Transition metal-mediated approaches

A transition metal-mediated approach to amidation of glycal substrates can lead to the formation of 2-deoxy-2-trifluoroacetyl-amido derivatives. Thus, when a solution of (saltmen)Mn(N) is added to a mixture of trifluoroacetic anhydride (TFAA) and glycal 107 followed by sequential treatment with thiophenol and BF3 Et20, the 2-A-trifluoroacetamido thioglycoside 108 can be obtained in good yield and excellent diastereoselectivity (O Scheme 53) [91]. 

Construction of the Benzintidazole Core Using Transition Metal-Mediated Approaches... 

As already documented for other azoles, methods for transition metal-mediated approaches to imidazoles with different substitution pattern have been disclosed recently, (a) In the Pd-catalyzed multicomponent approach, N-alkylimines, acid chlorides and N-tosylimines are coupled in the presence of a phosphine, DIPEA/LiCl, carbon monoxide, and a specific Pd(0) catalyst to give tetrasubstituted N-alkylimidazoles 48 [398] ... 

A collection of such methods has been given J. A. Davies, R. J. Staples, Electrochemical Approaches to Transition Metal Mediated C-H Bond Activation, in Selective Hydrocarbon Activation (J. A. Davies, P. L. Watson, J. F. Liebman, A. Greenberg, Edits.), p. 379 ff, VCH Publishers, New York 1990. 

Abstract The transition metal mediated conversion of alkynes, alkenes, and carbon monoxide in a formal [2 + 2+1] cycloaddition process, commonly known as the Pauson-Khand reaction (PKR), is an elegant method for the construction of cyclopentenone scaffolds. During the last decade, significant improvements have been achieved in this area. For instance, catalytic PKR variants are nowadays possible with different metal sources. In addition, new asymmetric approaches were established and the reaction has been applied as a key step in various total syntheses. Recent work has also focused on the development of CO-free conditions, incorporating transfer carbonylation reactions. This review attempts to cover the most important developments in this area. 

Although a large number of 2-oxygenated tricyclic carbazole alkaloids have been isolated from different natural sources, only a few syntheses of this class of alkaloids were reported. This is mainly due to the lack of general methods, as well as the difficulty associated with the known synthetic methods, to build up the required substitution pattern. Prior to 1990, some total syntheses of 2-oxygenated carbazoles were reported and were covered in the earlier treatises by Kapil (1), Husson (2), and Chakraborty (3) in Volumes 13, 26, and 44 of this series. Since 1990, the only total syntheses which appeared in the literature for this class of carbazole alkaloids were transition metal-mediated or -catalyzed approaches, respectively. These general approaches offered a series of 2-oxygenated carbazole alkaloids (8,10). 

A Fe S-based catalytic transition metal complex has not yet been found, but the present status of quantum chemical calculations encourages further work into this direction. Finally, the complexity of this task assures that results are obtained, which can be generalized for transition-metal-mediated catalysis in general. These results are of methodological and chemical nature since they provide improved quantum chemical methods and approaches for the calculation of structural motifs, which do also occur in many other catalytic systems. 

It is however clear that the use of carefully ion-exchanged amorphous supports can lead to catalysts that have superior regioselectivities in propene dimerization compared with many homogeneous systems. The approach may well be more generally applicable to, for example, other areas of transition metal mediated catalysis and this aspect is now attracting our attention. 

Numerous transition metal-mediated [2 + 2+2] cycloadditions have been utilized in the synthesis of pyridines . Selective cyclotrimerization of alkynes with nitriles leads to pentasubstituted pyridines 310 with minimal formation of benzenoid byproducts (Scheme 157) <20000L3131>. Different alkynes can be utilized in the same strategy if a sequential approach is used (Scheme 158) <2000JA4994>. 

Transition Metal-Mediated Nitrenoid-Transfer Approach... 

This review focuses on the transition metal mediated polymerization of isocyanides and 1,2-diisocyanobenzenes. Other approaches, including acid-catalyzed polymerization and main-group metal mediated polymerization, are also included, but only if they are closely related to the above topic. 

Other approaches to indoles based on transition metal-mediated couplings have been outlined in Sections 4.2.2 and 4.2.4. Nicolaou et al. recently described a novel solid-phase synthesis of substituted indolines and indoles [414]. 

An extremely powerful approach developed by Thorp and coworkers over the last decade uses a transition-metal mediator to facilitate the catalytic oxidation of DNA. " In a typical assay, RuCbpyXa is electrochemically oxidized to Rulbpyls , which rapidly oxidizes guanine residues in DNA to... 

Many methods for the preparation of both the a- and )8-C-glycosides have been developed, and new approaches are still appearing in the literature. This chapter covers the literature from the end of 1994 to early 1999, with other inclusions as deemed appropriate. Following the organization of earlier work [1], we select the mode reactivity of the anomeric center to define the category. The chapter discusses free radical chemistry. Cl anions, Wittig chemistry-cyclization chemistry, transition metal mediated chemistry, sigmatropic chemistry, and approaches based on cationic chemistry. The chapter focuses on the most recent and novel developments in the field, but all the relevant references are included whenever possible. 

As for the second type of approach, the transition-metal mediated nitrenoid transfer to olefins represents a very concise route to the azdridine structure very often, however, an excess of the olefinic subsrate is required for preparatively useful yields. In this arena, Andersson and coworkers <97TL6897> have studied the copper-catalyzed aziridination of olefins using [N-(arenesulfonyl)imino]phenyliodinanes (136) as nitrene precursors, and have reported on conditions which give good to excellent yields of aziridines 137 without the constraint of having to use an excess of alkene. 

In recent years, significant research effort has been directed toward identification of new methods for benzimidazole synthesis. Recent advances, particularly in transition metal-mediated reactions, have ushered in novel synthetic approaches for the construction and functionalization of this useful ring system. The present overview of new transition metal-mediated synthetic approaches focuses primarily on methodologies reported from 2008 to early 2012 that may soon find broader application in drug synthesis. 

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