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

Abstract

The difference of the reactivity for S_N2 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/321G 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 LUMO_c-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 AFB₁oxide has little influence on the geometry around the reaction center and the activation energy.