Benzopyrylium salts Flavylium

Phenyl-l-benzopyrylium salts (flavylium salts) are important natural products (WillstAtter 1913). Numerous red, blue and violet pigments of blossoms and leaves are derived from the 3,5,7-trihydroxy-2-phenyl-l-benzopyrylium system 5 which has additional OH functions on the 2-phenyl substituent ... 

No simple examples are known of electrophilic or radical substitution of either heterocyclic or homocyclic rings of benzopyrylium salts flavylium and l-phenyl-2-benzopyrylium salts nitrate in the substituent benzene ring. Having said this, the cyclisation of coumarin-4-propanoic acid may represent Friedel-Crafts type intramolecular attack on the carbonyl-O-protonated form i.e. on a 2-hydroxy-1-... 

I Q I 2-Phenyl-l-benzopyrylium salts (flavylium salts) are important natural products... 

There are numerous examples of benzopyrylium salts, benzopyransand benzopyranones, and frequently they have trivial names that reflect their long history (see Box 5.1). Many are natural products, and frequently these compounds contain hydroxy or alkoxy groups (sometimes in the form of a sugar residue). Polyhydroxylated natural products based upon 2-phenylbenzopyrylium (flavylium) salts and with ether linkages to sugars are called anthocyanins, whereas without their sugars they are known as anthocyanidins. 

Nucleophilic addition to benzopyrylium salts is another route to chromenes, but the attack may occur at the positions 2 or 4, thus giving rise to chrom-3-enes (30) or chrom-2-enes (31). Thus NaBH4 reduces simple flavylium salts to flav-2-ene121 (which, in its turn, behaves as a nucleophile and gives rise to dimeric products110) whereas 3-substituted... 

Benzopyrylium salts are reduced by sodium borohydride to the 4/T-chromene but LAH yields the 2//-isomer. The reduction products of flavylium salts vary with the conditions used even the method of isolating the product has to be carefully controlled since, for example, 4/f-flavenes may be isomerized by contact with acid to 2//-flavenes. Flavylium... 

Activated phenols react readily with aryl ethynyl ketones in the presence of HIT), to give flavylium salts, 3-deoxyanthocyanidins. The method has been extended to aryl di(ynones) to produce dicationic 2,2 -(phenylene)bis(benzopyrylium) salts (Scheme 21) <07EJO2438, 07SL1067>. 

Much more work has been done on 1-benzopyryliums than on 2-benzopyryliums, because of their relevance to the flavylium (2-phenyl-l-benzopyrylium) nucleus, which occurs widely in the anthocyanins, and much of that work has been conducted on flavylium itself. As with pyrylium salts, benzopyrylium salts usually add nucleophiles at the carbon adjacent to the oxygen. 

Benzopyrylium salts (chromylium salts) are coloured and possess longwave UVA IS maxima at 385 nm. A bathochromic shift is observed in the presence of a 2-phenyl substituent, i.e. for 2-phenyl-1-benzopyrylium ions (flavylium ions). H-NMR data (see pyrylium ion, p 223) show the influence of the positively charged oxygen which reduces the electron density on C-2 and C-4 5 = 9.75 (2-H), 8.40 (3-H) and 8.75 (4-H) (CF3COOD). 

These red, blue or violet colored benzopyrylium and flavylium salts (Formula 18.22) occur in the form of glycosides, the anthocyanins, in most... 

In this section the formation of benzopyrylium (632), isobenzopyrylium (633) and xanthy-lium (634) salts is covered in addition to the parent system. There is no separate treatment of flavylium salts, which are considered as phenyl derivatives of benzopyryliums. 

Oxidative general breakdown of flavylium salts was utilised in early structural work on the natural compounds. Baeyer-Villiger oxidation is such a process, whereby the two halves of the molecule can be separately examined (after ester hydrolysis of the product). Flavylium salts can be oxidised to flavones using thallium(III) nitrate, and benzopyrylium itself can be converted into coumarin with manganese dioxide. ... 

Catalytic hydrogenation of flavylium salts is generally straightforward and results in the saturation of the heterocyclic ring. Lithium aluminium hydride reduces flavylium salts, generating 4H-chromenes, unless there is a 3-methoxyl, when 2//-chromenes are the products. 2-Benzopyryliums add hydride at C-l. °... 

The first pyrylium salt without hydroxy or alkoxy substituents made by the protonation or alkylation of 4-pyrones was obtained in 1911 by Baeyer and Piccard fi"om 4-pyrones and Grignard reagents. Isolation of these compounds was often difficult, and their characterization was confiised by a lack of a clear understanding of their electronic structure. However, there was much interest in benzopyrenes (coumarins, flavones, and chromones) and benzopyrylium (flavylium, chromylium) salts isolated fi om plants (anthocyanins, anthocyanidines) to which they conferred a large diversity of colors. 

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