| SPAAC |
• No external catalyst required |
• Relatively slower reaction kinetics |
61 and 80
|
| • Precisely selective |
| • Biocompatible |
• Possibility of cyclooctyne interaction with nucleophiles in living systems |
| • Inert towards physiological environment |
| Staudinger ligation |
• Highly selective approach for chemical labelling in vivo
|
• Low stability of iminophosphoranes/azaylides |
64 and 81
|
| • Synthesis of a library of bioconjugates implemented in pharmaceutical research |
• Vulnerability of phosphorous compounds towards moisture and/or air |
| Thiol–ene reaction |
• Homogenous polymer network via regulated step-growth and chain-growth processes |
• Oxygen inhibition, complex volume relaxation, and stress buildup |
67 and 68
|
| • Rapid and uniform synthesis of thiol–ene networks under ambient air conditions |
| IEDDA |
• No coupling reagent or catalyst required |
• Sensitivity of trans-cyclooctene and 1,2,4,5-tetrazine to acids, thiols, copper ions, and bases |
71 and 82
|
| • Fast reaction kinetics in aqueous environment |
| • Better kinetics when compared with the conventional DA reaction |
• Difficult to investigate the reaction in vivo with smaller biomolecules, such as peptides |
| • Enhanced stability of reaction owing to electron-withdrawing groups attached with tetrazine reagent |
• Adopting such an approach at a clinical level presents a logistical difficulty |
| • Water, biological media and organic solvents can all be used for the reaction |
| Tetrazole ligation |
• Reactivity-based tool in biological systems owing to the biocompatible light source activation and genetically encodable alkene reporters |
• Limited biological relevance of reactions initiated by UV light in the range of 300–360 nm owing to phytotoxicity of the radiations to the living cells |
74 and 83
|
| • Precise control over both spatial and temporal aspects owing to activation only in the presence of specific light wavelengths |
| Oxime ligation |
• Reaction proceeds under mild acidic conditions in aqueous systems |
• Hydrolytic instability of oximes, like other condensation products such as imines and hydrazones in aqueous media |
77
|
| • Compatible with the majority of biomolecule functionalities |
• Difficult synthesis and storage of aldehyde or amino-oxy functionalized biomolecules |
| • The only side-product formed is water |
• Susceptibility of aldehydes to spontaneous oxidation or self-coupling |
