Syntheses of [carbonyl-11C]2-(2-benzoylphenoxy)-N-phenylacetamide from [11C]carbon monoxide by the Suzuki and the Stille reactions

Syntheses of [carbonyl-11C]2-(2-benzoylphenoxy)-N-phenylacetamide, a radiolabeled inhibitor of human immunodeficiency virus type 1 (HIV 1) reverse transcriptase, were achieved by applying palladium-mediated cross-coupling reactions with insertion of [11C]carbon monoxide. Our interest was focused for the present on a comparison of the Stille and Suzuki methods, using trimethylphenylstannane or phenylboronic acid as alternative coupling reagents, respectively. The Suzuki variant gave a much higher amount of [11C]CO radioactivity trapped in the reaction mixture, but a significant loss of product occurred due to adsorption phenomena on the potassium carbonate present in the heterogeneous reaction mixture. The labeled product was isolated in only 20% yield (based on trapped [11C]CO, not corrected for decay). According to Stille, the reaction provided a product that could be isolated more easily but it did not increase the final yield of the target compound due to a low trapping efficiency for [11C]CO. Both methods were performed in an overall synthesis time of 30 min, starting from [11C]CO2, and gave a product with a specific radioactivity of at least 30 GBq/μmol. The Stille method as well as the Suzuki reaction allowed the synthesis of a radiochemically pure product in aqueous acetonitrile.