Carbocyclizations chemoselectivity

As terminal alkynes and ethynyl alcohols are the convenient sources to generate ruthenium vinylidene and allenylidene intermediates, many carbocyclizations have been achieved via nucleophilic addition and other activations at the two intermediates. Most reported carbocyclizations appear to be synthetically useful, not only because of their chemoselectivities but also because of their tolerance toward organic functional groups. Additional examples of catalytic carbocyclization based on ruthenium vinylidenes are still growing, and on the basis of the concepts developed here one can expect to see many new applications in the near future. 

Cyclopropene, methylenecyclopropane and their derivatives have proved to be valuable reagents in transition metal-catalyzed cycloaddition reactions. Small and medium carbocycles can be prepared by this method. The chemoselectivity observed in some of these reactions is quite remarkable. In addition, high degree of regio- and stereoselectivity is obtained in most cases. In particular the new [3+2] cycloaddition described here and which involves methylenecyclopropane and its derivatives as trimethylenemethane synthones, shows great synthetic promise as a method for constructing fivemembered rings. 

The importance of controlling the direction of ring closure for the unsymmetrical di-ester condensation has prompted researchers to introduce different chemoselective methods for this reaction. The mono-thiol version of the condensation was first developed for that particular purpose by Yamada, Hosaka, and co-workers in 1981. An example is the ring formation of compounds 22. The ring closure occurs using an appropriate base such as lithium diispropylamide (LDA) in dry THF at -30 °C in the case of X = CH2 or sodium hydride at room temperature in case of X = N (or S) to form the corresponding carbocyclic or heterocyclic rings 22a-c in 74-77% yields. 

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