Pimarane skeleton

Diterpene phytotoxins sphaeropsidins A—F, tri- and tetracyclic unrearranged pimarane skeleton, are isolated from Sphaeropsis sapinea, a fungus that causes a canker disease of Italian cypress. Sphaeropsidin A is the major toxic substance showing nonhost-selective phytotoxic activity326 (Figure 36). 

About 80% of the diterpene acids of the genus Finns have an abietane skeleton, with abietic acid (structure 3) as the predominant component. Other diterpene acids have a pimarane skeleton (structure 4). All compounds with the abietane skeleton are distinguished by the presence of an isopropyl group. 

A group of diterpenes with the pimarane skeleton (1). In R, the methyl group at C-10 is cis to the ethyl group at C-13, whereas in the isopimaranes it is in the transposition. Both enantiomers of not only the R but also of the isopimaranes occur in nature. R are formed by a further cyclization of the labdane skeleton. Examples are annonalide, momilactones A and B (both growth inhibitors for plants ), leucophleol and ent-8(14),15-pimaradiene. In the past the R were also known as sandaracopimaranes. 

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. 

Canonica, L., B. Rindone, and C. Scolastico A new diterpene with pimarane skeleton. Tetrahedron Lett. 4801 (1969). 

Schmitz, F.J., D.P. Michaud, and P.G. Schmidt Marine Natural Products Par-guerol, Deoxyparguerol, and Isoparguerol. New Brominated Diterpenes with Modified Pimarane Skeletons from the Sea Hare Aplysia dactylomela. J. Am. Chem. Soc. 104, 6415 (1982). Erratum J. Am. Chem. Soc. 106, 3385 (1984). 

Schmitz, F.J., Michaud, D.P., and Schmidt, P.G. (1982) Marine natural products parguerol, deoxy-parguerol and isoparguerol. New brominated diterpenes with modified pimarane skeletons from the sea hare Aplyda dactylomda. J. Am. Chem. Soc., 104, 6415-6423. 

Lastly, tedanol isolated from a Caribbean Tedania ignis collected in the mangroves of Sweeting Cay, Grand Bahama Island, is a brominated and sulfated diterpene with ent-pimarane skeleton. This original diterpene displays potent anti-inflammatory activity by inhibition of COX-2 (cyclooxygenase 2) expression thus, tedanol appears as a non-steroidal anti-inflammatory (Costantino et al, 2009b). 

Mono-, sesqui-, and some diterpenoids are found in marine and terrestrial flora. They are, therefore, not always unambiguous tracers for higher plant sources. However, diterpenoids with the abietane and pimarane (Fig. 1), and less common phyllocladane and kaurane, skeletons are predominant constituents in resins and supportive tissue of coniferous vegetation (Coniferae), which evolved in the late Paleozoic (200—300 million years ago). Diterpenoid biomarkers have been characterized in... 

In general, very little has been written about diterpenes from the Latin American Celastra-ceaeas these structures are not often found. Abietriene type diterpenes have been the general rule in the Celastraceae although the chemical study of the minor constituents of Orthosphenia mexicana and Rzedowskia tolantonguensis did enable pimarane type diterpenes to be isolated and chemically characterized [43,45] and the second of these species afforded a series of new diterpenes with an isopimarane skeleton, described for the first time in the Celastraceae. 

Regardless of its origin (gum, wood or tall oil), rosin is mainly composed (90-95 pa- cent) of diterpenic mono-carboxylic acids, commonly known as resin acids whose generic formula is C19H29COOH. The remaining components are essentially made up of neutral compounds, the nature of which depends on the specific origin of the rosin [5]. The most common resin acids found in pine rosin are derived from the three basic tricyclic carbon skeletons abietane, pimarane and isopimarane and the less common bicyclic labdane skeleton (Fig. 4.1). 

The most common pimarane-type acids are pimaric, isopimaric and sandaracopimaric acids (Fig. 4.3). Compared with the abietic counterparts, not only is the basic skeleton diffa-ent but, more importantly, the double bond system is now not conjugated, a fact which reduces significantly the possible chemical exploitation of these compounds. 

Ginkgolides (e.g., ginkgolide-B, 444), from the root-bark of Ginkgo biloba, have a novel carbon skeleton. Biosynthetic experiments have shown that their genesis apparently involves a seconor rearranged pimarane and C-methylation with methionine (293). 

In addition to the /-abietane skeleton, other tricyclic e /-pimarane derived ring skeletons have been reported (Figure 15) Cleistanthanes and isocleistan-thanes were detected in Brickellia eupatoriedes (JP) and erythroxanes were identified in Helichrysum refluxum (450). The absolute configuration (5R, lOR) of the Brickellia cleistanthanes is identical to that reported for the cleistanthanes of the Euphorbiaceae, but antipodal to those (5S, lOS) of the Velloziaceae (Vellozia) (Figure 16 [p. 428] 427). 

Figure 13. Proposed biogenetic routes for sandaracopimaranes (normal-pimaranes) and related skeletons.

Figure 15. Proposed biogenetic routes for tricyclic e /-pimarane, e /-abietane, erythroxane, cleistanthane and isocleistanthane diterpene skeletons derived from ent-labdane precursors.

Most of the approximately ninety Brickellia (Eupatorieae Alomiinae) species inhabit the North American deserts, although three species are South American disjuncts. The diterpene chemistries of the ten investigated taxa (Table 6 [p. 441] Figure 19 [pp. 457-461]) are remarkably uniform. Normal-labdanes often characterized by novel 2a,3o -hydroxylation/esterification occur in all diterpene-producing species. In B. eupatoriedes, these 2a,3a-substituted normal-labdanes co-occurred with cleistanthanes and other eAtf-pimarane-derived skeletal types. Production of normal-labdanes together with tri- and polycyclic ewMabdane-derived skeletons is an often-repeated theme in the diterpene chemistries of Compositae taxa. 

Fotir of these are isoprenologs of skeletons already known (pyronane, drimane, pimarane). A new carbon skeleton whose representatives are currently known only in species of the genus Ircinia is an isoprenolog of a sesquiterpenic skeleton. Isomers of oplopane and oppositane are both present in species of Laurencia (see Figure 13.14). 

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