Gunther Fischer

Bio: Gunther Fischer is an academic researcher. The author has contributed to research in topics: Tautomer & Dimroth rearrangement. The author has an hindex of 6, co-authored 13 publications receiving 186 citations.

Recent Progress in 1,2,4-Triazolo[1,5-a]pyrimidine Chemistry

Abstract: Publisher Summary 1,2,4-Triazolo[1,5-a]pyrimidines (TPs) play an important role in pharmaceutics, agrochemistry, photography, and other fields. They attract attention in synthetic, analytical, and theoretical chemistry. Isomeric 1,2,4-triazolo[4,3-a]pyrimidines and partially reduced TPs are mentioned only when they are precursors, potential intermediates, or reaction products of TPs. By far, the most TP syntheses are condensations of dinucleophilic 5-amino-1,2,4-triazoles (ATs) with 1,3-bifunctional synthons as in the formation of TP 2. Quantum-chemical calculations on TPs, particularly oxo compounds, were performed by several groups. A new aromaticity index was calculated for heterocyclic compounds. Comparison of the index of 2-ribosyl-5,7-dimethyl TP with that of the isomeric compound suggests that the driving force for the Dimroth rearrangement of the latter substance is the increase in aromatic character. The chapter discusses the molecular dimensions, molecular spectra, thermodynamic properties, tautomerism, and betain and mesoionic structures of TPs. The reactivity of TPs are described including triazolopyrimidines as bases and acids, ring alkylation, electrophilic reactions at carbon ring atoms, nucleophilic addition, ring cleavage, nucleophilic substitution of functional groups at the rings introduction and transformation of individual substituents, and reactivity of side chains. The pharmaceutical and agrochemical uses are described. The uses in information recording are also discussed.

1,2,4-Triazolo[1,5-a]pyrimidines

Abstract: Publisher Summary The formation of 1,2,4-triazolo [1,5-a] pyrimidines (TPs) was first reported by Biilow and Haas Over the next 50 years, only a few papers and patents on TPs were published, which were included in Mosby's comprehensive monograph on “Heterocyclic Systems with Bridgehead Nitrogen Atoms” Most syntheses of TPs start either from a 1,2,4-triazole derivative or from a pyrimidine residue and need annulation of a second heterocyclic ring Preferably 5-amino-1,2,4-triazoles (AT) and 2-hydrazinopyrimidines (HP) are used as starting synthons In the synthesis of TPs from HPs, intermediates are believed to exist and have been isolated in many cases These products and other acylated HPs may serve as convenient starting materials for cyclization reactions Acylated HPs are also prepared from 2-alkylthiopyrimidines and acylhydrazines and, further, cyclized in situ to triazolopyrimidines Many TPs have been tested as potential growth inhibitors of microorganisms, because they are isomeric to purines and in some cases can be converted to nucleoside analogs

Chapter 3 Azulenes Fused to Heterocycles

Abstract: Publisher Summary This chapter discusses azulenes, its occurrence and synthesis, structure, reactivity, and application. This chapter is restricted to azulenes directly fused to heterorings showing maximum unsaturation or to the corresponding tautomers. Most syntheses under review are based on azulene, cycloheptatriene or cyclopentadiene derivatives and consist in cyclizations to form the heterorings. Deliberate dehydrogenation of natural hydroazulenes and other sesquiterpene derivatives, sometimes after preceding dehydration, hydrogenation, and reduction, has been the first synthetic path to azulenes. The pharmaceutical uses include Linderazulene and its dihydro compound that are found to exhibit antitumor, antifungal, and immunostimulating activities. They are also used in information recording. Polymethine dyes containing the azulenium moiety are suitable as charge generators for electrophotography, especially for semiconductor laser-recording media showing both high sensitivity in recording and stability in reading.

Tropones, Tropolones, and Tropylium Salts with Fused Heterocyclic Rings: Part 2: Structure, Reactivity, and Application

Abstract: Publisher Summary The chapter discusses the structure, reactivity, and application of tropones, tropolones, and tropylium. X-ray diffraction, molecular spectra, aromaticity and stability, tautomerism, and polarographic reduction of tropones, tropolones, and tropylium are described in the chapter. The recognition, isolation, and characterization of troponoid systems have been facilitated by certain chemical properties that usually occur in both monocyclic and fused derivatives. Most tropolones give sparingly soluble, yellow or orange sodium salts, green cupric chelates, and colored ferric complexes. Valence isomerization and dimerization are common reactions in the photochemistry of troponoids. The acidity of parent tropolone lies between those of acetic acid and phenol; benzotropolones are only as acidic as phenol. Parent tropone forms very stable salts with acids; its basicity has its origin in the high stability of the hydroxytropylium ion formed by protonation. Chelate-complexes of tropolones are discussed in the chapter. Parent tropone on halogenation, affords addition products and by subsequent dehydrohalogenation affords halotropones substituted at 2- or 2,7-positions. Cyclocondensations onto the seven-membered ring are illustrated in the chapter by the reactions of pyridotropolones. Monocyclic tropylium salts on oxidation usually undergo ring contraction, benzotropylium salts give benzotropones. The troponoid system is included in many natural products, including alkaloids and antibiotics. Among heterocyclic derivatives, pyrrolotropones, tricyclic thieno- or pyrrolo-benzotropones, and other compounds have been claimed in patent applications. The pharmaceutical and photographic uses of tropones, tropolones and tropylium are discussed in the chapter.

Cascade reactions in total synthesis.

Abstract: The design and implementation of cascade reactions is a challenging facet of organic chemistry, yet one that can impart striking novelty, elegance, and efficiency to synthetic strategies. The application of cascade reactions to natural products synthesis represents a particularly demanding task, but the results can be both stunning and instructive. This Review highlights selected examples of cascade reactions in total synthesis, with particular emphasis on recent applications therein. The examples discussed herein illustrate the power of these processes in the construction of complex molecules and underscore their future potential in chemical synthesis.

Kaskadenreaktionen in der Totalsynthese

Abstract: Die Entwicklung und Umsetzung von Kaskadenreaktionen hat zu beachtlichen neuartigen, eleganten und effizienten Synthesestrategien gefuhrt. Als besonders anspruchsvoll erwies sich die Anwendung von Kaskadenreaktionen in der Naturstoffsynthese, doch gerade hier winken verbluffende und zugleich aufschlussreiche Ergebnisse als Belohnung. In diesem Aufsatz werden ausgewahlte Kaskadenreaktionen in der Totalsynthese erlautert, wobei neuere Anwendungen besonders hervorgehoben werden. Die erorterten Beispiele sollen die Leistungsfahigkeit dieser Prozesse beim Aufbau von komplexen Molekulstrukturen veranschaulichen und ihr Potenzial fur zukunftige chemische Synthesen verdeutlichen.

Recent developments in solvent-free multicomponent reactions: a perfect synergy for eco-compatible organic synthesis

Abstract: Multicomponent reactions have gained significant importance as a tool for the synthesis of a wide variety of useful compounds, including pharmaceuticals. In this context, the multiple component approach is especially appealing in view of the fact that products are formed in a single step, and the diversity can be readily achieved simply by varying the reacting components. The eco-friendly, solvent-free multicomponent approach opens up numerous possibilities for conducting rapid organic synthesis and functional group transformations more efficiently. Additionally, there are distinct advantages of these solvent-free protocols since they provide reduction or elimination of solvents thereby preventing pollution in organic synthesis “at source”. The chemo-, regio- or stereoselective synthesis of high-value chemical entities and parallel synthesis to generate a library of small molecules will add to the growth of multicomponent solvent-free reactions in the near future. In this review we summarized the results reported mainly within the last 10 years. It is quite clear from the growing number of emerging publications in this field that the possibility to utilize multicomponent technology allows reaction conditions to be accessed that are very valuable for organic synthesis. Therefore, diversity oriented synthesis (DOS) is rapidly becoming one of the paradigms in the process of modern drug discovery. This has spurred research in those fields of chemical investigation that lead to the rapid assembly of not only molecular diversity, but also molecular complexity. As a consequence multi-component as well as domino or related reactions are witnessing a new spring.

Metal-Catalyzed [6 + 3] Cycloaddition of Tropone with Azomethine Ylides: A Practical Access to Piperidine-Fused Bicyclic Heterocycles

Abstract: The first metal-catalyzed [6 + 3] cycloaddition of tropone with azomethine ylides has been developed. With the use of a chiral ferrocenylphosphine–copper(I) complex as the catalyst, the asymmetric variant of the [6 + 3] cycloaddition has also been successfully achieved. The reactions proceeded smoothly under mild conditions, affording piperidine-fused bicyclic heterocycles in moderate to high yields with good to excellent diastereo- and enantioselectivies. The procedures are operationally simple and the catalysts are cheap and readily accessible, thus providing a practical approach to piperidine-fused bicyclic heterocycles.