Theoretical study of radical and neutral intermediates of artemisinin decomposition.

Four artemisinin reductive decomposition routes A, B1, B2, and B3 with 13 species (QHS, 1/2, 3, 4, 5, 5a, 6, 7, 18, 18a, 19, 20, and 21) were studied at the B3LYP/6-31G** level. Structures of the species were analyzed in terms of geometrical parameters, Löwdin bond orders, partial atomic charges and spin densities, electronic and free energies, and entropy. Searches in the Cambridge Structural Database for high-level quality artemisinin-related structures were also performed. Principal Component and Hierarchical Cluster analyses were performed on selected electronic and structural variables to rationalize relationships between the routes. The A and B1 routes are possibly interconnected. Structural and electronic features of all species show that there are two clusters: A-B1 and B2-B3. The latter cluster is thermodynamically more favorable (DeltaDeltaG is -64 to -88 kcal mol(-1)) than the former (DeltaDeltaG is -58 to -59 kcal mol(-1)), but kinetical preference may be the opposite. Along the artemisinin decomposition routes, especially B2 and B3, larger structural changes including formation of branched structures and CO2 release are related to increased exothermicity of the conversions, weakened attractive oxygen-oxygen interactions, and increased entropy of the formed species. The intermediate 4 definitely belongs to some minor artemisinin decomposition route.