New Brassinosteroid Analogs with 23,24-Dinorcholan Side Chain, and Benzoate Function at C-22: Synthesis, Assessment...

The synthesis and biological evaluation of brassinosteroids (BRs) analogs with chemical modification in the side alkyl chain is a matter of current interest. Recently, a series of BR analogs with phenyl or benzoate groups in the alkyl chain have been reported. The effect of substitution in the aromatic ring on the biological activities of these new analogs has been evaluated, and the results suggest that the bioactivity is enhanced by substitution with an F atom. In this context, we have synthesized, characterized, and evaluated a series of new analogs of 23,24-bisnorcholenic type in which the benzoate group at the C-22 position is substituted with an F atom at "ortho or para" positions. Plant growth-promoting activities were evaluated by using the rice lamina inclination test and bean second internode biotest. The results obtained with both bioassays indicate that the compound with an F atom in the para position on the aromatic ring is the most active BR analog and in some cases is even more active than brassinolide. The docking study confirmed that compounds with an F atom adopt an orientation similar to that predicted for brassinolide, and the F atom in the "para" position generates an extra hydrogen bond in the predicted binding position.