Ajmalan

Ajmalan is a parent hydride used in the IUPAC nomenclature of natural products and also in CAS nomenclature.[1] It is a 20-carbon alkaloid with six rings and seven chiral centres.

The conventional representation of the ajmalan skeleton, with numbering

The name is derived from ajmaline, an antiarrhythmic alkaloid isolated from the roots of Rauvolfia serpentina[2] which is formally a dihydroxy-derivative of ajmalan. The –an ending indicates that ajmalan is partially saturated. Ajmaline itself is named after Hakim Ajmal Khan, a distinguished practitioner of the Unani school of traditional medicine in South Asia.[3]

The absolute configuration of the seven chiral carbon atoms in ajmalan is defined by convention, as is the numbering system.[1] The stereochemistry is the same as that in naturally occurring ajmaline, and corresponds to (2R,3S,5S,7S,15S,16R,20S) using conventional numbering.

Ajmalan can be systematically named as

(1S,4S,5S,7S,8R,16S,17R)-4-ethyl-9-methyl-2,9-diazahexacyclo[14.2.1.02,7.05,18.08,16.010,15]nonadeca-10,12,14-triene

or as

(2S,3S,5S,6aS,11aR,11bS,12R)-4H,11H-3-ethyl-11-methyl-1,2,3,5,6,6a,11a,11b-octahydro-2,5,6a-(epiethane[1,1,2]triyl)indolo[2,3-c]quinolizine.

Note that the numbering of the atoms in the systematic names is different from the conventional numbering of ajmalan.[4]

The ajmalan skeleton is similar to those of certain other alkaloids, and ajmalan could also be given the following semisystematic names:

(2β,5β,16R,20β)-1-methyl-1,2,19,20-tetrahydro-5,16-cyclo-16a-homo-17-norakuammilan;
(2β,5β,7β,16R,20β)-1-methyl-2,7-dihydro-5,16:7,17-dicyclocorynan;
(2β,7β,16R,20β)-1-methyl-2,7,19,20-tetrahydro-7,17-cyclosarpagan;
(2β,3α,7β,20β)-1-methyl-2,7,19,20-tetrahydro-3,4:7,17-dicyclo-22-norvobasan;
(2β,5β,7β,16R,20β)-1-methyl-2,7-dihydro-5,16:7,17-dicyclo-17-secoyohimban.

However, the relative complexity even of these names justifies the use of ajmalan as a defined parent hydride in alkaloid nomenclature.

References
  1. "Revised Section F: Natural Products and Related Compounds (IUPAC Recommendations 1999)", Pure Appl. Chem., 71 (4): 587–643, doi:10.1351/pac199971040587.
  2. Siddiqui, S.; Siddiqui, R. H. (1931). J. Indian Chem. Soc. 8: 667–80.
  3. Ahmed Nasim Sandilvi (2003). "Salimuzzaman Siddiqui: pioneer of scientific research in Pakistan. Archived 2007-09-27 at the Wayback Machine" Daily Dawn, 2003-04-12. Retrieved on 2007-07-19.
  4. The numbering is also different between the von Baeyer name and the fusion name, given the different conventions which apply to the two methods of nomenclature.
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