Convergent synthesis of ¹³N-labelled Peptidic structures using aqueous [¹³N]NH₃

Nitrogen-13 has a 10-min half-life which places time constraints on the complexity of viable synthetic methods for its incorporation into PET imaging agents. In exploring ways to overcome this limitation, we have used the Ugi reaction to develop a rapid one-pot method for radiolabelling peptidic molecules using [¹³N]NH₃ as a synthetic precursor.

Carrier-added [¹³N]NH₃ (50 μL) was added to a solution of carboxylic acid, aldehyde, and isocyanide in 2,2,2-TFE (200 μL). The mixture was heated in a microwave synthesiser at 120 °C for 10 min. Reactions were analysed by radio-HPLC and radio-LCMS. Isolation of the target ¹³N-labelled peptidic Ugi compound was achieved via semi-preparative radio-HPLC as demonstrated for Ugi1.

Radio-HPLC analysis of each reaction confirmed the formation of radioactive products co-eluting with their respective reference standards with radiochemical yields of the crude products ranging from 11% to 23%. Two cyclic γ-lactam structures were also achieved via intra-molecular reactions. Additional radioactive by-products observed in the radio-chromatogram were identified as ¹³N-labelled di-imines formed from the reaction of [¹³N]NH₃ with two isocyanide molecules. The desired ¹³N-labelled Ugi product was isolated using semi-preparative HPLC.

We have developed a one-pot method that opens up new routes to radiolabel complex, peptidic molecules with ¹³N using aqueous [¹³N]NH₃ as a synthetic precursor.