Recent Developments in the [5+2] Cycloaddition

The [5+2] cycloaddition allows the synthesis of a diversity of complex highly functionalized seven-membered products in a single step. These cycloadducts can readily be further manipulated synthetically for use in the synthesis of a number of complex natural products and important biologically active products containing seven-membered rings. In addition to the common and highly efficient [5+2] cycloadditions of (oxido)pyrylium and (oxido)pyridinium ions with various pi-systems, providing an easy access to a wide range of novel heterocyclic seven-membered rings exhibiting an oxygen or nitrogen bridge, the metal-catalyzed [5+2] cycloadditions still attract a great attention and have become one of the most popular ways of constructing seven-membered compounds. Among the most important reactions are metal-catalyzed (hetero) [5+2] cycloadditions of vinyl-substituted three-membered rings, rhodium-catalyzed [5+2] cycloadditions of 3-acyloxy-1,4-enynes, and metal-catalyzed [5+2] cycloadditions of ortho-vinylphenols and ortho-vinyl/arylanilines.