Chapter VI - Basic synthetic methods and formal syntheses

Abstract

This chapter describes the two different approaches to the synthesis of steroids: total synthesis and partial synthesis. The total synthesis includes the synthesis of the whole steroid molecule from simple precursors. The partial synthesis relies on the transformation of starting materials that already possess the tetracyclic fragment characteristic for steroids. All syntheses of brassinosteroids (BS) are partial syntheses, which imply the introduction of the necessary functional groups into the tetracyclic fragment and the construction of the side chain. The implementation of the first task is based mainly on the experience accumulated during the syntheses of other steroids, first of all the ecdysteroids. For the construction of the characteristic side chain of BS, new methods have to be developed. The first problem that has to be solved in partial syntheses of BS is the choice of an appropriate starting material, preferably one with the same carbon skeleton. It is important to have functional groups in the starting compound, which can be transformed into those that are characteristic for BS. For large-scale preparations, the availability of the starting material is a determining factor. The most suitable starting compounds for partial syntheses are sterols.