EPIMERIZATION AT C-5 OF BRASSINOLIDE WITH SODIUM METHOXIDE AND THE BIOLOGICAL-ACTIVITY OF 5-EPI-BRASSINOLIDE IN THE RICE LAMINA INCLINATIONTEST

Brassinolide 1 easily epimerized at C-5 by treatment with sodium metho xide in refluxing methanol via two intermediary methyl esters, and the subsequent re-lactonization with acid led to the formation of 5-epi-b rassinolide 2 (42%) and 6(6a-->3a)abeo-5-epi-brassinolide 3 (18%), alo ng with recovery of 1 (37%); 3 was quantitatively converted to a 94:6 equilibrium mixture of 2 and 3 by prolonged treatment with acidic resi n at 60 degrees C. The NMR experiments allowed the conformations of th e A/B ring moiety of 1 and 2 and the corresponding part of 3 in soluti on to be elucidated. Biological activity of 2 in the rice lamina incli nation test was less than 1/1000 compared with 1, providing clear evid ence that the A/B trans fused ring junction of brassinosteroids is an essential structural factor for high biological activity.