Syntheses of 5-phenyl-2-pyridinamine, a possibly carcinogenic pyrolysis product of phenylalanine, and some of its putative metabolites

Abstract

5-Phenyl-2-pyridinamine (PPA) is a pyrolysis product of phenylalanine, the presence of which has been demonstrated in broiled sardines. Since PPA is mutagenic in the Ames test and is structurally related to the aminobiphenyls, it has to be considered as potentially carcinogenic. In this study procedures for the synthesis of PPA and its possible metabolites were developed to make them available for biological studies. PPA was synthesized in one step from 2,5-pyridinediamine. However, this method is only suitable for the preparation of small amounts. Larger quantities were synthesized starting from 5-nitro-2-pyridinamine in four steps. PPA was also prepared via a six-step synthesis, starting from 6-amino-3-pyridinecarboxamide. This route was also used for the synthesis of tritiated PPA ([³H]PPA) and 2-nitro-5-phenylpyridine, the latter being the precursor of the two putative proximate carcinogenic metabolites, viz. the hydroxylamine and the hydroxamic add of PPA. In the course of these multi-step syntheses a new method for the preparation of unsymmetrical biaryls was worked out. The following possible metabolites were also synthesized: N-(5-phenyl-2-pyridinyl)acetamide in the course of the synthesis of PPA starting from 5-nitro-2-pyridinamine, and both 5-(4-hydroxyphenyl)-2-pyridinamine (4'-OH-PPA) and N-[5-(4-hydroxyphenyl)-2-pyridinyl]acetamide starting from 5-(4-aminophenyl)-2-pyridinamine. After incubation of PPA in suspensions of freshly isolated hepatocytes from rats pretreated with polychlorinated biphenyls (Aroclor 1254) or PPA itself, the presence of 4'-OH-PPA was demonstrated.