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

Abstract In order to probe the ligand recognition sites of brassinosteroid (BR)-receptors, B-ring modified brassinolides (BLs), i.e. 6a-carbaBL ( 3 ), 6a-carba-B-homoBL ( 4 ), the C-5 epimers of compounds 3 and 4 ( 6 and 7 , respectively), and 6-deoxo-6a-oxo-6a-carbaBL ( 8 ), were prepared from castasterone ( 2 ) 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 ( 1 ) (100%)> 2 (13%)> 3 (0.3%)≫ 4 , 6 , 7 , 8 (0%). Using the rescue test of hypocotyl length of the BR deficient mutant, Arabidopsis det2 , the corresponding order was: 1 (100%)> 8 (44%)> 2 (28%)> 3 (4%)> 6 (0.5%)> 4 (0.1%), and 7 (inhibition). The markedly different activities of 8 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.