A new synthesis of brassinosteroids with a cholestane framework based on a highly functionalized starting material

• Boric acid was used for selective protection of 22,23-diol in 24-epicastasterone.
• Synthesis of C-22 aldehydes starting from 24-epicastasterone was proposed.
• Claisen rearrangement was used for the side chain construction.
• 28-Norcastasterone and 28-norbrassinolide were prepared.
• Trideuterated analogues of 27C brassinosteroids were synthesized.

A new route to the synthesis of minor brassinosteroids with a cholestane framework (28-norcastasterone and 28-norbrassinolide) has been proposed. It makes use of commercially available 24-epicastasterone as a starting material. In addition, [26,26,26-2H3]-28-norcastasterone and [26,26,26-2H3]-28-norbrassinolide have been prepared as tools for analytical applications. The key steps were regioselective manipulations of functional groups in 24-epicastasterone, oxidative cleavage of 22,23-diol group and Claisen rearrangement.

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