Bioactivity of brassinolide methyl ethers

Four new derivatives of brassinolide (RR) were prepared by single or multiple methylation of the hydroxyl groups at C-2, C-3, C-22 and or C-23 to afford (22R,23R,24S)-2 alpha,3 alpha,23-trihydroxy-22-methoxy-7-oxa-7a-homo-5 alpha-ergostan-6-one (22-MeBR), (22R,23R,24S)-2 alpha,3 alpha,22-trihydroxy-23-methoxy-7-oxa-7a-homo-5 alpha-ergostan-6-one (23-MeBR),(22R,23R,24S)-2 alpha,3 alpha-dihydroxy-22,23-dimethoxy-7-oxa-7a-homo-5 alpha-ergostan-6-one (diMeBR), and (22R,23R,24S)-2 alpha,3 alpha,22,23-tetramethoxy-7-oxa-7a-homo-5 alpha-ergostan-6-one (tetraMeBR). The formation of methyl ethers is expected to block glycosylation of the free hydroxyl groups, which should prevent potential metabolic deactivation of these compounds by plants when. they are applied exogenously. The above BR derivatives were subjected to the rice leaf lamina inclination assay, in comparison with BR, to test for brassinosteroid activity. Whereas tetraMeBR and 23-MeBR showed negligible and very low activity, respectively, even at the high dosage of 1000 ng/plant, 22-MeBR showed weak activity relative to BR at low doses (1-10 ng/plant), and appreciable activity at high doses (1000 ng/plant). Of the four new compounds, diMeBR was the most active, comparable to 24-epibrassinolide (24-epiBR) at low to medium doses, and comparable to BR at doses of 1000 ng/plant. Thus, diMeBR is a good candidate for further investigation of persistence. As is the case for BR, a strong synergistic effect was observed when 22-MeBR or diMeBR were administered together with the auxin, indole-3-acetic acid. (C) 1998 Elsevier Science Ltd. All rights reserved.