Mono- and dimethylbenzyls as new protecting groups for no-carrier-added nucleophilic aromatic radiofluorination

In this work, we tested the applicability of several methyl substituted benzyl groups as an alternative to the methyl group for the protection of the hydroxyl groups in the nucleophilic aromatic radiofluorination. As a model synthesis, the no-carrier-added (n.c.a.) preparation of 2-[F-18]fluoro-3-hydroxy-6-methylpyridine from O-protected 3-hydroxy-6-methyl-2-nitropyridine was chosen. Conditions for acidolytic and hydrogenolytic cleavage of heteroaryl esters were studied. Among various protecting groups tested, 4-methylbenzyl and 2,4-dimethylbenzyl groups proved to be the best by resulting in about 70% yields of [F-18]-labelled product after hydrolysis with 32% HCl at 120 A degrees C for 10 min. Furthermore, 4-methylbenzyl ester cleaved readily under catalytic transfer hydrogenation condition using ammonium formate and 10% Pd/C in boiling methanol to give 2-[F-18]fluoro-3-hydroxy-6-methylpyridine in radiochemical yield of 75% within a reaction time of 10 min. Conditions for the cleavage of both 4-methylbenzyl and 2,4-dimethylbenzyl esters are well suited for the implementation into an automated synthesis module.