Introduction

In 1955, a first indication of the chemical basis for the increased resistance of rye plants towards pathogenic fungi was discovered: benzoxazolin-2(3H)-one (acronym: BOA).78 Soon, it was found that BOA had two plant precursors, a benzoxazinoid acetal glucoside and its aglucone.39,79 Thus, BOA is the final product of a degradation process of a (2R)-2-P-D-glucoside of 2,4-di-hydroxy-2H-1,4-benzoxazin-3(4H)-one (acronym: DIBOA-Glc) which undergoes enzymatic cleavage to its aglucone 2,4-dihydroxy-2H-1,4-benzoxazin-3(4H)-one (acronym: DIBOA) followed by a chemical fission into BOA and formic acid as shown in Figure 4.1.

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, and last, but not least, Poaceae, mainly in cereals, like maize, rye, and wheat. In most cases, reported benzoxazinoids were discovered by chance. Usually, no systematic search has been undertaken for them.

ß-glucosidase

OH OH

-HCOOH

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