Terpenoid nomenclature

The terpenoids are divided into groups and sub-groups according to the pathway by which nature synthesised them and hence, by their skeletal structures since these arise directly from the biosynthesis. As described above, the first basis for classification is the number of isoprene units which make up the terpenoid. The names for these groups are shown in Table 1.1. The next classification depends on whether the skeleta remain as open chains or have been cyclised giving one, two or more rings. Families of terpenoids possessing the same skeleton are named after a principal member of that family, usually either the most common or 

In the case of cyclic terpenoids, the letters a, p and often refer to the location of the double bond in isomeric olefins. In these cases, the letter a indicates an endocyclic trisubstituted double bond, p refers to 

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Implicit ia the base names are the absolute configurations at carbons 8 and 12 and the iadicated numbering systems. Derivatives of these parent stmctures are named according to terpene and steroid nomenclature rules (see Steroids Terpenoids). The lengthy and awkward nature of the chemical abstract systematic nomenclature (12) for these compounds has resulted ia the development (13) and use of simplified nomenclature based on common names. 

From these stractural features it is interesting to note that each molecule of chlorophylls a and b consists of a hydrophilic part (tetrapyrrole macrocycle) and a hydrophobic portion (long terpenoid chain of phytol esterifying the acid group at C-17). Figure 2.1.2 shows the structures and nomenclature of chlorophylls a and b and their major breakdown derivatives. 

In 1965 Seshardri et al. described the isolation of an unknown terpenoid acid B obtained from the lichens of Lobaria retigera in the western Himalayas [8]. (Note that this annotation bears no relation to the subsequent nomenclature later defined by Corey and Shibata.) Four collections had been made in the summer of 1962 from under the Rutba plants in the Valley of Flowers (12,500 feet) and on the way to Hemkund Lokpal (13,500 feet) and from underneath rocks and from pine trees in Ganghariya (10,000 feet). The samples were subjected to a series of increasingly polar extractions (petroleum ether, diethyl ether, acetone). The unknown terpenoid acid B was present in all petroleum ether extracts, except that of the sample obtained from the Valley of Flowers, in compositions ranging from 0.47 % to... 

Semisystematic names. Examples of semisystematic parents are corynoxan and labdane. DNP makes widespread use of semisystematic names for terpenoids and steroids (where skeletons such as labdane have not been used in CAS nomenclature since 8C1). lUPAC gives tables of recognised semisystematic parent skeletons and directions for introducing new ones, but in view of the discovery of ever more structurally complex types of natural products, most authors have avoided this route. Semisystematic parents can be modified by operators such as nor-, abeo. 

Geometrical isomers are those in which the distinction lies in the relative placement of substituents across a double bond or ring. The original nomenclature for describing such isomers used the prefix cis- to refer to an isomer in which two substituents lie on the same side of a double bond and trans- for those in which the substituents are on opposite sides. Therefore, geraniol is considered to be a trans-isomer because the main terpenoid skeleton runs across the double bond next to the alcohol function. Nerol, in which both of the chain residues are on the same side of the double bond is, correspondingly, the m-isomer. Exactly the same... 

The biosynthesis of the T. proceeds according to the isoprene rule . Acyclic hydrocarbons formed in this way can be converted to a multitude of compounds by substitution, oxidation, cyclization, rearrangement, etc. reactions accordingly a large number of T. (>40000 have been described to date) occurs in nature. Included among the T. are not only the hydrocarbons but also the alcohols, ketones, aldehydes, and esters (alternative name terpenoids) derived from them. For nomenclature and technical use of T., see the individual entries and Lit.. ... 

It is clear from the mechanism shown in Figure 6.14 that terpenoid structures will always contain a multiple of ve carbon atoms when they are rst formed. The rst terpenoids to be studied contained 10 carbon atoms per molecule and were called monoterpenoids. This nomenclature has remained and so those with 5 carbon atoms are known as hemiterpenoids those with 15, sesquiterpenoids those with 20, diterpenoids and so on. In general, only the hemiterpenoids, monoterpenoids, and... 

Systematic names for terpenoids, based on the general rules of nomenclature of organic compounds (324), are usually cumbersome and are seldom used unless the structures are sufficiently simple (212). 

The first terpenoids to be studied in detail contained two isoprene units and were called mono terpenoids. Consequently, the basic nomenclature system is based on 10 carbon units rather than the 5 of isoprene and this is shown in Table 8.1. 

The mechanism of a reaction determines what elementary steps and what transition states are involved in the reaction. In the case of short-chain terpenoid biosynthesis, the mechanism involves carbocations (see Nomenclature). Figure 4.13 displays the mechanism, where the arrows indicate electron transfer. Thus, first, an allylic carbocation is... 

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