Current Organic Chemistry - Volume 13, Issue 12, 2009

In this issue of Current Organic Chemistry, the emphasis is placed on the ever-changing field of the fungi as intriguing source of bioactive natural products and as efficient biocatalysts. I have been very pleased with the positive response to the first issue, which must be attributed to the high quality papers of the contributing authors. In my opinion this issue maintains that high level of quality and interest. So, the present issue features four chapters written by experts in the fields of biotechnology and bioorganic of fungi. The first review is illustrative, written by Warley de S. Borges, Keyller B. Borges, Pierina S. Bonato, Suraia Said and Monica T. Pupo, present an authoritative account on recent research data about endophytes as sources of novel and bioactive natural products, as producers of enzymes and their use on biotransformation processes. The occurrence of 2,6-cyclofarnesanes in the nature is scarce and their presence has only been described in fungi, where they are considered as taxonomic markers. Most of these cyclofarnesanes present interesting biological activities. In the second contribution, Barrero et al., provide a comprehensive review about the natural occurrence, biological properties, biosynthesis and chemical synthesis of these compounds. The biotransformation of sesquiterpenoids is an important tool in the structural modification of these compounds, especially for natural products with complex structures, due to their significant regio- and stereo-selectivity. The third review “Biotransformations of Sesquiterpenoids by Rhizopus species” provided by A.Garcia-Granados et al., offers a comprehensive and up-to-date review on the biotransformation of sesquiterpenoids by diverse Rhizopus species.

Endophytes exhibit a complex web of interactions with host plants and with other endophytic microorganisms and therefore they have been intensively studied over the last several years as prolific sources of new and bioactive natural products. In fact, an impressive number of natural products have been produced by endophytic microorganisms. In addition, some studies have shown endophytes to be good producers of useful enzymes to improve industrial processes. More recently, endophytes have also received attention as biocatalysts in the chemical transformation of natural products and drugs. Results have shown their ability to modify chemical structures with a high degree of stereospecificity. Some reactions are similar to those catalyzed by mammal phase I metabolism; therefore endophytes could be used as models for drug metabolism studies. This paper summarizes recent data on endophytes research as a source of novel and bioactive natural products, as producers of enzymes and their use on biotransformation processes.

The occurrence of 2,6-cyclofarnesanes in Nature is scarce and their presence has been only described in microscopic fungi. However this class of compounds constitutes an interesting group of natural products since they present specific bioactivities. This review moves on to the advances achieved in this kind of structures. These advances are presented in three parts. The first one presents the structure and biological properties of these cyclopentane derivatives. In fact, these compounds could be divided in two groups, namely, chokols, isolated from the stroma of the timothy Phleum pratens inFfected by the endophytic fungus Epicloe typhina, and cyclonerodiol and derivatives, chemically related to chokols, and found to be widespread in fungi. Both, chokols and cyclonerodiol have been described as antifungal, although other biological activities attributed to these compounds are also discussed. The second part considers the biosynthetic studies justifying the formation of cyclonerolidol from cyclization of nerolidyl pyrophosphate. The third and more extended part of the review is devoted to the achievements made in different synthetic strategies directed at some of cyclopentane sesquiterpenes.

Biotransformations of sesquiterpenoids by Rhizopus species have been used to provide new derivatives with potential biological activities. In these fungal transformation processes, the main action of the Rhizopus genus has been the introduction of hydroxyl groups into remote positions of the molecules, which is difficult to achieve by chemical means. Other reactions carried out with stereo-selectivity by these microorganisms have been: epoxidations of double bonds, hydrogenations of the exocyclic C-C double bond of the lactone rings, and reductions of carbonyl groups giving Salcohols. Several reactions have also been performed by these fungi with regio-selectivity, such as oxidations of hydroxyl groups and deacetylations. In some biotransformation processes, a carbon skeleton rearrangement of the substrate has taken place.

One of the major problems facing the industrialized world today is the contamination of soils, ground water, sediments, surfacewater and air with hazardous and toxic chemicals. The application of microorganisms which degrade or transform hazardous organic contaminants to less toxic compounds has become increasingly popular in recent years. This review, with approximately 300 references covering the period 2005-2008, describes the use of fungi as a method of bioremediation to clean up environmental pollutants.

Xanthones or 9H-xanthen-9-ones (dibenzo-γ-pyrones) comprise an important class of oxygenated heterocycles, with prenylated derivatives being the most abundant group. The prenylated xanthones are found to possess a wide range of important biological and pharmacological activities. As a consequence of this, synthetic strategies leading to new and more complex molecules have been widely explored in the past years. In our literature survey, from January 1963 to March 2009, a total of 93 synthetic prenylated xanthones were described and 24 of them were firstly obtained from natural sources. Thus, the main purpose of this review is to report synthetic methods to obtain prenylated xanthones, such as simple prenylation, Claisen rearrangement and/or cyclization. We also discuss the application of new methodologies to the synthesis of prenylated derivatives, such as microwave assisted organic synthesis, heterogeneous catalysis with Montmorillonite K10 Clay as catalyst and the combination of heterogeneous catalysis with microwave irradiation. Furthermore, different approaches and methodologies used to synthesize bioactive natural prenylated xanthones, like α-mangostin, are also discussed. Finally, biological activities of prenylated xanthones are also briefly referred.