You are presented with multiple paper abstracts, alongside the associated literature review and chapter titles. Given the provided information, you are going to do a literature review. Please generate a coherent literature review that synthesizes the content, draws connections between different works, highlights major findings, and identifies any gaps in the existing research.

Literature Review Title: Small Molecule Radiopharmaceuticals – A Review of Current Approaches 
Chapter Title:  Tetrazines (Tetrazine-Trans-Cyclooctene Ligation) (41, 42) 
1. Abstract of Cited Paper (BIB001):  The increasing application of positron emission tomography (PET) in nuclear medicine has stimulated the extensive development of a multitude of novel and versatile bioorthogonal conjugation techniques especially for the radiolabeling of biologically active high molecular weight compounds like peptides, proteins or antibodies. Taking into consideration that the introduction of fluorine-18 (t(1/2) = 109.8 min) proceeds under harsh conditions, radiolabeling of these biologically active molecules represents an outstanding challenge and is of enormous interest. Special attention has to be paid to the method of 18F-introduction. It should proceed in a regioselective manner under mild physiological conditions, in an acceptable time span, with high yields and high specific activities. For these reasons and due to the high number of functional groups found in these compounds, a specific labeling procedure has to be developed for every bioactive macromolecule. Bioorthogonal strategies including the Cu-assisted Huisgen cycloaddition and its copper-free click variant, both Staudinger Ligations or the tetrazine-click reaction have been successfully applied and represent valuable alternatives for the selective introduction of fluorine-18 to overcome the afore mentioned obstacles. This comprehensive review deals with the progress and illustrates the latest developments in the field of bioorthogonal labeling with the focus on the preparation of radiofluorinated building blocks and tracers for molecular imaging. 
2. Abstract of Cited Paper (BIB002):  The inverse electron-demand Diels-Alder (IEDDA) cycloaddition between 1,2,4,5-tetrazines and strained alkene dienophiles is an emergent variety of catalyst-free ‘click’ chemistry that has the potential to have a transformational impact on the synthesis and development of radiopharmaceuticals. The ligation is selective, rapid, high-yielding, clean, and bioorthogonal and, since its advent in 2008, has been employed in a wide variety of chemical settings. In radiochemistry, the reaction has proven particularly useful with 18F and has already been utilized to create a number of 18F-labeled agents, including the PARP1-targeting small molecule 18F-AZD2281, the αvβ3 integrin-targeting peptide 18F-RGD, and the GLP-1-targeting peptide 18F-exendin. The inherent flexibility of the ligation has also been applied to the construction of radiometal-based probes, specifically the development of a modular strategy for the synthesis of radioimmunoconjugates that effectively eliminates variability in the construction of these agents. Further, the exceptional speed and biorthogonality of the reaction have made it especially promising in the realm of in vivo pretargeted imaging and therapy, and pretargeted imaging strategies based on the isotopes 111In, 18F, and 64Cu have already proven capable of producing images with high tumor contrast and low levels of uptake in background, nontarget organs. Ultimately, the characteristics of inverse electron-demand Diels–Alder click chemistry make it almost uniquely well-suited for radiochemistry, and although the field is young, this ligation has the potential to make a tremendous impact on the synthesis, development, and study of novel radiopharmaceuticals.

Literature Review:
Tetrazine-trans-cyclooctene ligation (TTCO ligation), introduced in 2010, is the inverse electron demand of the Diels-Alder (IEDDA) cycloaddition between a cyclooctene and a 1,2,4,5-tetrazine under the release of nitrogen BIB001 . Here, the tetrazine functionalized biomolecule is reacted with an 18 F-labeled cyclooctene (more preferred for radiolabeling). The approach has the advantages of fast reaction rates even without catalyst making it suitable for 11 C-labeling reaction (78), nonreversibility because of nitrogen release, broad tolerance range, both aqueous and organic based high yields. Its mechanism and the application are covered in the review BIB002 . In short, the reaction has been applied for labeling peptides (RGD, GLP-1, exendin), small molecules PARP1-targeting small molecule and DOTA derivatives [refer review BIB003 ].