Carotol was first isolated by scientists Asahina and Tsukamoto in 1925. It is one of the primary components found in

carrot seed oil Carrot seed oil is the essential oil extract of the seed from the carrot plant '' Daucus carota''. The oil has a woody, earthy sweet smell and is yellow or amber-coloured to pale orange-brown in appearance. The pharmocologically active constitue ...

comprising approximately 40% of this

essential oil An essential oil is a concentrated hydrophobic liquid containing volatile (easily evaporated at normal temperatures) chemical compounds from plants. Essential oils are also known as volatile oils, ethereal oils, aetheroleum, or simply as the o ...

. This

sesquiterpene Sesquiterpenes are a class of terpenes that consist of three isoprene units and often have the molecular formula C15H24. Like monoterpenes, sesquiterpenes may be cyclic or contain rings, including many unique combinations. Biochemical modificat ...

alcohol is thought to be formed in carrot seeds (''Daucus carota L., Umbelliferae'') during the vegetation period. Additionally, studies have shown that carotol may be involved in

allelopathic Allelopathy is a biological phenomenon by which an organism produces one or more biochemicals that influence the germination, growth, survival, and reproduction of other organisms. These biochemicals are known as allelochemicals and can have ben ...

interactions expressing activity as an antifungal, herbicidal and insecticidal agent.

Biosynthesis

It has been proposed that there is a direct cyclisation of farnesyl pyrophosphate (FPP) to the carotol (carotane backbone). This type of cyclisation is unconventional for the typical chemistry of sesquiterpenes. The only other proposed mechanism requires a complex ten-membered ring with a methyl migration. This latter reaction, regardless of how plausible it may appear to be on paper, is energetically undesired, and through the diligent work of M. Soucek and coworkers, it was shown that the cyclization from FPP to carotol is the most probable biosynthesis route. The formation of farnesyl pyrophosphate is via the

mevalonate pathway The mevalonate pathway, also known as the isoprenoid pathway or HMG-CoA reductase pathway is an essential metabolic pathway present in eukaryotes, archaea, and some bacteria. The pathway produces two five-carbon building blocks called isopentenyl ...

. An additional five-carbon IPP unit is added in the same manner to GPP. The cyclization of FPP proceeds via a synchronous reaction of trans-antiparallel additions. This leads to the trans carbocation intermediate. Further cyclisation occurs yielding the 5 and 7-membered ring carbocation. This is followed by a 1,3-hydride shift and subsequent deprotonation to the diene. It is suggested that the carbocation elimination is directed to retain the proper structure of the isopropyl group. Based on the work of Soucek it is then proposed that a stereospecfic hydration will then take place leading to the enzymatic introduction of the hydroxyl group.

References

{{Reflist Sesquiterpenes Tertiary alcohols Azulenes