Coal-tars isoquinoline

Replacing one carbon atom of naphthalene with an a2omethene linkage creates the isomeric heterocycles 1- and 2-a2anaphthalene. Better known by their trivial names quinoline [91-22-5] (1) and isoquinoline [119-65-3] (2), these compounds have been the subject of extensive investigation since their extraction from coal tar in the nineteenth century. The variety of studies cover fields as diverse as molecular orbital theory and corrosion prevention. There is also a vast patent Hterature. The best assurance of continuing interest is the frequency with which quinoline and isoquinoline stmctures occur in alkaloids (qv) and pharmaceuticals (qv), for example, quinine [130-95-0] and morphine [57-27-2] (see Alkaloids). 

Alkyl Isoquinolines. Coal tar contains small amounts of l-methylisoquinoline [1721-93-3] 3-methylisoquinoline [1125-80-0] and 1,3-dimetliylisoquinoline [1721-94-4J. The 1- and 3-methyl groups are more reactive than others in the isoquinoline nucleus and readily oxidize with selenium dioxide to form the corresponding isoquinoline aldehydes (174). These compounds can also be obtained by the hydrolysis of the dihalomethyl group. The 1- and 3-methyhsoquinolines condense with benzaldehyde in the presence of zinc chloride or acetic anhydride to produce 1- and 3-styryhsoquinolines. Radicals formed by decarboxylation of carboxyUc acids react to produce 1-aIkyhsoquinolines. 

Coal-tar distillates -quinoline from [QUINOLINES AND ISOQUINOLINES] (Vol 20)... 

The simplest member of the quinoline family is quinoline itself. It is a hygroscopic, yellowish oily liquid that is slightly soluble in water, soluble in alcohol, ether, carbon disulfide, and readily soluble in many other organic solvents. It can be obtained through the distillation of coal tar [2]. Quinoline can be prepared from aniline with acrolein under heated sulfuric acid (the Skraup synthesis [3-6], shown in Scheme 1) [ 1]. Isoquinoline was also isolated from coal tar in 1885 [2]. Both of these bases have been known about for a long time. 

Isoquinoline, C9H7N, is present in coal-tar. Its odor is similar to that of quinoline it melts at 21° and boils at 237°. 

Quinoline and isoquinoline are heterocycles in which a benzene ring and a pyridine ring are fused through carbon. The isomeric heterocycles 1- and 2-azanaphthalene, better known by their trivial names quinoline and isoquinoline, have been the subject of extensive studies since their discovery in the extracts of coal tar at the beginning of nineteenth century. Since then their heterocyclic ring systems were found incorporated in several hundreds of natural products and were used as pharmacophore units in dozens of pharmaceuticals especially anti-bacterials, better known under the general name of Quinolones . ... 

A second important quinoline synthesis, from o-aminobenzaldehyde and acetaldehyde, was discovered by P. Friedlaender. Isoquinoline was discovered in coal tar, and its constitution established, by S. Hoogewerff and W. A. van Dorp, by oxidation to phthalic acid and cinchomeronic acid (pyridine-3 4-dicarboxylic acid). It was synthesised by Gabriel (see p. 839). 

Coal tar is a complex mixture consisting almost exclusively of aromatic compounds. The main components of coal tar are naphthalene, phenanthrene, fluoranthene, pyrene, acenaphthene, anthracene, the heterocyclics carbazole, quinoline and isoquinoline, phenol and benzofuran-derivatives, as well as sulfur compounds such as thianaphthene. Olefinic compounds are also present in coal tar (Table 3.1). The total number of constituents is estimated at 10,000. 

Coal tar also contains 0.2% isoquinoline, which is recovered in the same manner as quinoline. 

Quinoline and isoquinoline in the quinoline fractions of coal tar are determined by treating the sample with phthalic anhydride to remove quinaldine as quinophthalone/ )... 

Isoquinoline, mp 26 °C, bp 243 °C, is a colorless substance with a pleasant smeU. Isoquinoline occurs in coal-tar and in bone-oil for its basicity see p. 405. 

We can imagine the fusion of a benzene ring to pyridine in either of two ways, giving us quinoline and isoquinoline (1- and 2-azanaphthalene, according to our systematic nomenclature). Both are liquids with high boiling points. Many of their derivatives are found in nature or have been synthesized in the search for physiological activity. Like pyridine, quinoline and isoquinoline are readily available from coal tar. 

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