Application of Cross Metathesis in Diene and Polyene Synthesis

Olefin metathesis has become a useful method in organic synthesis, rivaling palladium catalyzed C-C bond forming reactions as well as more traditional olefination processes. Olefin cross metathesis (CM) emerged later than ring closing metathesis and ring opening metathesis polymerization due to poor selectivity control. However, after the discovery of more active second generation catalysts and elaboration by Grubbs of an empirical model to predict the outcome of these reactions, cross metathesis has found wide application in the synthesis of various alkenes including those that are sterically demanding or highly functionalized. In recent years enyne cross metathesis and metathetic coupling between alkene and diene or polyene were widely tested as alternative, convenient methods of diene and polyene preparation. In this review the application of ene-diene CM, ene-polyene CM, diene-polyene CM, ene-yne CM and alkene CM combined with other reactions in the synthesis of conjugated dienes and polyenes is presented. The aim of this article is to demonstrate not only the scope of CM application in the synthesis of conjugated dienes and polyenes but also its limitations associated mainly with problems of regio- and stereoselectivity control and how the encountered difficulties led to a better understanding of these processes and the discovery of more effective reaction conditions.