Facile synthesis of high specific activity 4-[1-14C]butyl-1,2-diphenylpyrazolidine-3,5-dione (phenylbutazone) using nucleophilic substitution

Authors: Singh, Anuradha; Hakk, Heldur; Lupton, Sara

Submitted to: Journal of Labelled Compounds and Radiopharmaceuticals
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/11/2017
Publication Date: 4/1/2018
Citation: Singh, A., Hakk, H., Lupton, S.J. 2018. Facile synthesis of high specific activity 4-[1-14C]butyl-1,2-diphenylpyrazolidine-3,5-dione (phenylbutazone) using nucleophilic substitution. Journal of Labelled Compounds and Radiopharmaceuticals. 61(4):386-390. https://doi.org/10.1002/jlcr.3597.
DOI: https://doi.org/10.1002/jlcr.3597

Interpretive Summary: Occasionally the need exists to study the metabolism, environmental fate, or transport of a chemical at trace levels to determine risks to the environment, animals, and/or humans. Such investigations are facilitated by the use of ‘tracer’ chemicals labeled with radioactive carbon-14 which is easily measured in environmental or animal matrices. Synthesis of ‘tracer’ radioactive chemicals, especially in minute quantities, is fraught with difficulty. We developed a new synthetic approach to synthesize a radiotracer version of the anti-inflammatory drug phenylbutazone. Phenylbutazone is sometimes used illegally in food animals and can cause violative residues in meat or milk. The new synthetic reaction can be performed under mild conditions with simple clean up procedures and high yields. The tracer-labeled phenylbutazone product was successfully used in a subsequent food safety study involving milk products.

Technical Abstract: Metabolism, environmental fate, and low concentration food residue studies would be facilitated by the use of radiolabeled test articles which can be readily quantified within complex matrices. However, radiochemical approaches for such studies require high specific activities to allow analytical detection of correspondingly low masses of test article. The synthesis of high specific activity (>50 mCi/mmol) [14C] radiolabeled phenylbutazone presents a challenge using existing methodology, mainly due to the low solvent volumes required and vigorous refluxing needed to close the pyrazolidinedione ring. Herein we report on the significant modification of an existing method that allows the synthesis of low masses of high specific activity (>50 µCi/µmole) [14C]-phenylbutazone under mild conditions with simple purification and high yield. The closure of the pyrazolidinedione ring of 1,2 diphenyl-3,5-pyrazolidinedione was accomplished as a first step with unlabeled 1,2-diphenylhydrazine and diethyl malonate in 32% yield under gram-scale conditions, which avoided the challenges of low solvent use and vigorous refluxing. Low mass of high specific activity [14C]-butyl bromide was then added via a nucleophilic substitution reaction as a final step. Yields ranged from 65-92% during multiple synthetic attempts with unlabeled butyl bromide, and were greatly influenced by reaction stoichiometry and the selection of base.