Ring Synthesis of Chromones

1 From ortho-Hydroxyacyl-Arenes with Esters 

Most syntheses of chromones require the prior construction of a l-(orf/to-hydroxyaryl)-l,3-diketone, or equivalent, and it is in the manner by which this intermediate is generated that the methods differ. 

Claisen condensation between an ester and the methylene adjacent to the carbonyl of the acylarene produces a l-(ortft(9-hydroxyaryl)-l,3-diketone. The Claisen condensation can be conducted in the presence of the acidic phenolic hydroxyl by the use of excess strong base triethylamine as solvent and base can also be utilised. Alternatively, the process is conducted in two steps first, acylation of the phenolic hydroxyl, and secondly, an intramolecular ° base-catalysed Claisen condensation, known as the Baker-Venkataraman rearrangement a synthesis of flavone itself is illustrative.  

The use of diazabicycloundecene (DBU) allows the whole sequence to be conducted without isolation of intermediates, as shown in the example below.  

The production of a 2-unsubstituted chromone by this route requires the use of triethyl orthoformate. Reaction with dimethylformamide dimethyl acetal, followed by an electrophile, bromine in the example, gives 3-substituted chromones.  

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Ring synthesis of chromones from ortho-hydroxyacylarenes and esters... 

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