Acyclic diterpenoids

Geranylgeranoic acid is a synthetic acyclic diterpenoid acid having 4 ene-bonds, which itself is unlikely to be an oxidant or prooxidant in cells. It is now known, however, that geranylgeranoic acid induces a hyper-production... 

The diterpenoids, which contain 20 carbon atoms, are represented by acyclic, monocyclic, bicyclic, tricyclic, and tetracyclic structures. Over 5,000 naturally occurring diterpenoids, many of which frequently occur in plant families Araliaceae, Aster-aceae, Cistaceae, Cupressaceae, Euphorbiaceae, Leguminosae, Labiatae, and Pinaceae, are known (32). The acyclic diterpenoid... 

The most important acyclic diterpenoid is phytol. It forms part of the chlorophyll-a molecule and is present in many other chlorophylls (see Section 2.4.4).The saturated analogue of phytol, dihydrophytol (orphytanol), is present in a variety of bacterial glyceride ether lipids (Section 2.4.1b). 

Figure 9.9 Acyclic diterpenoids reported to be present in olive oil.

Diterpenoids. Two acyclic diterpenoids have been reported to be present in the alcohol fraction isolated from olive oil. These are phytol (at a concentration of 120-180 mg/kg) which probably originates from chlorophyll, and geranyl-geraniol (Figure 9.9). 

As mentioned previously, the acyclic diterpenoids and cembranoids from Eremophila species are unique in containing internal c -double bonds. The derivation of the two classes of compounds from a common precursor seems reasonable. Thus, the acyclic hydroxy diacid (95) and the all-c -cembratriene (108) must branch from the same intermediate. Nevertheless, at least one acyclic diterpene (96) contains a rroni-double bond and cyclization of its precursor would lead to cembrenes displaying a 3,4-tra/u double bond. No such examples have yet been found but a clue to their formation might be obtained from a comparison of the two cembrenoids, 107 and 109, which differ in configuration at C3 and C4. Their possible origins from acyclic precursors which differ in the geometry of the 2,3-double bond are shown in Scheme 47. 

The two novel acyclic diterpenoids (241) and (242) have been isolated... 

Fig. 7.9 Examples for acyclic diterpenoid glycosides (above left), cembrenoids (below left), and labdenoids (above and below right) from the Solanaceae...

Ortalo-Magne, A., Culioli, G., Vails, R., Pucci, B., and Piovetti, L. (2005) Polar acyclic diterpenoids from Bifurcaria bijurcata (Fucales, Phaeophyta). Phytochemistry, 66, 2316-2323. 

Diterpenoids Acyclic Diterpenoids (535) 13-Hydroxygeranyllinalool 13- 0-(6 - O- P-L-fucosyl)- P-D-glucoside Arachniodes maximowiczii Ohwi C32H54.O11 oil -35 256... 

Sesquiterpenoid 130 and the diterpenoids 131-133 are some of the simplest acyclic marine isocyano-related compounds. The latter series represent the... 

IPP and DMAPP lead to geranylpyrophosphate (GPP), which is an immediate precursor of monoterpenes. The formation of nerylpyrophosphate (NPP) from GPP gives rise to a wide range of acyclic, cyclic, bicyclic or tricyclic skeletons. Reactions like rearrangement, oxidation, reduction and hydration via various terpene cyclases result in the formation of numerous terpene derivatives. Condensation of GPP and IPP leads to farnesylpyrophosphate (FPP), the immediate precursor of sesquiterpenoids. Likewise, FPP and IPP are conducive to diterpenoids. 

The ability to synthesize diterpenes is universal to plants, since phytol, the acyclic parent compound of the series, is present in ester attachment in the chlorophyll molecule and hence occurs in all green plants. Gibberellic acid is also widespread in the plant kingdom as a growth hormone. Besides phytol and gibberellic acid, the remaining diterpenoids are very restricted in occurrence and usually occur within one or only a few plant... 

The red algae (Rhodophyta) are the most prolific sources of halogenated organics in the marine environment, and this topic was recently reviewed (Fenical, 1975). At least six orders, representing some ten families of red algae are now known to produce a wide variety of structure types from halo-methanes (C,) to halogenated products derived from squalene (C30). Within this group are aromatic and acyclic compounds produced from acetate (poly-ketide) biosynthesis and monoterpenes (Cio), sesqui- (Cu) and diterpenoids (C2o)- The structures of well over 200 compounds have now been firmly established. 

SuKH Dev and R. Misra, CRC Handbook of Terpenoids. Diterpenoids. Vol. I. Acyclic and Monocyclic Diterpenoids, 1-70, CRC Press, Boca Raton, FL, 1985. 

Diterpenoids refer to those compounds having a two consecutive terpenoid structure (20Q, usually in crystal state. They can be classified into two subgroups acyclic and cyclic diterpenoids, for example, phytol (a naturally linear diterpenoid used in preparation of vitamin E and Kl) and vitamin A (monocyclo-diterpenoid rich in fish oil). The most important diterpenoids in terms of bioactivities are dicyclo- and tricycle-diterpenoids, such as paclitaxel (Taxol) (details please see in Sect. 4.3 of this chapter below) and its analog docetaxel (Taxotere) for various cancer treatment, and ginkgoUdes against the aggregation of platelet. 

Most abundant are instead cyclic diterpenoids that originate by cyclization reactions of GGPP from both sides of the molecule tail (alcohol end) or head (isopropyhdene end). Cyclization generally involves carbocations and proceeds by a concerted addition mechanism generating different cyclic systems, due to the folding of the acyclic substrate chain on the specific enzyme and originating rich and wide array of cyclic systems [6]. 

Although the number of aroma compounds derived from acyclic carotenoids is much inferior to that of the mono- and bicyclic compounds, some of them can also be considered as breakdown products from genuine mono-, sesqui- and diterpenoids. The importance of the aliphatic isoprenoids (282) to (291) in the formation of total flavors of certain foodstuffs is not less than that of the cyclic compounds, the three methyl ketones (282), (287) and (290) which are related to the main tomato pigment lycopene were observed in tomato flavor (75). The hexahydro derivative (291) from coffee (595), jasmine oil (722) and green tea 438) is perceived as flowery and warm and can be considered as an oxidative biodegradation product of phytol and phyta-diene. 6-Methyl-3,5-heptadien-2-one (283), with a grassy and cinnamonlike aroma 438) [detected in tomato 668), the essential oil of Hama-metis leaves 383), Ceylon tea (722) and passion fruit (777)], and pseudo-ionone (288) [also isolated from passion fruit (777)] are believed to be formed from two different dehydrolycopenes. Compounds other than carotenoids, such as solanesol or squalene, can also be considered... 

---