Fused ring names

(a)

furan (trivial, retained) oxole (Hantzsch-Widman)

Knowing that the structure (a) is called furan, let’s name the structure (b).

(b)

2-nitrofuran (substitutive)

Easy: 2-nitrofuran.

Keeping that in mind, what kind of structure do you think corresponds to the name ‘2-benzofuran’?

(c)

2-benzofuran (fused ring) benzo[c]furan (fused ring) isobenzofuran (trivial)

Not so easy now, eh? Although the names ‘2-nitrofuran’ and ‘2-benzofuran’ look similar, they are built according to very different principles. While the former is a simple substitutive name — meaning that one hydrogen atom of furan (a) is substituted at the position 2 by a nitro group — the latter is a fused ring name. In 2-benzofuran (c), the benzene ring is not a substituent; rather, it is fused to the furan ring. According to the IUPAC recommendations [1, FR-1.1.1], the two rings are called fused (or ortho-fused) when they have two atoms and one bond in common. A fused ring name is built of a “parent component”, for instance ‘furan’, and a “fusion prefix” (in fact a combining form), e.g. ‘benzo’.

What about the ‘2’ in ‘2-benzofuran’? IUPAC refer to this number as a locant [1, FR-2.2.8]:

The locants quoted correspond to the numbering of the bicyclic structure, which follows FR-5.4.

Have a look at the numbering of (c) according to the aforementioned rule:

(c)

What we see is that the oxygen atom in 2-benzofuran is found at the position 2, as opposed to the oxygen atom in 1-benzofuran which is found at the position 1:

(d)

1-benzofuran (fused ring) benzo[b]furan (fused ring)

As you can see, the use of locant numbers here is quite unlike to that of substitutive nomenclature. ‘2’ in ‘2-benzofuran’ does not tell us to which atoms of furan ring the benzene ring is fused. It tells us indirectly that the rings should be fused in such a way that furan’s oxygen atom would find itself at the position 2.

An alternative method is to use italic letters a, b, c, etc. for the sides of the parent ring [1, FR-4.1]. Thus, the side (1,2) is a; the side (2,3) is b, etc.^* For naming purposes, these “letter locants” imply directionality. In case of furan, the side a (1,2) is not equivalent to the side e (5,1) and the side b (2,3) is not equivalent to the side d (4,5):

(a)

Thus the structures (c) and (d) can be called benzo[c]furan and benzo[b]furan^†, respectively. The bit in square brackets is referred to as “fusion descriptor” [2].

In fact this latter naming method is preferred by IUPAC for the structures like (e) and (f):

(e)	(f)

5H-dibenzo[b,f]azepine (Hantzsch-Widman + fused ring) dibenzo[b,k][1,4,7,10,13,16]hexaoxacyclooctadecane‡ (replacement + fused ring) dibenzo-18-crown-6 (Pedersen)

For more complex fused ring systems, combinations of numerical and letter locants are employed. Consider the structure (g).

(g)	(h)	(i)

1H-imidazo[4,5-c]quinoline (fused ring) 1H-imidazole (trivial, parent hydride) quinoline (trivial, parent hydride)

It could be described as a fusion of 1H-imidazole (h) and quinoline (i). More specifically, the atoms 4 and 5 of 1H-imidazole are fused with the atoms 3 and 4 (aka side c) of quinoline, respectively. The resulting name is 1H-imidazo[4,5-c]quinoline, where ‘[4,5-c]’ is a fusion descriptor.

You might have noticed that I am not a big fan of locants. In my view, the fewer locants the name has the better. And fused ring names provide excellent examples to illustrate my point.

(j)

psoralen (trivial) 7H-furo[3,2-g]chromen-7-one (fused ring) 7H-furo[3,2-g][1]benzopyran-7-one‡ (fused ring)

To name (j) systematically, we first take furan (a) and fuse it with 2H-chromene (k):

(k)

2H-chromene (trivial, parent hydride) 2H-1-benzopyran (fused ring)

More specifically, the atoms 3 and 2 of furan are fused with the atoms 6 and 7 (aka side g) of chromene, respectively. [Alternatively, we can use the systematic name 2H-1-benzopyran instead of chromene; here, the locant ‘1’ refers to the numbering of the oxygen atom, in the same fashion we’ve seen in 1-benzofuran (d).] However, once it is done, the whole skeleton gets renumbered according to the rule FR-5.4. And so, the chromene position 2 becomes the new position 7, and the oxo group is added to this new position, hence ‘7H-furo[3,2-g]chromen-7-one’ (or ‘7H-furo[3,2-g][1]benzopyran-7-one’^‡).

(j)

Names constructed thus contain locants meaning disparate things. But we shouldn’t let this relatively minor annoyance obscure the big picture. Stripped of locants, the systematic name ‘furochromenone’ or, even more informatively, ‘furobenzopyranone’, already gives us much better idea of the structure (j) than its trivial name, ‘psoralen’.

*	What happens if there are more than 26 sides? Then the additional sides are marked as a1, b1, c1 and so on, then a2, b2, c2, etc. [1, FR-4.1]. No, it’s not elegant; hopefully, I’ll never use letter locants like these.
†	The lower locants are preferred, so benzo[b]furan is preferred to benzo[d]furan.
‡	The IUPAC recommends to use square brackets around the locants thus [1, FR-4.8]: When a component requires the citation of locants (e.g. in a Hantzsch-Widman name) these are always quoted in square brackets. Note that these numbers in brackets only refer to the numbering of the component and have no significance in the final numbering of the complete fused ring system. Why the locants which are “free” in parent component names (such as 1,4,7,10,13,16-hexaoxacyclooctadecane) should be put in square brackets in fused ring names (e.g. dibenzo[b,k][1,4,7,10,13,16]hexaoxacyclooctadecane) is not exactly clear.

References

1. Moss, G.P. (1998) Nomenclature of fused and bridged fused ring systems (IUPAC Recommendations 1998). Pure and Applied Chemistry 70, 143—216.
2. Hellwich, K.-H., Hartshorn, R.M., Yerin, A., Damhus, T. and Hutton, A.T. (2020). Brief guide to the nomenclature of organic chemistry (IUPAC Technical Report). Pure and Applied Chemistry 92, 527—539.