak activity although the (+)-ABA-like analogue is more potent than its enantiomer. The anti-transpirant response increases over time, which may indicate that the analogue is being metabolized to ABA in vivo.