Current Organic Chemistry - Volume 10, Issue 12, 2006

It is a real pleasure for me to serve as the executive guest editor for Current Organic Chemistry's special issue on recent developments in the synthesis of heterocycles by transition-metal catalyzed cyclization reactions. The importance of heterocycles in many field of Science (including Organic Chemistry, Inorganic Chemistry, Bioorganic Chemistry, Biological Chemistry, Agricultural Chemistry, Industrial Chemistry, Pharmaceutical Chemistry, Medicinal Chemistry, and Material Science) can hardly be overemphasized, and justifies a long lasting effort to work out new synthetic protocols for their production. A particularly attractive approach is based on transition-metal catalyzed heterocyclization reactions of suitably functionalized substrates, which can allow the regioselective synthesis of highly functionalized heterocycles starting from readily available starting materials under mild and selective conditions. During the last years, this approach to the synthesis of heterocycles has acquired a growing importance, as testified by the increasing number of publications, including reviews, devoted to this kind of Chemistry. This volume will cover a wide range of topics related to transition-metal catalyzed synthesis of heterocycles, ranging from cyclocarbonylation to ring-closing metathesis. The first review, by Professor G. Balme (University of Lyon 1 "Claude Bernard") and co-authors D. Conreaux, D. Bouyssi, and N. Monteiro, deals with the Pd-catalyzed construction of heteropolycyclic molecules through an ordered sequence of steps, in which palladium intervenes in at least two sequential different transformations. The use of hydrosilylation, silylformylation, silylcarbocyclization, and cyclohydrocarbonylation reactions for the synthesis of heterocycles is illustrated in the review by Professor I. Ojima (State University of New York at Stony Brook) and co-author G. Varchi. The next review, by Professor B. Schmidt (University of Dortmund) and co-author J. Hermanns, reports on the recent advances on the synthesis of functionalized heterocycles by RCM (Ring Closing Metathesis) of polyunsaturated substrates. The review by Professor G. Vasapollo (University of Lecce) and co-author G. Mele is focused on the use of carbon monoxide as a building block for synthesizing heterocycles bearing a carbonyl group into the cycle (cyclocarbonylation). The versatility of Pd-based catalytic methodologies for the synthesis and functionalization of benzo[b]furans is illustrated in the review by Professor S. Cacchi (University of Rome 1 "La Sapienza") along with co-authors G. Fabrizi and A. Goggiamani. The recent developments on the transition-metal-catalyzed intramolecular enyne cyclization reaction leading to heterocycles have been reviewed by Professor Z. Zhang (Shanghai Jiaotong University) together with co-authors G. Zhu, X. Tong, F. Wang, X. Xie, and L. Jiang. The final review, by Professor F. Ragaini (University of Milan) along with co-authors S. Cenini, E. Gallo, A. Caselli and S. Fantauzzi, illustrates the utility of the reductive carbonylation reaction of nitroarenes for the one-step synthesis of heterocyclic derivatives. Finally, I would like to thank all the authors and the editorial staff for making this issue possible.

This review concerns the preparation of heteropolycyclic systems in a single operation using palladium complexes and is restricted to methods where the palladium intervenes in at least two sequential different transformations.

The design and development of highly efficient catalytic methods for the synthesis of carbocycles and heterocycles has been the subject of extensive study, because of their relevance to life sciences and material sciences. Among various available synthetic methods, the transition-metal-catalyzed cyclization reactions has attracted much attention for the transformation of simple starting materials into heterocyclic scaffolds, which can be further elaborated into more specific targets. Accordingly, this review describes recent advances in the synthesis of heterocycles by means of intramolecular hydrosilylation, intramolecular silylformylation, silylcarbocyclization (SiCaC), and cyclohydrocarbonylation reactions.

In most cases where ring closing metathesis is applied to the synthesis of heterocycles, α,ω-dienes are used as precursors. If substrates containing more than two double bonds are subjected to a metathesis reaction, carba- or heterocycles bearing additional exocyclic alkene functionality result, or multiple ring closing processes occur. This offers interesting and potentially very useful synthetic perspectives. On the other hand, selectivity problems need to be addressed as the cyclization of substrates with more than two double bonds available for olefin metathesis may result in constitutional isomers or stereoisomers. This review highlights problems and opportunities evolving from ring closing metathesis of tri-, tetra-, and polyenes as a strategy for the selective synthesis of functionalized heterocycles. The chapter on RCM of trienes is subdivided according to the symmetry of the metathesis precursor. The following two chapters deal with the double or multiple RCM of tetra- or polyenes. These processes are further classified according to the preferred cyclization mode. Finally, the application of cascade or domino metathesis reactions to the synthesis of heterocycles will be discussed. These processes can be classified into those where exclusively C-C-double bonds take part in the metathesis reaction, and those where one or more C-C-triple bonds are involved.

The synthesis of heterocyclic compounds such as lactones, lactams, pyrrolidinones and others can be achieved by cyclocarbonylation reactions catalyzed by transition metal complexes. The combination of palladium salts with certain different ligands turned out to be the most efficient catalyst systems to accomplish such cyclocarbonylation reactions. New synthetic strategies and novel approaches devoted to the preparation of such compounds having different ring size still remain a stimulating area of academic and industrial research.

In the past decades, the great potential of palladium catalysis has been widely used in the de novo construction of the functionalized benzo[b]furan ring and in the selective functionalization of the preformed benzo[b]furan system providing a broad range of new and versatile procedures. The de novo construction of the benzo[b]furan system generally involves the assembly of the furan nucleus on a benzenoid scaffold via intramolecular or intermolecular cyclization of compounds containing oxygen nucleophiles and carbon-carbon triple bonds or carbon-carbon double bonds. Other less frequently used de novo syntheses of the benzo[b]furan system are based on the intramolecular Buchwald/Hartwig C-O bond forming process and the construction of the benzenoid ring on a furan scaffold. The functionalization of the preformed benzo[b]furan system is usually based on the functionalization via benzofuryl halides or triflates and the functionalization via organometallic derivatives such as benzofurylstannanes, benzofurylboronic acids and benzofurylzinc compounds. Functionalization via direct activation of C-H bonds has also been described.

This review summarizes the recent developments in transition metal-catalyzed 1,n-enyne cyclization reactions that have been reported in the literature from January 1, 2002 to April 30, 2005. The cyclizations described herein include four parts according the reaction mechanism, i. e. cyclization initiated by oxidative cyclometallation; by activation of the triple bond to form a vinyl metal species; by electrophilic activation of the triple bond; and by activation of the double bond. Enyne cyclizations that do not follow the above reaction mechanism are not included.

The application of reductive carbonylation of nitroarenes to the synthesis of fine chemicals is described. The review focuses on the results reported from 1996 onwards and is mainly divided in two parts, respectively describing the intra-molecular cyclization reactions of nitroarenes bearing in the ortho position a suitable functional group and the intermolecular reactions of nitroarenes with external olefins and alkynes. Benzo-fused heterocycles such as indoles and quinolinones are obtained by the first group of reactions, whereas allylic amines, oxazines, pyrroles and indoles are the products of inter-molecular reactions. Mechanistic studies are also discussed in a separate section. x