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)

Let’s have a look at cycloartane (a) and lanostane (b). The only structural difference between these two structures is the bond between carbon atoms 9 and 19 in (a), and this bond creates one extra ring (in addition to the existing four) compared to (b). More extra rings created by this method will be indicated by ‘dicyclo’, ‘tricyclo’ and so on.

The inverse operation, viz. ring cleavage, is indicated by the prefix ‘seco’ (from the Latin word secāre, “to cut” — cf. section, sector, segment, etc.) Consider testolic acid (c), a metabolite of androstenedione (d):

(c)	(d)

testolic acid (trivial) 13-hydroxy-3-oxo-13,17-secoandrost-4-en-17-oic acid (substitutive + ‘seco’) androstenedione (trivial) androst-4-ene-3,17-dione (substitutive )

In its systematic name, 13-hydroxy-3-oxo-13,17-secoandrost-4-en-17-oic acid, the ‘13,17-seco’ bit indicates the bond scission between carbons 13 and 17. Note that there is also a decrease in the number of rings from four to three.

(e)	(f)

icajine (trivial) 19-methyl-16,19-secostrychnidine-10,16-dione (substitutive + ‘seco’) strychnidine (trivial, fundamental parent)

As you may have guessed, the ‘cyclo’ and ‘seco’ operations are respectively accompanied by the removal or addition of the implicit hydrogens. The bonds created or broken by these operations may involve heteroatoms as well as carbons. For example, in the systematic name of icajine (e), 19-methyl-16,19-secostrychnidine-10,16-dione, the ‘16,19-seco’ indicates the bond cleavage between carbon-16 and nitrogen-19 of strychnidine (f).

To sum up:

• ‘Cyclo’, ‘seco’, ‘homo’, ‘nor’ and skeletal replacement naming methods are parts of skeletal modification nomenclature;
• Both ‘nor’ and ‘seco’ operations also could be considered subtractive;
• Both ‘cyclo’ and ‘seco’ operations affect the number of rings;
• ‘Cyclo’, ‘seco’ and ‘nor’ operations may involve heteroatoms as well as carbon atoms;
• Both ‘homo’ and ‘seco’ operations imply addition of the appropriate number of hydrogen atoms, while ‘nor’ and ‘cyclo’ imply removal of the appropriate number of hydrogens.
• The names generated with these operations, based on trivial names of fundamental parents, are much shorter than it would otherwise be possible.

References

1. 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-4.3. Bond formation.
2. Ibid., Rule RF-4.4. Bond cleavage.