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The synthesis of [S-(R,S)]-4-[[methyl[2-methyl-3-(1-oxopropoxy)-3,4-diphenylbutyl]amino]-1-oxobutoxy]-2,5-pyrrolidinedione (propoxyphene active ester, 2) is described. This was used as an intermediate to prepare a propoxyphene immunogen, [S-(R,S)]-4-[methyl[2-methyl-3-(1-oxopropoxy)-3,4-diphenylbutyl]amino]-1-oxobutyl-Bovine Thyroglobulin (3). This immunogen was then used to generate antibodies which demonstrate good cross-reactivity to d-propoxyphene, d-norpropoxyphene, and other propoxyphene metabolites. In addition, these antibodies were shown to have very low cross-reactivity to methadone, a structurally related compound. The introduction of an aminomethyl benzoate spacer into the propoxyphene active ester (2), followed by the activation of the carboxylic acid, provided for a more stable active ester (5). This stable active ester, together with the antibodies generated from the propoxyphene immunogen, has led to the development of an immunoassay based on the Kinetic Interaction of Microparticles in Solution (KIMS).