Heterocyclic compounds are a class of compounds that consist of carbon atoms and non-carbon atoms altogether forming a cyclic skeletal structure. These non-carbon atoms are collectively referred to as a heteroatom. The common heteroatoms include nitrogen, oxygen, and sulfur. Lactones, lactams, cyclic ethers that we’ve already learned are heterocyclic compounds. With similar properties of congeneric open-chain compounds, they will be discussed in corresponding chapters. This chapter will mainly discuss heterocyclic compounds with relatively stable ring system and with a certain degree of aromaticity, also known as aromatic heterocyclic compound. There are many kinds and a large number of heterocyclic compounds widely distributed in nature. Many natural heterocyclic compounds in animals and plants play an important physiological role in the body. For example, heterocyclic structure exists in chlorophyll in plants, hemoglobin in animal blood, alkaloids and glycosides as the active ingredient in herbs, some antibiotics and vitamins, certain composition of amino acids in protein and bases in nucleotide. About half of the existing medicines have heterocyclic structures. Thus, heterocyclic compounds play an important role in organic compounds especially in organic medicines. The most common and also the most important heterocyclic compounds are that with five-membered or six-membered heterocyclic ring and their fused heterocyclic compounds. Therefore, according to the size of the ring, heterocyclic compounds can be divided into five-membered compounds and six-membered heterocyclic compounds. E.g., furan, thiophene, pyrrole, imidazole, triazole and their fused heterocyclic compounds, indole, benzimidazole and diphenylene-oxide are all five-membered heterocyclic compounds, while pyridine, pyrimidine and their fused heterocyclic compounds, quinoline and quinazoline are all six-membered heterocyclic compounds. According to the number of hetero atoms, five-membered compounds and six-membered heterocyclic compounds can be divided into one-hetero-atom-containing heterocyclic compounds and two-hetero-atom-containing and several-hetero-atom-containing heterocyclic compounds. E.g., furan, thiophene, pyridine, indole, quinoline, dibenzofuran, etc. are one-hetero-atom-containing heterocyclic compounds; imidazole, pyrimidine, benzimidazole, quinazoline, etc. are two-hetero-atom-containing heterocyclic compounds; purine, comprising more than two hetero atoms, is a several-hetero-atom-containing heterocyclic compound. According to the form of the ring in the molecule, heterocyclic compounds can be divided into single-heterocyclic compounds and fused heterocyclic compounds. E.g., furan, pyridine, pyrimidine, imidazole, etc. are single-heterocyclic compounds and indole, quinoline, dibenzofuran, purines, etc. are fused heterocyclic compounds.