Isoprene rule, basis

Terpenoids are widespread natural products that are formed from C5 isoprene units leading to their characteristic branched chain structure. Terpenoids are divided into families on the basis of the number of isoprene units from which they are formed. Thus there are monoterpenoids (Cio), sesquiterpenoids (C15) diterpenoids (C20), sesterterpenoids (C25), triterpenoids (C30) and carotenoids (C40). The isoprene units are normally linked together in a head-to-tail manner. However, the C30 triterpenoids and C40 carotenoids are formed by the dimerization of two Ci5 and C20 units, respectively. Hence, in these cases the central isoprene units are linked in a head-to-head manner. The presence of tertiary centres in the isoprenoid backbone of the terpenoids facilitates skeletal rearrangements in the biosynthesis of these natural products. As a consequence, on first inspection some structures appear not to obey the isoprene rule. 

A compound named y-terpinene, an isomer of a-terpinene, is isolated along with the a isomer from the oils of a number of plant products. Upon ozonolysis, followed by an oxidative workup, yterpinene yields two compounds, one of which is 4-methyl-3-oxopentanoic acid. The second compound is an acid which decarboxylates upon heating and gives acetone. Two structures consistent with the data are possible for yterpinene. What are they On the basis of the isoprene rule, one of them can be discarded as highly improbable. What is the structure of y-terpinene ... 

On the basis of the isoprene rule, which states that all terpenes are formally divisible into isoprene units, we can discard structure E. Therefore D is the correct structure for yberpinene ... 

Sesquiterpenes (sesquiterpenoids). A structurally highly diverse class of terpenoids with 15 carbon atoms skeleton derived biosynthetically from famesyl pyrophosphate (FPP) ( famesol, isoprene rule, ter-penes). More than 70 different ring systems are formed by enzyme-catalyzed cyclization of the linear parent structure these cyclic structures can be further modified by 1,2- and 1,3-hydride shifts, renewed cycliza-tions, hydroxylations, and other subsequent reactions. S. are widely distributed in plants, fiingi, and animals but are less common in bacteria. Specific biosynthetic routes are often characteristic for certain organisms. Thus, basidiomycetes preferentially use humulene as the basis for the syntheses of protoilludanes, illu-danes, lactaranes, hirsutanes, and related S. skeletons. Individual S. systems are also known for liverworts and marine organisms. In addition, liverworts often contain the optical antipodes of S. known from plants. 

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