Thiophene, chemically known as 1-thia-2,4-cyclopentadiene, is a heterocyclic compound and a thioether. At room temperature, thiophene is a colorless, foul-smelling, tear-producing liquid. Thiophene is naturally present in petroleum at levels up to several percent. Industrially, it is used in the denaturation of ethyl alcohols. Like furan, thiophene is aromatic, only slightly less aromatic than benzene. One of the 2 pairs of lone electrons of the sulfur atom is conjugated to 2 double bonds, forming an off-domain π-bond.
Thiophene was originally discovered as an impurity of benzene. Indigo was mixed with sulfuric acid and crude benzene to form a blue dye, which was early thought to be a reaction of benzene, until the German chemist Victor Meyer isolated thiophene and discovered the existence of this heterocyclic compound. Thiophene occurs naturally in petroleum, coal tar and shale oil, and can be present in petroleum at levels up to several percent.
Thiophene is similar to benzene in that both are aromatic. One of the 2 pairs of lone electrons of its sulfur atom is conjugated to 2 double bonds, forming an off-domain π-bond. Thiophene is only slightly less aromatic than benzene and more stable than furan and pyrrole, due to the longer C-S bond length, which eases the ring tension. The hydrogen at the 2-position of thiophene is also readily substituted with metals, yielding derivatives such as mercury and sodium. The thiophene ring system is stable to oxidizing agents, for example, oxidation of alkyl-substituted thiophene can form thiophene carboxylic acids. Reduction of thiophene with sodium metal in liquid ammonia and methanol solutions yields dihydrothiophene, as well as certain ring-opening compounds.
The types of thiophene can be mainly divided into α-thiophene derivatives (including α-chloromethylthiophene, thiophene-α-acetic acid, etc.), and β-thiophene derivatives (including β-thiophene formaldehyde, β-methylthiophene, etc.).
α-Thiophene derivatives are widely used in the synthesis of pharmaceuticals, pesticides, dyes, chemical reagents, polymer additives, etc. For example, antibiotics with thiophene ring have better efficacy than their phenyl congeners. Some new anti-inflammatory and analgesic drugs, such as sulforaphane, tibulonic acid, thiophanic acid and more than 10 other effective anti-inflammatory and analgesic drugs are all derivatives of thiophene. Thiophene derivatives can also be used to synthesize hundreds of drugs such as the anthelmintic thienopyridine, the anticholinergic cyclohexanemethanol, and the diuretic azothiamides. Generally, thiophene-α-acetic acid is mainly used in the synthesis of more than 20 kinds of antimicrobials such as Pioneerin I, Pioneerin II, Cefoxitin, etc. It is also used in the synthesis of many pharmaceutical products such as cardiovascular drugs, lipid-lowering drugs, anti-ulcer drugs, platelet agglutination inhibitors, cardiovascular diastolic drugs, 5-lipoxygenase inhibitors, etc.
β-thiophene derivatives, like a-thiophene derivatives, are mainly used in drug synthesis and play an important role in the development of new drugs due to their specific activity. In recent years, many new drugs containing β-thiophene derivatives have been marketed as potent drugs with novel structures, such as the latest series of antibiotic drugs, which contain β-thiophene derivatives. For example, Lornoxicam, synthesized from β-chlorothiophene-2-carboxylic acid, is one of the best anti-inflammatory and analgesic drugs in the world today.
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