Abstract
This paper presents a review on kaurane diterpenes and their glycoside derivatives, covering aspects of their occurrence, biological activities and the synthesis of these natural products and their analogues. First, it shows and classifies diterpenes, in accordance with the already established structural criteria in the literature. Then, kaurane diterpenes are presented, focusing on their chemical structures, occurrence in the plant kingdom and their main, recently described, biological activities. Moreover, the most significant works, published between 1964 and November 2006, which describe the total synthesis or structural transformations of some kaurane diterpenes, including either semisynthetic and/or microbiological methodologies, are consisely reviewed. At this point, some general considerations on glycosides are introduced, and kaurane glycosides are presented and discussed on the basis of their toxic importance and occurrence in the plant kingdom, having focused on related aspects of their biological activities and the relationships between these activities and the structural factors of their molecules. Finally, the principal methods of glycosidation by enzymatic and chemical processes are both presented, and a few papers on the synthesis of kaurane glycosides are succinctly discussed.
Highlights
Kauranes represent an important group of tetracyclic diterpenes and their structures are constituted by a perhydrophenantrene unit (A, B and C rings) fused with a cyclopentane unit (D ring) formed by a bridge of two carbons between C-8 and C-13 (Figure 3) [4]
Kaurenoic acid (1), an ent-kaurane diterpene that possesses a wide spectrum of bioactivities such as antiinflammatory, antibacterial, antifungal and moluscical properties, among others [6], is not commercially available, but it is relatively abundant in some species belonging to the Wedelia [40], Mikania [41], Annona [42] and Xylopia [43] genera, which has motivated its quantification in these plant species [42,43,44,45,46,47], enabling the use of all of them as natural sources of this diterpene
Diverse biological activities have been described for the kaurane diterpenes and many of them, including plant growth regulating, antimicrobial, antiparasitic, insect antifeedant, cytotoxic, antitumour, anti-HIV, steroidogenic, antifertility, hipotensive and antiinflamatory activities, among others, are referred to in the review published by Ghisalberti in 1997 [6]
Summary
The broad spectrum of biological activities presented by the kaurane diterpenes has motivated countless studies of structural modifications of the kaurane skeleton aiming to obtain new potentially bioactive substances. These structural transformations have been achieved either chemically [54,7172] or microbiologically, using microorganism cultures [73,74,75].
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