Heterocycles constitute the largest and the most diverse family of organic compounds Among them, aromatic heterocycles represent structural motifs found in a great number of biologically active natural and synthetic compounds, drugs, and agrochemicals Moreover, aromatic heterocycles are widely used for synthesis of dyes and polymeric materials of high value 1 There are numerous reports on employment of aromatic heterocycles as intermediates in organic synthesis 2

Although, a variety of highly efficient methodologies for synthesis of aromatic heterocycles and their derivatives have been reported in the past, the development of novel methodologies is in cuntinious demand Particlularly, development of new synthetic approaches toward heterocycles, aiming at achieving greater levels of molecular complexity and better functional group compatibilities in a convergent and atom economical fashions from readily accessible starting materials and under mild reaction conditions, is one of a major research endeavor in modern synthetic organic chemistry Transition metal-catalyzed transformations, which often help to meet the above criteria, are among the most attractive synthetic tools

Several excellent reviews dealing with transition metal-catalyzed synthesis of heterocyclic compounds have been published in literature during recent years Many of them highlighted the use of a particular transition metal, such as gold,3 silver,4 palladium,5 copper,6 cobalt,7 ruthenium,8 iron,9 mercury,10 rare-earth metals,11 and others Another array of reviews described the use of a specific kind of transformation, for instance, intramolecular nucleophilic attack of heteroatom at multiple C–C bonds,12 Sonogashira reaction,13 cycloaddition reactions,14 cycloisomerization reactions,15 C–H bond activation processes,16 metathesis reactions,17 etc Reviews devoted to an application of a particular type of starting materials have also been published Thus, for example, applications of isocyanides,18 diazocompounds,19 or azides20 have been discussed In addition, a significant attention was given to transition metal-catalyzed multicomponent syntheses of heterocycles21 Finally, syntheses of heterocycles featuring formation of intermediates, such as nitrenes,22 vinylidenes,23 carbenes, and carbenoids24 have also been reviewed

The main focus of the present review is a transition metal-catalyzed synthesis of aromatic monocyclic heterocycles The organization of the review is rather classical and is based on a heterocycle, categorized in the following order: (a) ring size of heterocycle, (b) number of heteroatoms, (c) type of heterocycle, and (d) a class of transformation involved A brief mechanistic discussion is given to provide information about a possible reaction pathway when necessary The review mostly discusses recent literature, starting from 200425 until the end of 2011, however, some earlier parent transformations are discussed when needed

Transition Metal-Mediated Synthesis of Monocyclic Aromatic Heterocycles

No copper needed: In recent years, a large number of metal-free click reactions have been reported based on thiol-ene radical additions, Diels–Alder reactions, and Michael additions. In this Minireview, special attention is given to the advantages and limitations of the different methods to evaluate whether they have the potential to surpass the overwhelming success of the copper(I)-catalyzed azide-alkyne cycloaddition.

The overwhelming success of click chemistry encouraged researchers to develop alternative “spring-loaded” chemical reactions for use in different fields of chemistry. Initially, the copper(I)-catalyzed azide-alkyne cycloaddition was the only click reaction. In recent years, metal-free [3+2] cycloaddition reactions, Diels–Alder reactions, and thiol-alkene radical addition reactions have come to the fore as click reactions because of their simple synthetic procedures and high yields. Furthermore, these metal-free reactions have wide applicability and are physiologically compatible. These and other alternative click reactions expand the opportunities for synthesizing small organic compounds as well as tailor-made macromolecules and bioconjugates. This Minireview discusses the success and applicability of new, in particular metal-free, click reactions.

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Click Chemistry beyond Metal-Catalyzed Cycloaddition

L-Tryptophan (Trp) metabolism through the kynurenine pathway (KP) is involved in the regulation of immunity, neuronal function and intestinal homeostasis. Imbalances in Trp metabolism in disorders ranging from cancer to neurodegenerative disease have stimulated interest in therapeutically targeting the KP, particularly the main rate-limiting enzymes indoleamine-2,3-dioxygenase 1 (IDO1), IDO2 and tryptophan-2,3-dioxygenase (TDO) as well as kynurenine monooxygenase (KMO). However, although small-molecule IDO1 inhibitors showed promise in early-stage cancer immunotherapy clinical trials, a phase III trial was negative. This Review summarizes the physiological and pathophysiological roles of Trp metabolism, highlighting the vast opportunities and challenges for drug development in multiple diseases.

Tryptophan metabolism as a common therapeutic target in cancer, neurodegeneration and beyond

Synthesis of heterocycles via electrophilic cyclization of alkynes containing heteroatom.

Cu(I)-catalyzed azide–alkyne 1,3-dipolar cycloaddition (CuAAC), popularly known as the “click reaction”, serves as the most potent and highly dependable tool for facile construction of simple to complex architectures at the molecular level. Click-knitted threads of two exclusively different molecular entities have created some really interesting structures for more than 15 years with a broad spectrum of applicability, including in the fascinating fields of synthetic chemistry, medicinal science, biochemistry, pharmacology, material science, and catalysis. The unique properties of the carbohydrate moiety and the advantages of highly chemo- and regioselective click chemistry, such as mild reaction conditions, efficient performance with a wide range of solvents, and compatibility with different functionalities, together produce miraculous neoglycoconjugates and neoglycopolymers with various synthetic, biological, and pharmaceutical applications. In this review we highlight the successful advancement of Cu(I)...

Cu-Catalyzed Click Reaction in Carbohydrate Chemistry

Benzyne click chemistry: synthesis of benzotriazoles from benzynes and azides.

The relative reactivity of various functional groups toward alkyne electrophilic cyclization reactions has been studied. The required diarylalkynes have been prepared by consecutive Sonogashira reactions of appropriately substituted aryl halides and competitive cyclizations have been performed using I2, ICl, NBS and PhSeCl as electrophiles. The results indicate that the nucleophilicity of the competing functional groups, polarization of the alkyne triple bond, and the cationic nature of the intermediate are the most important factors in determining the outcome of these reactions.

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Competition studies in alkyne electrophilic cyclization reactions.

A large number of functionally substituted 2-alkyn-1-one O-methyl oximes have been cyclized under mild reaction conditions in the presence of ICl to give the corresponding 4-iodoisoxazoles in moderate to excellent yields. The resulting 4-iodoisoxazoles undergo various palladium-catalyzed reactions to yield 3,4,5-trisubstituted isoxazoles, including valdecoxib.

The synthesis of highly substituted isoxazoles by electrophilic cyclization: an efficient synthesis of valdecoxib.

Presently provided are IDO inhibitors and pharmaceutical compositions thereof, useful for modulating an activity of indoleamine 2,3-dioxygenase; treating indoleamine 2,3-dioxygenase (IDO) mediated inimunosuppression; treating a medical conditions that benefit from the inhibition of enzymatic activity of indoleamine-2,3-dioxygenase; enhancing the effectiveness of an anti-cancer treatment comprising administering an anti-cancer agent; treating tumor-specific immunosuppression associated with cancer; and treating immunosupression associated with an infectious disease.

Fused imidazole derivatives useful as ido inhibitors

Pyrido[1,2-a]indoles are known as medicinally and pharmaceutically important compounds, but there is a lack of efficient methods for their synthesis. We report a convenient and efficient route to these privileged structures starting from easily accessible 2-substituted pyridines and aryne precursors. A small library of compounds has been synthesized utilizing the developed method, affording variously substituted pyrido[1,2-a]indoles in moderate to good yields.

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Jesse Waldo

Papers

Benzyne click chemistry: synthesis of benzotriazoles from benzynes and azides.

Competition studies in alkyne electrophilic cyclization reactions.

The synthesis of highly substituted isoxazoles by electrophilic cyclization: an efficient synthesis of valdecoxib.

Fused imidazole derivatives useful as ido inhibitors

Synthesis of pyrido[1,2-a]indole malonates and amines through aryne annulation.