Current Organic Chemistry - Volume 13, Issue 1, 2009

From a viewpoint of synthetic organic chemistry, the recent developments and applications of zirconium compounds as promoter and catalyst for many important organic reactions including addition reactions such as Michael reaction, aldo reaction, Strecker reaction and carbometalation, cycloadditions, reduction reactions, cyclization, rearrangement reactions such as Ferrier rearrangement, Fries rearrangement, the sliding cyclohexane rearrangement, Beckman rearrangement and the rearrangement of alkoxybenzyl allyl ethers into aldehydes, the formation and cleavage of common protecting groups such as carbonyl compounds, carboxylic acids, amino groups and hydroxy groups, were reviewed. The use zirconium compounds including ZrCl4, ZrOCl2·8H2O, Zr(SO4)2·4H2O and Zr(HSO4)4, generally, allows the reaction to be carried out under mild conditions and affords the corresponding products in excellent yields with high regio-, chemo- and stereo-selectivity.

In the last decades Group 6 Fischer Carbene complexes have demonstrated their ability as an interesting tool in asymmetric synthesis. The use of this family of organometallic compounds permitted the access to a wide array of optically active and occasionally enantiopure molecules. In this sense, carbo- and heterocycles of different sizes -from three to eighth membered rings- along with open chain compound, have been synthesized. Additionally, the use of Fischer carbene complexes has, in some cases, dramatically simplified the access to some natural or biologically active molecules only accessible following other tedious methodologies. The synthesis of optically active molecules from Group 6 Fischer carbene complexes will be reviewed here from two points of view: from chiral and from achiral carbene complexes. Among the enantiopure carbene complexes a distinction between amino an alkoxycarbenes has been formulated. On the other hand, asymmetric synthesis with chiral alkoxycarbenes has been divided in three groups according to the position of the inductor of chirality: i) in the stabilizing group, ii) in the carbene ligand, iii) in the coordination sphere ligands. Finally, all the sections along this review have been subdivided according to the reaction type.

Avoiding volatile and toxic organic solvents during each particular phase of synthetic protocols should become important goal of chemical synthesis designers in academia and particularly in the industrial research community. Solvent- free reaction conditions (SFRC) are becoming a widely used experimental technique for the selective and efficient introduction of halogen atoms into organic compounds. The different approaches to the preparation of halosubstituted organic molecules using equimolar amount of the substrate and reagent under SFRC are reviewed and evaluated. Fluorination of various types of organic compounds under SFRC using fluorine gas, N-F reagents, and hydrogen or metal fluorides is compiled. Chloro-, bromo- or iodofunctionalisation under SFRC using molecular halogens, N-halosuccinimides, hydrogen or others halides is reviewed. Oxidative halogenations of comprehensive types of organic compounds under SFRC are evaluated.

Last years have witnessed enormous progresses in glycomic field, mainly as a consequence of the crucial role carbohydrates have shown in biological systems. While up to a few years ago attention was mainly focused on the use of easily available D-sugars, a recent interest has emerged around their L-enantiomers, as they have been found to be key components of several bioactive compounds, whether in the form of oligosaccharides, glycopeptides, terpene glycosides or other clinically useful agents. However, L-sugars (L-hexoses especially) are rather rare in nature and not easily accessible from inexpensive sources. As demand for their synthesis in considerable amount and high purity is more and more pressing, intense efforts have been addressed to the development of new and general methodologies for their construction. This review covers the synthetic routes to L-hexoses, mainly those coming from the new century. Methodologies for monosaccharide assembly will comprise de novo approaches, based on carbon chain elongation, hetero Diels-Alder reaction, asymmetric dihydroxylation up to the most recent amino acid-catalyzed aldol addition - as well as D-sugar manipulation strategies, including epimerization by chemical or enzymatic methods. Application of such protocols for the construction of biologically relevant oligosaccharides and natural products will be also briefly mentioned.

L-Ascorbic acid, commonly known as vitamin C is well-known in chemistry since long back. It has tremendous medical applications in several diseases. However, application of this chiral molecule in organic synthesis has been neglected earlier. In the later part of twentieth century application of ascorbic acid has gained momentum in organic synthesis of different molecules of biological importance and of chemotherapeutic significance. We have given an account of the history, chemistry, biochemistry and biosynthesis of ascorbic acid and application of this small molecule in organic synthesis. The application of ascorbic acid in accessing chiral synthons has also been described.