title: Synthesis and bioactivity of C-2 and C-3 methyl ether derivatives of brassinolide.
abstract: The following six novel methyl ether derivatives of brassinolide were prepared and their brassinosteroid activity was measured by means of the rice leaf lamina inclination bioassay: 2-O-methylbrassinolide, 3-O-methylbrassinolide, 2,22,23-tri-O-methylbrassinolide, 3,22,23-tri-O-methylbrassinolide, 2-O-methyl-25-methoxybrassinolide and 3-O-methyl-25-methoxybrassinolide. Brassinolide was used as a standard for comparison. All six compounds were also tested in the presence of 1000 ng of indole-3-acetic acid (IAA), an auxin that synergizes the effects of brassinosteroids. The 2-O-methyl- and 3-O-methylbrassinolide derivatives showed weak activity at high doses, which was enhanced by IAA, especially in the case of the 3-O-methyl derivative. Similarly, the 2,22,23-tri-O-methyl- and 3,22,23-tri-O-methyl derivatives displayed weak bioactivity on their own, but significantly stronger activity when applied with IAA. The 3-O-methyl and 3,22,23-tri-O-methyl analogues plus IAA were comparable in bioacivity to brassinolide alone, but were less active than brassinolide plus IAA. In each case, O-methylation at C-2 resulted in a greater loss of activity than O-methylation at C-3 under the same conditions. The relatively strong activity of 3,22,23-tri-O-methylbrassinolide in the presence of IAA is especially noteworthy as it indicates that free hydroxyl groups at C-3, C-22, and C-23 are not essential for bioactivity. Finally, 2-O-methyl- and 3-O-methyl-25-methoxybrassinolide were essentially inactive alone, and showed only a modest increase in bioactivity when coapplied with IAA.
authors.affiliation: Department of Chemistry, University of Calgary, Calgary, AB, Canada T2N 1N4. tgback@ucalgary.ca
authors.forename: Thomas G
authors.initials: TG
authors.lastname: Back
authors.forename: Loeke
authors.initials: L
authors.lastname: Janzen
authors.forename: Richard P
authors.initials: RP
authors.lastname: Pharis
authors.forename: Zhaohua
authors.initials: Z
authors.lastname: Yan
mesh_terms: D001681:Biological Assay; D060406:Brassinosteroids; D002777:Cholestanols / Q000138:chemical synthesis* / Q000737:chemistry / Q000494:pharmacology; D007210:Indoleacetic Acids / Q000494:pharmacology; D008738:Methyl Ethers / Q000138:chemical synthesis* / Q000737:chemistry / Q000494:pharmacology; D012275:Oryza; D018515:Plant Leaves / Q000187:drug effects* / Q000254:growth & development; D013260:Steroids, Heterocyclic / Q000138:chemical synthesis* / Q000737:chemistry / Q000494:pharmacology
chemical_list: D060406:Brassinosteroids; D002777:Cholestanols; D007210:Indoleacetic Acids; D008738:Methyl Ethers; D013260:Steroids, Heterocyclic; C030737:indoleacetic acid; C023623:brassinolide