A new synthesis of castasterone and brassinolide from stigmasterol. A concise and stereoselective elaboration of the side chain from a C-22 aldehyde

Abstract

Brassinolide (1) and castasterone (2) were prepared from aldehyde 7, in turn available from stigmasterol (3). The addition of (E)-1-propenyllithium to 7, followed by diastereoselective Sharpless epoxidation of allylic alcohol 8 using (L)-(+)-diethyl tartrate, and regioselective epoxide-opening of (threo) epoxy alcohol 15 with isopropylmagnesium chloride in the presence of a catalytic amount of cuprous cyanide afforded diol 17. The epoxidation and cuprate addition steps were investigated in greater detail with the simpler model aldehyde 4. Hydrolysis of 17 provided 2, which was converted preferentially into 1 or its regioisomer 22 by Baeyer–Villiger oxidation with trifluoroperoxyacetic acid or peroxyseleninic acids, respectively.