A theoretical investigation into the formation of sesquiterpene episulfides

A theoretical investigation using both density functional theory and post-Hartree-Fock ab initio methods has been carried out on the episulfidation of the sesquiterpenes germacrene D, beta-caryophyllene, and a-humulene. The calculations show that formation of the sesquiterpene episulfides mintsulfide, caryophyllene-6,7-episulfide, caryophyllene-3,6-episulfide, humulene-6,7-episulfide, and humulene-9, 10-episulfide, by reaction with elemental sulfur, are all exothermic, with formation of mintsulfide the most exothermic. Of the caryophyllene episulfides, caryophyllene-3,6-episulfide is more stable than caryophyllene-6,7-episulfide, consistent with experimental observations. Formation of humulene-6, 7-episulfide involves a lower energy diradical intermediate than formation of humulene-9,10-episulfide, consistent with the early preferential formation of humulene-6,7-episulfide observed experimentally.