Current Organic Synthesis - Volume 6, Issue 1, 2009

Enzymes are very versatile catalysts in organic chemistry, although in many occasions they present very low selectivity towards non-natural or artificial compounds. A simple strategy has recently been discovered which allows a greater enhancement of the enzyme selectivity. This strategy -based on the conformational changes of these enzymes during catalysis- consists in the preparation of a biocatalysts library using different immobilization protocols that may permit to immobilize the enzymes via different orientations, with different rigidity or generating different environments. This review examines the way in which different enzymes such as lipases, acylases, β-galactosidases, epoxyhydrolases, phosphorylases, hydroxynitrile lyases, sulfatases, glycosyltranferases, or tyrosynases have shown a huge variation with this solid phase strategy. This methodology has permitted to obtain highly enantioselective biocatalysts in asymmetric reactions, synthesis of β-lactams antibiotics or transglycosylation reactions.

Cyanohydrins are valuable intermediates in organic synthesis. They enable an extensive chemistry that can be implemented starting either from the nitrile or from the hydroxy functionality. A number of challenges relevant to the enzyme-catalysed synthesis of chiral cyanohydrins have been addressed in recent years. In this review we will discuss three topics: (1) The enantioselective synthesis of cyanohydrins catalysed by Hydroxynitrile Lyases (Oxynitrilases); (2) The preparation of enantiopure cyanohydrins catalysed by hydrolases/ lipases via (dynamic) kinetic resolutions and (3) the (in situ) conversion of the enantiopure cyanohydrins, extending the use of cyanohydrins as building blocks in organic synthesis.

Chiral 3,4-difunctionalized pyrrolidines, which can be derived from L-(+)- and D-(-)-tartaric acids, are useful and versatile building blocks in total synthesis, catalyst preparation as well as supramolecular and medicinal chemistry. The preparation of these azacycles, possible transformations and their application are described herein.

Several classical methods were described for the selective synthesis of cinnamaldehyde derivatives and 3-arylpropionic acids but their applicability remains limited due to the difficult and costly synthesis of the starting materials. Since few years, palladium catalyzed procedure has been reported. Whatever they use homogeneous or heterogeneous palladium catalysts, these catalytic methods offer viable environmental friendly alternatives to stoechiometric methods particularly since they generally required a single reaction step starting from simple substrates. Some of these new methods are really considered as green alternatives since they use heterogeneous easily separable (and recyclable) palladium catalysts. This review article summarizes the literature on this area. It is ended by mechanistic considerations.

The science of molecule-based materials is an exciting subject in light of its fundamental scientific interest and potential applications. Considerable efforts are now being invested in developing strategies for the design and preparation of new materials involving an interplay or synergy between electrical conductivity and magnetic properties. This review examines approaches for the preparation of organic conducting ferromagnets from organic spin systems in the form of stable organic radicals that are covalently appended to organosulfur donors. The concept of a spin-polarized donor and spin alignment via itinerant electrons is addressed, together with an overview of the most important classes of stable organic radicals. A full comprehensive discussion of progress and achievements in this area spanning the last 15 years is presented.

In the last years, the use of pyrocarbonates and particularly di-tert-butyldicarbonate (Boc2O) has found a renaissance in organic synthesis. Together with the classical couple Boc2O/DMAP, new synthetic strategies have been developed in the presence of environmentally benign Lewis Acids. So new syntheses of protected Boc-amines and alcohols have appeared in literature. Moreover, the use as activator in the synthesis of esters and anhydrides has received attention from different research groups. Finally an interesting and unexpected synthesis of t-butyl ethers has appeared. This review aims to cover all the literature since 1998, but not the application to the synthesis of natural products.