Current Organic Chemistry - Volume 10, Issue 16, 2006

The recent developments of a new chemistry of sulfur dioxide in which this substance adds to 1,3-dienes, acting as a dienophile, are reviewed. Experimental data and theoretical results are analyzed and discussed in order to elucidate the mechanisms through which hetero-Diels-Alder and cheletropic cycloaddition reactions proceed. Discussions about the concerted vs. multi-step nature of the mechanism, the regio- and stereoselectivity aspects, the role played by Lewis acids as catalysts, solvent and substituent effects, and conformational preferences of the resulting sultines and sulfolenes are carried out. The important finding that sulfur dioxide promotes its hetero-Diels-Alder and cheletropic additions to dienes is analyzed in details. Structural, thermodynamic and kinetic features affecting the competition between the two reactions are presented and discussed. Particularly, the suitability of a mechanism in which, by an appropriate rearrangement, sultines might give rise to sulfolenes is analyzed in the light of experimental and computational studies. It is shown that the combined experimental/theoretical research carried out to develop this new chemistry of sulfur dioxide constitutes a synergistic approach bearing excellent fruits.

Although biocatalysts still only account for a relatively small fraction of the total catalyst market, an increasing number of companies are beginning to supplement their range of metal and chemical catalysts with biocatalysts. Consequently, researchers have been trying to take advantage of new technical developments in order to make biocatalysis a viable alternative to traditional synthetic options. Particularly, application of biologically derived catalysts to the synthesis of agrochemicals has grown rapidly in recent years. This review, with approximately 180 references, deals with the use of isolated enzymes and microorganisms as catalysts for the preparation of pesticides and their precursors, covering the period previous to December 2004. An overview of the industrial production of agrochemicals is also included.

Ladderanes are hydrocarbons consisting of two or more fused cyclobutane rings. These molecules and their derivatives are of fundamental interest to physical organic chemists due to their strain and electronic properties. Recently, the astonishing discovery that some bacterial lipids contain ladderane substructures has revitalized interest in this class of molecules. This review describes the synthesis, reactivity, applications, and biology of ladderanes.

Cyclic peptides are a special bioactive group of compounds with interesting pharmacological and biochemical properties. Different natural sources are mentioned in this paper, but interest is focused on those cyclopeptides isolated from higher plants and higher fungi. Cyclopeptides occur in higher plant species, which belong to the following families: Annonaceae, Araliaceae, Asteraceae, Caryophyllaceae, Euphorbiaceae, Fabaceae, Labiatae, Linaceae, Olacaceae, Rhamnaceae, Rubiaceae, Rutaceae, Schizandraceae, Solanaceae, and Violaceae. Basidiomycetes is the group of higher fungi, which has shown to be rich in cyclopeptides, e.g., Amanita, Conocybe, Lepiota, Galerina, and Omphalotus species. Usually these compounds are associated to a toxic response in mammals, as well as to cytotoxic, insecticidal, antimalarial, estrogenic, sedative, nematicidal, antimicrobial, immunosuppressive, and enzyme-inhibitory activities. Structures are elucidated by chemical methods and spectroscopic techniques. Configurational and conformational analyses of some bioactive cyclopeptides are discussed. Conformational studies of the natural cyclopeptides and their derivatives are related to the stereochemical requirements for these bioactive compounds.

Isohexenylnaphthazarins (IHN), commonly known as Alkannins and Shikonins (A/S), are lipophilic red pigments. They are found in the outer surface of the roots of at least a hundred and fifty species that belong to the genera Alkanna, Lithospermum, Echium, Onosma, Anchusa and Cynoglossum of the Boraginaceae family. The chiral pairs A/S are potent pharmaceutical substances with a well-established and wide spectrum of wound healing, antimicrobial, antiinflammatory, antioxidant, anticancer and antithrombotic biological activity. For organic chemists uninitiated in the chemistry of quinones, the structures of alkannin (1) and shikonin (2) may look misleading simple. However, in spite of great efforts over many years by several research groups worldwide, a much needed viable synthetic route to these enantiomers has remained elusive until very recently. The value of A/S motivated biotechnologists to develop the world’s first manufacturing process utilizing plant cell cultures. The research in this area has provided a wealth of knowledge to the field of biotechnology. In addition, great insights into the biosynthesis of these natural products and to our understanding of plant secondary metabolism in general, has been gained from this work. The last years there has been extensive scientific research in many areas throughout the disciplines of chemistry and biology and more specifically in cancer chemotherapy and a number of papers have appeared in the literature. Significant research has been conducted on A/S effectiveness on several tumors and on their mechanism of anticancer action. The aim of this paper was to review the recent advances in chemistry, biology, biotechnology and biosynthesis of alkannins and shikonins.