Multicomponent reactions (MCRs) designated as ABB' are defined as those reactions that introduce into the final product one molecule of component A and two molecules of component B in a chemo-differentiating manner. This ability to discriminate the incorporation of component B ensures that these processes maintain the advantages of using multicomponent reactions in diversity-oriented molecular construction. Furthermore, they benefit from the fact that only two reagents need to be mixed together. Component B can be therefore considered to be a privileged building block, and the reactions in which it participates, chemo-differentiating multicomponent reactions. Among the reduced set of compounds capable of acting as such building blocks, we discuss the use of ketenes, terminal conjugated alkynoates, enolisable carbonyl compounds, cyclic enol ethers and cyclic enamines.