Brassinolide analogs

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. 

Subsequent to our use of butenolide anions in aldol reactions to construct sidechains of steroids, two groups have reported a similar method for synthesis of brassinolide or its analogs. Kametani and co-workers employed the dianion of 3-isopropyltetronic acid which was reacted with a 20-keto steroid. The aldol product was dehydrated and the product hydrogenated to furnish the brassinolide sidechain (28,29). Zhou and Tian have used the reaction of a C-22 aldehyde derived from... 

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. 

Figure 3 shows hypothetical biosynthetic pathway of brassinolide. Namely campesterol or its analogs will be converted to teasterone via several steps of oxidation, then successively oxidized to typhasterol, castasterone and brassinolide. In parentheses, biological activities in the rice lamina inclination assay are shown. The... 

Scheme 1 Structures of steroids used in this study. The stereochemistry of natural brassinolide compared to the synthetic analogs (BR-1 and BR-2) is indicated.

Figure 1 Comparison of the effect of two synthetic analogs (BR 1 and BR 2) of brassinolide on the growth (measured as increase of fresh weight - fw - in % of the untreated control) of transformed Nicoticuia tabacum callus-cultures. Culture period 3 weeks. CV <5%...

Table I Water content of transformed tobacco callus-cultures grown on brassinosteroid-containing medium for 3 weeks. The brassinosteroid employed (BR-1) is a synthetic 225,235,245-analog of the naturally occurring brassinolide. The concentration of BR-1 was varied from IO 10 M to KT6 M CV <5%...

The discovery of brassinolide by scientists at USDA led to the development of a new group of plant growth substances, termed brassinosteroids, which have the potential for increasing crop yields and biomass. The chapter summarizes the USDA s contributions to brassinosteroids, which include isolation and identification of brassinolide, synthesis of brassinolide and its analogs, and limited biological and field studies on brassinosteroids. An overview of the current data requirements for the registration of brassinosteroids as biochemical pesticides under the Federal Insecticide, Fungicide, and Rodenticide Act is presented. 

Early in our synthetic program for brassinolide and its analogs, we learned that there were certain structural requirements in the steroid molecule for eliciting the biological activity. These requirements include the following (l, 5-7) ... 

Our synthetic work on brassinosteroids showed that the stereochemistry in the steroid molecule is very important to retain the biological activity. We found that, of the 13 asymmetric centers in the molecule, only 3 asymmetric centers (at C-22, C-23, and C-24) can be altered. On the basis of this information, we prepared brassinolide and all the analogs (23) by using the general synthetic scheme (Figure 3). Additionally, we prepared compounds having different substituents (H, CH3, and C2H5) at C-24 (2, 7). 

Epicastasterone (13) is assumed to be the immediate biogenetic precursor of 24-epibrassinolide (12), analogously with the related couple castasterone (3) and brassinolide (1) in the (245 )-series. 4 In further experiments 3H labeled 24-epicastasterone (13) was fed to cell suspension cultures of Lycopersicon esculentum and the extract obtained four days after administration was shared into two halfs. One part was subjected to enzymatic hydrolysis (aglycone fraction) while the other one was further used in the glucosidic state. 

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