Brassinosteroids structure

In the following recent advances in new brassinosteroid structures, synthesis and metabolism with special consideration of results of our laboratory are discussed. 

Figure 28 Structures of three brassinosteroids first isolated from plants. Numbers of carbon atoms are shown in the structure of castasterone.

Figure 29 Structural varieties of naturally occurring brassinosteroids. 

Figure 31 Derivatives used for GC-MS analysis of brassinosteroids and their diagnostic ions for the side chain structures.

Figure 32 Structures of triazole inhibitors of brassinosteroid biosynthesis.

Figure 34 Structures of brassinosteroid receptors and current model for brassinosteroid signaling.

T. Yokota K. Mori, Molecular Structure and Biological Activity of Brassinolide and Related Brassinosteroids. In Molecular Structure and Biological Activity of Steroids M. Bohl, W. L. Duax, Eds. CRC Press Boca Raton, FL, 1992 pp 317-340. 

Brassinosteroids (BRs) are hydroxylated derivatives of cholestane and their structure variations comprise substitutions pattern on ring A, B and the C-17 side chain (Fig. 6.10). The BRs are classified as C27, C28/ or C29 BRs, depending on the substitutions and the length of the side chain. More than 70 BRs as well as more than 42 BR metabolites have been isolated and identified (Bajguz and Tretyn, 2003). 

During the decade since publication in 1979 of the structure of brassinolide (I), and the synthesis of the first brassinosteroid (2), also in 1979, events related to the "brassins project have taken some interesting turns. At this point (October 1990), there seems to be little interest in developing brassinosteroids for agricultural... 

Brassinosteroids are steroidal plant hormones which promote plant growth. To date the presence of over sixty kinds of brassinosteroids has been verified from various plant sources and thirty one of them fully characterized. In this chapter, the distribution and structural characteristics of the naturally-occurring brassinosteroids are discussed. 

All the naturally-occurring brassinosteroids are known to be derivatives of 5a-cholestane. Diverse structural variations thus come from the kinds and orientation of functionalities on the skeleton. 

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). 

Structural characteristics of unknown brassinosteroids in immature seed of P. vulgaris have been elucidated by GC/MS analyses. They are either stereoisomers of known brassinosteroids, or brassinosteroids that have been oxidized during metabolism [ 23-keto-brassinosteroids ( 34 ), brassinosteroids bearing either carbonyl (32 ) or hydroxyl ( 33 )], or brassinosteroids bearing an extra carbon atom [ CO ( 35 ) or COO ( 36 ) ] on the A ring ( Fig. 3 ). However, their complete structures remain to be characterized (30). 

Figure 2. Structures of brassinosteroid conjugates ftomP. vulgaris seed.

Figure 3. Possible structures of representative unknown brassinosteroids in P. vulgaris seed.

Some structure-activity relationships, synthesis of new brassinosteroids, and new sensitive bioassays for brassinosteroids are presented. 

Different Activities of Several Brassinosteroids in Two Bean Internode Tests. It is a well-known phenomenon that structurally closely related plant growth regulators exhibit different orders of activity in different bioassays. This makes it difficult to define the criteria of specificity of their hormonal action. The same situation exists in the case of BRst, and it can be documented by the comparison of different BRst in the two bioassays. 

Of special interest for practical applications, as well as for future receptor studies, are the structure-activity relationships of the brassinosteroids. As postulated (6) the structural requirements for a high activity are the following (22R.23R)-vicinal diol moiety (24S)-methyl- or ethyl group 7-oxa-lactone or 6-oxo functionality 3a-hydroxy group, 2a,3a-vicinai diol or 3a,4a-vicinal diol and A/B-trans-fused ring junction. 

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. 

For an interaction to occur with a putative brassinosteroid receptor, both stereospeciflcally arranged vicinal diol functions in the ring A, as well as in the side chain, can be assumed as important. In our synthetic program for modified brassinosteroids, we were especially interested in new types of analogs with the ring A- and B-seco structure to study the influence of the geometry of this molecular adaptation on biological activity. 

Microbial Transformation of Brassinosteroids. Until now, information concerning the microbial transformations of brassinosteroids has not been published. Such biotransformations could open novel pathways to additionally functionalized members for structure-activity investigations as well as provide information about possible metabolic processes of such compounds. Furthermore, the intermediate metabolites may have practical application. 

Figure 3. Structures of brassinosteroids in C. roseus crown gall cells of V208.

---