Nornames

A variant of subtractive nomenclature employs the prefix ‘nor’. Probably the most famous example is the neurotransmitter noradrenaline aka norepinephrine (a):

(a)	(b)

noradrenaline (trivial + subtractive) norepinephrine (trivial + subtractive) 4-[(1R)-2-amino-1-hydroxyethyl]benzene-1,2-diol adrenaline (trivial) epinephrine (trivial) 4-[(1R)-1-hydroxy-2-(methylamino)ethyl]benzene-1,2-diol

These names are derived from the trivial names of (b), adrenaline or epinephrine. In this particular example, the meaning of ‘nor’ is the same as ‘demethyl’.

‘Nor’ is an abbreviation of the word “normal”, first used in 1868 in “normal opianic acid” (6-formyl-2,3-dihydroxybenzoic acid) to indicate the completely demethylated form of opianic acid (6-formyl-2,3-dimethoxybenzoic acid) [1]. However, the modern IUPAC-recommended use of ‘nor’ is confined to the removal of one unsubstituted skeletal atom — any skeletal atom, not necessarily carbon — along with the implicit hydrogens from a parent structure; the position of the removed atom is denoted by its locant [2]. Since the methyl group carbon is not a part of the skeleton in (b), the names “noradrenaline” or “norepinephrine” shouldn’t really be recommended, but here you are.

(c)	(d)

(1S)-1,23-dihydroxy-24,25,26,27-tetranorcalciol (trivial + substitutive + subtractive) (1S,3R,5Z,7E)-24-nor-9,10-secochola-5,7,10(19)-triene-1,3,23-triol (substitutive) calciol (trivial) (3S,5Z,7E)-9,10-secocholesta-5,7,10(19)-trien-3-ol (substitutive)

If several skeletal atoms are lost, the corresponding multiplicative prefixes di, tri, tetra etc. are used together with ‘nor’. For instance, the structure (c) could be named as a derivative of calciol (d) which has lost four carbon atoms, hence the ‘tetranor’ bit.

(e)	(f)

7-norcholest-4-en-3-one (substitutive + subtractive) cholest-4-en-3-one (substitutive)

The prefix ‘nor’ is also used to indicate elimination of a carbon from a ring with subsequent ring contraction. For example, ‘7-nor’ in 7-norcholest-4-en-3-one (e) indicates the loss of carbon atom at the position 7 of cholestane skeleton, which implies the formation of a new bond, viz. the one between carbon-6 and carbon-8.

The ‘de’-type and ‘nor’-type subtractive naming systems are usually considered together, although their nature is quite different. Remember that the organic chemist’s idea of a molecule is a skeleton adorned by substituents? If ‘de’-type method, like substitutive nomenclature, deals with substituents, the ‘nor’-type method could be viewed as a variant of skeletal replacement nomenclature where a skeletal atom is replaced with a “zero” atom.

References

1. Gaddum, J.H. (1956) The prefix ‘nor’ in chemical nomenclature. Nature 177, 1046.
2. Giles, Jr., P.M. (1999) Revised Section F: Natural products and related compounds (IUPAC recommendations 1999). Pure and Applied Chemistry 71, 587—643. Rule RF-4.1. Removal of skeletal atoms without affecting the number of rings.