Effects on Bioactivity due to C-5 Heteroatom Substituents on Synthetic 28-Homobrassinosteroid Analogs

Five new 28-homobrassinosteroids have been synthesized, namely, (22R,23R)-5-fluoro-3α,22,23-trihydroxy-5α-stigmastan-6-one, (22R,23R)-5-fluoro-3β,22,23-trihydroxy-5α-stigmastan-6-one, (22R,23R)-5-fluoro-2α,3α,22,23-tetrahydroxy-5α-stigmastan-6-one, (22R,23R)-3α,5,22,23-tetrahydroxy-5α-stigmastan-6-one and (22R,23R)-3β,5,22,23-tetrahydroxy-5α-stigmastan-6-one. Their bioactivities were evaluated by the rice lamina inclination test. C-5α Fluorinated analogs showed excellent in vitro bioactivity, also revealed at low doses, while C-5α hydroxylated analogs resulted in an important decrease in bioactivity. Previously given explanations to justify the decreasing effect due to C-5α electronegative groups should be revised.