Structure-activity studies of brassinolide B-ring analogues

Six new analogues of brassinolide were prepared in order to investigate their structure-activity relationship: 7-azabrassinolide, 7-thiabrassinolide, 6-deoxybrassinolide, B-homocastasterone, 6-methylidene-castasterone and 6-methylidene-B-homocastasterone. These compounds were subjected to the rice leaf lamina inclination assay, in comparison with brassinolide and 24-epibrassinolide and/or castasterone to test for brassinosteroid activity. The activity of 7-azabrassinolide, 7-thiabrassinolide and 6-deoxybrassinolide was comparable to that of 24-epibrassinolide, but lower than that of brassinolide. B-Homocastasterone was less active than either brassinolide or castasterone. The B-ring carbocycles 6-methylidene-castasterone and 6-methylidene-B-homocastasterone were essentially inactive. These results indicate that neither the oxygen atom at the 7-position of brassinolide, nor its carbonyl group, is essential for activity. However, the complete absence of a polar functional group from the B-ring, as in 6-methylidenecastasterone and 6-methylidene-B-homocastasterone, results in a total loss of bioactivity. This inactivity persists even in the presence of an exocyclic methylidene group that flattens the B-ring to resemble that of brassinolide or castasterone by virtue of the sp(2)-hybridized carbon atom at C-6. Finally, the bioactivity of several, but not all, of the brassinosteroids, was significantly and synergistically increased by the simultaneous application of the auxin, indole-3-acetic acid. (C) 1998 Elsevier Science Ltd. All rights reserved.