Ring closing metathesis , carbene synthesis

Keywords Ruthenium-carbenes, Ruthenium-allenylidenes, Ring closing metathesis, Natural product synthesis, Fine chemicals. 

With the discovery by Grubbs of ruthenium carbene complexes such as Cl2(PCy3)2Ru=CHR, which mediate olefin metathesis under mild reaction conditions and which are compatible with a broad range of functional groups [111], the application of olefin metathesis to solid-phase synthesis became a realistic approach for the preparation of alkenes. Both ring-closing metathesis and cross-metathesis of alkenes and alkynes bound to insoluble supports have been realized (Figure 5.12). 

A common procedure used to forge carbocyclic systems is represented by the ring-closing metathesis reactions exploiting various Grubbs-type carbene catalysts. Based on this cyclization technology, Skaanderup and Madsen [71], on the way to polyhydroxy-nortropane calystegines, reported the asymmetric synthesis of aminated 6a-carbahexoseptanose derivatives 311. 

The metallo-carbene catalysts for ring-closing metathesis (RCM) reactions are ideal for this purpose. As a result of the mild reaction conditions and lack of competition with the coordination chemistry of the intermediate helical complex, the RCM strategy is particularly well adapted to the synthesis of copper(I)-complexed catenanes. A natural extension of this work was the preparation of a trefoil knot following the strategy depicted in Figure 11. 

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