Methods for construction of sidechain of brassinosteroids and application to syntheses of brassinosteroids

Abstract

This chapter presents the method of constructing side chain of brassinosteroids. Hyodeoxycholic acid is used as a starting material for the synthesis of brassinosteroids and for the A, B ring modification, a highly regioselective synthesis of the 7-oxalactone ring by ozone oxidation of the enolsilyl ether is developed, which can be regarded as a complement of the Baeyer-Villiger oxidation. For the construction of the side chain, the hyodeoxycholic acid is first degraded to the 20-carbaldehyde as ergosterol or stigmasterol. Various stereocontrolled methods are used for building of the dihydroxy sidechain with 22R, 23R, 24S-configuration. For economy, the intact side chain of hyodeoxycholic acid and (22E)-methylhyodeoxycholate is used as the starting material. For the synthesis of brassinosteroids from the intact sidechain of hyodeoxycholic acid, the β-alkylative 1,3-carbonyl transposition methods is employed as the key step. The very high enantioselectivity of the osmium catalyzed asymmetric dihydroxylation (ADH) of the olefin is applied to (22E)-methyl hyodeoxycholate, providing the natural (22, 23)-isomer as the major products. And then, the 22-olefin steroid from hyodeoxycholic acid is employed as the staring material for the synthesis of 23-aryl and 23-furyl brassinosteroids via the Heck reaction and asymmetric dihydroxylation.