Chapter VII - Syntheses of natural BS

Abstract

This chapter describes the partial synthesis of natural brassinosteroids (BS). Brassinolide is currently considered to be the most important synthetic target among BS due to its extremely high biological activity. Preparation of brassinolide is a relatively difficult task and a synthetic challenge. Brassinolide syntheses have been accomplished starting from pregnenolone, from hyodeoxycholic acid, from a mixture of sterols containing crinosterol, and especially from stigmasterol. Syntheses described in this group make use of 22-aldehydes as key intermediates, all of which are derived from stigmasterol. Crinosterol is an ideal starting material for the synthesis of brassinolide. In the synthesis of brassinolide starting from pregnenolone, all the necessary carbon atoms of the side chain are introduced via a chelation-controlled addition of the anion obtained from 3-isopropyltetronic acid 178 to ketone 400. Epibrassinolide is considered to be the most probable candidate for practical application of BS in agriculture. It combines important features such as high biological activity and relatively good availability. In many tests, its activity is comparable with that of brassinolide.