Synthesis of brassinosteroids of varying acyl side chains and evaluation of their brassinolide-like activity.

Brassinosteroids containing various side chain moieties were synthesized and their activity was determined as the reciprocal logarithm of the ED(50) (50% effective dose per plant in moles) in the rice lamina inclination assay using synergist indole-3-acetic acid (IAA). The introduction of a hydroxyl group in the alpha-position to the carbonyl group of the ester structure significantly enhanced the activity. 2alpha,3alpha-Dihydroxy-17beta-[(2R,3S)-2-hydroxy-3-methylpentanoyl]oxy-B-homo-7-oxa-5alpha-androstan-6-one showed the highest activity, for which the pED(50) was determined to be 10.5 under synergistic conditions with IAA. Under identical conditions, the pED(50) values of brassinolide and castasterone were determined to be 13.6 and 12.3 respectively. With respect to the alpha-carbon of the acyl moiety, the R-form was 10 times more potent than the corresponding S-form. Substituting the terminal structure (Et) of the side chain to that of the most potent compound, brassinolide (i-Pr), did not increase the activity.