Chemical model studies on the monoamine oxidase-B catalyzed oxidation of 4-substituted 1-cyclopropyl-1,2,3,6-tetrahydropyridines.

Two catalytic pathways have been proposed for the flavoenzyme monoamine oxidase B (MAO-B)--one based on an initial single electron transfer (SET) step from the nitrogen lone pair and the second based on an initial alpha-carbon hydrogen atom transfer (HAT) step. The SET pathway is consistent with the mechanism based inactivation properties of various cyclopropylamines. The observation that MAO-B catalyzes the efficient oxidation of certain 1-cyclopropyl-4-substituted-1,2,3,6-tetrahydropyridines to the corresponding dihydropyridinium metabolites suggests that the catalytic pathway for these cyclic tertiary allylamines may not proceed via the putative SET generated aminyl radical cations. The present paper describes the chemical fate of a series of N-cyclopropyltetrahydropyridines examined under reaction conditions that model the SET and the HAT pathways. All of the test compounds were rapidly converted under HAT reaction conditions to their dihydropyridinium products. Although the test compounds also were oxidized rapidly under SET conditions, no evidence for dihydropyridinium product formation was observed. The products that were identified most likely were formed after cyclopropyl ring opening of the initially formed cyclopropylaminyl radical cation. The results are discussed in terms of the mechanism of MAO-B catalysis.