Natural ecdysteroids

As stated above, the insecticidal activity of natural ecdysteroid is very low. One of the reasons for the low insecticidal activity is probably the poor penetration of hydrophilic ecdysteroids through insect cuticle. Other reasons may be related to the metabolism and excretion. According to Blackford et al, (41), tomato moth larvae are able to feed on a diet containing 400-ppm 20E without any adverse effects on growth and development. Also, when a high dose of 20E was injected into the final instar larvae of the silkworm, Bombyx moriy the hemolymph 20E titer decreases immediately (42). These results indicate that 20E when applied exogenously into insects by either oral administration or injection can be easily metabolized and excreted. 

LC-NMR can be used to identify natural products in crnde plant extracts that usually consist of complex mixtnres. The crnde natural product extracts normally contain a great nnmber of closely related and difficult-to-separate compounds. The classical separation approach may become very tedious and time-consuming. The directly conpled HPLC-NMR presents an efficient separation techniqne together with a powerfnl spectroscopic method to speed up the identification process. LC-NMR has been nsed extensively for characterization of natnral prodncts. More recently, the combination of LC-NMR and LC-MS has been further developed in this field. Eor example, Wilson et al. have nsed combined on-flow NMR and electrospray ionization MS to characterize ecdysteroids in extracts of silene otites. After reversed-phase HPLC nsing D2O in acetonitrile-dj and UV detection, the LC flow was split 95 5 for the simnl-taneous detection by NMR and MS. The peaks of interest were analyzed by stop-flow NMR to give better quality spectra for structural assignment. 

The first application of LC-NMR-MS to natural products analysis was presented in 1999 [37]. The additional mass spectroscopic information allowed the identification of a further ecdysteroid in an extract of Silene otides which could not be identified by LC-NMR alone [24], Further applications of this double hyphenation dealt with the identification of napthodianthrones [38] and flavone glycosides [38,39] in natural products extracts. Taking the hyphenated technique one step further, the suitability of an integrated LC-UV-IR-NMR-MS system for natural products analysis has been assessed [40] - again using ecdysteroids as an example. Such systems still suffer from different requirements of the individual detectors (mainly in terms of sensitivity). However, the results obtained are promising. 

The structural variety of natural zoo-, phyto- and mycoecdysteroids is wide [78], but it does not encompass some structural features which would be desirable for certain applications or for certain SAR studies. There are a number of areas in which novel ecdysteroids would be very helpful ... 

As part of an extensive survey of natural products to detect those showing ecdysteroid agonist or antagonist activities in the Bn bioassay [170], it was found that the diterpene maocrystal E (8-8) showed agonist activity (EC50 = 5 x 10 6M). 

The ecdysteroid antagonists identified to date are all of rather low potency This may be a consequence of them all being plant natural products, which the plant is able to produce in high concentrations at low cost to itself [231],... 

In addition to their well established role in catalyzing the metabolism of a wide variety of naturally occurring and synthetic xenobiotics, cytochrome P-450-mediated mixed-function oxidases are of critical Importance in the biosynthesis and regulation of the major hormones (ecdysteroids and juvenile hormone) that control insect growth and development. The characteristics of the mixed-function oxidases involved in the synthesis of insect hormones are described and the possibility that the enzymes might represent potential targets for insect control is discussed. 

Two-dimensional displacement thin-layer chromatography (2D-D-TLC) of ecdysteroids was also introduced. Samples of natural origin (plant extracts) were investigated by 2D-D-TLC. The first dimensional run separated the ecdysteroids from the majority of the contaminating compounds (e.g., flavonoids) while the second dimensional run improved the separation, and concentrated the spots into sharp bands. 

Steroids, which are a class of compounds that occur in nature and in synthetic products, have a cyclopen-tanoperhydrophenanthrene skeleton. The carbon atoms and rings are labeled according to the scheme shown in Fig. 1. The following classes of compounds belongs to steroids sterols, bile acids, cardenolides, androgens, estrogens, corticosteroids, steroid sa-pogenins, steroid alkaloids, ecdysteroids, and vitamin D. 

Finally, since many natural product compounds have been investigated with various chromatographic modes and detection techniques, a selection of examples has been summarized in this chapter. This information has been compiled in the form of tables for well-researched classes of secondary metabolites selected from the major subgroups of isoprenoids (mono-, sesqui-, di-, and triterpenes iridoids and secoiridoids carotenoids saponins and ecdysteroids), of phenolics (coumarins, flavonoids, and isoflavonoids), and of alkaloids. 

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