Jannie P. J. Marais

Manuka Oil, A Natural Herbicide with Preemergence Activity

Abstract Natural herbicides approved in organic agriculture are primarily nonselective burn-down essential oils applied POST. Multiple applications are often required due to their low efficacy. To address this problem, the in vivo herbicidal activity of manuka oil, the essential oil distilled from manuka tree (Leptospermum scoparium J.R. and G. Forst), was tested on selected broadleaf and grass weeds. While manuka oil exhibited good POST activity when applied in combination with a commercial lemongrass oil–based herbicide, it ultimately demonstrated interesting PRE activity, providing control of large crabgrass seedlings at a rate of 3 L ha−1. Manuka oil and its main active ingredient, leptospermone, were stable in soil for up to 7 d and had half-lives of 18 and 15 d, respectively. The systemic activity of manuka oil addresses many of the current limitations associated with natural herbicides. Additionally, its soil persistence opens up a multitude of new possibilities for the use of manuka oil as a tool for weed management and may be a potential bridge between traditional and organic agriculture. Nomenclature: Leptospermone; 2,2,4,4-tetramethyl-6-(3-methyl-1-oxobutyl)-1,3,5-cyclohexanetrione, CAS 567-75-9; large crabgrass; Digitaria sanguinalis (L.) Scop. DIGSA.

Phytochemistry of the mopane, Colophospermum mopane.

The polyphenolic pool of the heartwood of the mopane, Colophospermum mopane Kirk ex J. Leonard, exhibits extreme diversity and complexity. It comprises a variety of monomeric flavonoids, e.g. flavan-3-ols, flavan-3,4-diols including the mopanols and peltogynols, flavonols, dimeric proanthocyanidins, e.g. proguibourtinidins, profisetinidins, promopanidins, propeltogynidins, and a variety of profisetinidin-type triflavanoids. The di- and tri-meric proanthocyanidins are accompanied by several functionalized tetrahydropyrano- and hexahydrodipyrano-chromenes (phlobatannins) that originate from the bi- and tri-flavanoids, respectively, via rearrangement of the pyran heterocycle(s). Owing to the predominance of the 5-deoxy (A-ring) flavan-3-ols, the chain terminating moieties in the biosynthesis of oligo- and poly-meric proanthocyanidins, the di- and tri-meric analogs also exhibit diversity as far as interflavanyl bonding positions are concerned. Such heterogeneity results from the reduced nucleophilicity of the A-rings of 5-deoxy flavan-3-ols, compared to the A-rings of the 5-oxy analogs (catechins), hence permitting alternative centers to participate in proanthocyanidin formation. Biomimetic-type syntheses were extensively utilized to unequivocally establish constitution and absolute stereochemistry of both the conventional and pyran ring rearranged-type di- and tri-meric compounds. Comprehension of the intricate mechanistic and stereochemical course of the pyran ring rearrangement reactions also contributed significantly to unambiguous structure elucidations. The aerial parts of the mopane are rich in essential oils that comprise mainly alpha-pinene and limonene, which are presumably responsible for the strong turpentine odor of the pods. The leaves also contain significant concentrations of beta-sitosterol and stigmasterol which are apparently the source of sterols in various organs of the mopane moth, Gonimbrasia belina. Three diterpenes, dihydrogrindelic acid, labd-13E-en-15-oate and dihydrogrindelaldehyde are present in the bark and seeds, the latter compound exhibiting significant cytotoxicity against a human breast cancer cell line.

Proanthocyanidins: Chemistry and Biology

This chapter summarizes progress in the chemistry and biology of the proanthocyanidins, a diverse group of naturally occurring plant polyphenols, emphasizing results emanating from studies in the post-CONAP-I era. Besides defining the terms proanthocyanidins and leucoanthocyanidins, differentiating between the proanthocyanidins and the bi- through hexa-flavonoids, and describing the proper nomenclature of the proanthocyanidins, the review also describes the origin and role of the nucleophilic and electrophilic monomeric flavonoid moieties in the biosynthesis of the proanthocyanidins, as well as the numerous synthesis protocols aimed at the stereoselective access of, for example, the flavans and flavan-3-ols. The section covering the natural occurrence and chemistry of the A- and B-type proanthocyanidins is dominated by a comprehensive discussion of the siginificant efforts devoted to development of synthesis protocols targeting the procyanidins up to the undecamer level in quantities sufficient for assessment of their in vitro biological activities. Ample references are provided for readers interested in the class of compounds termed nonproanthocyanidins with flavan or flavan-3-ol constituent units, especially the black tea pigments and the ‘complex tannins’, the latter comprising the two major classes of tannins as constituent units. The conformation section discusses the conformational flexibility of the pyran heterocycle and isomerism due to restricted rotation about the interflavanyl bond axis. Several references are provided to permit access to the considerable influence of HPLC–MS analyses on the structure elucidation of proanthocyanidins, as well as their profound effects on human health and nutrition.