DABCO-Catalyzed Nucleophilic Aromatic Substitution and Its Use in Improving the Synthesis of O²-Benzylcytosines as Substrates of CLIP-Tag

We describe the effect of 1,4-diazabicyclo[2.2.2]octane (DABCO), a nucleophilic tertiary amine, in the synthesis of O²-benzylcytosines in terms of both reaction efficiency and operational convenience. Benzylcytosines are substrates of the protein-labeling tool CLIP-tag. The applications of the synthesized CLIP-tag substrates in the fluorescent labeling of plasma membranes of live mammalian cells are demonstrated. For providing a fuller depiction of the function of DABCO in nucleophilic aromatic substitution (S_NAr) reactions involving nitrogen heterocycles, the efficiency and scope of DABCO-catalyzed substitutions on chlorinated pyrimidines are studied. The structures of single- and double-DABCO adducts of pyrimidine are characterized by single-crystal X-ray diffraction and NMR spectroscopy. The S_NAr reactivities of the DABCO adducts are assessed. Both the benefits and limitations of DABCO as a nucleophilic catalyst in S_NAr reactions are discussed.