Abietane

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Abietane
Names
IUPAC name
Abietane
Systematic IUPAC name
(4aR,4bS,7S,8aS,10aS)-1,1,4a-Trimethyl-7-(propan-2-yl)tetradecahydrophenanthrene
Other names
13α-Isopropylpodocarpane
Identifiers
3D model (
JSmol
)
ChEBI
ChemSpider
  • InChI=1S/C20H36/c1-14(2)15-7-9-17-16(13-15)8-10-18-19(3,4)11-6-12-20(17,18)5/h14-18H,6-13H2,1-5H3/t15-,16-,17-,18-,20+/m0/s1 checkY
    Key: STIVVCHBLMGYSL-ZYNAIFEFSA-N checkY
  • [H][C@]12[C@@](C[C@@H](C(C)C)CC2)([H])CC[C@@]3([H])C(C)(C)CCC[C@@]31C
Properties
C20H36
Molar mass 276.508 g·mol−1
Density 0.876 g/ml
Boiling point 338
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
☒N verify (what is checkY☒N ?)

Abietane is a

natural chemical compounds such as abietic acid,[1] carnosic acid, and ferruginol
which are collectively known as abietanes or abietane diterpenes.

Abietanes are found in the tissues and resins of certain higher plants, particularly gymnosperms.[2][3] Although the functions of terpenes are not fully understood, conifers appear to produce abietane diterpenoids as a form of defense against insect and microbial attack.[4][5] Some abietane diterpenoids, especially aromatic abietenes, are of interest to the pharmacology and natural products communities for their potential biological activities.[6] In the rock record, abietanes are commonly found in amber as well as in fossil wood, sometimes in the form of the mineral fichtelite. Additionally, abietanes are observed in sediments—both riverine and marine—and in coals, where they are often interpreted as geochemical biomarkers for terrestrial input from conifers.[2][7][4][8][9]

Chemical structure and properties

Abietane skeleton numbering scheme[6]

Abietanes are tricyclic 20-carbon

diterpenoids are synthesized from four five-carbon isoprene units. Abietanes are generally nonpolar, volatile, and less dense than water. The presence of one or more polar functional groups (typically a carboxylic acid or alcohol
) tends to increase the polarity and boiling point of a given abietane relative to its unsubstituted hydrocarbon form.

Biological sources and synthesis

Abietenes are synthesized from geranylgeranyl diphosphate via a copalyl diphosphate intermediate by class 2 diterpene cyclases and class 1 diterpene syntheses.[10]

In higher plants, abietanes are synthesized from geranylgeranyl diphosphate (GGPP) via a copalyl diphosphate (CPP) intermediate. First, GGPP is cyclized by a class II diterpene cyclase enzyme to CPP. The conformation of the GGPP molecule dictates the stereochemistry of the CPP intermediate after cyclization. The stereochemistry of the typical abietane skeleton suggests a GGPP precursor with its fused cyclohexyl rings in a chair-chair ("normal") conformation, although some abietanes with alternative stereochemistry may be cyclized from CCP isomers containing alternative combinations of boat and chair cyclohexane conformers. After the initial cyclization to CPP, which forms rings A and B in the abietane skeleton, the C ring is formed with the help of a class I diterpene synthase enzyme. Subsequent methyl migration and dehydrogenation steps yield the abietene isomers.[10]

Preservation and diagenesis

Proposed diagenetic pathway for abietic acid involving defunctionalization and aromatization to form retene[8]
Abietanes found in the rock record are typically interpreted as evidence of higher plants, particularly gymnosperms, in the deep past.

Diagenetic transformation of biomolecules is not fully understood, but several broad diagenetic patterns are hypothesized to affect the transformation of abietanes as they are heated and pressurized in sediments. The first of these patterns is defunctionalization. In particular, the reducing conditions of diagenesis are believed to cause abietanes to lose oxygen-containing functional groups, including carboxylic acids and alcohols, as well as methyl groups.[8] In addition to defunctionalization, abietanes likely undergo dehydrogenation and aromatization reactions to form more energetically stable systems of conjugated pi bonds in their characteristic three ring structure. The hypothesized diagenetic pathway of abietic acid is illustrative of these general patterns. Abietic acid is dehydrogenated to dehydroabietic acid, which then loses its carboxylic acid functional group to become dehydroabietin. Loss of the 5-Me group and further dehydrogenation form the aromatic 1,2,3,4-tetrahydroretene molecule. Final aromatization produces retene, a common biomarker molecule observed in sedimentary samples.[8]

Measurement techniques

Abietanes found in modern gymnosperm resins as well as in the rock record are separated and characterized by

trimethylsilyl groups by treatment with BSTFA prior to GC-MS analysis.[11] More oxidized abietane derivatives have been studied using thermally assisted methylation using tetramethylammonium hydroxide (TMAH) followed by GC-MS analysis.[12] MS-MS analysis has been used to elucidate fragmentation mechanisms for mass spectrum peaks of interest.[11] Mass spectra for abietic acid and some other common abietanes are publicly available in the NIST database.[13] The spectrum for abietic acid possesses characteristic peaks at m/z = 256 and 241.[11]

Use as a biomarker

Abietanes preserved in geological settings are typically interpreted to derive from

diterpenoids, including abietanes, in ancient resins and the rock record compared to modern conifer samples.[9]

Examples from archaeology

  • Abietenes from colophony, tar, and pitch have been identified in caulking used on ancient ships.[9]
  • Abietanes have been used to identify conifer resins associated with Egyptian mummies.[9]
  • The ratio of oxidation products of abietanes including dehydroabietic acid and de-7-oxo-dehydroabietic acid and 15-hydroxyl-7-oxo-dehydroabietic acid have been used to estimate the oxidation state of varnish on Vermeer's famous painting, "Girl with a Pearl Earring."[12]

Examples from geochemistry

  • Carbon isotopic measurements of abietanes and other di- and tri-terpenoids have been made in modern plants, as well as in ancient samples, where they reveal a carbon isotope excursion during the Paleocene-Eocene Thermal Maximum (PETM).[3][14]
  • Abietanes found in marine sediments have been used as evidence of ancient terrigenous input.[15]
  • Abietane diterpenoids have been attributed to resinous vascular plants in samples dating to the Jurassic.[7]

See also

References

  1. PMID 8302943
    .
  2. ^
    ISSN 0146-6380
    .
  3. ^
    ISSN 0016-7037
    .
  4. ^
    ISBN 978-0-08-098300-4
    , retrieved 2021-05-10
  5. S2CID 1506353
    .
  6. ^
    PMID 25643290
    .
  7. ^
    ISSN 0146-6380
    .
  8. ^
    OCLC 27895324
    .
  9. ^
    ISBN 978-0-521-83762-0
    , retrieved 2021-05-10
  10. ^
    PMID 20890488
    .
  11. ^
    S2CID 8264935
    .
  12. ^
    ISSN 0165-2370
    .
  13. ^ "Abietic acid". webbook.nist.gov. Retrieved 2021-05-22.
  14. S2CID 129625887
    .
  15. ISSN 0016-7037
    .
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