Cyclo and seco

The prefix ‘cyclo’ is used in chemical names to indicate a ring structure. In additive nomenclature, this prefix is usually italicised and followed by a hyphen, as we have seen for polynuclear entities such as cyclo-tri-μ-oxido-tris(dioxidotungsten). On the other hand, in substitutive nomenclature ‘cyclo’ is not italicised, there is no hyphen, and no other prefixes could be inserted between ‘cyclo’ and the root, as in cyclopropane.

Somewhat confusingly, this prefix is also employed in skeletal modification nomenclature when an additional ring is created [1]. In the names generated thus, ‘cyclo’ has to be preceded by the locants of the skeletal atoms that form a new bond.

(a)	(b)

cycloartane (trivial) 9β,19-cyclolanostane (‘cyclo’) lanostane (trivial, fundamental parent)

Homonames

Both skeletal replacement and ‘nor’-type subtractive naming methods can be considered subtypes of skeletal modification nomenclature. And there are more.

Consider homocysteine (a):

(a)	(b)

homocysteine (trivial + ‘homo’ addition) 2-amino-4-sulfanylbutanoic acid (substitutive) cysteine (trivial) 2-amino-3-sulfanylpropanoic acid (substitutive)

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’.