Macrocycles are often described as molecules and ions containing a ring of twelve or more atoms. Classical examples include the crown ethers, calixarenes,

porphyrin Porphyrins ( ) are a group of heterocyclic macrocycle organic compounds, composed of four modified pyrrole subunits interconnected at their α carbon atoms via methine bridges (=CH−). The parent of porphyrin is porphine, a rare chemical com ...

s, and cyclodextrins. Macrocycles describe a large, mature area of chemistry.

Synthesis

The formation of macrocycles by ring-closure is called macrocylization. Pioneering work was reported for studies on terpenoid macrocycles. The central challenge to macrocyclization is that ring-closing reactions do not favor the formation of large rings. Instead, small rings or polymers tend to form. This kinetic problem can be addressed by using high-dilution reactions, whereby intramolecular processes are favored relative to polymerizations. Some macrocyclizations are favored using template reactions. Templates are ions, molecules, surfaces etc. that bind and pre-organize compounds, guiding them toward formation of a particular ring size. The crown ethers are often generated in the presence of an alkali metal cation, which organizes the condensing components by complexation. An illustrative macrocyclization is the synthesis of (−)-

muscone Muscone is an organic compound that is the primary contributor to the odor of musk. The chemical structure of muscone was first elucidated by Leopold Ružička. It consists of a 15-membered ring ketone with one methyl substituent in the 3-positi ...

from (+)-

citronellal Citronellal or rhodinal ( C10 H18 O) is a mono terpenoid aldehyde, the main component in the mixture of terpenoid chemical compounds that give citronella oil its distinctive lemon scent. Citronellal is a main isolate in distilled oils from the ...

. The 15-membered ring is generated by ring-closing metathesis. : Uroporphyrinogen III skeletal

Occurrence and applications

One important application are the many macrocyclic antibiotics, the

macrolide The Macrolides are a class of natural products that consist of a large macrocyclic lactone ring to which one or more deoxy sugars, usually cladinose and desosamine, may be attached. The lactone rings are usually 14-, 15-, or 16-membered. Ma ...

s, e.g.

clarithromycin Clarithromycin, sold under the brand name Biaxin among others, is an antibiotic used to treat various bacterial infections. This includes strep throat, pneumonia, skin infections, '' H. pylori'' infection, and Lyme disease, among others. Clarith ...

. Many metallocofactors are bound to macrocyclic ligands, which include

s, corrins, and chlorins. These rings arise from multistep biosynthetic processes that also feature macrocycles. Macrocycles often bind ions and facilitate

ion transport In biology, a transporter is a transmembrane protein that moves ions (or other small molecules) across a biological membrane to accomplish many different biological functions including, cellular communication, maintaining homeostasis, energy produc ...

across hydrophobic membranes and solvents. The macrocycle envelops the ion with a hydrophobic sheath, which facilitates

phase transfer In chemistry, a phase-transfer catalyst or PTC is a catalyst that facilitates the transition of a reactant from one phase into another phase where reaction occurs. Phase-transfer catalysis is a special form of heterogeneous catalysis. Ionic r ...

properties. Macrocycles are often bioactive and could be useful for drug delivery.

Subdivisions

* Cryptand * Rotaxane * Catenane * Molecular knot * Effective molarity * Macrocyclic stereocontrol

Synthesis

Occurrence and applications

Subdivisions

See also

References

Further reading