Effect of chain length and ring size of alkyl and cycloalkyl side-chain substituents upon the biological activity of brassinosteroids. Preparation ofnovel analogues with activity exceeding that of brassinolide

A series of brassinosteroids with different alkyl or cycloalkyl substituent s in place of the isopropyl group at C-24 of brassinolide (1) were prepared by the CuCN-catalyzed addition of Grignard reagents to (threo-2R,3S,5 alph a, 22R,23R,24S)-23,24-epoxy-6,6-(ethylenedioxy)-2,3-(isopropylidenedioxy)-2 6,27-dinorcholestan-22-ol (9), followed by deketalization and Baeyer-Villig er oxidation. Compound 9 was employed as part of a 70:30 threo/erythro mixt ure of epoxides 9 and 10, from which the erythro-epoxide 10 was recovered i ntact after the Grignard additions. Thus, the corresponding n-dodecyl, n-he xyl, n-propyl, tert-butyl, cyclohexyl, cyclopentyl, cyclobutyl, and cyclopr opyl analogues of brassinolide were obtained. A rearrangement byproduct was observed during the preparation of the cyclopropyl-substituted brassinoste roid when ether was used as the solvent in the Grignard reaction, but could be avoided by the use of THF. A method for recycling the undesired erythro -epoxide 10 was developed on the basis of deoxygenation with tellurium and lithium triethylboro- hydride. The rice leaf lamina inclination assay was t hen used to measure the bioactivity of the products. In general, increasing activity was observed as the length or ring size of the C-24 hydrocarbon s ubstituent decreased. The novel cyclobutyl- and cyclopropyl-substituted ana logues of brassinolide (1) were ca. 5-7 times as active as 1 and thus appea r to be the most potent brassinosteroids reported to date. Further enhancem ent of the bioactivity of all of the above brassinosteroids, except that of the inactive n-dodecyl derivative, was observed when the brassinosteroid w as applied together with an auxin, indole-3-acetic acid (IAA). The synergy between the brassinosteroids and IAA thus increased the bioactivity of the brassinosteroids, including the cyclopropyl and cyclobutyl derivatives, by ca. 1-2 orders of magnitude.