Benzoxazinoids

Benzoxazinoids, which possess a l,4-benzoxazin-3-(4//)-one ring, are protective and allelophatic metabolites that are found in a wide range of species of the family Poaceae (Gramineae) of the monocot plants. The 2,4-dihydroxy-2//-l,4-benzoxazin-3(4//)-one (DIBOA) and its C-7-methoxy derivative 2,4-dihydroxy-7-methoxy-2//-l, 4-benzoxazin-3(4H)-one (DIMBOA) are the predominant representatives of benzoxazinoids in plant [373]. 

Benzoxazinoid biosynthesis is restricted to single isolated species within the families Ranunculaceae (i.e., Consolida orientalis), Lamiaceae (Lamium galeobdolon), and Plantaginaceae (Scoparia dulcis) [374, 375] and to several species in the Acan-thaceae Acanthus mollis, Aphelandra tetragona, Aphelandra squarrosa, Blepharis edulis) [376]. 

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Tabaglio V, Gavazzi C, Schulz M, Marocco A (2008) Alternative weed control using the allelopathic effect of natural benzoxazinoids from rye mulch. Agron Sustain Dev 28 397-401... 

Kanchanapoom T, Kamel MS, Kasai R, Picheansoonthon C, Hiraga Y, Yamasaki K (2001) Benzoxazinoid Glucosides from Acanthus ilicifolius. Phytochemistry 58 637... 

Benzoxazolin-2(3H)-one is a phytotoxic allelochemical resulting from a two step degradation and root exudation process based upon an acetal glucoside of the 2,4-dihydroxy-2H-1,4-benz-oxazin-3(4H)-one skeleton. Such benzoxazinoids or cyclic hydroxamic acids occur in Acanthaceae, Poaceae, Ranunculaceae, and Scrophulariaceae. Their aglucones and resulting benzoxazolinones act as a plant s... 

These early findings, together with the ability to act as plant resistance factors towards pests, led to a cascade of some hundred papers dealing in an interdisciplinary manner with all aspects of benzoxazinoids. This name seems to be more correct than the name cyclic hydroxamic acids, which has often been used to name this class of compounds. However, it is not exact from the structural point of view, because it omits the fact that also a variety of acetal glucosides with lactam units was found in plants. They also may be split enzymatically to form lactam aglucones, but cannot undergo chemical decay into benzoxazolin-2(3H)-ones. Benzoxazinoids occur in Acanthaceae, Ranunculaceae, Scrophulariaceae,... 

Enzymatic and chemical degradation of the benzoxazinoid acetal glucoside DIBOA-GIc. 

The biosynthetic origin of the other three benzoxazolinones mentioned in Figure 4.4 is not yet clear. At least other ways of formation than from the benzoxazinoid glucoside two step decay, like metabolic action of endogenous fungi on BOA, MBOA, or DMBOA, have to been considered. 

Finally, some leading references and reviews on other research topics of the benzoxazinoids shall be cited to allow an entrance in these related fields. 

The entire subject has been reviewed in detail several times.32,56,57,65 Recently, a new adsorptive method for the isolation of DIMBOA has been reported.49 The impact of benzoxazinoids on the western corn rootworm (Diabrotica virgifera LeConte) development has been studied.17 The allocation of a hydroxamic acid and biomass during vegetative development of rye has been investigated.27 Effects of benzoxazinoids from maize on survival and fecundity of aphids have been explored.13 DIMBOA concentrations have been measured in various isolines of wheat and corresponding plant introduction lines.55 The variation of the content of several benzoxazinoids in relation to the age and plant organ has been determined in maize plants.12... 

Baumeler, A., Hesse, M., and Werner, C. 2000. Benzoxazinoids - cyclic hydroxamic acids, lactams and their corresponding glucosides in the genus Aphelandra (Acanthaceae). Phytochemistry 53, 213-222... 

Hashimoto, Y. and Shudo, K. 1996. Chemistry of biologically active benzoxazinoids. Phytochemistry 43, 551-559... 

Kanchanapoom, T., Kasai, R., Picheansoonthon, C., and Yamasaki K. 2001. Megastigmane, aliphatic alcohol and benzoxazinoid glycosides from Acanthus ebracteatus. Phytochemistry 58, 811-817... 

Sicker, D., Hartenstein, H. and Kluge, M. 1997. Natural benzoxazinoids -Synthesis of acetal glucosides, aglucones, and analogues. In Pandalai, S. G. (Ed ), Recent Research Developments in Phytochemistry, Vol. 1. Research Signpost, Trivandrum, 203-223... 

Finally, consideration must be given to the availability of pure reference compounds from which to construct analytical calibration curves. Some compounds (such as phenolic acids) may be purchased commercially, but for others (such as benzoxazinoids) it will be necessary to first isolate and purify them from recognized natural sources described in the literature12 or to synthesise them.11... 

The exudation of allelochemicals by living plants is one of the basic tenets of allelopathy theory and would be an indispensable component of any harnessed crop capacity against weeds.9 The exudation of relatively large amounts of key phenolics and benzoxazinoids from wheat (Triticum aestivum) cultivars (and other Triticum species) may well serve as an easily measured indicator of valuable... 

Plants release allelochemicals as volatiles, leaf leachates, and root exudates. In addition, all the constituents of plant residues are eventually released into the environment through microbial decomposition. During this process, most allelochemicals lose their activity but some compounds, for example, benzoxazinoids (cyclic hydroxamic acids), can be activated after hydrolysis.1-3... 

It was the ability to act as a plant own source for the resistance of cereals towzirds pests, which immediately caused a really interdisciplinary research directed on the understanding of all aspects of this imique class of natural products, which is often named hydroxamic acids or cyclic hydroxamic acids. We will rather shortly call them benzoxazinoids or benzoxazinones for the following reasons. Though most of them contain a cyclic hydroxamic acid and some the related lactam, it is only by the direct combination with the cyclic hemiacetal unit that these compounds receive their unique bioactive properties. As mentioned below, the bioactivity is clearly enhanced by a donor substituent in position 7. Hence, to avoid misinterpretations of the features responsible for biological effects, the neutral terms benzoxazinones or benzoxazinoids, are better suitable to name the class. 

In the last decade, several other authors have been dealing with other topics of the benzoxazinoid research in detail, which will be cited here in form of leading references only, which allow an entrance in these fields. 

Recently, the position of benzoxazinoid glucosides within the realm of ca. 110.000 natural products known altogether has been categorised [61]. Hence, benzoxazinoids belong to a subdivision of about 1.000 hemiacetalether compounds. Their common feature is the existence of an ether like linkage arising from the formal combination of two former cyclic hemiacetals by dehydration (Fig. (2)). 

Besides of Poaceae, benzoxazinoid acetal glucosides have been isolated from various species of the Acanthaceae, Ranunculaceae, and Scrophulariaceae as well. In most cases reported the discovery of benzoxazinoids was made by chance, i.e. a systematic search for them among other species of the families mentioned has not been undertaken nor has such a search been done within other plant families. Therefore, it seems most likely, that benzoxazinoids will be constituents of other wild plants, too, which are not at all as deeply investigated as agriculturally used ones. One reason for this assumption is the circumstance that the benzoxazinoids protecting effect especially for a young plant should be a favouring selection criterion in the evolution of any plant equipped with this system. 

Benzoxazinoid acetal glucosides are compounds of low toxicity which have been found to be stable imder neutral conditions. They can undergo a two step degradation, an enzymatic followed by a chemical one. In the non-injured plant cell, the acetal glucosides and the enzyme P-glucosidase... 

As a result of the chemical degradation accompanied by the loss of formic acid (substituted) benzoxazolin-2(3/i)-ones are formed in a ring contraction reaction. The half life of DIMBOA in the juice of injured maize cells is about one day [74]. In fact, it was from this end of the cascade that benzoxazinoid acetal glucosides were detected starting with the discovery of benzoxazolin-2(3/f)-one (BOA) from rye [87]. Meanwhile, several substituted benzoxazolin-2(3/7)-ones (Fig. (5)) have been found in plants. 

The improvement results from using a catalytic transfer hydrogenation [113] for the reductive cyclization. The critical step of this method is the demethylation of the methyl acetal. This problem has been a main subject of a first review on the synthesis of benzoxazinoid aglucones [104]. 

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