Introduction:

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)

Coumarin (2H-chromen-2-one) is a pleasurably fragrant chemical compound (particularly, a Benzopyrones) found in numerous plants, notably in high concentration in the tonka bean (Dipteryx odorata), sweet woodruff (Galium odoratum), mullein (Verbascum spp.), vanilla grass (Anthoxanthum odoratum), sweet grass (Hierochloe odorata), Cassia cinnamon  (Cinnamomum aromaticum) and sweet clover. The name comes from a French term, coumarou, for the tonka bean.

General Physical and Chemical Properties of Coumarins:

Coumarin consists of a sweet odor, readily recognized as the scent of newly-mown hay and has been employed in perfumes since the year 1882. Sweet woodruff, sweet grass and sweet clover in specific are named for their sweet smell, which is because of their high content of this substance. It has been employed as aroma enhancer in pipe tobaccos and several alcoholic drinks; however it is usually banned as a flavorant food additive, due to concerns regarding hepatotoxicity coumarin causes in the animal models. In high concentrations in foods, coumarin is rather bitter-tasting appetite suppressant and is probably prepared by plants as a defense chemical to discourage predation.

Reactions of Coumarins: 

Resonance in the unsaturated lactone unit of coumarin provides a strong hint as to its likely reactivity. Therefore, the oxygen atom of the carbonyl group receives electron density both by the enone chromophore from the internal resonance of the lactone group (figure shown below).

Fig: Resonance structures of Coumarin

Nucleophilic addition takes place mostly at the carbon atom of the carbonyl group causing ring opening. Likewise, electrophilic reagents encompassing an element capable of making a strong bond to oxygen (oxophiles) bind to the oxygen atom of the carbonyl group; therefore silanes, for illustration, give 2-(o-silyl)benzopyrilium salts.

The other less oxophilic electrophiles provide C-6 substituted Coumarins, however it is uncertain whether the substrate for these reactions is the free coumarin or a cation made up by protonation or bonded via a Lewis acid at the carbonyl oxygen. Several typical reactions are illustated below (figure shown below).

Fig: Some Reactions of Coumarin

In the presence of a Lewis acid example: AlCl₃, bromine reacts by coumarin to form 6-bromocoumarin, though in the absence of Lewis acids, bromine adds across the 3,4-double bond to provide 3,4-dibromo-3,4-dihydrocoumarin. In the presence of pyridine a dehydrobromination reaction occurs, leading to the 3-bromocoumarin as the preferential product.

Synthesis of Coumarins:

One approach is to make use of a 2-hdroxybenzaldehyde to form all however two atoms of the molecule. The remaining atoms are supplied through malonic acid (Propain-1,3-dioic acid), that joins by the aldehyde in a Knoevanagel condensation step, before cyclisation (Lactonization) and decarboxylation take place (figure shown below).

Fig: Synthesis of Coumarin

Coumarins can as well be synthesized via the von Pechmann reaction from Phenol, malic acid and concentrated sulphuric acid.

Fig: Synthesis of Coumarins by von Pechmann Reaction

4-Methylcoumarins bearing hydroxy and some other electron-donating groups can be synthesized from the corresponding phenols via reaction with ethyl acetoacetate in the presence of sulphuric acid. The hydrolysis of ester group in the product then lets the lactone ring of the coumarin to form (figure shown below).

Fig: Synthesis of 4-Methylcoumarins

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