Synthesis and Biological Activity of 25-Methoxy-, 25-Fluoro-, and 25-Azabrassinolide and 25-Fluorocastasterone:  Surprising Effects of Heteroatom Substituents at C-25

Abstract

The CuCN-catalyzed addition of 2-propenylmagnesium bromide to (threo-2R,3S,5alpha,22R,23R,24S)-23,24-epoxy-6,6-(ethylenedioxy)-2,3-(isopropylidenedioxy)-26,27-dinorcholestan-22-ol (11a) afforded the corresponding Delta(25)-22,23-diol 12. This was converted into 25-methoxybrassinolide (7) by protection as the 22,23-acetonide 13, oxymercuration in methanol, Baeyer-Villiger oxidation, and deprotection. Similarly, the addition of pyridinium poly(hydrogen fluoride) to 13 and deprotection afforded 25-fluorocastasterone (8), which was converted into 25-fluorobrassinolide (9) by Baeyer-Villiger oxidation. Treatment of threo-epoxide 11a with Me(2)NMgBr, followed by Baeyer-Villiger oxidation of the corresponding tetraacetate and saponification, provided 25-azabrassinolide (10). Epoxide 11a is therefore a versatile intermediate for the synthesis of side-chain analogues of brassinolide (1). 25-Methoxybrassinolide (7) displayed strong activity in the rice leaf lamina inclination bioassay, which was significantly enhanced by the simultaneous application of an auxin, indole-3-acetic acid (IAA). Thus, the presence of a 25-methoxy substituent, like that of the previously reported 25-hydroxy group in the 24-epibrassinolide series, yields a molecule with potent biological activity. 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 with binding to a putative brassinosteroid receptor or that they prevent the in vivo enzymatic oxidation at C-25 that is required for bioactivity.