Improved synthesis of brassinolide

Abstract

Brassinolide has been synthesized from stigmasterol in an overall yield of 7%. The key step in the synthesis is aldol condensation of 2α,3α-isopropylidenedioxy-6-oxo-23,24-dinor-5α-cholan-22-al with 3-isopropylbut-2-enolide carried out at – 78 °C, which gives a product with 22R,23R stereochemistry in high yield. Catalytic hydrogenation of this product is highly stereoselective leading to the desired 24S stereochemistry in an intermediate which is readily transformed into brassinolide.