1.4. Heterocyclic compounds
When the ring system in an organic compounds contains at least one other element (e.g. N, O, S, B, Si) other than carbon, it is classified as heterocyclic [52]. At least one heterocyclic ring component is found in about 50% of known organic compounds.
The most common heterocyclic systems contain heteroatoms N, O or S and good examples of these are six-membered heterocycle pyridine and five-membered heterocycles pyrrole, furan and thiophene [52]. Pyridine is one of the most common and best known heterocycles. It is an excellent polar solvent and act as a donor ligand in metal complexes. In a nicotinamide, pyridine is a structural part of an important coenzyme, and the tobacco alkaloid nicotine is perhaps the best known naturally occurring pyridine derivative. Pyrrole occurs widely in nature and is a structural part of porphyrin haeme, chlorophyll and vitamin B12. Furan is found in coal tar, and some terpenoids such as rose oil, contain the furan ring in their chemical structure. The reduction of furan leads to the heterocyclic ether tetrahydrofuran (THF), which is widely used as an organic solvent. Thiophene, which is a sulfur containing heterocycle, occurs in nature in some plant products but its greater importance is as an ingredient in synthetic pharmaceuticals and dyestuffs.
N−Heterocycles were involved at the very beginning of life in the genesis of DNA and play an essential role in many living systems. The nucleic acid bases adenine (A), guanine (G), cytosine (C) and thymine (T) are derivatives of the aromatic N−heterocycles pyrimidine and purine [53,54]. The base-pairing of DNA by H-bonding is illustrated in Fig. 5. Proteins, which are linear chains of α-amino acids are other important biological macromolecules. One of the essential amino acids, histidine, contains a heterocyclic imidazole ring in its chemical structure (Fig. 5). Also histamine, which is a decarboxylation product of histidine has an important role in living systems, e.g. as a contracting agent of smooth muscles and as a substance involved in allergic reactions. The other important amino acid tryptophan (Fig. 5) and many naturally occuring alkaloids are indole derived structures [53]. In tryptophane the heteroaromatic indole residue has appeared preference for cation−π and π −π interactions [8,14].
Most of the important heterocycles mentioned here are aromatic [52,53]. According to the Hückel rule a monocyclic system containing 4n+2 π -electrons (n=0,1,2,… ) is classified as aromatic. Figure 6 shows the orbital structure of the five-membered aromatic system imidazole. Other criteria of aromaticity are bond lengths, ring current effects and chemical shifts in 1H NMR spectra [52]. All the heterocycles selected for study in this work are aromatic 6π -electron systems.
