Terpenoids chemically formed from

Figure 2. Structures of terpenoids chemically formed from linalool (1 ) at pH 3.5 (cf. Fig.1-4). (2) 2,4(8)-p-menthadiene (3) B-myrcene (4) a-phellandrene (5) cx-terpinene (6) limonene (7) B-phellandrene (8) (Z)-ocimene (9) y-terpinene (10) (E)-ocimene (11) p-cymene (12) terpinolene (13) (E,Z)-alloocimene (14) (E,E)-alloocimene (15) a-terpineol (16) 3,7-dimethyl-l-oct-ene-3,7-diol (17) 1,8-cineole (18) 2,2,6-trimethyl-2-vinyl-te-trahydropyran.

It is difficult to reconcile the unique chemical structure of tetrodotoxin with that of an animal product. Its structure is not related to that of other animal products by any readily recognized biosynthetic scheme. It is not a terpenoid, not obviously formed from amino acid or carbohydrate units, and apparently not constructed from acetate or propionate units. Nor does it resemble any of the various plant alkaloid patterns. It thus appears to be a very unlikely animal product to result from known biogenetic pathways. In this connection the metabolic incorporation of radioactive precursors using torosa and ]C. granulosa salamanders was studied by Shimizu et al. (47). They observed significant isotopic incorporation into amino acids and steroid metabolites, but they found no such incorporation associated with tetrodotoxin. 

Terpenoids are compounds derived from a combination of two or more isoprene units. Isoprene is a five carbon unit, chemically known as 2-methyl-1,3-butadiene. According to the isoprene rule proposed by Leopold Ruzicka, terpenoids arise from head-to-tail joining of isoprene units. Carbon 1 is called the head and carbon 4 is the tail . For example, myrcene is a simple 10-carbon-containing terpenoid formed from the head-to-tail union of two isoprene units as follows. 

Terpenes are obtained either by processing wood in the kraft process in paper production or by collecting resins and turpentine from conifers. The scale of produced terpenoids in comparison with fats and oils is small. Applications for terpenes are in pure form as solvents, as odorous substances, or in dyes. Most terpenoids contain double bonds which are readily available to perform chemical reactions. A widespread component of turpentine is a-pinene, from which many fragrances are produced. A further often-used resource is myrcene, which is obtained by pyrolysis of (3-pinene. Myrcene is an important base chemical to produce, for example, the fragrances nerol and geraniol [7]. 

Males of neotropical euglossine bees (Apidae), called orchid bees, collect odoriferous substances from flowers of orchids and other plants. The floral scents of these species display relatively simple chemical compositions dominated by one or two major components, mostly terpenoids and aromatic compounds such as cr-pinene, 1,8-cineol, eugenol, -dimethoxybenzene (35), 2,3-epoxygeranyl acetate (36), nerolidol, 4-methoxycinnamaldehyde (37), and benzyl benzoate.113 Since the orchid bees have odor preferences, their collection of fragrances leads to specialized pollination of particular plant species. Male bees absorb the floral volatiles with their tarsal hairs, form species-specific bouquets, and finally accumulate them in their hind tibial pouches. These bouquets have potential roles in courtship displays and marking territories.114 115... 

The alarm chemicals are frequently in the form of terpenoids, simple ketones and aldehydes and can be released from the mandibular, anal, poison, frontal or Dufour s gland [12]. The origin and biosynthesis of those these chemicals are well known, and most of them seem to be natural products synthesized by the insect as opposed to sequestered compounds from plant material, as occurs with terpenes in insects [13]. 

This second volume, which reviews the alkaloid literature from July 1970 to June 1971, approaches more closely the standard Specialist Periodical Report originally envisaged by the Chemical Society and adopts a form which, with minor variations, will very probably be followed in subsequent volumes. Once again the whole field of alkaloid chemistry has been reviewed, with the exception of the Steroidal Alkaloids of the Solanum and Veratrum Groups. The omission of these groups in the first volume was deliberate their inclusion in the second volume was intended, but proved to be impracticable, and we hope to remedy this omission in the third volume. It is fortunate, however, that this particular area can quite properly be discussed in a volume devoted to alkaloids or in one devoted to steroids and for a brief review of recent developments in this subject the reader is meanwhile referred to the Specialist Periodical Report on Terpenoids and Steroids, Volume One (Senior Reporter Dr. K. H. Overton). 

Despite great diversity in form and function, the terpenoids are unified in their common biosynthetic origin. The biosynthesis of all terpenoids from simple, primary metabolites can be divided into four overall steps (a) synthesis of the fundamental precursor IPP (b) repetitive additions of IPP to form a series of prenyl diphosphate homologs, which serve as the immediate precursors of the different classes of terpenoids (c) elaboration of these allylic prenyl diphosphates by specific terpenoid synthases to yield terpenoid skeletons and (d) secondary enzymatic modifications to the skeletons (largely redox reactions) to give rise to the functional properties and great chemical diversity of this family of natural products. As bacosides are triterpenoid derivatives, they may probably foUow the common biosynthetic pathway of terpenoid production [40]. 

This complexity of EOs phytochemistry led to a certain inconsistency of the chemical composition of an essential oil. In fact, several factors influence the balance of the compounds within EOs. Terpenoids and isoprenoid are synthesized through secondary metabolism of the plant. Monoterpenes are biosynthesized in plastid via two 5-carbon precursors, that is, isopentenyl pyrophosphate and dimethylallyl pyrophosphate, which condense to give the monoterpenes (10-carbon). The sesquiterpenes (15-carbon) are formed via the mevalonate pathway in the cytosol. Phenylpropanoids are derived mainly from the shikimate pathway [5]. 

It turns out that they are chemically related to each other, as you can imagine from the chemical structures shown in Fig. 12.1. They are called terpenes and terpenoids. They can be regarded as derivatives from a five-carbon compound called isoprene (2-methyl butadiene). Two isoprene molecules combine to form mono-terpene (ten-carbon compound). The fragrant oils mentioned above are all the derivatives of mono-terpene. Terpenes are derived from a common metabolic intermediate of glucose, acetyl-CoA (coenzyme A). By the way, a tri-terpene (which three terpene molecules combine to form) called squalene leads to the formation of steroids, and if you connect a large number of isoprene in a linear fashion, you will get natural rubber (Chap. 5). 

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