Synthesis and Reactions of Dibenzopyrilium Salts, Chemistry tutorial

Introduction:

There are many illustrations of benzopyrilium salts, Benzopyrones and benzopyranones and often they have trivial names that reflect their long history (figure shown below).

1526_Structures of Benzopyrilium Salts, Coumarins, Chromones.jpg

Fig: Structures of Benzopyrilium Salts, Coumarins, Chromones and Flavonoids

Most of the natural products and frequently such compounds have hydroxyl or alkoxy groups (at times in the form of a sugar residue). Polyhydroxylated natural products based on 2-phenylbenzopyrilium (or Flavylium) salts and with ether linkages to sugars are known as anthocyanins, while without their sugars they are termed as anthocyanidins.

General Properties of Benzopyrilium Salts:

Anthocyanins, in association by other compounds, like flavones, are responsible for the colour of several flowers. The anthocyanin found in rose petals is cyanin; it can be isolated as its chloride. The corresponding anthocyanidin, cyanidin, exists as the pentahydroxy salt in the acidic media, however as the pH increases it provides first a quinone and then an anion. Each of such forms has different colours (figure shown below).

742_Effect of pH on Cyanidin.jpg

Fig: Effect of pH on Cyanidin

Other natural products, in the group are built up via ring fusions of various kinds of ring systems. They comprise the garden insecticide rotenone (at times sold in crude form as derris powder, the pulverised bark of the plant Derris elliptica).

Synthesis of Pyrilium Salts:

Salts of the type R3O+X- are usually unstable and can't be isolated unless X- is an especially stable (that is, non-nucleophilic) anion like BF4-. Though, the oxonium salts derived via oxidation of the pyran nucleus are somewhat stable as the cation methods aromatic stabilization energy which is lost whenever an anion bonds covalently to it. The parent member of the series, the Pyrilium ion is made up by acidification of the sodium salt of glutaconic aldehyde at low temperatures (figure shown below).

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Fig: Synthesis of Pyrilium Salts

Synthesis of Anthocyanidin and Cyanidin Chloride:

The glycosides of a number of 2-arylbenzopyrlium salts take place naturally as plant coloring matters, the anthocyanins. The parent salts, the anthocyanidins are synthesized from salicylaldehydes and acetophenones via an acid-catalyzed aldol reaction followed via ring closure in the ethylacetate solution.

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Fig: Synthesis of 2-arylbenzopyrlium Salts (anthocyanidins)

Cyanidin chloride has been manufactured as follows:

81_Synthesis of Cyanidin Chloride.jpg

Fig: Synthesis of Cyanidin Chloride

The general process of synthesis of the three ring systems is illustrated in the figure by using phenols and 1,3-dicarbonyl compound as starting materials.

2210_Synthesis of 2-Benzopyrilium  Salts, Chromone and Coumarin.jpg

Fig: Synthesis of 2-Benzopyrilium Salts, Chromone and Coumarin

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