Introduction to Heterocyclic Compounds

Introduction to Heterocyclic Compounds

• Heterocyclic compounds are cyclic compounds containing at least two different elements in their ring structure. Carbon and another element (nitrogen, oxygen, or sulfur) frequently form these rings.
• Many biologically significant molecules (amino acids, vitamins, hormones) contain heterocyclic rings.

Nomenclature Principles

• Basic nomenclature:

  • The parent heterocyclic ring name is determined by ring size and the heteroatom.
  • Common prefixes: Azetidine (4-membered ring with nitrogen), Pyrrolidine (5-membered ring with nitrogen), Piperidine (6-membered ring with nitrogen), Pyrrole (5-membered ring with nitrogen), Furan (5-membered ring with oxygen), Thiophene (5-membered ring with sulfur).

• Numbering:

  • Ring atoms are numbered to give the heteroatom the lowest possible number.
  • Numbering starts at the point of highest priority substitution.
  • Substituents are named and numbered sequentially.

• Specific examples:

  • Pyridine: A 6-membered aromatic heterocycle with nitrogen. Substituents are named and numbered.
  • Pyrimidine: A 6-membered aromatic heterocycle with two nitrogens. Substituents are named and numbered.
  • Purine: A fused double ring with two nitrogens; a crucial component of DNA. Substitution numbering is essential.

Common Heterocyclic Rings

• Pyrrole: A five-membered aromatic heterocycle with one nitrogen atom. Its high aromaticity contributes to stability.
• Pyridine: A six-membered aromatic heterocycle with one nitrogen atom. Structurally related to benzene.
• Furan: A five-membered aromatic heterocycle with one oxygen atom. Oxygen's presence influences physical and chemical properties.
• Thiophene: A five-membered aromatic heterocycle with one sulfur atom. Sulfur alters reactivity compared to carbon-only rings.

Naming Complex Heterocycles

• Fused Rings: Two or more rings sharing common atoms form a fused ring system. Nomenclature reflects this.
• Naming fused rings: Numbering logically incorporates ring and substituent numbering.

Other Key Considerations

• Aromatic vs. Non-aromatic: Pi electron conjugation affects aromaticity and thus properties.
• Reactivity: The heteroatom's nature significantly impacts compound reactivity. Examples include pyridine nitrogen's electron acceptance and furan oxygen's nucleophilicity.
• Biological significance: Heterocyclic rings are present in many biologically active molecules; understanding their structures and properties is vital in drug design and biochemistry.