I am preparing to do an experiment that involves synthesizing dibenzo(h,k)-4-oxatricyclo(,6)undec-8,10- diene-3, 5-dione via a Diels-Alder reaction.

Could someone please explain the nomenclature for this molecule? Specifically, what do "h", "k" and,6 mean?

Below is the reaction: enter image description here

  • 2
    $\begingroup$ I'm not very good with organic chemistry but this got me thinking, and after checking this I think the [h,k] denotes which positions the oxatricyclo~ group is bonded to the "main" ring. Maybe this is clearer: i.imgur.com/Iudam83.png $\endgroup$
    – Molx
    Commented Sep 27, 2015 at 2:48
  • $\begingroup$ @Molx Interesting, but unfortunately, it's not even wrong. $\endgroup$
    – mykhal
    Commented Nov 28, 2018 at 2:23

2 Answers 2


The systematic (but not necessarily preferred) name tricyclo[,6]undecane describes the structure of octahydro-1⁠H-4,7-ethanoindene (see this answer on how to name tricyclo compounds):


Expanding the name from tricyclo[,6]undecane to 4-oxatricyclo[,6]undec-8,10-diene-3,5-dione adds the functional groups, which leads to the structure of 3a,4,7,7a-tetrahydro-4,7-etheno-2-benzofuran-1,3-dione:


According to the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book), isomers of fused polycyclic ring systems are distinguished by lettering each peripheral side of the parent component using the italic letters a, b, c, etc., beginning with a for the side numbered ‘1,2’, b for ‘2,3’, etc.
For example:

azulene    benzo[a]azulene    dibenzo[e,h]azulene

Accordingly, fusion of two benzene rings to the above-mentioned parent structure 4-oxatricyclo[,6]undec-8,10-diene-3,5-dione at the relevant sides yields the name dibenzo[h,k]-4-oxatricyclo[,6]undec-8,10-diene-3,5-dione.
(Note that this name does not necessarily correspond to the preferred IUPAC nomenclature and notation.)


Regarding Loong's correct answer, unsaturated fused polycyclic hydrocarbon azulene atom numbering1 (top left), and peripheral bond (edge) letter “numbering” for fusion nomenclature (top right).

Below is the skeleton of your product (without the two fused benzenes, before the dibenzo[…]… nomenclatoric operation), atom numbering2 (bottom left).

However, letter “numbering” of edges (bonds) is not used for polycyclic saturated hydrocarbon nomenclature, as the names are constructed in a different way3 (also there's no single continuous edge loop defined). But if someone would try to do that similarly for unsaturated polycyclic ring nomenclature, he/she would come with bond letter-ing, including those h,k (bottom right).


It's really strange that this very notable chemical seller uses this weird, wrongly constructed name as (the only) product name (webarchived) (or did you see it elsewhere?); see e.g. PubChem record for alternatives. (By the way, I think that a decent name, although maybe not preferred as well, is 3,4-(9,10-dihydroanthracene-9,10-diyl)oxolane-2,5-dione, maybe yet better with bridging nomenclature using prefixes like benzeno, furano, epipyrano, …: 11,15-dihydro-9,10-[3,4]furanoanthracene-12,14-dione (see e.g. examples in BB2013 P-

  1. Note that the non-peripheral, fusion atom numbers are appended with Latin letters, but these are unrelated to edge (bond) letters.
  2. Note the different numbering for saturated polycyclic hydrocarbons, starting at “fusion” (wrong term in this context), or bridges junction; also no letters added anywhere.
  3. The dibenzo derivative would be named as an unsaturated pentacyclic hydrocarbon, pentacyclo[,7.09,14.015,19]nonadeca‐2(7),3,5,9(14),10,12‐hexaene.

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