Tandem [4+2]/[3+2] Cycloadditions

This chapter focuses on tandem cycloadditions that are initiated by a [4 + 2] cycloaddition step that generates a 1,3-dipole or a precursor to a 1,3-dipole, which then engages in a [3 + 2] cycloaddition. Compared to the cycloadditions of nitroalkenes, tandem cycloadditions of 1,3,4-oxadiazoles consist not of two, but of three elementary reactions. The major synthetic applications of the tandem cycloadditions of 1,3,4-oxadiazoles include construction of oligomeric ribbon-like or cavity molecules as well as syntheses of natural products. The chapter refines the definition of a tandem process and its three variants: cascade, consecutive, and sequential. The tandemcycloaddition of nitroalkenes is a very complex process consisting of a number of elementary reactions. The examples of the tandem cycloadditions of nitroalkenes reviewed here demonstrate a well-developed strategy that can and has been applied for syntheses of many natural and unnatural compounds.