Fanie R. van Heerden

Antimalarial activity of plant metabolites

This review covers the structures of compounds with antiplasmodial activity isolated from plants and is organized according to plant family. A total of 170 structures has been reviewed from 186 references found in the literature up to December 2000.

Palladium-catalysed substitution reactions of geminal allylic diacetates

Abstract The Pd(0)-catalysed substitution of allylic 1,1-diacetates by both carbon and oxygen nucleophiles is described. The products isolated resulted from either single or double substitution reactions.

Tremorgenic mycotoxins from Penicillium crustosum: isolation of penitrems A–F and the structure elucidation and absolute configuration of penitrem A

The isolation and characteristics of six tremorgenic mycotoxins, penitrems A–F from cultures of Penicillium crustosum are reported. The assignment of structure (2) to penitrem A is based on a detailed study of its high-field 1H and 13C n.m.r. spectra. The conformation and relative configuration of penitrem A was deduced from the observed proton–proton nuclear Overhauser effects (n.O.e.) and the magnitude of the proton–proton coupling constants. The chirality of C-25 was determined as S by the ‘partial resolution’ method of Horeau and penitrem A must therefore have the (12R, 14S, 15R, 18S, 19R, 22S, 23S, 24R, 25S, 26R, 28S, 31R, 32S) absolute configuration.

Cytotoxic isoflavones from Erythrina indica

Bioassay-directed fractionation of the CH(2)Cl(2)-MeOH (1:1) extract of the stem bark of Erythrina indica, has resulted in the isolation of two new isoflavone derivatives named indicanines D and E together with 11 known compounds including: six isoflavones (genistein, wighteone, alpinumisoflavone, dimethylalpinumisoflavone, 8-prenyl erythrinin C, and erysenegalensein E), one cinnamate (erythrinassinate B), two pentacyclic triterpenes (oleanolic acid and erythrodiol), and two phytosterols (stigmasterol and its 3-O-beta-D-glucopyranoside). The structures of the new compounds were elucidated by means of spectroscopic analysis. The in vitro cytocidal activity against KB cells of some of the isolated compounds is also reported.

Structure and synthesis of some complex pyranoisoflavonoids from the bark of Dalbergia nitidula Welw. ex Bak.

Exhaustive examination of the flavonoid content of the bark of Dalbergia nitidula revealed, amongst other substances a mixture of eight novel and complex (3S)-isoflavans [leiocin, leiocinol, nitidulin, nitidulan, heminitidulan] and (6aS,11aS)-pterocarpans [nitiducarpin, hemileiocarpin, nitiducol]. These insoflavonoids all possess the equivalent of prenyl or geranyl side-chains, which, with one exception, are cyclized to stereochemically related 2H-pyran moeities during biogenesis. Structures determined by physical means were substantiated for the isoflavans by a single synthesis.

Tremorgenic mycotoxins from Penicillium crustosum. Biosynthesis of penitrem A

The biosynthesis of penitrem A has been studied with both 13C- and 2H-labelled precursors, viz. [1-13C]-, [2-13C]-, [1,2-13C2]-, and [1-13C,2-2H3]-acetate, [2-13C]-, [2,3-13C2]-, [2-2H2]- and [5-2H2]-mevalonate. The results show that penitrem A is derived from tryptophan, which contributes the indole moiety of the metabolite, geranylgeranylpyrophosphate, and two isopentenylpyrophosphate units. A 1,2-bond migration, involving the 2,3-bond of a mevalonate unit, occurs in the course of the biosynthesis and results in the observation of a one-bond (C,C) coupling between two [1-13C]acetate-derived carbon atoms and between two [2-13C]acetate-derived carbon atoms. Analysis of the one-bond (C,C)coupling constants in [2-13C]acetate-derived penitrem A showed that [1,2-13C2]acetate was formed during the fermentation. Although loss of water from an hydroxyisopropyl group to form the isopropenyl function present in penitrem A should proceed with retention of the stereochemical integrity of the two methyl groups, isomerization of the double bond causes equal distribution of 13C label between C-36 and C-38 and precludes any stereochemical deductions.

The major phenolic compounds in the leaves of Cyclopia species (honeybush tea)

Abstract The phenolic compounds of the leaves of Cyclopia species (tribe Podalyrieae) are of both chemotaxonomic and commercial interest, as the leaves are used to brew a herbal drink known as honeybush tea. Despite the commercial importance of Cyclopia, virtually nothing was known about the chemistry of the leaves prior to the present work. Methanolic extracts from leaves of 22 species were screened for the presence and distribution of phenolic compounds. Three major constituents of the leaves were identified as mangiferin (a xanthone) and glycosides of the flavanones hesperitin and isosakuranetin. The combination of these three compounds is a unique character for Cyclopia, as none of them are present in any of the other genera of the tribes Podalyrieae and Liparieae. Various combinations of the three compounds occur in the different infrageneric groups, but the species are remarkably similar. These results are thus of chemotaxonomic significance at the generic rather than infrageneric level.

A prenylated xanthone from Allanblackia floribunda.

A new prenylated xanthone, named allanxanthone A, was isolated from the stem bark of Allanblackia floribunda in addition to known compounds, 1,5-dihydoxyxanthone, 1,5,6-trihydroxy-3,7-dimethoxyxanthone, stigmasterol and stigmasteryl-3-O-beta-D-glucopyranoside. The structure of the new compound was assigned as 1,3,5-trihydroxy-2-(3-methylbut-2-enyl)-4-(1,1-dimethylprop-2-enyl) xanthone, by means of spectroscopic analysis. The 13C NMR spectral data of 1,5-dihydroxyxanthone is reported here for the first time as well as the in vitro cytotoxic activity of xanthone metabolites against the KB cell line.

Tremorgenic mycotoxins from Penicillium crustosum. Structure elucidation and absolute configuration of penitrems B–F

The assignment of structures (2)–(6) to penitrems B–F, tremorgenic mycotoxins isolated from cultures of Penicillium crustosum, is based on a detailed study of their high field 1H and 13C n.m.r. spectra. The relative configuration of penitrems B–F was deduced from the magnitude of the proton–proton coupling constants. The absolute configuration of these metabolites is based on comparisons with that of penitrem A and on the absolute configuration of penitrem D determined by the ‘partial resolution’ method of Horeau.

Application of the Suzuki-Miyaura reaction in the synthesis of flavonoids.

The application of the Suzuki-Miyaura reaction in the synthesis of flavonoids, an important class of natural products, is reviewed. This reaction has not only been employed to provide access to flavonoid nuclei, but has also been applied to the synthesis of dimeric flavonoids and in the synthesis of libraries of flavonoid derivatives for biological activity studies. The classes of flavonoids that are discussed are the chalcones, flavones, isoflavones, neoflavones, biflavones and derivatives of flavonoids obtained by C-C bond formation via the Suzuki-Miyaura reaction.