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The 2- and 4-quinolones are a class of bicyclic molecules, organic chemical structures that are related to the heteroaromatic coal tar isolate quinoline, see image at right. Specific quinoline molecules substituted with an hydroxyl functional group at carbons 2 and 4 (C-2 and C-4, see image) are most often observed in isomeric forms termed 2- and 4-quinolones, respectively (tautomers, relationships explained further below). The relative importance of 4-quinolones has increased with the discovery that such structures that also bear a carboxylic acid (-COOH) and other functional groups at particular sites on the ring have very potent bacteriocidal activities, inhibiting of a broad spectrum of Gram negative and Gram positive DNA gyrase and topoisomerase enzymes. Hence, they are very useful in antibacterial therapy. An example of such a 4-quinolone is "cipro" (ciprofloxacin, image at right), where the atoms of quinoline can be traced within this related structure. Ciprofloxacin is a "second-generation" fluoroquinolone antibacterial (see below), introduced by Bayer AG and still in wide use as the second decade of the new millennium begins. ==Relationship, hydroxyquinolines and quinolones== In the chemical synthesis of specific quinolines, the nitrogen-containing ring is often "closed" as a part of the synthetic process, and in some syntheses, doing so can install a hydroxyl group (an –OH functional group) on carbons adjacent to or across from the ring nitrogen (i.e., the C-2 and the C-4 positions, see quinoline structure above). An example of such a synthesis is the Camps cyclization, which, depending on exact starting materials and reaction conditions, can give both 2-hydroxyquinolines (B) and 4-hydroxyquinolines (A) as shown. thumb . Such hydroxy-substituted quinolines are, most often, not distinguished from a particular isomeric form termed a tautomer (see Prof. Reusch's further definition in the Notes).〔Per Prof. Reusch (Michigan State), tautomers are "rapidly interconverted constitutional isomers, usually distinguished by a different bonding location for a labile hydrogen atom ... and a differently located double bond. See www2.chemistry.msu.edu:80/faculty/reusch/VirtTxtJml/react2.htm . Retrieved 13 April 2011.〕 In this particular rapid isomerization, the labile proton (H+) migrates from the hydroxyl group to the ring nitrogen, and the double bond (pi-electron density) migrates from within the ring to the carbon-oxygen bond to form a carbonyl group. These tautomerizations are represented in the images at right by the same process in closely related structures. (The tautomerizations also take place when the adjacent all-carbon ring is absent.) When represented as the carbonyl-containing tautomer—often the most stable form, so the one observed in structure measurements such as room temperature NMR—the pair of hydroxy-substitituted quinolines shown above are formally referred to as 2-quinolones (oxygen adjacent to the ring N) or 4-quinolones (oxygen across from the ring N), as the case may be. Hence, ciprofloxacin (shown above) is 4-quinolone. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「2- and 4-Quinolones」の詳細全文を読む スポンサード リンク
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