The isolation of avenacins A-1, A-2, B-1, and B-2, chemical defences against cereal ‘take-all’ disease. Structure of their ‘aglycones’, the avenestergenins, and their anhydro dimers

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Abstract
The isolation is described of the four pre-formed antifungal compounds, the avenacins, from oat roots. These prevent attack by the wheat pathogen Gaeumannomyces graminis var. tritici(Ggt): the latter is the causative fungus of ‘take-all’ disease. The four avenacins A-1, A-2, B-1, and B-2 have the same trisaccharide attachment and on deglycosation give as ‘aglycones’ avenestergenins A-1, A-2, B-1, and B-2. A-1 and B-1 contain an esterifying N-methylanthranilate, A-2 and B-2 a benzoate. All four contain a triterpene core, the A-series being related to the B- by possession of an extra 23-hydroxy group. The structure of avenestergenin A-1 is examined in detail using 1H n.m.r. with spin decoupling and Cosy plot, n.O.e. difference spectroscopy and 2D linkage of 13C and 1H resonances. This and mass spectral work, leads to complete structural and stereochemical proposals for the avenestergenins. After considerable difficulties, an X-ray structure of avenestergenin A-2 has been achieved. The four avenestergenins form a set of eight anhydro dimers, investigated by n.m.r. and f.a.b. mass spectra. These result from formation of 1,3-dioxane rings using the C-3 and C-23 hydroxy groups of one molecule of the A series with the C-30 aldehyde of another molecule of either the A or the B series.