Organische Chemie an fester Phase

Monitoring Reactions on Solid Phases with Raman Spectroscopy

The benefits of Raman spectroscopy were shown for the on-bead monitoring of diverse reactions. Raman spectroscopy was used for the development of new procedures on established linker systems, the real-time observation of several reactions on solid phases and the estimation of the reaction time for a new cleavage strategy. Selected conversions on solid phases, such as the on-bead conversion of functional groups and the attachment of novel building blocks, were demonstrated. Raman spectra were obtained after isolation and purification of the solid supports, but they were also measured directly in the reaction vessels. Even the detection of Raman-active functional groups in swollen polymer resins and in reaction mixtures was demonstrated, and allows real-time observation of the progress of diverse reactions on solid supports.

Electrochemical, Pulsed-Field-Gradient Spin-Echo NMR Spectroscopic, and ESR Spectroscopic Study of the Diffusivity of Molecular Probes inside Gel-Type Cross-Linked Polymers

The permeability of five gel-type synthetic resins, obtained by polymerization of 1-vinylpyrrolidin-2-one cross-linked with N,N'-methylenebisacrylamide (1, 2, 3, 4, and 5 wt %) and swollen by N,N-dimethylformamide (DMF), has been analyzed. The diffusion of 2,2,6,6-tetramethyl-4-oxo-1-piperidinyloxyl (TEMPONE) was studied by ultramicroelectrode voltammetry. Similar measurements were performed for solutions of non-cross-linked poly(vinylpyrrolidone) in DMF. To provide information on the rotational mobility of TEMPONE and the translational mobility of DMF, electron spin resonance (ESR) spectroscopic and pulsed-field-gradient spin-echo nuclear magnetic resonance (PGSE-NMR) spectroscopic experiments, respectively, were carried out. Comparative analysis of the results obtained by electrochemical, ESR spectroscopic, and PGSE-NMR spectroscopic measurements showed that diffusivity inside the polymer framework is significantly affected by the extent of cross-linking, the size of the diffusing probe, and the presence of electrolytes.

Combining Enabling Techniques in Organic Synthesis: Continuous Flow Processes with Heterogenized Catalysts

The concepts article describes enabling techniques (solid-phase assisted synthesis, new reactor design, microwave irradiation and new solvents) in organic chemistry and emphasizes the combination of several of them for creating new synthetic technology platforms. Particular focus is put on the combination of immobilized catalysts as well as biocatalysts with continuous flow processes. In this context, the PASSflow continuous flow technique fulfils both chemical as well as chemical engineering requirements. It combines reactor design with optimized, monolithic solid phases as well as reversible immobilization techniques for performing small as well as large scale synthesis with heterogenized catalysts under continuous flow conditions.

Knotting and threading of molecules: chemistry and chirality of molecular knots and their assemblies

How and why do molecules tangle or thread? Investigations of molecular knots (knotanes) may shed some light on the mechanisms of (supra)molecular templation and the folding of molecules that result in intertwining. The topological chirality of these fascinating molecules leads to new types of isomerism and paves the way to nanosized molecular motors. Their preparation and derivatization makes high demands on modern synthetic methods and analytical separation since molecular knots are formed in a more or less planned design based on metal coordination or hydrogen-bonding patterns. This Review describes the development of templation techniques for the synthesis of knotanes and their chiral resolution as well as their selective functionalization and use as building blocks in the synthesis of higher knotane assemblies. Such assemblies can possess linear, branched, or even macrocyclic structures which, on the one hand, introduce unprecedented isomeric compositions that arise from multiple topological stereogenic units and, on the other, define new types of artificial macromolecules beyond polymers and dendritic species.

Combinatorial chemistry: libraries from libraries, the art of the diversity-oriented transformation of resin-bound peptides and chiral polyamides to low molecular weight acyclic and heterocyclic compounds

Combinatorial chemistry has deeply impacted the drug discovery process by accelerating the synthesis and screening of large numbers of compounds having therapeutic and/or diagnostic potential. These techniques offer unique enhancement in the potential identification of new and/or therapeutic candidates. Our efforts over the past 10 years in the design and diversity-oriented synthesis of low molecular weight acyclic and heterocyclic combinatorial libraries derived from amino acids, peptides, and/or peptidomimetics are described. Employing a "toolbox" of various chemical transformations, including alkylation, oxidation, reduction, acylation, and the use of a variety of multifunctional reagents, the "libraries from libraries" concept has enabled the continued development of an ever-expanding, structurally varied series of organic chemical libraries.

Highly functionalized organomagnesium reagents prepared through halogen-metal exchange

Organomagnesium reagents occupy a central position in synthetic organic and organometallic chemistry. Recently, the halogen-magnesium exchange has considerably extended the range of functionalized Grignard reagents available for synthetic purposes. Functional groups such as esters, nitriles, iodides, imines, or even nitro groups can be present in a wide range of aromatic and heterocyclic organomagnesium reagents. Also various highly functionalized alkenyl magnesium species can be prepared. These recent developments as well as new applications of organomagnesium reagents in cross-coupling reactions and amination reactions will be covered in this Review.

Tailoring Ultraresins Based on the Cross-Linking of Polyethylene Imines. Comparative Investigation of the Chemical Composition, the Swelling, the Mobility, the Chemical Accessibility, and the Performance in Solid-Phase Synthesis of Very High Loaded Resins

Ultraresins have been prepared from polyethyleneimines and cross-linking molecules and have been provided with various degrees of cross-linking. The total nitrogen loading and the loading with secondary and with tertiary amines have been determined in all products. Nitrogen loadings of the novel resins were up to 15 mmol/g, reactive secondary amines up to 13.8 mmol/g. In addition to the exceptionally high loading, the novel resins displayed efficient swelling volumes in polar and nonpolar solvents. The mobility of resin-bound species as determined by EPR-spectroscopy, depending on the amount of cross-linker, indicated good flexibility and reactivity of this resin type. The novel, high-loaded resins have been investigated subsequently in solid-phase synthesis. The Rink amide linker and two different hydroxy linkers (hydroxyacetamide, HMPB) have been attached to the resin. Despite the high loadings, the secondary amines were easily accessible and could be functionalized exhaustively. Reactivity of the linker-coupled resins was found to be closely related to the resin composition. Increased resin cross-linking led to reduced swelling, reduced mobility, and reduced reactivity in the synthesis of a medium-sized model peptide. As the result of the systematic investigation of structure-property relations in Ultraresins, a support material was identified that combined high reactivity and a mobility in the range of the extremely flexible Tentagel supports. In the optimized Ultraresin, >95% of all available secondary nitrogens could be coupled with Rink linker or with the small 2-hydroxyacetamide anchor, resulting in loadings from 2.7 to 6.8 mmol/g, respectively. A resin with an attached HMPB linker and spacer delivered analytically pure peptides in solid-phase synthesis, fully exploiting the exceptionally high loadings.

The Chemistry of Isocyanides, their MultiComponent Reactions and their Libraries

The first century of isocyanide chemistry, which was then still a rather empty part of Organic Chemistry, began in 1859. In 1958 isocyanides became generally available by dehydration the formylamines. One year later the four component reaction of isocyanides (U-4CR) was introduced. This one-pot reaction is accomplished just by mixing amines, carbonyl compounds, suitable acids and isocyanides. Most chemical reactions have their own ”scope and limitation”, whereas the U-4CR can convert almost all combinations of educts into their products. Until 1995 this chemistry was moderately used, but since then a new era of the U-4CR and its unions with further reactions have become increasingly popular, particularly as libraries. In industry this chemistry became one of its most often used methods of finding new desirable products. In contrast to most other areas of chemistry, isocyanide chemistry is not yet exhausted and still much progress can be expected there.

Click Chemistry: Diverse Chemical Function from a Few Good Reactions

Examination of nature's favorite molecules reveals a striking preference for making carbon-heteroatom bonds over carbon-carbon bonds-surely no surprise given that carbon dioxide is nature's starting material and that most reactions are performed in water. Nucleic acids, proteins, and polysaccharides are condensation polymers of small subunits stitched together by carbon-heteroatom bonds. Even the 35 or so building blocks from which these crucial molecules are made each contain, at most, six contiguous C-C bonds, except for the three aromatic amino acids. Taking our cue from nature's approach, we address here the development of a set of powerful, highly reliable, and selective reactions for the rapid synthesis of useful new compounds and combinatorial libraries through heteroatom links (C-X-C), an approach we call "click chemistry". Click chemistry is at once defined, enabled, and constrained by a handful of nearly perfect "spring-loaded" reactions. The stringent criteria for a process to earn click chemistry status are described along with examples of the molecular frameworks that are easily made using this spartan, but powerful, synthetic strategy.

Tracelessness Unmasked: A General Linker Nomenclature

Nowadays it is rare to find an issue of a major chemistry journal without at least one article on solid-phase synthesis. This is hardly surprising: the technique promises an end to arduous work-up procedures and the ability to facilitate the creation of vast libraries of compounds using combinatorial techniques. No longer is the technique only of interest to those involved in peptide synthesis: an enormous variety of product classes have now been prepared on and isolated from the solid phase. It is the "linker" which is the focus of this article. The linker's ultimate function is to release a product from the support into solution: it does this, without exception, with a chemical change to the product at the former linkage site. Some linkers, apparently, are "traceless". But what, in fact, is "tracelessness"? Twenty years ago, in a climate where cleavage of a linker resulted in formation of a polar carboxylic acid as the vestige of the support, the concept was attractive. Today the chemist is faced with a myriad of novel linkers which have the ability to release products bearing most major functionalities at the former linkage site and we will argue here that the term "traceless", although currently in widespread use, is meaningless. Instead, we propose a new categorization of linkers based on the functionality they release upon cleavage, and suggest a nomenclature to underpin this categorization. We anticipate that the article will also serve to highlight areas of linker technology in need of further research.

An enzyme-labile safety catch linker for synthesis on a soluble polymeric support

The development of new and broadly applicable linker groups which are stable under a variety of reaction conditions and allow release of the desired products from the solid support under very mild conditions is of great interest in organic synthesis and combinatorial chemistry. We describe an enzyme-labile safety-catch linker which releases alcohols and amines through i) enzymatic cleavage of an amino group and ii) subsequent lactam formation. The linker group was investigated on different polymeric supports: TentaGel. PEGA, CPG-beads and the soluble polymer POE-6000. From these linker-polymer conjugates 2-methoxy-5-nitrobenzyl alcohol was released by penicillin G acylase catalysed cleavage of a phenylacetamide and attack of the liberated benzylamine on the neighbouring ester group in ortho position. The model study revealed that only in the case of soluble POE-6000 conjugate high yields for the cleavage could be achieved. In the case of the other solid supports the enzyme does not have access to the interior of the polymer matrix. The application of the POE-6000 linker conjugate was investigated for various esters in Pd0-catalysed Heck-, Suzuki- and Sonogashira reactions as well as in a Mitsunobu reaction and cycloadditions. These studies proved that the linker is stable under a broad variety of reaction conditions and that the enzymatic method allows for release of the desired product alcohols under extremely mild conditions at pH 7 and 37 degrees C. In addition, the enzymatic reaction proceeds with complete chemoselectivity, that is other esters or amides are not attacked by the biocatalyst. In addition to alcohols amines can also be cleaved by means of the enzyme-initiated two-step process. In these cases the higher stability of amides as compared to esters requires warming to 60 degrees C to induce cyclization and release of the desired product.

Dendrimers and hyperbranched polymers as high-loading supports for organic synthesis

Polymeric supports have become a big necessity for automated synthesis and combinatorial chemistry, yet, the loading capacities of most polystyrene resins are very limited (typically < 1.5 mmol x g(-1)). Dendrimers and hyperbranched polymers have been discussed for this application and now became readily available. These soluble polymers can either be used directly as high-loading supports for substrates, reagents, and catalysts or alternatively in hybrid polymers linked to conventional polystyrene resins.

Development of an easy-to-use mass spectrometric technique to monitor solid-phase reactions on polystyrene supports

By using mass spectrometry as an analytical tool to characterise substituted, cross-linked polystyrene resins, it is possible to directly monitor the progress of the solid-phase reactions performed on these resins without prior cleavage of the resin-bound molecules. Therefore, this is a true on-resin analytical method. The mass-to-charge ratios observed in the mass spectra are readily assigned to fragments of the polymer that include the chemically bound substituents. This is the first time that the formation and breaking of bonds have been directly observed on the polymeric support. Furthermore, the relative intensities of the signals in the mass spectra provide a measure of the completeness of the reaction. Because these measurements are rapidly performed without further chemical transformations or cleavage procedures, and because only minimal amounts of material are needed, this technique could become the solid-phase equivalent of thin-layer chromatography used in classical liquid-phase chemistry.

Cs(2)CO(3)-promoted efficient carbonate and carbamate synthesis on solid phase

[reaction: see text] Mild and efficient preparation of alkyl carbonates and carbamates on solid supports is described herein. Alcohols or amines were coupled with Merrifield's resin through a CO(2) linker in the presence of cesium carbonate and tetrabutylammonium iodide (TBAI).

Multicomponent domino reactions for the synthesis of biologically active natural products and drugs

A main issue in modern synthetic organic chemistry, which deals with the preparation of natural products, pharmaceuticals, diagnostics, agrochemicals, and other important materials, is the improvement of efficiency, the avoidance of toxic reagents, the reduction of waste, and the responsible treatment of our resources. One of the ways to fulfill these goals is the development and use of domino processes, which consist of several bond-forming reactions and which allow the highly efficient synthesis of complex molecules starting from simple substrates. Herein, the combination of several catalytic bond-forming transformations is clearly most appropriate. The synthesis of the enantiopure alkaloid (-)-hirsutine 22, which has a strong inhibitorial effect on influenza A viruses, was accomplished using a biomimetic domino Knoevenagel-hetero-Diels Alder-solvolysis-hydrogenation process. In a similar way the alkaloids (+)-dihydrocorynantheine 23 and (-)-dihydroantirhine 24 as well as heterosteroids 62, D-homosteroids 65 and 68, and azasteroids 25 are prepared. In addition, novel steroid alkaloids 26 are accessible by a combination of the formation of an iminium salt, a hydride shift, and an alkylation. The anti-leukemic pentacyclic (-)-cephalotaxine 27 is obtained by a combination of two Pd-catalyzed reactions.