Limonene is a chiral molecule. That is, it exists in two forms as mirror images (e.g., human hands are an example of chirality). Most chiral molecules are found in one form in nature, and D-limonene is the naturally occurring form of limonene. We are not aware of any synthetic limonene preparations on the market as D-limonene is found in citrus fruits and a variety of other plants. Furthermore, the compound is a terpene and is not particularly easy to produce synthetically.