IUPAC BlueBook 2013 states:

P-25.3.3 Numbering fused ring systems

Fused ring systems with retained names, systematic names, or fused names are systematically numbered in the same manner. Anthracene, phenanthrene, acridine, carbazole, xanthene and its chalcogen analogues, purine, and cyclopenta[a]phenanthrene are exceptions; traditional numberings are retained.


All except the last one, cyclopenta[a]phenanthrene are trivial retained names, this one is systematically created name (P-25).

All except cyclopenta[a]phenanthrene have the numbering explicitly depicted (in P-25.1.1 or P-25.2.1; anthracene and phenanthrene in errata).

It's natural and expected that there can be different numberings for the same compound for different naming system chosen (e.g. decahydronaphthalene vs bicyclo[4.4.0]decane – fusion carbons C-4a and C-8a vs C-1 and C-6).

So, what is (your qualified opinion on) the correct numbering of cyclopenta[a]phenanthrene? In other words, for example, what is the correct name for this compound:

Fig. - formula

  1. 1H-cyclopenta[a]phenanthrene
  2. 3H-cyclopenta[a]phenanthrene
  3. 17H-cyclopenta[a]phenanthrene

or other one?

(1.) would be correct systematic numbering (if I'm not mistaken; used by current MarvinSketch), (2.) a historic alternative one, (3.) would be for steroid (gonane, alternative name sterane in pre-IUPAC 1951 steroid nomenclature) numbering.

If it's (3.) that would be, in my knowledge, only exception from systematic numbering for systematically created name, however extraordinary claims require extraordinary evidence, which is not seen in the book. And, the book can have yet uncorrected mistakes.

Curiously, as gonane numbering is well defined (Appendix 3), most unambigous name seems to be 1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16-hexadecadehydrogonane (or gona-1,3,5(10),6,8,11,13,15-octaene).


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