A short convergent synthesis of the side chains of brassinolide, cathasterone, and cryptolide

A new approach to steroidal derivatives of the campestane series, containing the 22 alpha-hydroxy-, 22 alpha,23 alpha-dihydroxy-, and 22 alpha-hydroxy-23-ketone moieties characteristic of brassinolide and its congeners, has been developed. The key step is the coupling of a steroidal C-22 aldehyde with an anion derived from a specially synthesized thioacetal-containing chiral synthon. The cathasterone and cryptolide side chains are prepared by reductive or hydrolytic thioketal removal, respectively. The brassinolide side chain is obtained by DIBAL-H reduction of the TBS-protected 22 alpha-hydroxy-23-ketone. (c) 2012 Elsevier Ltd. All rights reserved.