Isoprene rule

Head and tail position for isoprene
Isoprene

Isoprene contains 5 carbon atoms
Isoprene

Monoterpenes contain 10 carbon atoms
Myrcene , the red wavy line, marks the junction between the two isoprene units
Limonene , the red marked serpentine lines mark the connection points between the two isoprene units

Sesquiterpenes contain 15 carbon atoms
Candines, the wavy lines marked in red , mark the connection points between the three isoprene units

The German chemist Otto Wallach first recognized the isoprene rule . It was first formulated by the Croatian chemist Leopold Ružička ; it is an empirical rule in natural product chemistry . Accordingly, many natural substances are oligomers or polymers of isoprene (2-methyl-1,3-butadiene, empirical formula: C ₅ H ₈ ).

Monoterpenes (C ₁₀ ) made up of two isoprene units - each with five carbon atoms (C ₅ ) - and sesquiterpenes (C ₁₅ ) made up of three isoprene units are simple examples of such natural products. Diterpenes (C ₂₀ ) composed of four isoprene units and carotenoids (C ₄₀ ) composed of eight isoprene units are more complex examples. Dolichol (C ₁₀₀ ) therefore contains 20 isoprene units. Rubber is a polyterpene with the empirical formula (C ₅ H ₈ ) _n with n = 8000-30000 and can be pyrolytically degraded to isoprene.

The discovery of natural substances that do not obey the isoprene rule - such as lanosterol - led Leopold Ružička to reformulate the biogenetic isoprene rule in 1953 . Accordingly, the apparent exceptions to the isoprene rule are based on subsequent reactions ( isomerizations , alkylations , dealkylations). It was also recognized that the oligomerization of isoprene can also be carried out via the head-to-head or tail-to-tail link, in addition to the usual head-to-tail link.

Chemical-historical importance

From a chemical-historical point of view, the isoprene rule is of considerable importance, since the determination of the constitution of numerous natural substances was only possible in this way.

Individual evidence

1. ^ Albert Gossauer: Structure and reactivity of biomolecules , Verlag Helvetica Chimica Acta, Zurich, 2006, pp. 124-133, ISBN 978-3-906390-29-1 .
2. ↑ ^{a b} Otto-Albrecht Neumüller (Ed.): Römpps Chemie-Lexikon. Volume 3: H-L. 8th revised and expanded edition. Franckh'sche Verlagshandlung, Stuttgart 1983, ISBN 3-440-04513-7 , p. 1966.
3. ^ Hans Beyer and Wolfgang Walter : Organische Chemie , S. Hirzel Verlag, Stuttgart, 1984, p. 627, ISBN 3-7776-0406-2 .
4. ↑ Otto-Albrecht Neumüller (Ed.): Römpps Chemie-Lexikon. Volume 3: H-L. 8th revised and expanded edition. Franckh'sche Verlagshandlung, Stuttgart 1983, ISBN 3-440-04513-7 , pp. 2064-2069.
5. ↑ Leopold Ruzika: The role of fragrances in my chemical life's work , Helv. Chim. Acta 54 (1971) 1753-1759.
6. ↑ JW Cornforth : Olefin alkylation in biosynthesis , Angew. Chem. 80 ( 1968 ) pp. 977-985, DOI: 10.1002 / anie.19680802302 .