5.13 (4+3) Cycloadditions

Abstract

Seven-membered rings form the basic skeleton of many relevant polycyclic products, and therefore there is a great interest in the development of practical and rapid strategies for their assembly. Among the different constructive strategies, (4+3) cycloadditions represent a particularly useful alternative because of the ready availability of four-carbon partners (dienes) and three-atom precursors. Many developments in the area, in particular those involving the use of oxyallyl cations as three-atom components, were achieved in the 1970s and 80s, and are collected in the first edition of Comprehensive Organic Synthesis in 1991. However, many other relevant advances, including new (4+3) cycloaddition tactics, have been reported since then. This chapter covers and discusses these advances from both a mechanistic and synthetic perspective. The different cycloaddition strategies have been organized on the basis of similar mechanistic features and/or common hypothetical intermediates. A section discussing ‘formal’ (4+3) annulations that involve sequential processes occurring via well-defined and even stable organic intermediates have also been included. Finally, a number of synthetic applications are also collected, so that the reader can perceive the current synthetic potential of the technology.