Formation of the steroidal C-25 chiral center via the asymmetric alkylation methodology

Synthesis of brassinosteroids with a keto group in the side chain

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.

The development and use of a general route to brassinolide, its biosynthetic precursors, metabolites and analogues

A new method for the construction of steroid side chains through the addition of lithium salts of dithianes to a C-22 aldehyde was developed. It was applied for the preparation of brassinolide and related brassinosteroids.