Effects on bioactivity due to C-5 heteroatom substituents on synthetic 28-homobrassinosteroid analogs

Five new 28-humobrassinosteroids have been synthesized, namely, (22R,23R)-5 -fluoro-3 alpha,22,23-trihydroxy-5 alpha-stigmastan-6-one, (22R,23R)-5-fluo ro-3 beta,22,23-trihydroxy-5 alpha-stigmastan-6-one (22R,23 R)-5-fluoro-2 a lpha,3 alpha,22, 23-tetrahydroxy-5 alpha-stigmastan-6-one, (22R,23R)-3 alph a,5,21,23-tetrahydroxy-5 alpha-stigmastan-6-one and (22R,23R)-3 beta,5,22,2 3-tetrahydroxy-5 alpha-stigmastan-6-one. Their bioactivities were evaluated by the rice lamina inclination test. C-5 alpha Fluorinated analogs showed excellent in vitro bioactivity, also revealed at low doses, while C-5 alpha hydroxylated analogs resulted in an important decrease in bioactivity. Pre viously given explanations to justify the decreasing effect due to C-5 alph a electronegative groups should be revised. (C) 2000 Elsevier Science Ltd. All rights reserved.