Fluorous synthesis: a fluorous-phase strategy for improving separation efficiency in organic synthesis

Towards the nonstick egg: designing fluorous proteins

Anyone who has made scrambled eggs will have had cause to praise the properties of Teflon. Teflon's highly chemically inert and nonstick nature derives from the perfluorinated polymer polytetrafluoroethylene. Perfluorocarbons have unique and valuable physical properties not found in nature. By incorporating fluorine into proteins, it might be possible to produce biological molecules with novel and useful properties.

Combinatorial liquid-phase synthesis of structurally diverse benzimidazole libraries

An expedient liquid-phase synthesis for construction of the diverse benzimidazole libraries is described. Nucleophilic aryl substitution of poly(ethylene glycol)-supported 4-fluoro-3-nitrobenzoic acid 3 with several primary amines under basic conditions, followed by Zn/NH(4)Cl mediated nitro group reduction, gave the PEG bound diamines 5. Subsequent cyclization of immobilized o-phenylenediamine 5 using thiocarbonyldiimidazole (TCD) or thiophosgene in dichloromethane furnished benzimidazole-2-thiones 6. Treatment of 6 with alkyl halides and benzylic halides in the presence of triethylamine provided 1-substituted-2-alkylthio-5-carbamoylbenzimidazoles on the support. The desired products 8 were severed from the PEG under mild conditions in high yield and high purity.

Polyaromatic Scavenger Reagents (PAHSR): A New Methodology for Rapid Purification in Solution-Phase Combinatorial Synthesis

A new method of purification of solution-phase combinatorial libraries has been developed. Development of a chemically inert polyaromatic anchor with a reactive "scavenger reagent" (PAHSR) allows unreacted reagents and impurities to be removed from a reaction by absorption of the PAHSR to charcoal and simple filtration.

Combinatorial chemistry as a tool for drug discovery

The question 'will combinatorial chemistry deliver real medicines' has been posed [96]. First it is important to realise that the chemical part of the drug discovery process cannot stand alone; the integration of synthesis and biological assays is fundamental to the combinatorial approach. The results presented in Tables 3.1 to 3.8 suggest that so far smaller directed combinatorial libraries have obtained equivalent results to those obtained previously from traditional medicinal chemistry analogue programs. Unfortunately, because of the long time it takes to develop pharmaceutical drugs there are no examples yet of marketed drugs discovered by combinatorial methods. There are interesting examples where active leads have been discovered from the screening of the same library against multiple targets (e.g. libraries 13, 39, 43, 66, 71 and 76). It is now possible to handle much larger libraries of non-oligomeric structures and the chemistry required for such applications is becoming available. Whether combinatorial approaches can also be adapted to deal with all the other requirements of a successful pharmaceutical (lack of toxicity, bioavailability etc.) is open to question but there are already examples such as cassette dosing [235-237]. However we can still be optimistic about the possibility of larger libraries producing avenues of investigation for the medicinal chemist to develop into real drugs. Combinatorial chemistry is an important tool for the medicinal chemist.

Recent applications of polymer-supported reagents and scavengers in combinatorial, parallel, or multistep synthesis

There has been a wealth of recent reports concerning support-bound reagents and scavengers in the solution-phase synthesis of compound libraries and natural products. Important advances in 1999 include the continued development and use of novel reagents for heterocycle synthesis, the increased use of catch-and-release purification, and the development of increasingly sophisticated techniques to allow sequestering of many types of impurities from desired compounds. These techniques have all been combined to enable the complicated multistep synthesis of natural products and of libraries of novel drug-like molecules, without conventional purification.

Synthesis of fluorous fullerene adducts: reversible solubilization of fullerenes in perfluorinated solvents

Fullerene (C60) Diels-Alder adducts with perfluoroalkylated 1,3-cyclopentadiene (1a,b) were synthesized and studied. The perfluoroalkylated cyclopentadiene was found to be less reactive toward C60 than cyclopentadiene itself, possibly because of the electron-withdrawing effect of the side chain. Because of the same effect, the temperature of the retro-Diels-Alder reaction for the fluorinated adducts was lower (70 degrees C) than the reported value (95 degrees C) for the cyclopentadiene adducts of C60. Higher adducts of the fluorous diene and C60 were found to be soluble in perfluorohexane and to show visible partitioning between organic (toluene) and fluorous phases. Also, the Diels-Alder addition of the fluorous diene was accompanied by extensive oxidation of the fullerene core, as revealed by MALDI-TOF data.

Fluoroacrylate-bound fluorous-phase soluble hydrogenation catalysts

[reaction: see text] Fluoroacrylate polymer-bound hydrogenation catalysts are described. N-Acryloxysuccinimide-containing fluoroacrylate polymers were converted into phosphine ligands and subsequently into analogues of Wilkinson's catalyst by amidation of N-acryloxysuccinimide active ester residues and Rh exchange. The resulting catalysts have excellent activity and can be reused following fluorous biphasic liquid/liquid separation and extraction.

Highly versatile solid phase synthesis of biofunctional 4-aryl-3,4-dihydropyrimidines using resin-bound isothiourea building blocks and multidirectional resin cleavage

A series of pharmacologically active, functionalized 4-aryl-3,4-dihydropyrimidine-5-carboxylates (DHPMs) are prepared by a versatile novel solid phase approach. In the key step, a polymer-bound thiouronium salt is condensed with unsaturated 1-ketoesters. The resulting polymer bound 1,4-dihydropyrimidines are cleaved from the resin employing multidirectional resin cleavage strategies.

Phase-trafficking reagents and phase-switching strategies for parallel synthesis

Strategies for the parallel synthesis of small molecule arrays are reviewed with an emphasis on those approaches that integrate the sciences of organic synthesis and product purification. The use of phase-trafficking reagents (PTRs) and phase-switching strategies are highlighted, which allow the rapid, parallel synthesis of small molecule arrays with built-in purification determinants.

Parallel synthesis with fluorous reagents and reactants

Fluorous reagents and reactants provide emerging new options in parallel synthesis because they are easily prepared, reacted, and then separated from the desired reaction products. Some background information on fluorous reaction and separation techniques is provided, and the utility of fluorous reagents and reactants is illustrated with a number of case studies that feature both liquid-liquid and solid-solid separations.

Solution-phase combinatorial chemistry

The use of solution phase techniques has been explored as an alternative to solid-phase chemistry approaches for the preparation of arrays of compounds in the drug discovery process. Solution-phase work is free from some of the constraints of solid-phase approaches but has disadvantages with respect to purification. This article will also illustrate some of the advances made in recent years in solution phase array chemistry including using supported reagents and simple extractive protocols for the effective preparation of high quality samples.

Organic-Fluorous Phase Switches: A Fluorous Amine Scavenger for Purification in Solution Phase Parallel Synthesis

The synthesis of the fluorous amine scavenger [(C(6)F(13)CH(2)CH(2))(3)SiCH(2)CH(2)CH(2)](2)NH and its successful application in the automated solution phase parallel synthesis of a urea library are described. Ureas were made by robotic synthesis from organic amines and excess isocyanates. The amine scavenger reacts with excess isocyanate, and the fluorous tag serves to solubilize the resulting adduct in the fluorous phase so it can be removed by fluorous-organic extraction. Organic urea products are isolated in high yields and purities after liquid-liquid extraction. Preliminary biological evaluation shows that several of the ureas have ion channel modulation abilities. In contrast to polymer and ionic quenching methods, the fluorous quench works whether the product is soluble or insoluble in the reaction medium, and ionizable functional groups are tolerated in the products.

Solid-phase oligosaccharide synthesis and related technologies

Research efforts directed at the development of methodologies effective for solid-phase synthesis of oligosaccharides have resulted in a number of impressive achievements. In addition, closely related technologies, such as soluble polymer-supported synthesis and fluorous synthesis of the same class of molecules, have proved to be quite promising.

Solution-phase combinatorial synthesis of ureas using nitrophenylcarbamates

Nitrophenylcarbamates were reacted with various amines to yield ureas. A high throughput purification of these crude products was achieved by using polymer bound scavengers.

"Propylene spaced" allyl tin reagents: a new class of fluorous tin reagents for allylations under radical and metal-catalyzed conditions

A new generation of propylene-spaced fluorous allyltin reagents [(Rf(CH2)3)3SnCH2CH = CH2] is described. These succeed in radical allylations where their lower homologs (ethylene-spaced) fail, and they provide improved performance in transition metal catalyzed allylations. The reagents and byproducts are readily separated by simple fluorous-organic liquid-liquid or solid-liquid extractions.

A mixed resin bed for the quenching and purification of tetrabutylammonium fluoride mediated desilylating reactions

A polymer-bound calcium sulfonate resin is prepared and used to sequester tetrabutylammonium fluoride. The simultaneous use of the calcium sulfonate resin with a sulfonic acid resin is used for the quenching and purification of desilylating reactions involving tetrabutylammonium fluoride as the reagent. Employment of this resin workup technique eliminates the need for a liquid-phase extractive protocol allowing the procedure to be easily automated.

Chemically-tagged Mitsunobu reagents for use in solution-phase chemical library synthesis

A general method for high-throughput product purification of Mitsunobu reactions is described. Tagged phosphine and azodicarboxylate reagents are used to synthesize individual library members in solution-phase. Workup and purification are easily accomplished by post-reaction sequestration of the tagged reagents and reagent byproducts by a complementary functionalized ion exchange resin. The reagents are utilized in a 3 step library synthesis.

Solid-phase extraction on C18 silica as a purification strategy in the solution synthesis of a 1-thio-beta-D-galactopyranoside library

A novel strategy for the purification of carbohydrate-based chemical libraries synthesized in solution was developed. Purification of reaction products was accomplished by means of solid-phase extraction enabled by protecting the 2-, 3-, 4-, and 6-hydroxyl groups of a galactose derivative as their hydrophobic O-laurates. The presence of multiple O-laurates allowed adsorption of reaction products onto C18 silica while reagents and by-products were washed away with MeOH. Products were quantitatively eluted with pentane. Purification of products using solid-phase extraction offers the combined advantages of solution synthesis (normal solution reactivity and ease of reaction monitoring) with those of solid-phase synthesis (facile product isolation permitting the use of large excesses of reagents). To demonstrate the utility of the hydrophobic recovery-procedure, tetra-O-lauroyl-beta-D-galactopyranose-1-thiol was subjected to high-yielding reactions with a panel of Michael-acceptors and an alpha-chloro ketone. The resulting ketone adducts were then either reduced to the alcohols or reductively aminated with a selection of amino acids to give 30 different 1-thio-beta-D-galactosides as mixtures of four diastereomers after removal of protecting groups. At each step, the product was separated from the reagents and their by-products by simple adsorption onto C18 silica, washing with MeOH and elution of product with pentane. After completion of the combinatorial chemistry sequence, the O-laurates were cleaved by methanolysis and the product methyl laurate in turn removed from the desired water-soluble products by C18 adsorption. Individual library members were thus conveniently produced on 10-30 mg scales at purity levels of > 90%. One of the 1-thio-beta-D-galactosides thus produced was found to be a competitive inhibitor of the beta-galactosidase from E. coli with Ki value of 1.7 microM.