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Can someone explain the nomenclature of 7-amino-4-methylcoumarin, pictured below (from the Sigma-Aldrich website)?
The alpha-carbons of the methyl- and amino-groups here are separated by a minimum of three C's, whereas the numbers 4 and 7 are separated by two digits. What am I missing?
A quick perusal of the internet reveals that there are at least four numbering schemes used for coumarin derivatives. In each case, elision, duplication or out-of-sequence numbering creates a gap that accounts for the odd numbering of substituents.
Some numbering schemes do not concern themselves with the carbons shared between the rings, as seen here (Fig. 1). This is common in PAH numbering schemes and is recommended by IUPAC1.
As seen here, 4 is followed by 4a and 8 is followed by 8a.
Alternatively, the a modifier can be replaced with a prime, as seen here (Fig. 2).
This coumarin-like structure denotes the fused ring carbons 9 and 10.
(1) International Union of Pure and Applied Chemistry; NOMENCLATURE OF FUSED AND BRIDGED FUSED RING SYSTEMS; Pure and Applied Chemistry; 1998, 70, pp. 143-216
This is one of the tricky parts in the nomenclature of fused cyclic systems. Excerpt from page 180 of the 1998 IUPAC Nomenclature of fused and bridged fused ring systems (I have added bold to the piece of importance to answer your question):
The numbering of the peripheral atoms in a particular orientation starts from the uppermost ring. If there is more than one uppermost ring the ring furthest to the right is chosen. Numbering starts from the non-fusion atom most counterclockwise in the ring selected, and numbering proceeds in a clockwise direction around the system, including fusion heteroatoms. Fusion carbon atoms are not included; each fusion carbon atom is given the same number as the immediately preceding position modified by a roman letter a, b, c, etc. If heteroatoms are identified by replacement nomenclature all atoms retain the numbers of the parent hydrocarbon.
If one considers coumarin, this is not the IUPAC nomenclature but the numbering rule remains the same. Below is an example of the numbering scheme for coumarin which summarizes why the substituents are numbered by this manner on coumarin.
