heterocyclic

Examples of heterocyclic in the following topics:

• Nomenclature

  • Consistent use of "etine" and "oline" as a suffix for 4 & 5-membered unsaturated heterocycles is prevented by their former use for similar sized nitrogen heterocycles.
  • Polycyclic compounds incorporating one or more heterocyclic rings are well known.
  • The two quinolines illustrate another nuance of heterocyclic nomenclature.
  • Heterocyclic rings are found in many naturally occurring compounds.
  • In the diagram below, some other examples of heterocyclic natural products will be displayed.

• Some Polycyclic Heterocycles

  • Heterocyclic structures are found in many natural products.
  • Derivatives of the simple fused ring heterocycle purine constitute an especially important and abundant family of natural products.
  • Sulfur heterocycles are found in nature, but to a lesser degree than their nitrogen and oxygen analogs.
  • Two members of the B-vitamin complex, biotin and thiamine, incorporate such heterocyclic moieties.
  • These are shown together with other heterocyclic B-vitamins in the following diagram.

• Five-Membered Rings

  • Many other procedures leading to substituted heterocycles of this kind have been devised.
  • Indole is probably the most important fused ring heterocycle in this class.
  • Other possible reactions are suggested by the structural features of these heterocycles.
  • The most important condensed ring system related to these heterocycles is indole.
  • Whether the indole nitrogen is substituted or not, the favored site of attack is C-3 of the heterocyclic ring.

• Four-Membered Rings

  • Several methods of preparing four-membered heterocyclic compounds are shown in the following diagram.
  • Reactions of four-membered heterocycles also show the influence of ring strain.

• Nomenclature and Structure of Amines

  • Many aromatic and heterocyclic amines are known by unique common names, the origins of which are often unknown to the chemists that use them frequently.
  • There is a systematic nomenclature of heterocyclic compounds, but it will not be discussed here.
  • The chloro aziridine can invert, but requires a higher activation energy to do so, compared with larger heterocyclic amines.
  • The rough estimate of angle strain is made using a C-N-C angle of 60º as an arbitrary value for the three-membered heterocycle.

• Biosynthesis of Tetrapyrroles

  • Porphyrins are heterocyclic macrocycles composed of four modified pyrrole subunits interconnected at their α carbon atoms via methine bridges (=CH-).
  • Several other heterocycles are related to porphyrins.

• Three-Membered Rings

  • Oxiranes (epoxides) are the most commonly encountered three-membered heterocycles.
  • The second diagram above shows four additional examples of three-membered heterocycle reactivity or intermediacy.

• Six-Membered Rings

  • The chemical reactivity of the saturated members of this class of heterocycles: tetrahydropyran, thiane and piperidine, resemble that of acyclic ethers, sulfides, and 2º-amines, and will not be described here. 1,3-Dioxanes and dithianes are cyclic acetals and thioacetals.
  • Fully unsaturated six-membered nitrogen heterocycles, such as pyridine, pyrazine, pyrimidine and pyridazine, have stable aromatic rings.
  • The fused ring heterocycles quinoline and isoquinoline provide additional evidence for the stability of the pyridine ring.
  • By contrast, the heterocyclic ring in both compounds undergoes preferential catalytic hydrogenation to yield tetrahydroproducts.

• Other Aromatic Compounds

  • The aromatic heterocycle pyridine is similar to benzene, and is often used as a weak base for scavenging protons.
  • Furan and pyrrole have heterocyclic five-membered rings, in which the heteroatom has at least one pair of non-bonding valence shell electrons.
  • The second and third compounds are heterocycles having aromatic properties.
  • The last compound is imidazole, a heterocycle having two nitrogen atoms.

• A Useful Mnemonic for Regioselectivity

  • A novel group of [4s + 2s] cycloaddition reactions leading to five-membered heterocyclic adducts are classified as dipolar cycloadditions.