Diterpenes, monocyclic

The name Ciereskane was given in tribute to Professor Leon S. Ciereszko. Figure 20.29 Main diterpenic monocyclic carbon skeletons of Octocorallia. 

The tricyclic substance 18A and 18B are both potential synthetic intermediates for synthesis of the biologically active diterpene forskolin. These intermediates can be prepared from the monocyclic precursors shown. Indicate the nature of the reactions involved in these transformations. 

We ve classified more than 550 diteipenoids (listed in Table 3 whith corresponding sources) from all the reported Salvia species which we ve divided into 2 categories. The first one contains the monocyclic and bicyclic diterpenoids including labdanes and clerodanes and related diterpenes. The second category concerns tricyclic and tetracyclic diterpenes including pimaranes and abietanes and related diterpenes. The clerodane type diterpenes are mainly represented by american species while abietane type are found in european and asian species. 

In our knowledge, 2,6-dimethyl-10-(p-tolyl)undeca-2,6-(E)-diene (1) is the alone monocyclic diterpene, it was reported from S. dorisiana [39]. 

Crawford5 has reported a synthesis of the monocyclic diterpene r/r-artemisene (15), in which selective metalation is used in two of the four steps. Thus (+ )-limonene (3) is converted into the metalated derivative (9) reaction of (9) with paraformaldehyde... 

Cycloaddition (cf. 12,163). In the presence of CpCo(CO)2, the monocyclic enync (Z)-l cyclizcs to the tetracyclic diene (2) as a 2 1 mixture of epimers. The product has the carbon skeleton present in the ditcrpenc stemodin (3), and can be converted in three steps to an intermediate in a total synthesis of the diterpene. This cycloaddition fails with (E)-l. 

The occurrence of monocyclic diterpenes with a prenylbisabolane skeleton is quite uncommon and it is regarded as an interesting chemotaxonomic feature of this plant species. 

The biogenesis of pimarane, the parent compound of many polycyclic diterpenes, is assumed to arise from Ko-geranylgeranylpyrophosphate After dissociation of the pyrophosphate anion, the remaining acyclic allylic cation undergoes a 1,3-sigmatropic hydrogen shift and thereby cyclizes to a monocyclic carbenium ion which, itself, isomerizes to the ionic precursor of the pimarane skeleton. 

Various bi- and tricyclic diterpenes are derived from the monocyclic cembrane (Table 4). Casbane, for example, is simply 2,15-cyclocembrane another bond between C-6 and C-10 leads to lathyrane, from which the iatrophanes arise by opening the C-l-C-2 bond. 

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. 

Fig. 22.6. Diterpenes derived from a monocyclic precursor (modified from Adolf and Hecker, 1977 used with permission of the copyright owner. Laser Pages Publishing, Ltd., Israel).

Cembranes, cembranoids monocyclic diterpenes isolated from several plants, in particular from the gum resins of pines. C. have also been found in marine coelenterates and insects. 

Quayle employed an ATRC to approach the eunicellin core, a diterpene isolated from soft corals off the coast of Banyuls-sur-Mer. Treatment of geraniol trichloroacetate 29 with the copper catalyst induced cyclization to yield lactone 30 in 72% yield (Scheme 25.14). The monocyclic lactone is formed at room temperature, but then harsher reaction conditions (84°C) are required because of the less activated nature of the a-dichloro moiety of 31. The resulting dia-stereoisomeric chlorides undergo spontaneous dehydrochlorination to give 32 as a 2 1 diastereoisomeric mixture at carbon ( ). 

Mori, K. and Uno, T. (1989) Synthesis and structure revision of bifurcarenone, a unique monocyclic diterpene in combination with a hydroquinone C7 unit as an inhibitor of mitotic cell division. 

Table 20.11 Distribution of the main monocyclic diterpenic carbon skeletons"...

---