Trifluoroacetyl as a protecting group for HYNIC: stability in the presence of electrophiles and application in the synthesis of 99mTc-radiolabelled peptides

TrifluoroacetylHYNIC-peptides have recently been shown to label directly with 99mTc as efficiently as their non-trifluoroacetylated analogs. In this work, the trifluoroacetyl (Tfa) moiety has been evaluated as a protecting group for HYNIC against reaction with strong electrophiles. Fmoc-(trifluoroacetylHYNIC)-lysine, the chosen model starting material, was found to be resistant against acetaldehyde and benzylchloroformate challenges, at 1 mol equiv and a 1000 M excess, respectively. In contrast, the Fmoc-(HYNIC)-lysine derivative, with a free hydrazine group, was quantitatively converted to the corresponding hydrazone after a 1 h incubation with acetaldehyde. Fmoc-(trifluoroacetylHYNIC)-lysine was also found to be stable over a wide pH range (3.6–10) to the acetaldehyde challenge. High efficiency 99mTc-radiolabelling (99%) was achieved in the presence of acetaldehyde using Fmoc-(trifluoroacetylHYNIC)-lysine, as compared to a poor radiolabelling yield (34%) obtained with the non-trifluoroacetylated analog. These findings firmly establish the trifluoroacetyl group as a convenient and effective protecting group for HYNIC, and as a promising alternative to currently available labelling strategies.