Structures of the oat root resistance factors to ‘take-all’ disease, avenancins A-1, A-2, B-1 and B-2 and their companion substances

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Abstract
It is shown that avenestergenins, having a 12-oxo group, are not true aglycones of the avenacin series: the latter are 12, 13β-epoxides. Acid hydrolysis would be expected to lead to a 13α, 12-ketone, not the 13β, 12-ketone of the avenestergenins, and the chemistry of the process is modelled using the 12α, 13α- and 12β, 13β-epoxides from 3β-benzoyloxyolean-12-ene and isolating the 13α, and 13β, 12-ketones. The former is readily converted into the latter under acid conditions similar to those employed for hydrolysis of the avenacins. Search of the oat extractives has resulted in isolation of the true free aglycone of the avenacin A-1 series, named epoxyavenagenin A-1. By combination of f.a.b. m.s., methylation, 13C and 1H n.m.r. techniques, the trisaccharide chain of all four avenacines is shown to be [β-D-glucopyranosyl(1 → 4)]-[β-D-glucopyranosyl(1 → 2)]-α-L-arabinopyranosyl attached at the triterpene 3-β-hydroxy group. This completes structural and stereochemical details for avenacins A-1, A-2, B-1, and B-2. As minor components of healthy oat root extract, two compounds formulated as glucoavenacin A-1 and deglucoavenacin A-1 have been isolated. The latter is of particular interest as, along with bis-deglucoavenacin A-1, they are detoxified products of avenacin A-1 formed by the highly virulent Gaeumannomyces graminis var. avena which can attack oat roots.