Current Organic Synthesis - Volume 6, Issue 3, 2009

Transition metal-catalyzed [2+2] cycloadditions between bicyclic alkenes and alkynes provides an efficient method for the construction of cyclobutene rings. Various transition metal complexes, including those of cobalt, nickel, ruthenium, rhodium, and rhenium, have been used to catalyze [2+2] cycloadditions between bicyclic alkenes and alkynes. The purpose of this review is to summarize and discuss the various aspects of transition metal-catalyzed [2+2] cycloaddition between bicyclic alkenes and alkynes, including the development of metal catalysts, the mechanisms of the cycloadditions catalyzed by different metal catalysts, the reactivity of the alkene and the alkyne components in the cycloaddition, the compatibility of different functional groups on the alkene and the alkyne components, the chemo- and regioselectivity of the cycloadditions between unsymmetrical substrates, and the asymmetric induction studies using chiral auxilliaries on the alkyne component in addition to the use of using chiral ligands. The scope and limitations of each catalyst in the [2+2] cycloadditions will also be discussed.

Due to their relevance in biological processes, molybdenum-catalyzed oxygen-transfer reactions have attracted considerable interest. In this context, some dioxomolybdenum(VI) complexes, with the cis-[MoO2]2+ core, that mimic oxotransferases, have been found to be efficient catalysts for several oxidation and reduction reactions in organic synthesis. More recently, the utility of these molybdenum( VI) complexes has been extended to other processes such as hydrosilylation reactions and, due to the amphoteric character of the Mo=O unit, to some Lewis acid-Lewis base conjugate catalyzed reactions.

Palladium(0)-catalyzed allylation of soft nucleophiles is a powerful synthetic methodology for the construction of carboncarbon or carbon-heteroatom bonds. The possibility of controlling the regioselectivity is one of the most interesting aspects of this chemistry. In this review, the different factors that can affect the regiochemistry of the nucleophilic attack will be discussed.

Three-membered heterocyclic rings offer a powerful combination of reactivity, stability, availability, and atom economy. In fact, the asymmetric ring opening of allylic epoxides with nucleophiles of different kinds, as will be discussed in this Review, offers the possibility of generating valuable chiral non-racemic building blocks in a very simple manner and in a stereodefined fashion.

Because the renewed interest of β-lactams in organic and medicinal chemistry, research in this field continues in producing new appealing progresses. This review tries to call the attention on synthetic aspects related to the generation of the four-membered heterocyclic ring, published in the last few years, with special accent in stereoselective synthetic approaches. The compendium will cover methods for achieving diastereo- and enantioselectivity during [2+2] cycloadditions, with the Staudinger reaction still as the first choice method, but also important achievements have been published for the imine-enolate condensation, the carbonylative cycloaddition, and the Kinugasa reaction. Significant success was also obtained in the synthesis of innovative 2-azetidinones through the formation of a single bond. In this respect, the C3-C4 bond formation, through intramolecular cyclization of linear epoxy or halo derivatives, and the carbenoid C-H insertion are among the most effectively exploited procedures. In addition, the adaptation of most procedures to solid-phase methodologies has facilitated the generation of molecular diversity based on the β-lactam skeleton.