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The importance of the two chiral centers at C-3 and C-7 in the molecular structure of jasmonic acid i n plant responses was investigated. We separated methyl jasmonate (MeJA) into (3R)-and (3S)-isomers with a fixed stereochemistry at (2-3, but epimerization at C-7 is possible. The four isomers of the nonepimerizable analog 7-methyl MeJA were synthesized. These six esters and their corresponding acids were tested in three bioassays: (a) senescence in sunflower (Helianfbus annuus) cotyledons; (b) proteinase inhibitor I1 gene expression in transgenic tobacco (Nicofiana fabacum) with P-glucuronidase as a biochemical reporter; and (c) seed germination in Brassica napus and wheat (Trificum aesfivum). The esters and acids had similar activities in the three assays, with the ester being more effective than its acid. The (3R)-stereochemistry was critical for jasmonate activity. Although activity was reduced after substituting the C-7 proton with a methyl group, the analogs with (3R,7R)-or (3R,7S)-stereochemistry were active i n some of the assays. Although the four isomers of 7-methyl MeJA were inactive or only weakly active in the senescence assay, they could overcome the senescence-promoting effect of (3R)-MeJA. The strongest antagonistic effect was observed with the (3R,7S)-isomer.There is increasing evidence for the involvement of JA and its ester, MeJA, in regulating a wide range of plant physiological processes, including senescence (Ueda and

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This paper is dedicated to Professor Arthur S. Perlin on the occasion of his 67th birthday NANCY LAMB and SUZANNE R. ABRAMS. Can. J. Chem. 68, 1151 (1990) The abscisic acid analogs ( 4-hydroxy-4-(5-hydroxy-3-methylpent-3-en-1-ynyl)-3,3,5-trimethylcyclohexanones were synthesized from a common precursor, (-)-(6R)-2,2,6-trimethylcyclohexan-1,4-dione, which was readily prepared by the fermentation of oxoisophorone with bakers' yeast.