The dimerization of α-phellandrene: Implications for Boswellia chemotaxonomy

InformationFrankincense oleogum resin essential oils, rich in α-phellandrene, often contain small amounts of several α-phellandrene dimers.The desert habitats of frankincense (Boswellia spp.) trees expose the resins to both heat and sunlight, which could promote the dimerization reactions of α-phellandrene.We hypothesized that thermally and/or photochemically-allowed cycloaddition reactions account for the formation of αphellandrene dimers.In this study, the profiles of α-phellandrene dimers from several Boswellia species were analyzed by GC-MS and compared.In addition, both thermal and photochemical reactions of α-phellandrene were carried out and the reaction mixtures were also analyzed by GC-MS.These results, coupled with density functional theory (DFT) calculations (ωB97X-D/6-31G*), suggest that photochemically allowed [2 + 2] cycloaddition reactions are important contributors to the formation of α-phellandrene dimers in frankincense essential oils.These results also contribute to the body of Boswellia chemotaxonomic markers, particularly between B. sacra and B. carteri.