Quantum mechanical approach for structure elucidation of novel halogenated sesquiterpene lactones

Abstract

The stereochemistries of halogenated lumisantonin were rationalized by four distinct methods. In the first method the activation energies for the halonium ion intermediates and the reaction energies for all candidate structures were calculated. In the second approach the coupling constants were calculated and compared with the experimental values. The best matches for the calculated and experimental coupling constants were achieved by the modified procedure. In the third approach the computed 1H and 13C NMR chemical shifts were compared with the experimental values and the mean absolute errors (MAE) were measured. In the fourth method the adapted DP4 probabilities were evaluated and the determined stereochemistries for the sesquiterpene lactones (3a and 4a) were in agreement with the first three protocols.