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

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.