Kevin D. Barrow

Fusicoccin. Part III. The structure of fusicoccin H

The structure (IIa) of fusicoccin H, a minor phytotoxic glycoside produced by Fusicoccum amydali, has been determined by degradative studies and by chemical correlation with the known fusicoccin series. Feeding experiments have shown that fusicoccin H can act as a precursor of fusicoccin. This strongly suggests that fusicoccin is a diterpenoid and not a degraded sesterterpenoid.

Biosynthesis of flavoglaucin. Stereochemistry of aromatic isoprenylation

Incorporation studies with [1,2-13C]acetate have established the regular polyketide nature of flavoglaucin modified by isoprenylation. The aromatic isoprenylation occurs without any change in stereochemistry of the olefin in the dimethylallyl moiety.

Fusicoccin. Part IV. The structure of fusicoccin J

The structure (III) of fusicoccin J, a minor glucoside produced by Fusicoccum amygdali, has been determined by its synthesis from fusicoccin (Ia). Feeding experiments have established that fusicoccin J can act as an efficient precursor of fusicoccin. This suggests that the new fusicoccin is a late intermediate on the biosynthetic pathway.

Fusicoccin. Part I. The nature of the substituent groups

Degradative studies of fusicoccin, a phytotoxic glycoside produced by Fusicoccum amygdali resulted in a revised molecular formaula C36H56O12 and the characterisation of all the oxygen substituents. Thus fusicoccin was shown to be a glycoside with one methoxy, one acetoxy, and two hydroxy aglycone substituents, the sugar moiety consisting of an α-glucoside carrying one acetyl group at C-3 and a 1,1-dimethylallyl ether at C-6. The efficient incorporation of DL-[2-14C]mevalonic acid lactone into fusicoccin indicated a polyisoprenoid structure.

Fusicoccin. Part II. The constitution of fusicoccin

Chemical evidence is presented which largely establishes the constitution of fusicoccin and of its aglycone. The preparation of a suitable heavy-atom derivative has enabled the complete structure of the aglycone to be determined by X-ray crystallography. Combining this result with chemical evidence has afforded the complete structure of fusicoccin.

Biosynthesis of illudosin, a fomannosane-type sesquiterpene, by the Basidiomycete Omphalotus nidiformis

Sites of incorporation of 13C-labelled acetates and [1-13C,2-2H3]acetate into illudosin 1, a fomannosane type sesquiterpene produced by Omphalotus nidiformis, have been determined by 13C NMR spectroscopy. The biosynthesis of illudosin was as expected, apart from the presence of some labelled species with two deuteriums attached to C-6 in illudosin which is also labelled at C-5 with 13C. This unusual rearrangement was also observed in the co-metabolite illudin M. A mechanism is proposed which involves the formation of a series of carbocations at the active site of the cyclase enzyme that can be reprotonated (or deuterated) non-stereospecifically.

Biosynthesis of echinulin. The stereochemistry of aromatic isoprenylation

The incorporation of [1,2-13C]acetate into echinulin has shown that the aromatic isoprenylation at the 5- and 7-positions occurs without any change in stereochemistry around the double bond of the dimethyl-allyl group and chemical degradation of echinulin, obtained by feeding [5(R)-3H]- and [5(S)-3H]mevalonate, to isopentylamine and oxidation with pea seedling diamine oxidase has shown that the isoprenylation reactions at positions 5- and 7- proceed with inversion of configuration at the allylic pyrophosphate.

Fusicoccin. Part V. The biosynthesis of fusicoccin from [1-13C]- and [2-13C]-acetate

The positions of labelled atoms in fusicoccin biosynthesised from [1-13C]- and [2-13C]-acetate have been determined by 13C n.m.r. spectroscopy. The results are consistent with fusicoccin being formed by direct cyclisation of a precursor such as geranylgeraniol pyrophosphate.