Chromones ring opening with amines

Rather more reliable and certainly better documented is the dehydrobromination of 3-bromochroman-4-ones, achieved by treatment with an amine (5UA4205). In some instances the ring-opened enamine (482) has been isolated and subsequently cyclized to the chromone (65JCS3610). 

Primary and secondary amines lead to enaminones 3 by Michael addition and subsequent ring-opening on treatment with acids, enaminones 3 regenerate the chromone system ... 

Thus, the reaction of 2-R -chromones with amines usually starts with the attack by the amino group on the C-2 atom. In the case of secondary amines or in the presence of a substituent at the 5-position, the reaction can stop after 1,4-nucleophilic addition however, in most cases, it is accompanied by pyrone ring opening giving the corresponding aminovinyl ketones, whose structural features and subsequent transformations provide a variety of products. An exception is the reaction of 2-R -chromones with 2-aminoethanol pointing to the possibility of an attack by the amine on the carbonyl group. 

The pyrone ring is opened by heating with aqueous alkalis [24,25], amines [151-155] or hydrazine [13, 157]. Much use has been made of the alkaline hydrolysis of chromones in the determination of structure [185, 186] for example, bromination of 7-hydroxychromone-2-carboxylic acid was shown to proceed at C-8 by degrading the product to 3-bromo-2,4-dihydroxyaceto-phenone (85) [25]. 

Chromones 230 smoothly react with indole and A-methylindole in refluxing pyridine resnlting in the formation of 244 as mixtures of Z- and fi-isomers [124]. These reactions include the nucleophilic 1,4-addition of the amine or indole with concomitant opening of the pyrone ring and subsequent intramolecular cyclization of the intermediate at the R CO group [125]. 

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