R. Marthinus Horak

Structure elucidation of the fumonisins, mycotoxins from Fusarium moniliforme

The structures of the fumonisins, a family of structurally related mycotoxins isolated from cultures of Fusarium moniliforme, were elucidated by mass spectrometry and 1H and 13C n.m.r. spectroscopy as the diester of propane-1,2,3-tricarboxylic acid and either 2-acetylamino- or 2-amino-12,16-dimethyl-3,5,10,14,15-pentahydroxyicosane as well as in each case the C-10 deoxy analogue; in all cases both the C-14 and C-15 hydroxy groups are involved in ester formation with the terminal carboxy group of propane-1,2,3-tricarboxylic acid.

Metabolites of Aspergillus ustus. Part 3. Structure elucidation of austalides G–L

The structure elucidation of austalides G–L, based on a study of their 1H and 13C n.m.r. spectra and chemical derivatization, is described.

Metabolites of Aspergillus ustus. Part 1. Application of the heteronuclear selective population inversion (SPI) n.m.r. technique to the structure elucidation of the austalides A–F, novel ortho ester meroterpenoids

The isolation and characteristics of 12 biosynthetically related metabolites, austalides A–L from cultures of Aspergillus ustus are reported. The structure elucidation of the austalides A–F is based on a detailed study of their high-field 1H and 13C n.m.r. spectra and especially on the three- and two-bond (C,H) connectivity pattern as determined by heteronuclear 13C-{1H} selective population inversion (SPI) experiments. The conformation and relative configuration of austalides A and D were deduced from the observed proton–proton nuclear Overhauser effects (n.O.e.s) and the magnitude of the proton–proton coupling constants. Base-catalysed hydrogen–deuterium exchange on the diketone derivative (18), obtained by Jones oxidation of austalide F, leads to the regio- and stereo-specific incorporation of deuterium atoms at C-12, C-18, and C-21. A mechanism is proposed to explain the incorporation of a deuterium atom at C-21, a position γ to the C-13 carbonyl group.

Structures of the austalides A–E, five noval toxic metabolites from Aspergillus ustus

The structures of austalides A–E, five new mycotoxins isolated from cultures of Aspergillus ustus, are based on the X-ray crystallographic study of austalide A, chemical derivatization, and correlation of their 13C and 1H n.m.r spectral data.

Stereochemical course of ring formation in fumitremorgin B and verruculogen, metabolites of Penicillium verruculosum: investigation into the loss of stereochemical integrity of the geminal methyl groups

Incorporation studies of [2-2H3,2–13C]acetate and different (2H,13C)-labelled rnevalonolactones into verruculogen established the stereochemical course of ring C formation in fumitremorgin B, which results in the loss of stereochemical integrity of the C-22 methyl groups, and of the formation of the eight-membered peroxide ring at C-25 in verruculogen.

Metabolites of Aspergillus ustus. Part 2. Stereoelectronic control in the acid-catalysed hydrolysis of the ortho ester moiety in austalides A–F

Attempted acetylation of the C-13 hydroxy function in austalides B (2) and D (4) with either acetic anhydride and pyridine or 4-dimethylaminopyridine fails. Acetylation occurs under more forcing conditions using acetic anhydride and catalytic amounts of perchloric acid, but the major products are in each case δ-lactone derivatives. It is shown that the formation of a δ- instead of an Iµ-lactone from the ortho ester moiety is favoured because initial cleavage of the C(17)–O(16) bond is stereoelectronically assisted by the two other oxygen atoms, O-30 and O-35, since each has a lone-pair orbital antiperiplanar to this bond.

Biosynthesis of verruculogen, a tremorgenic metabolite of Penicillium verruculosum: stereochemical course of peroxide ring formation

The stereochemical course of the formation of the eight-membered peroxide ring in verruculogen, a tremorgenic metabolite of Penicillium verruculosum, was established by incorporation of 13C-, 2H-, and 18O-labelled precursors.

Absolute configuration and biosynthesis of the austalides, meroterpenoid metabolites of Aspergillus ustus: mode of cyclisation of the farnesyl moiety

A study of the fate of the hydrogen atoms in the biosynthesis of austalide D using both (13C,2H)- and 2H-labelled mevalonolactones established, in conjunction with absolute configuration as determined by an X-ray crystallographic study, the stereochemical course of cyclisation of the farnesyl-derived moiety.

Stereochemical studies on the biosynthesis of viridicatumtoxin: evidence for a 1,3-hydride shift in the formation of the spirobicyclic ring system

A study of the fate of the hydrogen atoms in the biosynthesis of viridicatumtoxin using (13C,2H)-labelled mevalonolactones established that a 1,3-hydride shift occurs in the formation of the spirobicyclic ring system.

Metabolites of Aspergillus ustus. Part 4. Stable-isotope labelling studies on the biosynthesis of the austalides

Incorporationof [1-13C]-, [1,2-13C2]-and [1-13C,2,2,2-2H3]-acetate, and (3RS)-[2-13C]mevalonolactone into austalide D(2), a metabolite of Aspergillus ustus, shows that the austalides are derived from 6[(2E,6E)farnesyl]-5,7-dihydroxy-4-methylphthalide. The proposed mechanism for the subsequent cyclisation and oxidative modifications of the farnesyl moiety, consistent with the relative stereochemistry of the austalides and the structures of the cometabolites, austalides J (4), K (5), and L (6), is supported by the incorporation of austalide K into austalide D. The addition of ethanol to growing cultures of A. ustus severely inhibits normal metabolite production and induces the formation of a new metabolite, ethyl 6-ethyl-2,4-dihydroxy-3-methylbenzoate (11). The biosynthetic origin of this metabolite was studied using [1-13C]- and [1-13C,2,2,2-2H3]acetate and (2S)-[methyl-13C]methionine as precursors.