Synthesis of brassinosteroids with a keto group in the side chain

• Cyclic mono- and dicarbonate of epibrassinolide were prepared.
• An effective cleavage of 22,23-cyclic boric ester was achieved.
• A procedure for monobenzylation of 22,23-diols was reported.
• Two variants of partial oxidation of 22,23-diols were reported.

The aim of this work was to prepare 24-epicryptolide and 22-dehydro-24-epibrassinolide as possible metabolites of 24-epibrassinolide. The main synthetic problem to be solved was the differentiation of functional groups in brassinosteroids. Distinguishing 2α,3α-diol function from another diol group in 24-epibrassinolide was achieved via selective hydrolysis of 2α,3α-cyclic carbonate or via regioselective reaction of boric acid with the functional groups in the side chain. The hydroxyl at C-23 was more reactive than the 22-OH in the oxidation with bromine in the presence of bis(tributyltin) oxide and in the benzylation reaction that resulted in the predominant formation of the corresponding α-hydroxy ketone derivatives with the ratio ranging from 4:1 to 1.5:1.

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