2-Phenyl-1-benzopyrylium

Their structure, flavylium cation, includes two benzene rings bonded by an unsaturated cationic oxygenated heterocycle, derived from the 2-phenyl-benzopyrylium nucleus. Five molecules have been identified in grapes and wines, with two or three substituents (OH and OCH3) according to the substitution of the lateral nucleus (Figure 6.9). 

When 2-phenyl-l-benzopyrylium salts are treated with sodium azide, 2-azido-l-benzopyrans... 

The trivial names flavene (10), isoflavene (11) and flavylium (12) are in widespread use and are retained here. However, these compounds will usually be considered as the phenyl derivatives of 2H-chromene or benzopyrylium rather than in separate sections. 

The alternative fragmentation by pathway B results in loss of the phenyl substituent giving the benzopyrylium species (12d), which decomposes further to give (12e). 

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. 

Phenyl-2H-1 -benzopyran) (3-Pheny I-2H-1 -benzopyran) (2-Phenyl-1 -benzopyrylium)... 

Isoflavylium 1-Benzopyrylium, 3-phenyl- Norpinane Bicyclo[3.1.1]heptane... 

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. 

No simple examples are known of electrophilic or radical substitution of either heterocycUc or homocyclic rings of benzopyrylium salts flavyhum and l-phenyl-2-benzopyrylium salts nitrate in the substituent... 

This type of reaction was first used by Dilthey and Fischer69 in connection with the mechanism of the type in Section II, D, 2. By analogy with the well-known formation of benzopyrylium salts from methyl(ene) ketones and o-hydroxybenzaldehyde or o-hydroxy-acetophenone,340 benzoylacetone (84, R = Me, R = H, R" = Ph) and acetophenone (85, RIV = Ph, R" = H) afforded in the presence of acetic anhydride and sulfuric acid, 2-methyl-4,6-diphenylpyrylium sulfoacetate. The interesting feature of this reaction is that a unique product is obtained (i.e., only the carbonyl group of the diketone adjacent to the phenyl enters the condensation, although two such groups are available) R and R" could be reversed, but they are not.69-69 Similarly, dibenzoylmethane (84, R = R" = Ph, R =H) afforded 2,4,6-triphenylpyrylium with acetophenone in 80% yield.69... 

Benzopyrylium Salts. - Dichloromethoxybutane has been used in the cycliz-ation of the benzyl phenyl ketone (134) to give (after addition of perchloric acid) the pyrylium salt (135). The latter reacted normally with methylamine to form the isoquinolinium salt (136). A complex series of transformations has been suggested to account for the formation of the pyrylium salt (138) by the reaction of the chromone (137) with malononitrile-acetic anhydride. ... 

Coumarin 13 (27/ -l-benzopyran-2-one), the 1-benzopyrylium ion 14 and the flavylium ion 15 (2-phenyl-l-benzopyrylium ion) derived from flavane (2-phenyl 2i/-chromene) are important derivatives of 277-chromene. They are discussed in chapters 6.7 and 6.8. 

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). 

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 ... 

They usually occur as glycosides (anthocyanins). Dilute acid causes hydrolysis to give a sugar and the corresponding aglycon 2-phenyl-1-benzopyrylium salt (anthocyanidin) [32]. 

The colour of anthocyanidin is pH dependent. The cyanidin system occurs at pH 3 as a red 2-phenyl-1-benzopyrylium ion 6 at pH = 8, deprotonation leads to the quinonoidal violet coloured base 7. In a stronger basic medium (pH = 11), it is converted into the blue anion 8 which forms complexes with A1 or Fe ions (E.Bayer 1966). The blue colour of certain blossoms, e.g. the cornflower, is due to the... 

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-... 

The benzopyrylium ion structure is widespread in nature. It is frequently found substituted with a 2-phenyl group, and such structures (3.59) are known as flavylium ions. Various hydroxylated forms, the anthocyanidins, are usually responsible for the beautiful colors of flowers and fruits. An example of such a structure is the reddish-brown pelargonidin chloride (3.60). 

Pyrylium salts. In contrast to the non-aromatic py-rans, their oxidized products the P. s. possess an aromatic electron sextet. P. s. belong to the oxonium salts but are, in contrast to most other oxonium salts of ethers, stable, especially when the 2,4,6-positions are substituted. The widely distributed natural products, the anthocyanidins (see anthocyanins), see also flavo-noids, are isolated as 2-phenyl-1-benzopyrylium chlorides. 

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