Bicyclic Diterpenes

Carboxylative TMM q cloaddition has also been realized with 3-methoxytropone and precursor (56) to produce an epimeric mixture of acids (122), which was employed in a synthetic study of the bicyclic diterpene sanadaol (123). The use of bi-dentate ligand tpdp (12) and high pressure did not improve the reaction. However, the addition of MesSnOAc as a co-catalyst did produce a better yield of (122) (Scheme 2.33) [16]. 

An example of stereocontrol by high pressure is given by the regio- and diastereoselective synthesis of hydrophenanthrenones [18] which are useful intermediates for synthesizing diterpenes and steroids, by EtAlCli-catalyzed cycloadditions of heteroannular bicyclic dienone 50 with (E)-piperylene (24) and 2,3-dimethyl-1,3-butadiene (51) (Scheme 5.4). 

Diels-Alder reaction of the furan derivative 148 with homochiral bicyclic enone 149 is the key step [56] in the total synthesis of the diterpenes jatropho-lone A and B, 151 and 152, respectively, isolated from Jatropha gossypiifolia L [57], Initial efforts to carry out the cycloaddition between 148 and 149 under thermal or Lewis-acid conditions failed due to diene instability. Application of 5kbar of pressure to a neat 1 1 mixture of diene and dienophile afforded crystalline 150 with the desired regiochemistry (Scheme 5.23). Subsequent aromatization, introduction of the methylene group, oxidation and methylation afforded (-l-)-jatropholones 151 and 152. 

Cationic cyclization. A key step in the synthesis of the diterpenes cafestol5 and atractyligenin4 involves a novel cation cyclization of bicyclic cyclopropanes to the tetracyclic systems of the diterpenes (equations I and II). Thus treatment of 1 with a slight excess of triflic anhydride and 2,6-lutidine effects cyclization to the rather unstable pentacycle 2 with the kaurene system. The related conversion of 3 to 4 can be effected with triflic anhydride and 2,6-di-r-butyl-4-methylpyridine in 1-nitropropane. 

The biosynthesis of diterpenes was created to produce molecular diversity. Literally hundreds of different macrocyclic, bicyclic and polycyclic diterpenes... 

The apparently latest total synthesis of a dolastane diterpene was published by Williams and coworkers in 1993 as a short communication (Fig. 16) [91]. (-)-Clavulara-l(15),17-dien-3,4-diol (129) was synthesized using a strategy that relied on the availability of the enantiomerically pure building block 162 from (+)-9,10-dibromocamphor (163) (Fig. 16). Cornerstones of the synthesis are a macrocyclization that afforded the 11-membered (A+B)C-ring (160) and a transannular cyclization that converted a bicyclic into a tricyclic ring system. Two of the seven chirality centres in the synthetic clavu-... 

Bicyclic diterpenes-clerodanes. An important group of Insect antifeedants are the clerodane diterpenoids, which have been isolated from several different plant families (13)(Figure 1). Particularly well studied are the antifeedant activities of caryoptln and clerodin, and their derivatives, from Clerodendron and Caryopteris, Verbenaceae, against the tobacco cutworm Spodoptera litura L. (25-27). 

Unknown until recently, and still far from understood. In 1982, Alfredo Ortega and his associates isolated a bicyclic diterpene, C23 H28O8, from material gathered in Oaxaca and named it salvinorin. Another group, led by Leander Valdes at the University of Michigan, independently isolated the same compound and named it divinorum. Because Ortega published first, the name salvinorin has precedence. Neither author tested salvinorin for human activity, but recent tests by Daniel Siebert and others, myself included, have proved the psychoactivity of salvinorin beyond further doubt. 

However, various bi- and tri-cyclic diterpenes have been isolated from other Salvia species.Extraction of the leaves of 5. divinorum has now yielded a novel bicyclic diterpene, salvinorin (1), C23H28O8 whose structural elucidation forms the subject of this paper. 

A pentacyclic diterpene 1 called dictyoxetane contains a most unusual subunit, a 2,7-dioxatricyclo[4.2.1.03>8 ]nonane. During model studies designed to provide access to this key subunit the bicyclic ether 2 was synthesised in the hope that Sn displacement would generate the unsaturated tricyclic oxetane. There was no reaction when 2 was treated with base. Reaction with a catalytic amount of p-toluenesulfonic acid in DMF at 75°C for 24 hours resulted only in formation of 4-methylacetophenone. The hydroxy mesylate 2 is also reported to decompose to 4-methylacetophenone on storage. 

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. 

Hirukawa, T., Shudo, T., Kato, T. Synthesis of secotrinervitanes, unique bicyclic diterpenes from termites. J. Chem. Soc., Perkin Trans. 1 1993,217-225. 

Unsymmetrical alkenes can be prepared by mixed intermolecular reactions if one of the components, commonly acetone, is used in excess (equation 83). As the isopropyl group is a common subunit of many terpenes this method provides a valuable route for its introduction. Pattenden and Robertson used such a reaction followed by a Grob-type fragmentation in their preparation of the daucenone (42) from the readily enolized ketone (43). The bicycle (42) was used as an intermediate for the synthesis of the diterpene ( )-isoamijiol (44 equation 84). Mixed couplings are not restricted to acetone, and almost any carbonyl may be used. For example, Paquette et al. employed the aldehyde (45) in a synthesis of ( )-a-vetispirene (46 equation 85). More complex mixed couplings are also possible. For example, the aldehydes (47) and (48) are coupled efficiently to the stilbene (49), which in turn is converted to phenan-threne alkaloids such as atherosperminine and thalictuberine (equation 86). ... 

Hydrophenanthrenes are known to be useful intermediates in the total synthesis of diterpenes and steroids. The Diels-Alder reaction of heteroannular bicyclic dienone 8 with ( )-piperylene (9a) and 2,3-dimethyl-l,3-butadiene (9b)... 

Bisabolene diterpenes Although this class of diterpenes was the last one to be discovered in Eremophila, its existence was predicted because of the occurrence of bicyclic and tricyclic diterpenes in this genus which, formally at least, could be derived from further cyclization of a putative intermediate containing this ring system (100). 

The decipiane diterpenes can be envisaged to arise from the same bisabolonium intermediate (205) but, in this case, by attack of X" on the Re-facs of Cll. This leads to the bicyclic intermediate (209) with the side chain in an axial orientation. If this is allowed to interact with the cyclohexene double bond, the decipiane tricyclic skeleton (210) can be generated as shown in Scheme 50. 

Cembranolide, a diterpene, is unique in that it is the only FPTase inhibitor isolated from a marine specimen, Lobophytum cristagalli (Order Alcyonacea, Family Alcyoniidae), that was collected at a 15m depth from Seychelles Island. The compound was isolated by silica gel chromatography followed by reverse phase HPLC and the structure of this 14/5-bicyclic diterpenoid (Figure 24) was elucidated by NMR and confirmed by X-ray crystallography [115]. 

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