Tropones, Tropolones, and Tropylium Salts with Fused Heterocyclic Rings : Part 2: Structure, Reactivity, and Application

Abstract

The chapter discusses the structure, reactivity, and application of tropones, tropolones, and tropylium. X-ray diffraction, molecular spectra, aromaticity and stability, tautomerism, and polarographic reduction of tropones, tropolones, and tropylium are described in the chapter. The recognition, isolation, and characterization of troponoid systems have been facilitated by certain chemical properties that usually occur in both monocyclic and fused derivatives. Most tropolones give sparingly soluble, yellow or orange sodium salts, green cupric chelates, and colored ferric complexes. Valence isomerization and dimerization are common reactions in the photochemistry of troponoids. The acidity of parent tropolone lies between those of acetic acid and phenol; benzotropolones are only as acidic as phenol. Parent tropone forms very stable salts with acids; its basicity has its origin in the high stability of the hydroxytropylium ion formed by protonation. Chelate-complexes of tropolones are discussed in the chapter. Parent tropone on halogenation, affords addition products and by subsequent dehydrohalogenation affords halotropones substituted at 2- or 2,7-positions. Cyclocondensations onto the seven-membered ring are illustrated in the chapter by the reactions of pyridotropolones. Monocyclic tropylium salts on oxidation usually undergo ring contraction, benzotropylium salts give benzotropones. The troponoid system is included in many natural products, including alkaloids and antibiotics. Among heterocyclic derivatives, pyrrolotropones, tricyclic thieno- or pyrrolo-benzotropones, and other compounds have been claimed in patent applications. The pharmaceutical and photographic uses of tropones, tropolones and tropylium are discussed in the chapter.