Abstract
The 6,7,8-trioxybicyclo[3,2,2]nonane model molecule has been used to study the reaction mechanism of the radical transformations in artemisinin. The transition state species and the details of the potential energy surfaces of the intramolecular 1,5-hydrogen shift and the homolytic cleavage of the C−C bond in artemisinin have been predicted at the B3LYP/6-31G(d,p) level. A low value of free energy of activation (6.4 kcal/mol) has been found for the intramolecular 1,5-hydrogen shift process. The similarity between the model molecule and artemisinin give the first theoretical support to the suggestion that the energy barrier would not be as high as in an open chain species. The structural details of the O-centered radical and the corresponding transition state revealed in this study indicate that the collinear prerequisite is not necessary and the nonlinear transition state is possible for the 1,5-H shift in artemisinin. Also, the critical distance between the transferred hydrogen atom and the receptor oxygen atom could be longer than 2.1 Å. The lifetimes of 3.4 × 1034 s and 8.3 × 1032 s at 30 K for the O-centered radicals suggest that it is possible to observe the O-centered radicals experimentally at low-temperature.
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