Epimerization at C-5 of brassinolide with sodium methoxide and the biological activity of 5-epi-brassinolide in the rice lamina inclination test

Abstract

Brassinolide 1 easily epimerized at C-5 by treatment with sodium methoxide in refluxing methanol via two intermediary methyl esters, and the subsequent re-lactonization with acid led to the formation of 5-epi-brassinolide 2 (42%) and 6(6a→3a)abeo-5-epi-brassinolide 3 (18%), along with recovery of 1 (37%); 3 was quantitatively converted to a 94∶6 equilibrium mixture of 2 and 3 by prolonged treatment with acidic resin at 60 °C. The NMR experiments allowed the conformations of the A/B ring moiety of 1 and 2 and the corresponding part of 3 in solution to be elucidated. Biological activity of 2 in the rice lamina inclination test was less than 1/1000 compared with 1, providing clear evidence that the A/B trans fused ring junction of brassinosteroids is an essential structural factor for high biological activity.

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