Current Organic Chemistry - Volume 9, Issue 3, 2005

The aldol reaction is regarded as one of the most powerful tools in organic synthesis for the formation of carbon-carbon bonds. Besides, this reaction allows the generation of many stereocenters in the final product, which makes it even more interesting from the synthetic organic chemist point of view. Consequently, an extensive number of methodologies for performing stereoselective aldol reactions have been developed over the last years. In this context, enantiopure α-amino acids, β-amino alcohols or closely related compounds have very often played successfully the role of chirality sources either as auxiliaries, ligands or catalysts in order to exert efficient stereocontrol in the addition step. In this review the use of such chiral reagents in asymmetric aldol reactions will be presented and the most important applications in the field of organic synthesis will be highlighted.

Among the natural or designed products containing the cyclobutane moiety, amino acids and nucleosides are relevant. Some of these compounds are receiving much attention due to their biological properties as antibiotics, and antiviral or antitumoral agents. Synthetic approaches to these compounds are considered in this review, based on [2+2]- photocycloadditions or on the use of suitable precursors bearing the pre-formed cyclobutane ring. Absolute chirality is determined by the use of chiral precursors, i.e. terpenes, through chemoenzimatic resolutions, or by using chiral reagents and catalysts. Functionalization of the carbocyclic ring, a feature of crucial relevance, is the other synthetic challenge to be considered. More of ninety recent works are referred.

Ring contraction of larger rings is one of the important tools in the repertoire of the methods for the synthesis of pyrroles and has long been described in the literature. This methodology affords pyrrole derivatives with varied substituents, which are otherwise difficult to synthesize. Some of the examples of ring contraction leading to pyrrole have already been exemplified in various reviews, but there is no review focusing only the ring contraction strategy. Herein, we wish to focus on the synthesis of pyrroles by this strategy. The methods are mainly classified on the basis of the type of the starting heterocycle used. The synthesis of fused pyrrole systems such as indoles is also included as the special case. Thus, we herein discuss the methods comprising of the extrusion of an atom or a group of atoms from the larger ring or the opening of the larger rings and recyclization to afford the pyrroles.

This paper reviews recent progresses of the organic reactions catalyzed by transition metals in supercritical carbon dioxide (scCO2) in our group. The reactions include oxidation, reduction, carbonylation, radical addition, C-C coupling and cycloaddition reactions. All the reactions can be performed in scCO2 with equal or even better results than in conventional organic solvents, owing to the high diffusion and low viscosity of scCO2. The addition of suitable amount of co-solvents in scCO2 is necessary in most reactions, because it not only improves the solubility of transition metal salts in scCO2, but also regulates the ratio of products and raises the selectivity as well. Our work demonstrates that scCO2 and its modifier can be used as an alternative solvent for toxic organic solvents, and thus offers an environmentally friendly medium for the development of new synthetic methods in green organic chemistry.

Enzymes have been used in non-aqueous media to make the reactions and the following process more efficient and greener. Here, recent progress in enzymatic reactions in supercritical carbon dioxide is reviewed. Hydrolysisesterification, oxidation-reduction and carboxylation are described, including highly enantioselective reactions.

Mass spectrometry has been used to study noncovalent protein interactions. Two typical ionizations permit the analysis of fragile biomolecules : matrix assisted laser desorption ionization (MALDI) and electrospray ionization (ESI). The gentleness of these techniques allows the detection of intact noncovalent complexes by mass spectrometry. However, the analysis success depends on the sample preparation and the instrumental parameter tuning both in MALDI-MS as well as in ESI-MS. ESI-MS seems to be the best analytical tool for this study as shown by the large number of published articles. Mass spectrometry becomes more and more attractive to obtain rapidly information about multimeric structures and interaction between host and guest.