Borneol

Borneol is a bicyclic organic compound and a terpene derivative. The hydroxyl group in this compound is placed in an endo position. The exo diastereomer is called isoborneol. Being chiral, borneol exists as enantiomers, both of which are found in nature: d-borneol and l-borneol.

Reactions

Borneol is oxidized to the ketone camphor.

Occurrence

The compound was identified and named camphre de Bornéo, or Borneo camphor in 1842 by the French chemist Charles Frédéric Gerhardt. Borneol can be found in several species of Heterotheca, Artemisia, Rosmarinus officinalis, Dryobalanops aromatica, Blumea balsamifera and Kaempferia galanga.
It is one of the chemical compounds found in castoreum. This compound is gathered from the beaver's plant food.

Synthesis

Borneol can be synthesized by reduction of camphor by the Meerwein–Ponndorf–Verley reduction. Industrially, a racemic mixture of camphor is used, leading to a racemic mixture of borneol and isoborneol. The chirality can be controlled by changing the chirality of camphor: -camphor gives -isoborneol and -borneol.
Reduction of camphor with sodium borohydride gives instead the diastereomer isoborneol.

Natural sources

Industrially, natural -borneol is produced from Cinnamomum burmanni and Cinnamomum camphora.
Natural -borneol occurs in Blumea balsamifera.

Biosynthesis

Borneol is synthesized using DMAPP as the starting material. DMAPP is then converted to GPP, which is acted upon by a bornyl diphosphate synthase to yield a bornyl diphosphate. A phosphatase then removes the phosphate groups, yielding borneol.
The chirality of borneol in a plant depends on the preferred chirality of the bornyl diphosphate synthase. Synthases for either chirality have been sequenced.
A downstream product is camphor of either chirality, a reaction catalyzed by -borneol dehydrogenase or -borneol dehydrogenase.

Uses

Whereas d-borneol was the enantiomer that used to be the most readily available commercially, the more commercially available enantiomer now is l-borneol, which also occurs in nature.
-Borneol from Dipterocarpus spp. is used in traditional Chinese medicine. An early description is found in the Bencao Gangmu.
Borneol is a component of many essential oils and it is a natural insect repellent. It also generates a TRPM8-mediated cooling sensation similar to menthol.
Laevo-borneol is used in perfumery. It has a balsamic odour type with pine, woody and camphoraceous facets.
Dextro-borneol is used in edaravone/dexborneol, a drug approved in China for stroke. It is approved in intravenous and sublingual forms. The intravenous combination was approved on the basis of trials showing it to be superior to edavarone alone.

Toxicology

Borneol may cause eye, skin, and respiratory irritation; it is harmful if swallowed. Acute exposure may cause headache, nausea, vomiting, dizziness, lightheadedness, and syncope. Exposure to higher levels or over a longer period of time may cause restlessness, difficulty concentrating, irritability, and seizures.

Skin irritation

Borneol has been shown to have little to no irritation effect when applied to the human skin at doses used in fine fragrance formulation. Skin exposure can lead to sensitization and a future allergic reaction even to small quantities.

Derivatives

The bornyl group is a univalent radical C₁₀H₁₇ derived from borneol by removal of hydroxyl and is also known as 2-bornyl. Isobornyl is the univalent radical C₁₀H₁₇ that is derived from isoborneol. The structural isomer fenchol is also a widely used compound derived from certain essential oils.
Bornyl acetate is the acetate ester of borneol.