"Click labeling" with 2-[18f]fluoroethylazide for positron emission tomography.

As an effort in the development of more flexible (18)F-labeling chemistry, we report herein on the use of the Cu(I)-catalyzed Huisgen cycloaddition, also known as the "click reaction", to form (18)F-labeled 1,2,3-triazoles. Nucleophilic fluorination of 2-azidoethyl-4-toluenesulfonate followed by distillation provided 2-[(18)F]fluoroethylazide in 55% radiochemical yield (decay-corrected). 2-[(18)F]fluoroethylazide was reacted with a small library of terminal alkynes in the presence of excess Cu(2+)/ascorbate or copper powder. The most reactive alkyne, N-benzylpropynamide provided nearly quantitative incorporation of 2-[(18)F]fluoroethylazide after 15 min at ambient temperature, whereas the majority of the alkyne substrates provided excellent yields of the corresponding (18)F-labeled 1,2,3-triazoles following heating to 80 degrees C. Using the method described, a model peptide was obtained in 92.3 +/- 0.3% (n = 3) radiochemical yield (decay-corrected) after purification by semipreparative HPLC.