Naturally occurring brassinosteroids

Figure 29 Structural varieties of naturally occurring brassinosteroids. 

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. 

Among all naturally-occurring brassinosteroids, brassinolide and castasterone are considered to be the most important brassinosteroids because of their wide distribution as well as their potent biological activity. 

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. 

Search for Naturally Occurring Brassinosteroid and Endogenous Levels in Plants. 

We are able to readily search for naturally occurring brassinosteroid using rice-lamina joint test [LJT] (1) as the bioassay method and GC/MS as the microanalytical method, which was established by Ikekawa and his co-workers as a microanalytical method for BS (2). Plant material was extracted three times in methanol or 80% methanol-H20. After the extract was evaporated under reduced pressure, the resulting aqueous residue was extracted with EtOAc at pH3 and the EtOAc fraction was partitioned with Na2C03-NaHC03 buffer at pH 10 to get neutral substances. 

Following the discovery of brassin in 1970 (1 ), the isolation and chemical characterization of the active ingredient brassinolide (BR) in 1979 (2) and the subsequent identification of a number of other naturally-occurring brassinosteroids (BS) (see 2) considerable research effort has been directed towards determining the taxonomic and morphological distribution and biological activity of these compounds in plants. 

Figure 2.21 Major members of the naturally occurring brassinosteroids. Reprinted with permission of Shokabo Publishing Co., Ltd...

Figure 2, Structure of naturally occurring brassinosteroids. Trivial and lUPAC equivalent names are given in Table I.

Takatsuto S (1991) Microanalysis of naturally occurring brassinosteroids. In Cutler HG, Yokota T, Adam G (eds) Brassinosimoids chemistry, bioactivity, and applications, vol 474, ACS Symposium Series. American Chemical Society, Washington, DC, pp 1391—1397... 

There are many naturally occurring compounds that have plant growth regulatory effects in whole plants or bioassays, and brassinosteroids are certainly potent - but should they have plant hormone status ... 

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%...

Dehydroteasterone (31), the first naturally occurring 3,6-diketo brassinosteroid from Distylium racemosum and Triticum aestivum, respectively, was synthesized from typhasterol (7)45 or teasterone (S)44 by oxidation of their corresponding isopropylidenedioxy derivatives 66 and 67, respectively, with pyridinium chlorochromate and subsequent deprotection (Scheme 7). 

Amongst natural products, the oxepin structure occurs only in senoxepin 19, a norsesquiterpene lactone of the groundsel Senecio platiphylla its structure has been established by synthesis [8]. Hydrogenated oxepins and oxepanones are often found in natural products, e.g. in the alkaloids strychnine (20, R = H) and brucine (20, R = OCH3), and in the brassinosteroids, e.g. the growth regulator brassinolid 21. 

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