3.18 The Ramberg–Bäcklund Rearrangement and the Eschenmoser Coupling Reaction

Abstract

The present review surveys the main relevant advances on the two carbon–carbon bond forming reactions, the Ramberg–Bäcklund rearrangement (RBR) and the Eschenmoser coupling reaction (ECR), after the publication of the first edition of Comprehensive Organic Synthesis in 1991. These two subjects were combined within one chapter due to their common mechanistic feature. Both proceed via the base-induced formation of an intermediate three-membered sulfur-containing heterocycle followed by sulfur-derived extrusion. However, if episulfones (thiirane-1,1-dioxides) are involved in the case of the RBR, episulfides (thiiranes) are the transient species in the ECR. With regard to the RBR, the isolation of previously postulated episulfone intermediate as well as its various important synthetic modifications are of special interest. A number of synthetic applications have been described, including preparation of natural products and complex bioactive molecules, synthesis of conjugated dienes, enynes, and polyenes and novel electrically conducting materials; it also provides the possibility of ring enlargement of cyclic ketones. As concerning the ECR, providing a convergent method for the synthesis of vinylogous amides and carbamates, valuable synthetic intermediates, emphasis is on its application to the natural product and bioactive molecule synthesis.