Runner R.T. Majinda

Eremostachiin: a new furanolabdane diterpene glycoside from Eremostachys glabra

The reversed-phase preparative HPLC analysis of the methanol extract of the rhizomes of Eremostachys glabra (Lamiaceae) led to the isolation of two furanolabdane diterpene glycosides, phlomisoside II, and β-D-glucopyranosyl-(1→2)-β-D-glucopyranosyl ester of phlomisoic acid (named eremostachiin), the latter being a novel natural product. The structure of the new compound has been elucidated unambiguously by HRMS and a series of 1D- and 2D-NMR spectroscopic techniques. The free radical scavenging activity of these compounds was assessed using the DPPH assay.

Recent results from natural product research at the University of Botswana

Marketed plants are very important items of trade in many parts of the world. The community uses these for a variety of purposes such as foods, cosmetics, flavors, spices, and medicines. It seems that plants that are used for medicinal purposes form the most common category. Four plants used for treatment of microbial infections, viz., Bolusanthus speciosus, Erythrina latissima, Crotalaria podocarpa, and Elephantorrhiza goetzei, were investigated, and these yielded several known and novel structures, some with appreciable antibiotic activity against the test organisms. The activity of some of the isolated plants and the parts of the plant from which these were obtained lend support to their traditional use. Bulbine abyssinica and B. capitata yielded phenylanthraquinones, some of which were shown to possess strong antiplasmodial activity. In addition, these yielded isofuranonaphthoquinones, which were also found to be weakly antiplasmodial and antioxidant. Scilla nervosa yielded several known and novel homoisoflavoinds of the 3-benzylchroman-4-ones and 3-benzylidinechroman-4-one type, as well as some stilbenoids. The homoisoflavonoids showed strong antitumor activity against various cancer cell lines. Rhus pyroides gave a novel bichalcone, which showed weak antifeedant activity, consistent with the observation by farmers that the plant was avoided by corn cricket. Results from investigated Dorstenia species originating from Cameroon, Ethiopia, and Tanzania yielded styrenes, coumarins, chalcones, and flavonoids. The chalcones and flavonoids showed various levels of prenylation or geranylation, and an observation made so far is that prenylated flavonoids are only found in Dorstenia species of African origin. The only example of a bis-geranylated chalcone is found in Dorstenia.

Antioxidant phenolic compounds from the leaves of Erica Arborea (Ericaceae).

Reversed-phase HPLC analyses of the methanol extract of the leaves of Erica arborea afforded a novel phenylpropanoid glucoside, named ericarborin, together with five flavonoids, dihydromyricetin 3-O-α-L-rhamnopyranoside, quercetin 3-O-β-D-glucopyranoside, quercetin 3-O-α-L-rhamnopyranoside, apigenin 7-O-β-D-glucopyranoside and apigenin 7-O-β-D-(6-O-acetyl-glucopyranoside). While the structure of ericarborin was determined by extensive 1D and 2D NMR analyses, the structures of all known flavonoids were determined by direct comparison of their spectroscopic data with respective literature data. The antioxidant properties of these compounds were assessed by the DPPH assay. The chemotaxonomic significance of these phenolic compounds has been discussed.

Flavonoid glycosides from Crotalaria podocarpa

Abstract The structures of two flavonoid glycosides isolated from the aerial parts of Crotalaria podocarpa have been established as apigenin 7-O-β- d -apiofuranosyl (1→6)-β-glucopyranoside and a new compound, acacetin 7-O-β- d -apiofuranosyl (1→6)-β- d -glucopyranoside. The structures of these flavonoid glycosides were determined using spectroscopic methods.

A new stilbene glycoside from Elephantorrhiza goetzei

A new stilbene glycoside, 5-methoxy-(E)-resveratrol 3-O-rutinoside (1) was isolated from the root bark of Elephantorrhiza goetzei, along with known compounds, namely gallic acid, (E)-resveratrol, (+/-)-catechin, (+/-)-gallocatechin and oleanene triterpenoids. The combined ethyl acetate-methanol extract exhibited high lethality against brine shrimp larvae (LC(50) 10.8 ppm) compared to the isolates, some of which were not active.

Distribution of sesquiterpene lactones and polyacetylenic thiophenes in Echinops

Abstract The distribution of sesquiterpene lactones, polyacetylenic thiophenes and their derivatives in the roots of nine Echinops species suggests three distinct groups.

A new flavan from Elephantorrhiza goetzei

Abstract A new compound, 3,3′,4′,5,6,7,8-heptahydroxyflavan ( 1 ), was isolated from the roots of Elephantorrhiza goetzei . The crude extract, and its n -butanol and residual water fractions showed antimicrobial activity. Semi-purified extracts showed higher activity and of the purified compounds only methyl gallate gave activity below 100 μg of sample.

Tylosema esculentum extractives and their bioactivity

The investigation of Tylosema esculentum (Morama) husks, cotyledons, and tuber yielded griffonilide 2, compound 1, griffonin 3, gallic acid 4, protocatechuic acid 5, β-sitosterol 6, behenic acid 7, oleic acid 8, sucrose 9, 2-O-ethyl-α-D-glucopyranoside 10, kaempferol 11 and kaempferol-3-O-β-D-glucopyranoside 12. The structures of the isolates were determined by NMR, HR-TOF EIMS, IR and UV-vis spectroscopy, and by comparison with literature data. The husk EtOAc and n-butanol extracts demonstrated >90% DPPH radical scavenging activity at concentrations of 25, 50 and 250 μg/mL. Furthermore the husk extracts showed higher total phenolic content (233 mg GAE/g). The extractives exhibited minimum inhibitory quantities of 50-100 μg or no activity against Staphylococcus aureus, Escherichia coli, Bacillus subtilis, Pseudomonas aeruginosa and Candida albicans. The tuber extracts were inactive against Caco-2 and Hela cell lines, while the husk extracts showed low activity against Caco-2 and Vero cell line with IC(50) values >400 μg/mL. The GC-MS analysis showed the beans and tuber non-polar (n-hexane) extracts major constituents as fatty acids.

Dihydro-β-agarofuran sesquiterpenes and pentacyclic triterpenoids from the root bark of Osyris lanceolata.

Three new dihydro-beta-agarofuran polyesters, 1alpha,9beta-difuranoyloxy-2-oxodihydro-beta-agarofuran (1), 1alpha,9beta-difuranoyloxy-2-oxo-3-enedihydro-beta-agarofuran (2), and 1alpha,9beta-difuranoyloxydihydro-beta-agarofuran (3), have been isolated from the CHCl(3) extract of the root bark of Osyris lanceolata, together with two known pentacylic triterpenoids, 4 and 5. Compounds 1-5 did not scavenge the DPPH radical within 30 min of reaction time. All five compounds displayed antifungal activity against Candida albicans. Compounds 1, 3, 4, and 5 showed antibacterial activity against the Gram-positive Bacillus subtilis and Staphylococcus aureus and Gram-negative Escherichia coli and Pseudomonas aeruginosa, with 4 and 5 being the most active. Compound 2 displayed weak antibacterial activity only against Escherichia coli.

Bioactive non-alkaloidal constituents from the genus Erythrina

The genus Erythrina is very rich in secondary metabolites particularly of the flavonoids class. A literature survey of non-alkaloidal secondary metabolites from Erythrina showed the presence of flavanones, flavonols, chalcones, cinnamoylphenols, stilbenoids, isoflavones, isoflavans, isoflavanones, pterocarpans, isoflav-3-enes, 3-phenoxychromones, coumastans, 3-phenyl-coumarins, lignans, cinnamate esters, simple phenolics, triterpenes, sesquiterpenes, long-chain carboxylic acids, and long-chain alcohols. The documented bioactivities of some of the isolated metabolites range from antimicrobial, anti-inflammatory, inhibition of platelet aggregation, tyrosinase inhibition, phospholipase A2 (PLA2) inhibitors, cyclooxygenase inhibitors, antioxidant, inhibitors of NA+/H+ exchange system, phospholipase C inhibitors, behavioural depression, muscle relaxation, β-adrenergic inhibition, diuretic, anticancer, cytotoxic, DNA-repair properties, oestrogenic or proestrogenic activities, antitrypanosomal, antiplasmodial, and anti-HIV activities. The bioactivity profile represented the various classes fairly reasonably but it became apparent that a number of these isolates have not yet been tested for biological activity. A very close agreement between ethno-medical use of the various Erythrina extract preparations (Table 1) and the results of biological activity (Table 2) was found, with the reported activities of pure isolates strongly supporting the documented ethno-medical uses and reported pharmacological activity. The most common activity exhibited by extracts and pure isolates was mostly microbial related. It was also interesting to note that some compounds showed high efficacy against resistant organisms, a very important aspect, since most used drugs especially antibiotics tend to produce resistance to certain strains of organisms. The survey showed furthermore that certain structural features were essential for certain activities and also that compounds that were found active were usually effective against not just one but several disease functions. One cannot help but surmise that more activities are yet to be reported for these same isolates. The challenge remains for researchers in the various pertinent disciplines to carry out more work on these to fill in the knowledge gaps that still exist.