BIOACTIVITY OF BRASSINOLIDE METHYL ETHERS

Four new derivatives of brassinolide (RR) were prepared by single or m ultiple 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 (2 3-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 a lpha,22,23-tetramethoxy-7-oxa-7a-homo-5 alpha-ergostan-6-one (tetraMeB R). 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 exoge nously. The above BR derivatives were subjected to the rice leaf lamin a inclination assay, in comparison with BR, to test for brassinosteroi d activity. Whereas tetraMeBR and 23-MeBR showed negligible and very l ow activity, respectively, even at the high dosage of 1000 ng/plant, 2 2-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-epibrassi nolide (24-epiBR) at low to medium doses, and comparable to BR at dose s of 1000 ng/plant. Thus, diMeBR is a good candidate for further inves tigation of persistence. As is the case for BR, a strong synergistic e ffect was observed when 22-MeBR or diMeBR were administered together w ith the auxin, indole-3-acetic acid. (C) 1998 Elsevier Science Ltd. Al l rights reserved.