Easy Preparation of Methyl 7-epi-Jasmonate and Four Stereoisomers of Methyl Cucurbate, and Assessment of the Stereogenic Effect of Jasmonate on Phytohormonal Activities.

To test the stereogenic effect of jasmonate on phytohormonal activities, methyl 7-epi-jasmonate (1b) and four stereoisomers of methyl cucurbate were easily prepared in racemic form: epimerization at the C-7 position of a commercially available methyl jasmonate (2b) with a base and subsequent fractional distillation gave a 46:54 mixture of 1b and 2b, whose reduction gave a mixture of methyl cucurbates (3-6), This synthetic chemistry was supplemented by molecular modeling and an NMR study on 1b and 2b. An assessment of the inhibitory activities of the prepared jasmonates on growth of the second leaf sheath of rice and on seed germination of cress clarified that the cis-configuration of the C-3 and C-7 side chains of jasmonate was an important factor for the high activities. In inhibiting the seed germination of cress, methyl 6-epi-cucurbate (4) exhibited activity that was markedly higher than the other compounds tested, showing that the stereochemistry at C-6 as well as at C-3 and C-7 was strictly recognized by this bioassay.