Dibenzopyrones, Pyrilium Salts and Other Benzopyrones, Chemistry tutorial

Introduction:

Coumarins (or Benzopyrones) are the simple molecules which comprise two structures of six member heterocyclic rings with two oxygen atoms. The Coumarins can be categorized as simple coumarin, furanocoumarins, pyranocoumarins and Coumarins substituted in the pyrone ring (table shown below). Simple Coumarins can experience hydroxylation, alkoxylation and alkylation to form its derivatives. For furanocoumarins, such compounds comprise of five-member furan ring linked to the coumarin structure. Pyranocoumarins is a compound which contained a linear or angular kind with the substituents on benzene and pyrone rings.

1384_Coumarins-benzopyrones.jpg

Fig: Coumarins-Benzopyrones

Coumarin is the chemical compound that is found naturally in several plants; however it can be synthetically prepared as well. It consists of a distinctive odor that has led people to use it as a food additive and ingredient in the perfume. Because of concerns regarding coumarin as a potential liver and kidney toxin, its utilization as a food additive is heavily restricted; however it is perfectly safe to eat foods that naturally encompass the compound.

The chemical name for coumarin is Benzopyrones. The distinctive sweet odor reminds numerous people of freshly cut grass or hay, and it has been employed in perfumes since the late 1800s. In a pure form, this compound consists of a crystalline structure, and it is stated to taste faintly such as vanilla. Whenever ingested, it acts as a blood thinner, and it as well appears to be efficient in treating some tumors. Coumarin consists of fungicidal properties as well. Though, other much safer substances can be employed for all of such purposes, however the compound is at times employed in combination with other blood thinners for the medical treatment.

One natural source of coumarin is Tonka beans, tropical beans that are acknowledged by the French as coumarou. To discharge their captive coumarin, the beans are soaked in alcohol and then fermented. The substance as well takes place in sweet clover, cherries, strawberries, bison grass, woodruff and apricots. Coumarin has traditionally been employed as a vanilla substitute in different foods, particularly tobacco, even though this usage is restricted in several countries.

Some of the traditional foods are made up with plants that comprise of coumarin, and it is clearly a significant flavor compound in the food as such foods are made up in a manner that concentrates the natural substance. In both Poland and Germany, plants such as woodruff are added to alcohol to form a beverage having a distinctive fresh, spring-like flavor that can probably be credited to the coumarin. Such products are not usually dangerous to consume, however people might wish for to avoid consuming high volumes of such foods; the similar goes for therapeutic coumarin or derivatives which might be prescribed via a doctor.

In plants, coumarin appears to act as the natural pesticide, cutting down on insects in such a way that the plant can grow undisturbed. Several chemicals in this family have been harnessed for their pesticide utilizations, and some related chemicals are even employed for larger pests such as rodents. Consumers might be familiar with one chemical in the family; warfarin is a popular anticoagulant that can be ingested or injected, based on the requirements of the patient.

Definition of Benzopyran:

Benzopyran is the polycyclic organic compound which results from the fusion of a benzene ring to a heterocyclic pyran ring. According to the IUPAC nomenclature it is known as chromene. There are two isomers of benzopyran which differ by the orientation of the fusion of the two rings compared to the oxygen, resultant in 1-benzopyran (chromene) and 2-benzopyran (isochromene) - the number represents where the oxygen atom is situated via standard naphthalene-like nomenclature.

The radical form of benzopyran is paramagnetic. The unpaired electron is delocalized over the entire benzopyran molecule, rendering it less reactive as compare to one would expect or else, an identical illustration is the cyclopentadienyl radical. Generally, benzopyran is encountered in the reduced state, in which it is partly saturated by one hydrogen atom, introducing a tetrahedral CH2 group in the pyran ring. Thus, there are numerous structural isomers owing to the multiple possible positions of the oxygen atom and the tetrahedral carbon atom:

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Fig: Structural isomers of chromene

Benzopyrones:

Benzopyrones might refer to either of two ketone derivatives of benzopyran that comprise the core skeleton of numerous flavonoid compounds:

  • Chromone (1-benzopyran-4-one)
  • Coumarin (1-benzopyran-2-one)

Some of the simple Benzopyrones have clinical medical value as edema modifiers. Coumarin and other Benzopyrones, like 5,6 Benzopyrones, 1,2 Benzopyrones, diosmin and others are termed to stimulate macrophages to degrade extracellular albumin, allowing faster resorption of the edematous fluids.

Naturally occurring coumarin is as well the base for different 4-hydroxybenzopyrone-based molecules that take place naturally dicoumarol and are made up synthetically warfarin and function as anticoagulants.

History of Coumarin:

The term 'Tonka' for tonka beans is taken from the Galibi (Carib) tongue spoken via natives of French Guiana (that is, one source for the plant); it as well appears in Tupi, the other language of the similar region, as the name of the tree. The old genus name, Coumarouna, was made up from the other Tupi name for tree, kumaru. The French term for Tonka bean, coumarou, is from this name.

Coumarin, named for coumarou was primary isolated from Tonka beans and sweet clover in the year 1820 by A. Vogel of Munich, who primarily mistook it for benzoic acid. As well in the year1820, Nicholas Jean Baptiste Gaston Guibourt (1790-1867) of France independently isolated coumarin; however he realized that it was not benzoic acid. In a following essay he presented to the pharmacy section of l'Académie royale de Médecine, Guibourt named the new substance 'coumarine'. In the year 1835, the French pharmacist A. Guillemette proved that Vogel and Guibourt had isolated the similar substance. Coumarin was first synthesized in the year 1868 by the English chemist William Henry Perkin.

Synthesis of Coumarin:

Coumarin can be made up by a number of name reactions by the Perkin reaction between the salicylaldehyde and acetic anhydride being a popular illustration. The Pechmann condensation provides the other route to coumarin and its derivatives; as does the Kostanecki acylation that can as well be employed to produce Chromones.

Natural occurrences of Coumarin:

Coumarin is mainly found naturally in numerous plants, notably in high concentration in the tonka bean (Dipteryx odorata), vanilla grass (Anthoxanthum odoratum), sweet woodruff (Galium odoratum), mullein (Verbascum spp.), sweet grass (Hierochloe odorata), cassia cinnamon (Cinnamomum cassia) not to be confused by true cinnamon ("Ceylon cinnamon", Cinnamomum zeylanicum that includes little coumarin), sweet-clover (Melilotus ssp.) and deertongue (Dichanthelium clandestinum). Coumarin is as well found in the extracts of Justicia pectoralis.

Associated compounds are found in some however not all the specimens of licorice.

Introduction to Pyrylium salt:

A Pyrylium salt is a salt having a Pyrylium cation or a derivative of it. The Pyrylium cation is a conjugated 6-membered carbon ring system having one carbon atom substituted by a positively charged oxygen atom. It is, similar to benzene, an aromatic compound.

A Pyrylium might be any derivative of the Pyrylium cation.

Chemical properties of Pyrylium salt:

The carbon to oxygen double bond is as well an oxonium ion, which because of aromatic stabilization is much less reactive as compare to ordinary oxonium ions. Though, the parent compound Pyrylium is unstable in neutral water such as oxonium salts. Pyrylium cations react by nucleophiles in the 2,4 and 6 positions and might yield in ring-opening reactions. The high electronegativity of the oxygen yields in the strongest single perturbation of one heteroatom in the six-membered ring.

Being aromatic, Pyrylium salts are simply made up from simple starting materials and their reactivity toward nucleophiles makes them helpful materials for getting other compounds by stronger aromatic character. Therefore, Pyrylium salts afford pyridines with ammonia, pyridinium salts with primary amines, phosphabenzenes with phosphine derivatives, pyridine-N-oxides with hydroxylamine, thiopyrylium salts with hydrogen sulfide and benzene derivatives with acetonitrile or nitromethane.

Synthesis of Pyrylium salt:

The Pyrylium salts with aromatic substituents like 2,4,6-triphenylpyrylium tetrafluoroborate can be obtained from the two moles of acetophenone and one mole of benzaldehyde in the presence of tetrafluoroboric acid and an oxidizing agent (that is, Dilthey synthesis). For Pyrylium salts with alkyl substituents like 2,4,6-trimethylpyrylium salts, the best technique employs the Balaban-Nenitzescu-Praill synthesis from tertiary butanol and acetic anhydride in the presence of tetrafluoroboric, perchloric or trifluoromethanesulphonic acids.2,4,6-Triphenylpyrylium salts are transformed by bases to stable a 1,5-enedione (pseudobase) however 2,4,6-trimethylpyrylium salts on treatment having hot alkali hydroxides afford an unstable pseudobase which undergoes an intramolecular condensation resulting 3,5-dimethylphenol. In warm deuterium oxide, 2,4,6-trimethylpyrylium salts experience isotopic exchange of 4-methyl hydrogen faster as compare for the 2- and 6-methyl groups, allowing the synthesis of regioselectively deuterated compounds.

Pyrones:

A Pyrylium cation having a hydroxyl anion substituent in the 2-position is not the zwitterionic aromatic compound however a neutral unsaturated lactone or a α-pyrone or pyran-2-one. Significant representatives of this class are the Coumarins. Similarly a 4-hydroxyl Pyrylium compound is an φ-pyrone or pyran-4-one to which group belongs a compound such as maltol.

Chromenylium ion:

The benzo-fused Pyrylium ion is as well termed as benzopyrilium or according to IUPAC Chromenylium ion (Formula: C9H7O, molar mass: 131.15 g/mol, exact mass: 131.04968983). This is basically the charged version of 1-benzopyran or chromene (IUPAC).

Flavylium ion:

In biology, the 2-phenylchromenylium ion is termed to as Flavylium. A class of flavylium derived compounds is anthocyanidins and anthocyanins, that is, pigments which are responsible for the colors of numerous flowers.

Naphthoxanthenium cation:

Higher fused derivatives of the Pyrilium as well exist. One good illustration is the Naphthoxanthenium derivative. This dye is highly stable, aromatic and planar. This absorbs in the UV and blue region and presents exceptional photophysical properties. This can be synthesized via chemical or photochemical reactions.

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