Of cembrene

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. 

Figure 10.3 Structure of cembrene A, cembrene C and isoincensole acetate. Reproduced from S. Hamm, E. Lesellier, J. Bleton, A. Tchapla, J. Chromatogr., A, 1018, 73 83. Copyright 2003 Elsevier Limited...

Synthetic endeavours have continued in the diterpenoid series. The cyclization of geranylgeranic acid chloride to derivatives of cembrene, and their conversion into cembrene itself, have been described. Some further extensions of the route to macrocyclic diterpenoids based on the intramolecular cyclization of epoxysulphides and the subsequent modification of the macrocylic product have been described. ... 

Yue et al. reported on the total synthesis of / -(—)-cembrene A by using the mod-ihed Wittig reaction and titanium-induced intramolecular carbonyl coupling as key steps the overall yield is 29% starting from -geranylacetone (164) and R-(+)-limonene (168) (Scheme 6-15). ... 

Li et al. reported on the total synthesis of cembrene C from geraniol (137) or trans, frani-famesol (91) by using the titantium-induced intramolecular coupling of the dicarbonyl precursor keto enal 176 as the key macrocyclization step (Scheme 6-17). 

An A-ray analysis of the diepoxide (135) derived from verticillol, which is obtained from the wood of Sciadopitys verticillata, has been used " to define the absolute configuration of this diterpenoid. The acid-catalysed cyclization of cembrene and isocembrenol, which leads to the formation of perhydrophenanth-rene derivatives, has been studied. 

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 ... 

A full paper on the isolation of cembrene-A (64) and (3Z)-cembrene A from a termite soldier, Cubitermes umbratus, has appeared. An 11,12-epoxycem-brene (65), which has been extracted from Greek tobacco, may act as a biogenetic precursor of some of the 8,11- and 8,12-epoxycembranoids which are also found in tobacco. 

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. ... 

The ring-forming step in a reported synthesis of cembrene was the nickel tetra-carbonyl-catalysed cyclization of acetoxy-dibromide (37 = H, = OAc) to the... 

Wender has applied his macroexpansion methodology to the synthesis of cembrene A in optically active form, starting with (+)-0 0... 

In the biosynthesis of cembrene A the terminal double bond is joined to the head of the molecule (Fig. 110). Cembrene derivatives occur in plants and animals. 

The intramolecular coupling reaction has allowed the synthesis of cembrene (CXV), a 14-membered diterpene (Dauben et al., 1974). 

Heartwoods of certain Taxus spp., especially that of T. baccata, contain several toxic diterpenoids, such as baccatin-IV (480) (264). About 30 compounds are known at present, and are invariably highly functionalized. The tricyclic framework of these compounds is readily rationalized by further cyclization of cembrene cation 473 (292). Verticillol (481), an extractive from the wood of Sciadopitys verticillata, corresponds to the bicyclic stage on the pathway to the taxane type (227). 

Consider the double bonds in the ring of cembrene as drawn in Figure 7.2. How many are cis and how many transl How many possible ring isomers are there for cembrene, presuming there are no stereochemical restrictions ... 

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