Current Organic Chemistry - Volume 8, Issue 18, 2004

Carbohydrates contain a high density of functional and stereochemical information in one molecular unit. They have been exploited as enantiopure starting materials in numerous ex-chiral pool syntheses. During the past decade, the potential of carbohydrates in terms of stereodifferentiating auxiliaries has been recognised. Glycosylamines proved to be efficient auxiliaries in stereoselective Strecker syntheses of α-amino nitriles and Ugi reactions yielding α-amino acid amides. Mannich-reactions of glycosyl imines with silyl ketene acetals resulted in the formation of β-amino acid derivatives with high diastereoselectivity. N-Glycosyl imines unfold their high potential in stereoselective reactions, in particular, in Mannich-Michael reaction sequences with 2-silyloxy-butadiene derivatives furnishing 2-substituted piperidinone derivatives with high asymmetric induction. On the basis of this efficient stereodifferentiation on glycosyl imines a number of alkaloids of the piperidine, indolizine, quinazoline, and decahydro-quinoline series have been synthesised in enantiomerically pure form. The N-glycosyl auxiliary has been demonstrated to control stereoselective protonation of enolates, for example, in alternative formation of cis-annulated perhydro-quinoline alkaloids typical for toxins of Dendrobates pumilio or trans-annulated analogues characteristic for toxins from Dendrobates histrionicus. Derivatives of amino sugars have been also investigated as components of enantioselective catalysts, e.g. palladium(0)-catalysed allylation reactions, allylic oxidation processes and Strecker syntheses.

Self-assembled monolayers (SAMs) of alkane thiols on gold and other metals are versatile constructs with which to study interfacial phenomena and reactions at surfaces. Surface properties of SAMs - e.g., wettability, stability in diverse environments, propensity to interact with or to resist adsorption of macromolecules -- depend on and can be controlled flexibly by the properties of the functional (head) groups in the w position of the alkyl chain. SAMs provide a basis for many important scientific and technological applications, ranging from micropatterning methods, through sensing, to biological recognition. Despite their importance, the literature on SAMs and the synthesis of molecules that constitute them remains scattered and often conflicting. The purpose of this Review is (i) to summarize the applications and physical properties of SAMs and (ii) to systematize the strategies of synthesis of ω-functionalized alkane thiols. Generic retrosynthetic scheme is developed that allows efficient synthetic planning. Issues related to the selection of appropriate protecting groups and the ways of introduction of the thiol functionality are discussed in detail, and illustrated with examples of syntheses of several complex alkane thiols.

At present time the main goal of important research programs is to create new biological and biomimetic materials for asking scientific questions and solving problems relevant to biology, e.g., pharmaceutical development, mutation detection, and DNA sequencing. Widespread approaches to this kind of research are based on design and synthesize structures, and then characterize the physical, structural or biological properties of the new materials. This research involves major experts and scientists in the chemistry of amino acids, peptides, sugars, nucleosides, oligonucleotides, bioconjugation, and particularly conformationally restricted amino acids have been the target of both synthetic and medicinal chemistry, since they can be applied to the design of relevant biologically active compounds.

Supramolecular complexes can be studied by a variety of spectroscopic methods and substantial information obtained by optical methods, by NMR spectroscopy, and by other techniques, clucidating structure and their stability, are reported in the scientific literature. However, very few data can be found concerning the rates by which these aggregates are formed or dissociated into separate species. This is due to the fact that association / dissociation takes place in a frequency range that is not accessible (or it is accessible only in very peculiar situations) either to optical techniques or to NMR. Thus distinct signals from the free and complexed species are detected with optical spectroscopies, while spectra representing concentration-weighted averages are observed by NMR since the exchange between free and complexed species is usually too fast in the NMR time scale. The association / dissociation processes, taking place in the time range between 10-5 and 10-9 seconds, can instead be studied very conveniently by EPR spectroscopy. Thus, this technique has been found among those more suitable to investigate the kinetics of association and dissociation in supramolecular chemistry and has provided valuable mechanistic information on the dynamics of these processes. In this review the results of our EPR studies on the supramolecular species formed in aqueous solutions with the guest benzyl tert-butyl nitroxide or related nitroxides and different host systems will be reported.

Catalysis by sol-gel doped materials -- chemical conversions mediated by porous glassy matrices containing actives entrapped within their internal huge porosity -- is now a developed chemical technology offering unique advantages in terms of catalyst stability, selective activity and versatility which include ease of materials production and application. In general, the performance of these materials is largely superior compared to other catalytic solids obtained by traditional support impregnation methods, while they offer entirely new possibilities prior unavailable in catalysis. Selective activities higher than in solution; mutually destructive catalysts in one pot; fast conversions in carbon dioxide; smooth reactions in water with hydrophobic catalysts; entrapped enzymes 10 times more active than in solution; asymmetric syntheses with full recovery of the precious catalyst: These are just a few examples of the chemical wonders made possible by these materials. The reactivity of sol-gel catalysts can indeed be controlled and tailored to meet the requirements of largely different chemical conversions either by varying their structural properties (surface polarity, porosity, surface area, morphology etc.) or even the chemical activity of the encapsulated active. The technology is being increasingly adopted by industry while intense researches are carried out worldwide in several academy and industry laboratories. In order therefore to prevent its rapid obsolescence, this review intends to outline the general findings emerging from research on catalysis by sol-gels to help the reader organic chemist in adopting a sol-gel catalyst as a viable and practical tool to achieve the desired products.