--%>
Assignment Help - homework help

  Synthesis of Chromones, Chemistry tutorial

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.

882_Synthesis of 2-substituted Chromones.jpg

Fig: Synthesis of 2-substituted Chromones

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

1308_Synthesis of 2-substituted Chromones from Salicylic Acid.jpg

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.

1376_Synthesis of 3-substituted Chromones.jpg

Fig: Synthesis of 3-substituted Chromones

Tutorsglobe: A way to secure high grade in your curriculum (Online Tutoring)

Expand your confidence, grow study skills and improve your grades.

Since 2009, Tutorsglobe has proactively helped millions of students to get better grades in school, college or university and score well in competitive tests with live, one-on-one online tutoring.

Using an advanced developed tutoring system providing little or no wait time, the students are connected on-demand with an expert at www.tutorsglobe.com. Students work one-on-one, in real-time with a tutor, communicating and studying using a virtual whiteboard technology.  Scientific and mathematical notation, symbols, geometric figures, graphing and freehand drawing can be rendered quickly and easily in the advanced whiteboard.

Free to know our price and packages for online chemistry tutoring. Chat with us or submit request at [email protected]