Terpenoids are a very prominent class of natural compounds produced in diverse genera of plants, fungi, algae and sponges. They gained significant pharmaceutical value since prehistoric times, due to their broad spectrum of medical applications. The fragrant leaves of Eucalyptus trees are a rich source of terpenoids. Therefore this review starts by summarizing the main terpenoid compounds present in Eucalyptus globulus, E. citriodora, E. radiata and E. resinifera and describing their biosynthetic pathways. Of the enormous number of pharmaceutically important terpenoids, this paper also reviews some well established and recently discovered examples and discusses their medical applications. In this context, the synthetic processes for (-)-menthol, (-)-cis-carveol, (+)-artemisinine, (+)-merrilactone A and (-)-sclareol are presented. The tricyclic sesquiterpene (-)-englerin A isolated from the stem bark of the Phyllanthus engleri plant (Euphorbiaceae) is highly active against certain renal cancer cell lines. In addition, recent studies showed that englerin A is also a potent and selective activator of TRPC4 and TRPC5 calcium channels. These important findings were the motivation for several renowned research labs to achieve a total synthesis of (-)-englerin A. Two prominent examples-Christmann and Metz-are compared and discussed in detail.
A systematic investigation of the mono-O-glycosylation of meso-, (S,S)-, and (R,R)-cyclohexane-1,2-diol with the D-glucose-derived 2-ketohexosyl bromide 5 is presented. In each instance, simple Königs−Knorr conditions not only elicit the exclusive formation of the respective β-2-ketoglycosides, but also effect their subsequent intramolecular hemiketalization to provide tricycles with a 1,5,10-trioxa-perhydroanthracene framework. The linkage geometries of the products − two each from the meso-(Ǟ 10, 12) and (S,S)-diols (Ǟ 16, 17), and only one from the (R,R)-isomer (Ǟ 26) − are determined in the hemiketalization step by an interplay of the anomeric effect and steric factors, favoring those isomers in which the pyran ring oxygen and the ketal-OH are in a trans-diaxial disposition. The most propitious case with respect to uniformity of reaction and yield (87%) turned out to the (R,R)-diolderived cis-cisoid-trans-fused 26 as steric and stereoelectronic factors operate concertedly in the hemiketalization
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