para-Quinone Methides as Acceptors in 1,6-Nucleophilic Conjugate Addition Reactions for the Synthesis of Structurally Diverse Molecules

para-Quinone methides (p-QMs) are naturally occurring molecules that have been finding increasing synthetic applications in the last few years. The presence of two electronically different exocyclic conjugate substituents in their structure, carbonyl and methylidene, leads to a pronounced reactivity owing to the polarization of the molecule. In this sense, those are prone to undergo the attack of nucleophiles in the terminal carbon exocyclic double bond, behaving as vinylogous electrophiles and generating 1,6-addition products. In this context, in the last few years the development of catalytic approaches for 1,6-nucleophilic addition reactions involving p-QMs has attracted considerable attention. Considering the extensive applications that such molecules have found in the last decades in 1,6-addition reactions, in this review we comprehensively discuss the historical development of this field, starting with early approaches on natural product synthesis, going through seminal non-stereoselective processes and progressing to cutting-edge asymmetric-catalyzed approaches.