Brassinolide structure-activity relationship

Structure-activity relationship Biosynthesis and Metabolism Biosynthesis of castasterone Biosynthesis of brassinolide from castasterone Regulation of biosynthesis Metabolism and its regulation Physiology and Signal Transduction Physiology Signal transduction Jasmonates and Oxylipins Introduction Chemistry Chemical structure Chemical stability... 

Brassinosteroids are grouped into C, Cg, and steroids like the typical phytosterols. These classifications result basically from different alkyl substitutions in the side chain, that is no substituent in steroids (14, 15 ), a methyl (1, 3,5, 7-9, 16-22,29) or an exomethylene ( 2,4, 6) at C24 in steroids, and an ethyl at C24 (12 )or an ethylidene at C24 ( 10,11, 13) or an exomethylene at C24 with a methyl at C25 ( 23-28 ) in steroids. The structure-activity relationship reveals that alkylation in the side chain is necessary for biological activity. More especially, the presence of a saturated alkyl ( a methyl or an ethyl ) at C24 and a methyl at C25 makes brassinosteroids biologically more active. Thus, 25-methylbrassinolide bearing both 24-methyl and 25-methyl functions has been chemically synthesized and it is more potent than brassinolide which, until now, had been considered to be the most potent brassinosteroid (44). 

However, some side chain epimers like 24-epibrassinolide 2, trisepi-brassinolide and (22S,23S)-homobrassinolide 3 show remarkable activity and are therefore used widely for biological experiments. Furthermore several structural analogs with a shortened side chain moiety are active in specific test systems (7). Thus, structure-activity relationships of brassinosteroids are a field of continual development and require further examination. 

Synthesis of Brassinosteroids. USDA scientists were the first to report the synthesis of various brassinolide analogs, some of which were later found to be present in plants. This work led to, in addition to identifying new brassinosteroids from plants, establishing the structure—activity relationship of brassinosteroids. The reasons for undertaking the synthesis were (a) to ascertain that brassinolide was indeed the active ingredient of the pollen, (b) to determine structure—activity relationships, and (c) to provide quantities of the active ingredient(s) sufficient for biological studies and field work. 

The chemical identify and biological activity has beeen verified through the use of synthetic brassinolide. A number of biologically active epimers and biologically inactive isomers were used in the study of structure activity relationships (11,12). 

Q brassinosteroids and the ecdysteroids are apparent and have led to some detailed examinations of the antiecdysteroid effects of brassinosteroids. The fact that both groups of compounds are exceedingly active in thdr respective domains makes them interesting models for testing in reciprocal systems and for structural modifications to pursue structure activity relationships. Again, only those brassinosteroids that are available in relatively abundant amounts have been evaluated and these include brassinolide, 22S,23S-homobrassinolide and 22S,23S-homocastasterone (Figure 6). These are similar in structure to ecdysone, 20- ... 

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