Cembrene diterpenes

Cyclization of polyenes. Treatment of fran -geranylgeranic acid chloride (1) with 1 eq. of stannic chloride in methylene chloride at -78° (1.5 hr.) effects cyclization to (2) in 71% yield. The reaction is of interest because it may be representative of the biogenesis of the 14-membered cembrene diterpenes. 

The GC-MS data (Figure 16.11) of the violet zone of B. carterii revealed that the unchanged diterpenes (verticillatriene, cembrene A, and cembrene C) and the nortriterpenes with carbohydrate structure originated from the pyrolyzed triterpenes (Figure 16.12) of the a- and (3-boswellic acids, named 24-norursa-3,12-diene (compound 7), 24-norursa-3,9(ll),12-triene (compound 8), 24-noroleana-3,12-diene (compound 9), and 24-noroleana-3,9(ll),12-triene (compound 10). 

For olibanum, six characteristic diterpenes were observed cembrene A (94), cembrene C (98), isoincensole acetate (104), a dimer of a-phellandrene (86) and two unidentified compounds (92) and (103). 

Frankincense, also called olibanum, is a natural oleo gum resin that exudes from incisions in the bark of Boswellia trees [46, 47]. Diterpenes like incensole or isoincensole and their oxide or acetate derivatives (see Figure 10.3) are characteristic biomarkers of olibanum [48]. Although diterpenoid hydrocarbons possessing the cembrane skeleton have been isolated from a variety of terrestrial and marine organisms, their occurrence and particularly that of cembrenes A and C (see Figure 10.3) is supplementary proof of the presence of olibanum in a sample. Optimisation of the SPME conditions was done with the aim of trapping these low volatile diterpenes. 

Sampling time 60 min extraction temperature 80°C. Monoterpenes a-pinene, p-myrcene, a-phellandreneand limonene sesquiterpenes a-cubebene, a-copaene, p-elemene, p-caryophyllene, a-humulene, y-muurolene, p-eudesmene and caryophyllene oxide diterpenes cembrene A and isoincensole acetate. 

The first observation is the similarity between the chemical compositions of both the Boswellia carteri and Boswellia sacra. For these three olibanum samples, a-pinene (2), (3-myrcene (8) and limonene (14) are the predominant monoterpenes. p-Caryophyllene (73) is the major sesquiterpene besides a-copaene (65), a-humulene (also called a-caryophyllene) (78) and caryophyllene oxide (95). The characteristic olibanum compounds isoincensole and isoincensole acetate (128) together with cembrene A (120) are the main diterpenes. 

Boswellia serrata olibanum has a chemical composition close to that of both the B. carteri and of B. sacra, but contains compounds that are absent in those from other Boswellia and could be used as markers methylchavicol (38), p-anisaldehyde (47), methyleugenol (70), isocaryophyllene (82), sesquiterpene 91, elemicin (92) and an unidentified diterpene (124) eluting between cembrene C (123) and verticilla-4(20),7,ll-triene (125). It is devoid of (5-caryophyllene (73), a-humulene (78), caryophyllene oxide (95) and bornyl acetate (50). 

Cembrene, C20H32, is a diterpene hydrocarbon isolated from pine resin. Cembrene has a UV absorption at 245 nm, but dihydrocembrene (C20H34), the product of hydrogenation with 1 equiv H2, has no UV absorption. On exhaustive hydrogenation, 4 equiv H2 react, and octahydrocembrene, C2UH4,. is produced. On ozonolysis of cembrene, followed by treatment of the ozonide with zinc, four carbonyl-containing products are obtained ... 

Cembrene, is a diterpene hydrocarbon isolated from pine resin. Cembrene... 

Pborbol and Its Relatives.— The Euphorbiaceae are a rich source of diterpenes. The hydrocarbon casbene (126), which is clearly related to the macrocyclic diterpene cembrene, has been isolated from Ricinus communis It may also be related to the parent hydrocarbon of the phorbol group of diterpenoids. 

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. 

A similar cyclization generates the 14-membered skeleton of cembrane from which other polycyclic diterpenes are derived. 3,7,11,15-Cembratetraene, better known as cembrene A, emerges directly from geranylgeranylpyrophosphate (Fig. 2) involving the 1,14-cyclization of the resulting allylic cation... 

Diterpenes Derived from Monocyclic Precursors Cembrene Taxanes... 

Several monocyclic diterpenes occur in nature. Cembrene (5), found in the resinous material from several gymno-sperms, is derived from the same intermediate leading to taxanes (such as 6 and 7) that are only found in the gymno-spermous family, Taxaceae (Sukh Dev and Misra, 1985) (Fig. 22.5). 

The monocyclic diterpene, cembrene (5), has been suggested to be formed from GGPP with a Z-olefin linkage (Newman, 1972) (Fig. 22.5). In view of other information, this compound is probably derived from a normal geranylg-eranyl-OPP precursor. 

The solid-state and solution conformations of the diterpene sarcophine (19) have been studied by AT-ray diffraction and nuclear Overhauser effects, respectively. The conformation in the solid-state is similar to that of cembrene, and in solution it is believed to be a fairly rigid system with the C-6—C-10 segment adopting a half-chair conformation. ... 

Formulas of terpenes in the scheme above are presented in two rows. In the bottom row, if the chemical bonds labeled in red are deleted the residues represent isoprene units. Based on the number of isoprene fragments, menthol is monoter-pene, a-selinene belongs to sesquiterpenes and cembrene as well as vitamin A are diterpenes. Menthol is the component of peppermint responsible for its refreshing odor. The aroma of celery originates from a-selinene while cembrene can be isolated from pine trees. The derivative of retinal has already been discussed in the section about the cis-trans isomerism in the biochemical process of vision. 

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