 
         
         Abstract
         
         
         
            
            
            
(4+3)-Annulations are incredibly versatile reactions which combine a 4-atom synthon
               and a 3-atom synthon to form both 7-membered carbocycles as well as heterocycles.
               We have previously reviewed transition-metal-catalyzed (4+3)-annulations. In this
               review, we will cover examples involving bases, NHCs, phosphines, Lewis and Brønsted
               acids as well as some rare examples of boronic acid catalysis and photocatalysis.
               In analogy to our previous review, we exclude annulations involving cyclic dienes
               like furan, pyrrole, cyclohexadiene or cyclopentadiene, as Chiu, Harmata, Fernándes
               and others have recently published reviews encompassing such substrates. We will however
               discuss the recent additions (2010–2020) to the literature on (4+3)-annulations involving
               other types of 4-atom-synthons.
            
            1 Introduction
            
            2 Bases
            
            3 Annulations Using N-Heterocyclic Carbenes
            
            3.1 N-Heterocyclic Carbenes (NHCs)
            
            3.2 N-Heterocyclic Carbenes and Base Dual-Activation
            
            4 Phosphines
            
            5 Acids
            
            5.1 Lewis Acids
            
            5.2 Brønsted Acids
            
            6 Boronic Acid Catalysis and Photocatalysis
            
            7 Conclusion
            
         
         Key words
annulation - (4+3)-annulations - organocatalysis - Lewis acids - Brønsted acids -
            phosphines - N-heterocyclic carbenes