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

II.2.1. Homogeneous Systems 2166 II.2.2. Heterogeneous Systems 2168 II.3. Hydride Transfer Reduction 2169 II.3.1. Homogeneous Systems 2169 II.3.2. Heterogeneous Systems 2171 II.4. Hydrosilylation 2172 III. C−O Bond Formation 2173 III.1. Epoxidation of Unfunctionalized Olefins 2173 III.1.1. Homogeneous Catalysis 2173 III.1.2. Heterogeneous System 2176 III.2. Dihydroxylation of Olefins 2178 III.2.1. Homogeneous Systems 2178 III.2.2. Heterogeneous System 2178 III.3. Ring Opening of Meso Epoxides 2180 III.4. Kinetic Resolution 2180 III.4.1. Terminal Epoxides 2180 III.4.2. Secondary Alcohols 2181 IV. C−H Bond Oxidation 2181 IV.1. Allylic and Benzylic Oxidation 2181 IV.2. Baeyer−Villiger-type Reaction 2181 IV.3. Wacker-type Cyclization 2182 V. S−O Bond Formation 2182 VI. C−N Bond Formation 2183 VI.1. Hydroboration/Amination 2183 VI.2. Enolate Amination 2183 VI.3. Aza-Claisen Rearrangement 2183 VI.4. Azide Synthesis 2184 VI.5. Aminohydroxylation 2184 VI.6. Aziridine Synthesis 2184 VI.7. C−N Bond Formation via S−N Bond Formation 2185

Nitrogen-containing ligands for asymmetric homogeneous and heterogeneous catalysis.

C2-Symmetric Chiral Bis(Oxazoline) Ligands in Asymmetric Catalysis

In recent years, the transition-metal catalyzed three-component coupling of an aldehyde, an alkyne and an amine, commonly called A3-coupling, has been established as a convenient and general approach towards propargylamines. Furthermore, the A3-coupling has found a broad application as a key step in the construction of various nitrogen-containing heterocycles, biologically active compounds and natural products. Several interesting modifications of the A3-coupling as well as different tandem reactions involving A3-coupling have been developed. This tutorial review aims to highlight the current achievements in the field of A3-couplings and related transformations.

A walk around the A3-coupling

The amount of research activity concerning alpha-methylene-gamma-butyrolactones and alpha-alkylidene-gamma-butyrolactones has increased dramatically in recent years. This Review summarizes the structural types, biological activities, and biosynthesis of these compounds, concentrating on publications from the past 10 years. Traditional approaches to alpha-methylene-gamma-butyrolactones and alpha-alkylidene-gamma-butyrolactones are then reviewed together with novel approaches, including those from our own research group, reported more recently.

The Renaissance of α‐Methylene‐γ‐butyrolactones: New Synthetic Approaches

A valuable pharmacophore, the 2-aminoimidazole moiety, can be accessed with a variety of substitution patterns through an addition–hydroamination–isomerization sequence (see scheme; R1,R4,R5=alkyl; R3=alkyl, aryl; R2=H, alkyl, aryl). The synthesis of the propargyl cyanamide precursors through a three-component coupling enables the preparation of this important heterocyclic core structure in just three steps.

Addition-hydroamination reactions of propargyl cyanamides: rapid access to highly substituted 2-aminoimidazoles.

This review will focus on the ability of the 2-aminoimidazole to occupy a unique subset of chemical space which makes it an ideal pharmacophore for the development of small molecule collections for discovery based research. These observations rely both on the use of 2-aminoimidazoles as building blocks in medicinal chemistry as well as the recent discovery of alkaloids from sponges of the genus Leucetta which exhibit a diverse range of biological activities around a relatively limited structural core. The preparation of these compounds will also be highlighted. In particular, marine natural products derived from sponges have provided valuable leads for therapeutic small molecules (3, 4). Surprisingly the large majority of these compounds have been isolated from organisms of the class Dermospongiae. In the mid 1980's chemists noted that the other major sponge class, Calcarea, had rarely been subject to chemical investigations. A flurry of efforts through the mid-1990's helped to establish biogenetic relationships among these sponges. Isolated to explore these inter- connections and not necessarily for specific biological responses the activities of these natural products have remained largely uncovered. Since these initial inves- tigations, an emerging structural class has recurrently been identified through bioassay guided isolation which contains the 2-aminoimidazole core. From the viewpoint of small molecule discovery this review will highlight alkaloids isolated from Leucetta sp. This small skeletal family has been shown to interrogate an incredibly diverse range of biological processes and thus represents an important discovery scaffold for both medicinal and discovery based research.

2-Aminoimidazoles from Leucetta Sponges: Synthesis and Biology of an Important Pharmacophore

Studies leading to a total synthesis of epothilones B and D are described. The overall synthetic plan was based on late-stage fragment assembly of two segments representing C1−C9 and C10−C21 of the structure. The C1−C9 fragment was prepared by elaboration of commercially available (2R)-3-hydroxy-2-methylpropanoate at both ends of the three-carbon unit. Introduction of carbons 1−4 containing the gem-dimethyl unit was achieved in a convergent manner using a diastereoselective addition of a stannane equivalent of a β-keto ester dianion. An enantioselective addition of such a stannane equivalent for a β-keto ester dianion was also used to fashion one version of the C10−C21 subunit; however, the fragment assembly (using bimolecular esterification followed by ring-closing metathesis) with this subunit failed. Therefore, fragment assembly was achieved using a Wittig reaction; this was followed by macrolactonization to close the macrocycle. The C10−C21 subunit needed for this approach was prepared in an efficient...

Total Synthesis of Epothilones B and D: Stannane Equivalents for β-Keto Ester Dianions

A methodology to generate 2-thio- and 2-oxoimidazoles through an addition−cyclization−isomerization reaction of propargylcyanamides with thiol and alcohol nucleophiles is described. In general, the reaction sequence allows for the rapid formation of highly substituted 2-thio- and 2-oxoimidazoles in good to excellent yields.

Robert L. Giles

Papers

Addition-hydroamination reactions of propargyl cyanamides: rapid access to highly substituted 2-aminoimidazoles.

2-Aminoimidazoles from Leucetta Sponges: Synthesis and Biology of an Important Pharmacophore

Total Synthesis of Epothilones B and D: Stannane Equivalents for β-Keto Ester Dianions

Synthesis of 2-thio- and 2-oxoimidazoles via cascade addition-cycloisomerization reactions of propargylcyanamides.

Catalytic asymmetric Diels–Alder reactions of α-thioacrylates for the preparation of norbornenone