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Menthol and the Nervous System

Page history last edited by ali zaraket 9 years, 5 months ago





[edit] Structure

Natural menthol exists as one pure stereoisomer, nearly always the (1R,2S,5R) form (bottom left corner of the diagram below). The eight possible stereoisomers are:

Structures of menthol isomers

In the natural compound, the isopropyl group is in the trans orientation to both the methyl and hydroxyl groups. Thus it can be drawn in any of the ways shown:

Menthol chair conformation Ball-and-stick 3D model highlighting menthol's chair conformation

In the ground state all three bulky groups in the chair are equatorial, making (−)-menthol and its enantiomer the most stable two isomers out of the eight.

There are two crystal forms for racemic menthol; these have melting points of 28 °C and 38 °C. Pure (−)-menthol has four crystal forms, of which the most stable is the α form, the familiar broad needles.

[edit] Biological properties

Menthol's ability to chemically trigger the cold-sensitive TRPM8 receptors in the skin is responsible for the well known cooling sensation that it provokes when inhaled, eaten, or applied to the skin.[1] In this sense it is similar to capsaicin, the chemical responsible for the spiciness of hot peppers (which stimulates heat sensors, also without causing an actual change in temperature).

Menthol has analgesic properties that are mediated through a selective activation of κ-opioid receptors.[2] Menthol also enhances the efficacy of ibuprofen in topical applications via vasodilation, which reduces skin barrier function.[3]

[edit] Occurrence

Mentha arvensis is the primary species of mint used to make natural menthol crystals and natural menthol flakes. This species is primarily grown in the Uttar Pradesh region in India.

(−)-Menthol (also called l-menthol or (1R,2S,5R)-menthol) occurs naturally in peppermint oil (along with a little menthone, the ester menthyl acetate and other compounds), obtained from Mentha x piperita.[4] Japanese menthol also contains a small percentage of the 1-epimer, (+)-neomenthol.

Biosynthesis of menthol was investigated in M. x piperita, and all enzymes involved in its biosynthesis have been identified and characterized.[5]

[edit] Production

As with many widely-used natural products, the demand for menthol greatly exceeds the supply from natural sources. Menthol is manufactured as a single enantiomer (94% ee) on the scale of 3,000 tons per year by Takasago International Corporation.[6] The process involves an asymmetric synthesis developed by a team led by Ryōji Noyori:

Menthol synthesis.pngAbout this image

The process begins by forming an allylic amine from myrcene, which undergoes asymmetric isomerisation in the presence of a BINAP rhodium complex to give (after hydrolysis) enantiomerically pure R-citronellal. This is cyclised by a carbonyl-ene-reaction initiated by zinc bromide to isopulegol which is then hydrogenated to give pure (1R,2S,5R)-menthol.

Racemic menthol can be prepared simply by hydrogenation of thymol, and menthol is also formed by hydrogenation of pulegone.

Comments (3)

Miss Yusser said

at 2:23 pm on May 1, 2011

I am impressed Abdallah :)....
A nice way to share the power point presentation you prepared.
But I think it would be better if you mention in one of the slides the name of the project, date and the name of our school with I DO9 since it is posted online so that you preserve your hardwork!! Best wishes
.... and one more thing I cannot see it full screen, if this is true to many of us, I think if u just use a larger size for the words and increase the number of slides , it would be clearer.... isn't it?? :)))) yalla good luck....

Eliane Metni said

at 9:19 pm on May 1, 2011

Yes, it is a neat way to share your PP! Well done.

Fatima Al-Zahraa Itani said

at 5:07 pm on May 2, 2011

Great work Abdullah..

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