Substitutional diversity

The combination of MCRs in one pot is not new. It was first introduced by Domling and Ugi who developed a 7-component reaction (7CR), that was basically a one-pot combination of a modified Asinger 4CR and U CR (Scheme 15) [100, 101]. In this 7CR, an a- or (3-bromo/chloro aldehyde 158, NaSH/NaOH, NH3, another aldehyde 161, an isocyanide 166, CO2, and a primary alcohol (solvent) are combined to afford 167 efficiently. However, NaSH/NaOH, NH3 and CO2 are invariable components in this reaction which limits the substitutional diversity (appendage diversity) and thereby the scope of the MCR. 

The diversity of chemical libraries may derive from the high substitutional, stereochemical and scaffold diversity of its members. Combinatorial chemistry is focused on the synthesis of chemical libraries with only high substitutional diversity. In general, combinatorial syntheses exploit a rather limited palette of chemical transformations to vary the substitution of library members. The synthesis of compound libraries with high substitutional diversity is described in Section 1.2.1. 

Synthesis of Compound Libraries with High Substitutional Diversity... 

Scheme 1.1 Schreiber s split-pool synthesis of 1,3-dioxanes with high substitutional diversity. Reagents and conditions (1) R SH or R R N (2) (i) ArCH(OMe)2, MejSiCl, HCl, dioxane (ii) PPTS, THF-MeOH (iii) piperidine (iv) Me3SiCl (3) amide, urea, thiourea or sulfonamide formation.

Although the diversity of asymmetric inverse-electron-demand hetero-Diels-Alder reactions has been weU established, examples of aU-carbon-based catalytic asymmetric versions have rarely been reported, and aU faU into the LUMO-lowering strategy. Based on previous applications of dienamine catalysis in asymmetric inverse-electron-demand hetero-Diels-Alder reactions, Li et al. extended this strategy to all-carbon-based asymmetric inverse-electron-demand Diels-Alder reactions (Scheme 1.28) [36]. The products of cyclohexene derivatives with substantial substitution diversity of electron-deficient dienes and crotonaldehyde were obtained with high diastereo- and enantioselectivities (up to 99% ee, dr up to 95 5). 

Different isotopically substituted thiazoles have been synthesized for diverse studies ... 

Substituted amides (not of the alkanolamide variety) are sold to diverse low volume markets. They have some utility ki polymers such as polyethylene, ethylene-vinyl acetate copolymers, acryUc polymers, PVC, polyamides, and polyesters. They have been found effective as pharmaceutical processkig aids, defoamers (qv), antimicrobials, pesticides, kisect repellents, dispersion stabilizers, and corrosion inhibitors. 

Steroids (1) are members of a large class of lipid compounds called terpenes that are biogenicaHy derived from the same parent compound, isoprene, C Hg Steroids contain or are derived from the perhydro-l,2-cyclopentenophenanthrene ring system (1) and are found in a variety of different marine, terrestrial, and synthetic sources. The vast diversity of the natural and synthetic members of this class depends on variations in side-chain substitution (primarily at C17), degree of unsaturation, degree and nature of oxidation, and the stereochemical relationships at the ring junctions. 

Many problems need to be solved before chemurgic materials can be economically used as feedstocks. Among these problems are the recovery, purification, and fractionation of the diverse materials. However, none of these problems are insurmountable. Serious concerns are the supply of the raw material, the relative costs of competitive materials, and competition with other uses for the raw materials. Competition is particularly significant because materials, such as wood, could easily be used in many cases for pulping or even higher value products, such as stmctural timber. Municipal soHd waste offers a substitute raw material with few other uses (33). 

The reactions (3)-(16) show additional regiospecific syntheses of substituted isoxazoles using a variety of starting components. They illustrate the introduction of diverse substituents into the isoxazole nucleus by choice of an appropriate starting material. 

A wide variety of /3-lactams are available by these routes because of the range of substituents possible in either the ketene or its equivalent substituted acetic acid derivative. Considerable diversity in imine structure is also possible. In addition to simple Schiff bases, imino esters and thioethers, amidines, cyclic imines and conjugated imines such as cinnamy-lidineaniline have found wide application in the synthesis of functionalized /3-lactams. A-Acylhydrazones can be used, but phenylhydrazones and O-alkyloximes do not give /3-lactams. These /3-lactam forming reactions give both cis and /raMS-azetidin-2-ones some control over stereochemistry can, however, be exercised by choice of reactants and conditions. 

All ihe circuits may carry current based on the diversity i actor. The loads may be substituted by space heaters. 

The market for PPVC film and sheet is only slightly less than for wire and cable insulation. Uses are as diverse as seepage barriers, factory doors, inflatables, baby pants, car trim, covering materials for book bindings and document cases and shower curtains. UPVC film has also been widely used for packaging of food, particularly for shrink-wrap purposes, but this market has been subject to some substitution by polypropylene. 

World production of unsaturated polyester resins in 1997 was of the order of 1.7 X 10 tonnes, with the USA accounting for about 45% and Western Europe 27%. Over 75% is used in reinforced plastics, with the rest being used for such diverse applications as car repair putties, cultured marble , wood substitution and surface coatings. The pattern of consumption in 1993 of reinforced polyesters in the USA was reported as ... 

The last group of substituted hydrocarbons produced by adding hydroxyl radicals to the hydrocarbon backbone are the compounds made when three hydroxyl radicals are substituted these are known as glycerols. The name of the simplest of this type of compound is just glycerol. Its molecular formula is 3115(011)3. Glycerol is a colorless, thick, syrupy liquid with a sweet taste, and has a flash point of 320°F, and is used to make such diverse products as candy and explosives, plus many more. Other glycerols are made, but most of them are not classified as hazardous materials. 

Nonfluonnated allenes also readily react with fluoroalkenes to give diverse fluonnated alkylidenecyclobutanes [727, 12S, 129, 130] (equations 55 and 56), except for tetramelhylallene, which rearranges to 2,4-dimethyl 1,3-pentadiene under the reaction conditions prior to cycloaddition (equation 57) Systematic studies of l,l-dichloro-2,2-difluoroethylene additions to alkyl-substituted allenes establish a two-step, diradical process for alkylidenecyclobutane formation [131, 132, 133]... 

Derivatives of the boranes include not only simple substituted compounds in which H has been replaced by halogen, OH, alkyl or aryl groups, etc., but also the much more diverse and numerous class of compounds in which one or more B atom in the cluster is replaced by another main-group element such as C, P or S, or by a wide range of metal atoms or coordinated metal groups. These will be considered in later sections. 

The AE reaction has been applied to a large number of diverse allylic alcohols. Illustration of the synthetic utility of substrates with a primary alcohol is presented by substitution pattern on the olefin and will follow the format used in previous reviews by Sharpless but with more current examples. Epoxidation of substrates bearing a chiral secondary alcohol is presented in the context of a kinetic resolution or a match versus mismatch with the chiral ligand. Epoxidation of substrates bearing a tertiary alcohol is not presented, as this class of substrate reacts extremely slowly. 

A synthetically powerful method, an approach based on cycloaddition chemistry, allows one to assemble the pyridine ring in one step. Not only is this method efficient, atom economy, but also its convergency allows for the preparation for highly substituted systems in which one can, in principle, control all five positions on the pyridine ring. A versatile example of this methodology is the Boger reaction. It has been applied to the synthesis of a very diverse set of targets. 

The von Richter cinnoline process was further extended to solid-phase synthesis. The route began from benzylaminomethyl polystyrene and the required diverse o-haloaryl resins represented by 21 were prepared from substituted o-haloanilines. A Pd-mediated cross-coupling reaction with 21 and the alkynes provided the alkynylaryl derivatives represented by alkyne 22. The von Richter cyclization reaction with hydrobromic or hydrochloric acid in acetone/HaO and cleavage from the resin occurred in the same step to furnish the cinnoline derivatives 23 in 47-95% yield and 60-90% purity (no yield reported for each entry). 

More recently, Tietze and Steinmetz (96SL667) used the patented polystyrene-resin methodology for for the solid-phase synthesis of a large number of diverse )3-keto esters 40a-h. These were reacted with phenylhydrazine in THF at room temperature to give hydrazones 41a-h that were then cychzed into 2-phenyl 5-substituted pyrazol-3-ones 42a-h by heating in toluene at 1(X)°C (Scheme 12). 

Of the several syntheses available for the phenothiazine ring system, perhaps the simplest is the sulfuration reaction. This consists of treating the corresponding diphenylamine with a mixture of sulfur and iodine to afford directly the desired heterocycle. Since the proton on the nitrogen of the resultant molecule is but weakly acidic, strong bases are required to form the corresponding anion in order to carry out subsequent alkylation reactions. In practice such diverse bases as ethylmagnesium bromide, sodium amide, and sodium hydride have all been used. Alkylation with (chloroethyl)diethylamine affords diethazine (1), a compound that exhibits both antihista-minic and antiParkinsonian activity. Substitution of w-(2-chloroethyl)pyrrolidine in this sequence leads to pyrathiazine (2), an antihistamine of moderate potency. 

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