Chromones reaction with alkali

Pyran-4-ones and their monobenzo derivatives undergo ring opening when treated with aqueous alkali. This reaction has been widely used in the investigation of structure of chromones, flavones and isoflavones but xanthones are little affected. The nucleophile attacks the 1,2-bond in a rate-determining step whose rate constant varies with the electron density in the ground state at C-2. A kinetic study of the reaction with several chromones supports the mechanism shown in Scheme 25 (79ACH(101)73). 

There are few cases in which free /3-aldehydo esters have been condensed successfully with ureas. Commonly, alkoxymethylene esters are used. The initial reaction leads to an acyclic intermediate that may require a separate treatment to induce ring closure. The reaction of a /3-keto ester with urea may be a two-step process in which case acid catalysis can be used in the formation of an acyclic intermediate, with ring closure effected by strong alkali. When the ester component is a lactone or chromone, the product contains a hydroxyalkyl <2000JME3837> or 2-hydroxyphenyl substituent <2004S942>, as shown by the synthesis of the 5-(2-hydroxyethyl)-4-pyrimidinone 657 and the 6-(2-hydroxyphenyl)-pyrimidine 659. 

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