Synthesis and biological activity of 25-methoxy-, 25-fluoro-, and 25-azabrassinolide and 25-fluorocastasterolle: Surprising effects of heteroatom substituents at C-25

The CuCN-catalyzed addition of 2-propenylmagnesium bromide to (threo-2R,3S, 5 alpha,22R,23R,24S)-23,24-epoxy-6,6-(ethylenedioxy)-2,3-(isopropylidenedio xy)-26,27-dinorcholestan-22-ol (11a) afforded the corresponding Delta(25)-2 2,23-diol 12. This was converted into 25-methoxybrassinolide (7) by protect ion as the 22,23-acetonide 13, oxymercuration in methanol, Baeyer-Villiger oxidation, and deprotection. Similarly, the addition of pyridinium poly(hyd rogen fluoride) Do 13 and deprotection afforded 25-fluorocastasterone (8), which was converted into 25-fluorobrassinolide (9) by Baeyer-Villiger oxida tion. Treatment of threo-epoxide 11a with Me2NMgBr, followed by Baeyer-Vill iger oxidation of the corresponding tetraacetate and saponification, provid ed 25-azabrassinolide (10). Epoxide 11a is therefore a versatile intermedia te for the synthesis of side-chain analogues of brassinolide (1). 25-Methox ybrassinolide (7) displayed strong activity in the rice leaf lamina inclina tion bioassay, which was significantly enhanced by the simultaneous applica tion of an auxin, indole-3-acetic acid (IAA). Thus, the presence of a 25-me thoxy substituent, like that of the previously reported 25-hydroxy group in the 24-epibrassinolide series, yields a molecule with potent biological ac tivity. On the other hand, 8-10 showed no bioactivity with or without IAA. This suggests that either the 25-fluoro and 25-aza substituents interfere w ith binding to a putative brassinosteroid receptor or that they prevent the in vivo enzymatic oxidation at C-25 that is required for bioactivity.