Chemistry Reference
In-Depth Information
undergo more elaboration (reduction, oxidation, derivatization, etc.) and “decora-
tions” that lead to terpenoid hydrocarbons, alcohols and their glycosides, ethers,
aldehydes, ketones, and carboxylic acids and their esters, which makes terpenoids
the most diverse class of plant natural products. It is noteworthy that over 40,000
different terpenoids have been isolated and characterized from natural sources
including plants (25, 26). Terpenoids also represent a functionally diverse class
of natural products. Although all the biological, ecological, and pharmacolog-
ical functions of terpenoids are yet to be fully understood, they are known to
have a variety of functions in the plant kingdom and in human health and nutri-
tion. Many plants are known to produce volatile terpenes for the purpose of
attracting specific insects for pollination or to keep away herbivorous animals;
some plants produce toxic or bitter-tasting terpenes known as antifeedants to
protect them from being eaten by animals. Most importantly, terpenoids also
play functional roles in plants as growth regulators (phytohormones) and sig-
naling compounds (sociohormones). Some important pharmacologically active
terpenoids include the sesquiterpenoid artemisinin with antimalarial activity (27),
the anticancer diterpenoid paclitaxel (Taxol
®
; Mead Johnson, Princeton, NJ) (28),
and the terpenoid indole alkaloids vincristine and vinblastine with anticancer
activity (29, 30).
1.4.1 Hemiterpenoids
Compared with other terpenoids, only a few true hemiterpenes are found in nature,
and over 90 of these occur as glycosides (31). The most noteworthy example is
isoprene (2-methylbut-2-ene), a volatile hemiterpenoid released by many trees.
Other natural hemiterpenes include prenol (3-methyl-2-buten-1-ol) in flowers
of
Cananga odorata
(Annonaceae) and hops (
Humulus lupulus
, Cannabaceae).
Its isomer, (
S
)-(
−
)-3-methyl-3-buten-2-ol is a constituent of the essential oils
of grapefruit, hops, and oranges. Another hemiterpenoid, 4-methoxy-2-methyl-
2-butanthiol, is responsible for the characteristic flavor of blackcurrant (
Ribes
nigrum
, Saxifragaceae). Numerous plant natural products with ester moieties
contain hemiterpenoid-derived carboxylic acid components such as 3-methyl-2-
butenoic acid and its isomers, angelic and tiglic acids, as well as its saturated
analog, isovaleric acid. The immediate biosynthetic precursors of hemiterpenoids,
IPP and DMAPP, are often used by plants as alkylating agents during the for-
mation of some natural products (meroterpenoids) of mixed biosynthetic origin.
1.4.2 Monoterpenoids
Monoterpenoids are responsible for fragrances and flavors of many plants and
thus their products are used in perfumery and as spices. To date over 1,500
monoterpenoids are known, and these constitute acyclic, monocyclic, and
bicyclic monoterpenoids (32), which occur in nature as hydrocarbons, alcohols,
aldehydes, and carboxylic acids and their esters. Several acyclic monoterpenoid
hydrocarbons are known, and these include trienes such as
β
-myrcene (
C1
),
