Interest in covalent organic frameworks as high-value materials has grown steadily since their development in the 2000s. However, the great advantage that allows us to obtain these crystalline materials—the reversibility of the bonds that form the network—supposes a drawback in terms of thermal and chemical stability. Among the different strategies employed for the stabilization of imine-based Covalent Organic Frameworks (COFs), cycloaddition and other related cyclization reactions are especially significant to obtain highly stable networks with extended π-delocalization and new functionalities, expanding even further the potential application of these materials. Therefore, this entry gathered the most recent research strategies for obtaining stable COFs by means of cyclization reactions, including the Povarov reaction and intramolecular oxidative cyclization reactions as well as some other recent innovative approaches.
Since the first reported synthesis of covalent organic frameworks (COFs) in 2005 by Yaghi and collaborators 
, these porous and crystalline materials have gained attention, importance, and applicability in different scientific and technological areas such as catalysis 
(including photo- 
and electrocatalysis 
, electronics 
, medicine 
, and food security 
, among others 
. The great relevance acquired by these materials is mainly due to the possibility of tuning different chemical or physical properties such as porosity, functionalities, and stability according to the specific needs, with imine-based COFs being of special interest 
. In this regard, stability is often one of the limiting factors for the applicability of COFs because the characteristic reversibility of the bonds formed in the polymerization reactions makes these materials more prone to chemical or thermal degradation. Therefore, the development of new COFs nowadays involves a thorough design toward stabilization to prevent degradation processes as much as possible.
The prevention of the degradation of imine-based COFs has been addressed by employing different strategies, such as the design and construction of materials with specific interlayer interactions that help maintain the integrity of the network. Other efforts have focused on preventing hydrolysis of the imine group, either by blocking it from nucleophiles or by creating intramolecular hydrogen bonding or hydrophobic environments around the imine group 
. However, the blocking of the -C=N- bond that seems to be gaining attention during the last few years is its irreversible transformation into another functional group. The conversion of the reversible imine linkages into irreversible moieties has also been used to simultaneously incorporate new functionalities into the network. Thus, this strategy has allowed the conversion of the imine linkages into amines using different methods such as the Leuckart–Wallach reduction 
, the thiol-ene click reaction 
, or the Strecker reaction 
. Other efforts have been focused on embedding the imine linkages into new heterocyclic rings, amplifying the π-electron delocalization of the lattices and, in many cases, adding new functionalities. These blocking bond steps initially were performed by the post-synthetic modification (PSM) of the COFs. However, one-pot (OP) syntheses, in which COF and its transformation are carried out in a cascade process, are gaining importance due to the savings in efforts and recurses that it entails.
The great interest generated by obtaining ultra-stable crystalline networks from different imine-based COFs by the formation of different cycles is evidenced by the growing number of publications focused on their synthesis and the evaluation of their structural, chemical, and electronic properties for later applications. This entry gathered the synthetic strategies, both by PSM and OP, for the transformation of reversible links of imine-based COFs into stabilized heterocyclic moieties by using cycloaddition or cyclization reactions. Particular attention will be paid to the most recent publications to reflect the state of the art in this field.