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Bioorganic SynthesisAn Introduction$
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Gary W. Morrow

Print publication date: 2016

Print ISBN-13: 9780199860531

Published to Oxford Scholarship Online: November 2020

DOI: 10.1093/oso/9780199860531.001.0001

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PRINTED FROM OXFORD SCHOLARSHIP ONLINE (oxford.universitypressscholarship.com). (c) Copyright Oxford University Press, 2021. All Rights Reserved. An individual user may print out a PDF of a single chapter of a monograph in OSO for personal use. date: 16 October 2021

The Terpenoid Pathway: Products from Mevalonic Acid and Deoxyxylulose Phosphate

The Terpenoid Pathway: Products from Mevalonic Acid and Deoxyxylulose Phosphate

4 (p.131) The Terpenoid Pathway: Products from Mevalonic Acid and Deoxyxylulose Phosphate
Bioorganic Synthesis

Gary W. Morrow

Oxford University Press

It was Otto Wallach (1847– 1931) who first coined the term “terpene” and made the observation that many plant-derived essential oils had chemical structures whose composition was based on multiples of a basic five-carbon unit. His work with turpentine and the organic products derived from it was consistent with earlier studies of natural rubber which had shown that its thermal decomposition released “isoprene” (2-methyl-1,3-butadiene) as the principal product. This led eventually to the formulation of the so-called biogenetic isoprene rule of Leopold Ruzicka (1887–1976) in 1953 which stated that “the carbon skeleton of the terpenes is composed of isoprene units linked in regular or irregular arrangement.” As it turns out, biosynthetic pathways to terpenes are found in nearly all organisms, producing a remarkable variety of different structural types, as we will soon see. In fact, something in excess of over 25,000 different terpenes with a wide variety of biological functions have been isolated from the plant kingdom over the years. Interestingly, while many terpenes are simple achiral compounds, others are chiral as can be seen in the case of α-pinene in Fig. 4.1. But unlike the naturally occurring L-amino acids and D-carbohydrates, different organisms may produce the same terpene product but in different enantiomeric forms. For example, limonene is formed by more than 300 plants, with the (+)-(R) enantiomer being the most widespread form as the major constituent of citrus peel essential oils (orange oil). As the most abundant of all terpenes, its pleasant citrus fragrance and flavor have led to its worldwide use in the food and fragrance industries and also as a botanical insecticide. A number of plants produce both enantiomers of limonene, while others produce only the (−)-(S)-enantiomer which possesses a strong pine smell reminiscent of turpentine. This obviously speaks to the chirality and enantioselectivity of our own olfactory receptor sites which can readily distinguish between the two enantiomers, thus signaling a different odor response in each case.

Keywords:   absinthe, bisabolene, cafestol, dammarenyl cation, epimerization, farnesene, geranial, hydrocortisone, iridoids

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