Terpenes carbon skeletons

The chemical interrelationships among these seemingly widely different types of compounds justify the recognized segregation of the terpenes from other fields of organic chemistry. The structural significance of the isoprene residue ( C-C-C) to the different types of terpene carbon skeletons has already been indicated. The simplest type of terpene is an open-chain olefinic structure formed by the union of two isoprene units. Such compounds are called the acyclic terpenes. From two units of isoprene or from the simple acyclic structures, under proper conditions, simple... 

Figure 19.54 Terpenic carbon skeletons bearing isonitrile and related groups.

The structural feature that distinguishes terpenes from other natural products is the iso prene unit The carbon skeleton of myrcene (exclusive of its double bonds) corresponds to the head to tail union of two isoprene units... 

Trees and shrubs contain a group of fragrant compounds called terpenes. The simplest terpene is isoprene. All other terpenes are built around carbon skeletons constructed from one or more isoprene units. Plants emit terpenes into the atmosphere, as anyone who has walked in a pine or eucalyptus forest will have noticed. The possible effect of terpenes on the concentration of ozone in the troposphere has been the subject of much debate and has led to careful measurements of rates of reaction with ozone. 

The possible occurrence of such major rearrangement of a compound s carbon skeleton, during the course of apparently unequivocal reactions, is clearly of the utmost significance in interpreting the results of experiments aimed at structure elucidation particularly when the actual product is isomeric with the expected one. Some rearrangements of this type are highly complex, e.g. in the field of natural products such as terpenes, and have often made the unambiguous elucidation of reaction pathways extremely difficult. The structure of reaction products should never be assumed but always confirmed as a routine measure lH and 13C n.m.r. spectroscopy have proved of enormous value in this respect. 

From their carbon skeletons terpenes can be seen as isoprene (C5) oligomers. Terpenes are classified according to their number of isoprene units as monoterpenes (C10), sesquiterpenes (Q5), diterpenes (C2o), etc. /1-Carotene (1) is a tetrater-pene. 

Hydroxy-4,4,6-trimethyl-2,5-hexadien-l-one, lanierone, 254, is a component of the complex aggregation signal of male Ipspini [468]. The carbon skeleton of 254 is the same as in isophorone 255, which has been identified as a volatile constituents of females of Ips typographus. Whether these compounds are degradation products of higher terpenes awaits further investigations. 

The male produced aggregation pheromone of the beech bark beetle, Taphrorychus bicolor is (lS,2T,5E)-bicolorin 261 [482,483]. Its carbon skeleton may represent a rearranged terpene. 

German for turpentine) and there are approximately 15000 terpenes. Terpenes are lipophilic, and the building blocks are five-carbon units with the branched carbon skeleton of isopentane. The basic units are sometimes called isoprene (F ig. 11.5fl), because heat decomposes terpenoids to isoprene. Depending on the number of C5 units fused, we distinguish mono- (Cio), sesqui- (C15), di- (C20), tri-(C30), tetra- (C40) and polyterpenoids [(Cs) , with n > 8]. Alpha-Pinene and bor-neol (Fig. 11.56) are examples of monoterpenes. 

Tsang and Fraser-Reid effected an unusual ring-forming reaction during synthetic studies aimed at the carbon skeleton of the trichothecene group of terpenes (Scheme 16) [33]. 

TERPENES AND TERPENOIDS. The class of organic compounds known as terpenes is characterized by the presence of the repeating carbon skeleton of isoprene ... 

Not only are the carbon skeletons of these substances divisible into isoprene units, but the terpene hydrocarbons are usually exact multiples of C5H8. An example is myrcene (Cl0H16), which occurs in the oils of bay and verbena and has a carbon skeleton divisible into two isoprene units. (Also see Exercise 3-19.)... 

Exercise 30-1 a. Write out all of the possible carbon skeletons for acyclic terpene and sesquiterpene hydrocarbons that follow the isoprene rule. Do not consider doublebond position isomers. 

Using the nor- type of names (nor- in the traditional terpene sense of complete stripping of methyl groups from the ring) for ring-carbon skeletons where convenient. 

Therefore, on the basis of their carbon skeletons, the bicyclic terpenes belong to the following condensed ring structures tbujane type to bicycloQ3.1.0 ]hexane carane type to bicyclo Q4.1.0 ]heptane pinane type to bicyclo Q3.1.1I]heptane and... 

For systematizing the nomenclature of bicyclic terpene hydrocarbons, a consideration of only the fundamental parent carbon skeletons larger than the unsubstituted ring systems (Chart 8) is not adequate. The fundamental unsubstituted parent ring... 

The parent structures, the terpene names, and the systematic bicyclo numberings recommended for the unsubstituted saturated ring carbon skeletons of these four fundamental bicyclic types are ... 

Thujane Group (Bicyclo Q3.1. O hexane Type). Name the thujane-type bicyclic terpene hydrocarbons (bicyclo 3.1. (T] hexane type) as thujanes, thujenes, and thu-jadienes, and base these names on the recommended fixed numbering of the thujane carbon skeleton. (Formula 46, Chart 10). For those derivatives which contain (1) fewer carbon atoms than thujane, (2) the same number as, or more carbon atoms than thujane but do not have a carbon skeleton identical to that of thujane, or (3) more carbon atoms than thujane and a carbon skeleton identical to that of thujane but contain substituents such that the principle of like treatment of like things will be violated by using the thujane, thujene, and thujadiene names, use the bicyclo 3.1 (T]-hexane-type names based on systematic bicyclo numbering (Formula 43, Chart 10). 

Allowing the addition of one carbon atom, or at most only two, to a terpene parent compound carbon skeleton for naming compounds as derivatives of that parent" (proposed by L. A. Goldblatt). This principle does not depend on a distinction between like and unlike groups, but is based merely on the number of carbon atoms in the substituents. Therefore, it allows such names as methylpinane, methylenepinane, (chlorometbyl)pinane, and (if two additional carbons are allowed) ethyl-pinane. The first of these four types of names are not allowed by either Proposal I or Proposal II. The other three types are allowed by I but not by II. 

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