Diterpenes structural types

Head-to-tail rearrangement of four isoprene units results in the formation of diterpenes (C20H32), as seen also in Fig. 4.2. Diterpenes are generally found in resins, e.g. pimaric acid and abietic acid. Some diterpenoids are also constituents of essential oils, e.g. phytol [3, 7-14, 37, 52, 53]. Like sesquiterpenes, diterpenes are heavier than monoterpenes therefore, they require more energy to go to the vapour phase. For this reason, longer distillation times are necessary for their recovery. The DNP lists 118 different structural types for diterpenoids [37]. Important diterpenes found in essential oils will be detailed. Some representatives of volatile diterpenes are as in Structure 4.32. 

Some of the major structural types encountered in diterpenes are shown helow. 

Four structural types of diterpenes are known from Helianthus labdanes, kauranes, atisiranes and trachylobanes (Figure 2), with the tetracyclic kaurane type being the most widespread in the genus. The majority of Helianthus diterpenes have an Ol-oriented carboxylic acid function attached to carbon 4. 

Structural and synthetic studies of the diterpene alkaloids of the Aconitum, Delphinium, and Garrya species continue to provide more insight into the chemistry of these complex bases. In recent years Japanese workers have isolated a series of isoprenoid alkaloids from plants of Daphniphyllum macropodum Miquel. Structural work on four new alkaloids representing three new structural types has been reported during this year. 

Abietanes. Structural type of diterpenes, e. g., abieta-7,13-diene, the most important representative is abietic acid. 

Sphenolobanes. Diterpenes of a new structural type isolated from species of the liverwort genus Anastro-phyllum. To date 13 different S. are known. 

Sesquiterpenes are derived from a 15-carbon intermediate, famesyl pyrophosphate (3), which is assembled by head-to-tail fusion of isoprene units (Cane, 1990). Biogenetic paths to most of the important structural types of sesquiterpenes have been proposed, but few have been studied in detail. The diversity of sesquiterpenes is much greater than that of monoterpenes, but also exceeds that of diterpenes and triterpenes. This situation presumably reflects the much greater conformational flexibility of the 10- or 11-membered rings as compared with that of 6-membered rings in the last two classes. 

Diterpenes related to e r-kaurene (5p,10a-configuration). Other Structural Types Derived from the enr-Kaurane Skeleton Gibberellins Grayanotoxins... 

Two main structural types of diterpene alkaloids are found in these genera. One group includes a series of comparatively simple and relatively nontoxic amino alcohols and esters modeled on a C20 skeleton. These alkaloids are not extensively oxygenated and contain, at most, one methoxyl... 

In Table 2, diterpene structures are identified by compound number and grouped with related structures belonging to the same skeletal type. The structural... 

Diterpenes are a large group of C20 compounds. Their main structural types are acyclic compounds, cyclic compounds, bicyclic diterpenes and tri- and tetra-cyclic diterpenes. The cosmetic use of this group of compounds is restricted to the antioxidant properties of the phenolic diterpenes extracted from rosemary and sage. 

Cimino, G De Rosa, D De Stefano, S and Minale, L. (1974b) Isoagafliolactone, a diterpene of a new structural type from the sponge Spongia officinalis. Tetrahedron, 30,645-649. 

The norditerpenoid alkaloids are complex, multi-cyclic Ciq diterpene alkaloids which are highly substituted with hydroxyl, methoxyl and ester groups. These alkaloids occur in members of the Ranunculaceae family (primarily in Aconitum spp. and Delphinium spp.) and many have been determined to be highly toxic to humans and animals. They occur as two skeletal structure types [36,37]. Three structural sub-types are associated with skeletal type 36, based upon C-7 substitution patterns and C-8, C15 unsaturation aconitine type (i.e. aconitine [38]) lycoctonine type (i.e. lycoctonine [36]) and pyrodelphinine type (i.e. pyrodelphinine [39]). Skeletal type 37 is designated the heteratisine type (i.e. heteratisine [37]) norditerpenoid alkaloids (Pelletier et al., 1984). Only a small number of pyrodelphinine and heteratisine type norditerpenoid alkaloids have been characterized and there is very limited toxicity data available for them (Benn and Jacyno, 1983). More than 300 aconitine and lycoctonine norditerpenoid alkaloids have been characterized (Pelletier et al., 1984, 1991). Aconitum spp. contain almost exclusively aconitine type alkaloids while Delphinium spp. contain primarily lycoctonine type with a few aconitine type alkaloids. 

Fig. 5 Molecular structures of tyrosinase-active diterpene (8-10) and napelline (11, 12) type alkaloids [49]...

Some representative Claisen rearrangements are shown in Scheme 6.14. Entry 1 illustrates the application of the Claisen rearrangement in the introduction of a substituent at the junction of two six-membered rings. Introduction of a substituent at this type of position is frequently necessary in the synthesis of steroids and terpenes. In Entry 2, formation and rearrangement of a 2-propenyl ether leads to formation of a methyl ketone. Entry 3 illustrates the use of 3-methoxyisoprene to form the allylic ether. The rearrangement of this type of ether leads to introduction of isoprene structural units into the reaction product. Entry 4 involves an allylic ether prepared by O-alkylation of a (3-keto enolate. Entry 5 was used in the course of synthesis of a diterpene lactone. Entry 6 is a case in which PdCl2 catalyzes both the formation and rearrangement of the reactant. 

The first Mexican substance searched for, found, and isolated based on old traditions, was the sesquiterpene hemandulcin from Lippia dulcis Trev. It was determined by a human taste panel to be more than 1000 times sweeter than sucrose. The structure of this sesquiterpene was determined and confirmed by chemical synthesis. It was nontoxic when administered orally to mice, and did not induce bacterial mutation.5 Further work has been carried out with some of the plants more frequently used (Table 12.3). The sesquiterpene alkaloids hippocrateine I, hippocrateine II, and emarginatine were identified in Hippocratea excelsa used in Mexican traditional medicine, and antimicrobial abietane type diterpenes were isolated from Salvia albocaerulea.6 7... 

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