Introduction:

Chromone is the parent member of a group of plant coloring pigments, the flavones (2-arylchromones) that are found both free and as glycosides. 2,3-substituted Chromones can be synthesized from salicylic acid derivatives and from 2-hydroxyphenyl ketones. 3-substituted Chromones can be synthesized through the condensation of the enolate of 2-hydroxyphenyl ketone and ethyl formate.

Synthesis of Chromones:

The synthesis of 2- and 3- substituted Chromones usually start from 2-hydroxyphenyl ketones.  In the first of two illustrations, a route to flavone is illustrated in the figure shown by employing 2-hydroxyacetophenone (2-hydroxyphenylethanone) and benzoyl chloride as starting materials. 

Primarily, the phenolic group of the acetophenone is O-acylated by benzoyl chloride, by using pyridine as a base (a Schotten-Baumann kind reaction). Under such conditions, the O-benzoyl derivative instantly enolises and is O-acylated again to result a dibenzoate. Without isolation, this product is cyclised via treatment with aqueous potassium hydroxide to result 2-hydroxy-2,3-dihydroflavone. Dehydration to flavone is then influenced by the action of glacial acetic acid having sulphuric acid.

Fig: Synthesis of 2-substituted Chromones

2-substituted Chromones can as well be synthesized from the salicylic acid derivatives (figure shown below). 

Fig: Synthesis of 2-substituted Chromones from Salicylic Acid Derivatives

An identical route to 3-substituted Chromones and iso flavone (R=Ph) relies on a Claisen-like condensation between the enolate of a 2-hydroxyphenyl ketone and ethyl formate (that is, methanoate) (figure shown).

This affords a 2-hydroxydihydrochromone which, as in the first illustration is subjected to the acid-promoted dehydration in the final step.

Fig: Synthesis of 3-substituted Chromones

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