Leslie Crombie

The phytoalexins of oat leaves: 4H-3,1-benzoxazin-4-ones or amides?

Abstract Synthetic evidence is presented that the major phytoalexin of oat leaves is not the 4-H-3,1-benzoxazin-4-one previously reported, but the corresponding amide. Other oat and carnation phytoalexins are prepared.

Pathogenicity of ‘take-all’ fungus to oats: Its relationship to the concentration and detoxification of the four avenacins

Abstract Data for inhibition of the growth of Gaeumannomyces graminis var. tritici (Ggt) and var. avenae (Gga), Phialophora radicicola and Fusarium avenaceum , caused by avenacins, are presented. The avenacins found in all oat species examined are sufficient in quantity to totally suppress growth of wheat ‘take-all’ (Ggt), even old roots containing 25 μg/g (fr. wt). Fungal variants that can also attack oats [var. avenae (Gga)] show considerable variations in their tolerance to avenacin A-1, ec 50 values being 5–80 μg/ml. Nevertheless, all Gga isolates maintained some growth at avenacin A-1 concentrations as high as 200 μg/ml and it is this ability to grow, albeit slowly, at high concentrations that is the critical difference between Gga and Ggt strains. The pathogenicity towards oats of a range of isolates of Gga is related to the fungicidal activity of avenacins. Gga pathogenicity is shown to increase with poor nutrition of the oat hosts (poor illumination, lack of minerals). Fungal detoxification of avenacins produces mono-deglucosylavenacin A-1, bis-deglucosylavenacin A-1 and, in one case, tris-deglycosylavenacin A-1. Ggt strains left avenacin A-1 almost unaffected giving only traces of mono-deglucosyl product. Gga strains bring about mono- and bis-deglucosylation whilst Fusarium avenaceum causes mainly bis-deglucosylation. Mono-deglucosylavenacin is shown to be less inhibitory to Gga than is avenacin A-1, whilst the bis-deglucosyl compound is still less inhibitory.

Distribution of avenacins A-1, A-2, B-1 and B-2 in oat roots: Their fungicidal activity towards ‘take-all’ fungus

Abstract Oat roots contain a group of four major in situ inhibitors of the ‘take-all’ fungus Gaeumannomyces graminis, avenacins A-1, A-2, B-1 and B-2: these are trisaccharide-bearing triterpenes esterified (A-1, B-1) with N-methylanthranilic acid or (A-2, B-2) benzoic acid. Tests using the more virulent var. avenae, which can attack oats as well as the more susceptible wheat, show the N-methylanthranilate esters to be considerably more fungicidal. Avenacin contents (0.22–1.0 mg/g dry weight) and composition (47–60 % A-1, 5–7 % B-1; 30–43 % A-2, 3–6 % B-2) for eleven species and varieties of oat roots (at 77 days) are recorded. Young roots of oats, Avena sativa (var. Peniarth) have a high content of A-1 (73 %) relative to A-2 (14 %) which gradually shifts to a more even distribution of A-1 (55 %) to A-2 (44 %) as the root ages. Total avenacins content of young (3 day) root tips is very high (12.8 mg/g dry wt) and can be estimated at ∼8 μg/root tip: the remainder of the young root has 5 mg/g dry wt of avenacins. The nutritional status of the var. avenae fungus is important in determining its vulnerability to avenacins. The latter provide oat roots with little defence against Fusarium attack.

Natural product chemistry and its part in the defence against insects and fungi in agriculture

This paper surveys our work on natural products as potential models for defensive substances against insect and fungal predators. Insecticides and repellents included are pyrethrins, rotenoids, lipid amides, phorbol esters, cordifolia germacranolides, nicandrenoids, mammeins, dihydroagarofuran esters, and cembrene diols. The fungal H-S toxins from Alternaria, and avenacins from oat roots are briefly considered. The avenacins provide an in-situ defence of oat roots against the destructive ‘Take-all’ fungus disease. © 1999 Society of Chemical Industry

Hydrozirconation methods for natural isobutylamides (anacyclin, pellitorine and its vinylogue) and synthons.

Abstract Hydrometallation of α,ω-diynes is used to make ω-acetylenic isobutylamide synthons: new syntheses of anacyclin, pellitorine and its vinylogous triene are described.

Natural products of Thailand high Δ1-THC-strain Cannabis. The bibenzyl-spiran-dihydrophenanthrene group: relations with cannabinoids and canniflavones

Since non-cannabinoids may influence the pharmacological profile of Cannabis-leaf drug, a detailed examination of the acidic fraction from leaf extractive has been made. Twelve non-cannabinoids have been isolated crystalline from a single high Δ1-THC-strain of Thailand Cannabis grown in Nottingham under controlled conditions: nine of the compounds were not previously known as natural products and their structures have been determined. The extractives comprise three bibenzyls, six spirans, two 9,10-dihydrophenanthrenes, and two prenylated flavones.The bibenzyls, spirans, and dihydrophenanthrenes may be linked together in a biogenetic scheme in which one-electron oxidation and reductive processes play important parts: the scheme is particularly supported by the discovery of a new spiran, cannabispiradienone, which holds a key position and undergoes a dienone–phenol rearrangement to give one of the new dihydrophenanthrenes. Relations between bibenzyl, cannabinoid, and flavone pathways are briefly considered.

Synthetic approaches to isobutylamides of insecticidal interest

Abstract Syntheses of 10,11-dehydro- and 10,11-( Z )-pipercide, piperovatin, and an ene-yne, and a perfluoro-isobutylamide, illustrate the utility of hydrozirconation methodology in this area.

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

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.

Deguelin cyclase, a prenyl to chromen transforming enzyme from Tephrosia vogellii

Abstract Seeds and plant parts of Tephrosia vogellii were investigated in order to provide systems for the study of prenyl to chromen transformation in rotenoids, as exemplified by the conversion of rot-2 -enonic acid into deguelin. No hydroxylated intermediate was found. A cell free preparation has been obtained from T. vogellii seedlings or seeds and shown to catalyse the reaction. The water soluble enzyme has been partially purified using ammonium sulphate precipitation, gel chromatography and ion exchange procedures. Data for the enzyme, named deguelin cyclase, are reported—optima for pH and temperature and Km (which indicates strong binding between enzyme and substrate). Results relevant to Mr determination are discussed. The enzyme has a requirement for oxygen, but not for cofactors. It is inhibited by chloride ion and chelating agents, particularly 1,10-phenanthroline. Deguelin cyclase can convert the 11-hydroxyrotenoid sumatrolic acid into α-toxicarol and lapachol into dehydro-α-lapachone, though the prenyl to chromen conversion is not general. It does not convert rot-2′-enonic acid into rotenone under the conditions studied. Deguelin cyclase seems not to belong to the P450 group and resembles more closely the non-heme iron protein isopenicillin N synthase.

Synthons for general routes to natural insecticidal lipid isobutylamides

Abstract Four routes to the general amide synthon (4) are described. Synthons (33,R=H) and (36), suitable for a similar approach to other groups of natural isobutylamides, are also prepared.