Tritiation of SR141716 by metallation–iodination–reduction: tritium‐proton nOe study

The central cannabinoid receptor antagonist SR141716, N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2, 4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide, was synthesized from commercially available starting materials. Condensation of an aryl hydrazine with a diketone followed by base promoted isomerization/cyclization of the intermediate anti-imine gave the pyrazole acid which was converted to the title hydrazide via its acid chloride. Facile iodination via metallation followed by in situ trapping with an iodine source gave a modest yield of the iodinated SR141716. The iodine was selectively reduced with tritium gas and catalyst while retaining the three aryl chlorine atoms in the structure. The tritiated SR141716 exhibited a tritium-proton nOe both definitively identifying the position of the tritium as well as the sought isomer of the diarylpyrazole. This work provides a direct method for the preparation of preferred iodinated aryl substrates that offer advantages where selectivity and high incorporation in catalytic reduction is sought. Copyright © 2002 John Wiley & Sons, Ltd.