Design, synthesis, and bioactivity of the first nonsteroidal mimetics of brassinolide

Ten novel compounds, each consisting of two subunits and a linker, were des igned with the aid of molecular modeling to resemble the natural steroidal phytohormone brassinolide. The mimetics were synthesized and subjected to t he rice leaf lamina inclination bioassay to test for brassinosteroid activi ty. Most of the mimetics displayed very weak or no bioactivity, but two wer e strongly active when coapplied with the auxin indole-3-acetic acid (IAA), which synergizes the activity of brassinosteroids. Thus, 1-(4,6 alpha ,7 a lpha -trihydroxy-5,6,7,8-tetrahydronaphthyl)-2-(6 alpha',7 alpha'-dihydroxy -5',6',7',8'-tetrahydronaphthyl)ethyne (4) and (E,)-1,2-bis[trans-(4a alpha ,8a beta)-4-oxo-6 alpha ,7 alpha -dihydroxy4a,5,6,7,8,8a-hexahydro-(3H)-na phthyl] ethylene (11) showed exceptional activity at doses as low as 0.01 n g and 0.001 ng/plant, respectively. These compounds are the first biologica lly active nonsteroidal brassinolide mimetics.