4-Nitrophenyl activated esters are superior synthons for indirect radiofluorination of biomolecules

Indirect radiolabelling has for a long time been the mainstay strategy for radiofluorination of biomolecules. Acylation of biomolecules through the use of an 18F-labelled activated ester is a standard method for indirect radiolabelling. However, the preparation of 18F-labelled activated esters is typically a complex and multistep procedure. Herein, we describe the use of 4-nitrophenyl (PNP) activated esters to rapidly prepare 18F-labelled acylation synthons in one step. Furthermore, we present a comparative study of PNP activated esters and the commonly utilised 2,3,5,6-tetrafluorphenyl (TFP) activated esters under direct radiofluorination conditions and demonstrate their relative acylation behaviour. We demonstrate the superiority of PNP esters under direct radiofluorination conditions with favourable acylation kinetics.