(+)- and (−)-α-Phellandrenes are widely distributed in nature. The (+)-isomer occurs in the oil of gingergrass, Manila elemi, bitter fennel, star anise, cinnamon, Spanish dill herb, and Boswellia serrata, among others. The (–)-hydrocarbon is found in the oil of E. dives, E. phellandra, E. cneorifolia, pimento, bay, and pepper. (±)-Phellandrene has also been described.
α-Phellandrene is used in scents, as a compenent of artificial essential oils, and as an attractive raw material for chemical syntheses. It is therefore of interest to review the significant phases of the chemistry of this p-menthadiene.
No technique other than fractional distillation is known to separate α-phellandrene from essential oils. Since the hydrocarbon partially decomposes on distillation at ordinary pressure, the fractionation should be conducted in vacuum. Even cautious distillation under diminished pressure does not lead to anything approaching quantitative isolation of the terpene owing to its tendency to polymerize.