Preparation, conformational analysis, and biological evaluation of 6a-carbabrassinolide and related compounds

In order to probe the ligand recognition sites of brassinosteroid (BR)-receptors, B-ring modified brassinolides (BLs), i.e. 6a-carbaBL (), 6a-carba-B-homoBL (), the C-5 epimers of compounds and ( and , respectively), and 6-deoxo-6a-oxo-6a-carbaBL (), were prepared from castasterone () via homologation with trimethylsilyldiazomethane and boron trifluoride etherate. Biological evaluation of these compounds using the rice lamina inclination (RLI) test gave the following order of potency (percent activity): BL () (100%)> (13%)> (0.3%)≫, , , (0%). Using the rescue test of hypocotyl length of the BR deficient mutant, Arabidopsis det2, the corresponding order was: (100%)> (44%)> (28%)> (4%)> (0.5%)> (0.1%), and (inhibition). The markedly different activities of in the two assay systems provide an evident experimental proof that the BR-receptors involved in RLI and in Arabidopsis hypocotyl elongation recognize BR molecules in different structure manners, at least in the B-ring region.