Ab Initio MO Study on the Nucleophilic Oxirane Ring Opening of Exo and Endo Aflatoxin B1 8,9-Oxide

Toshiya OKAJIMA* and Akane HASHIKAWA

Department of Chemistry, Faculty of Culture and Education, Saga University; 1 Honjo-machi, Saga-shi 840-8502 Japan

The difference of the reactivity for SN2 type oxirane ring opening of exo and endo Aflatoxin B1 (AFB1) 8,9-oxide (exo-1 and endo-1, respectively) was analyzed with ab initio molecular orbital theory. All stationary points including transition-state structures were optimized with no geometry constraint at the RHF/3-21G basis set, and energies were evaluate at Becke3LYP/3-21G level based on the RHF/3-21G geometries. The caluculation clarified the following three points: (1) the activation energy (ΔE) for endo attacking of NH3 molecule (the reaction with exo derivatives containing exo-1) is considerably smaller than those for exo attacking (the reaction with endo ones containg endo-1), (2) the reactivity for nucleophilic oxirane ring opening is controlled by the distortion of LUMOC-O of oxirane ring, which is probably caused by exo/endo relationship between oxirane ring and five-membered dihydrofurano ring (B) with respect to A ring, and (3) the remaining part (inclusing coumarin skeleton) of AFB1 oxide has little influence on the geometry around the reaction center and the activation energy.

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