Recent Results in the Synthesis of Brassinolide And Related Compounds

Brassinolide (1) isolated from the pollen of rape (Brassica napus), is a plant growth promoting steroid having a seven-memberd B-ring lactone and four successive chiral centers in the sidechain 1 . Brassinolide promotes both cell elongation and cell division and possesses a broad spectrum of biological activities compared with the known plant hormones. Brassinolide may find practical application in agriculture. Since the discovery of brassinolide, a number of related steroids, e.g. homobrassinolide (2) 2 , dolicholide(3) 3 , 25-methyldolichosterone( 4 )4a, 24-epi-brassinolide(5) 5 , typhasterol (6) 6 , have been isolated from higher plant and formed a new class of plant growth promotor. Their remarkable biological activities and novel chemical structure have led many laboratories to synthesize these natural products. An important problem in the construction of the sidechain is controlling the stereochemistry at C22, C23 and C24 (Fig. 1).