Chromones 3- hydroxy- from

Rajanna, K. C., Solomon, F., Ali, M. M., Saiprakash, P. K. Kinetics and mechanism of Viismeier-Haack synthesis of 3-formyl chromones derived from o-hydroxy aryl alkyl ketones a structure reactivity study. Tetrahedron 1996, 52, 3669-3682. 

However, chromones react differently, because the phenolic hydroxy group in the ring-opened intermediate is unreactive. Thus, isoxazoles (264) result from the reaction with hydroxylamine, and a pyrazole is formed with hydrazine. y-Pyrones also give pyrazoles with hydrazine. 

Coumarins and isocoumarins appear to be of varied origins. Simple coumarins, such as umbelliferone, are formed by the shikimic acid pathway in which hydroxylation of p-hydroxycinnamic acid occurs. Other coumarins, for example alternariol (690), are derived from a polyketide unit, as are a number of chromanones, chromones, pyranones and isocoumarins (B-78MI22400). The biosynthesis of 5-hydroxy-2-methylchromone has been shown to involve the chromanone (60JCS654). However, isocoumarins are also derived from the mixed acetate-shikimate route, through initial cyclization of the polyketide and subsequent lactonization. 

Both flavanones and 2 -hydroxychalcones are oxidised to flavones in the absence of solvent by In salts supported on silica gel <05TL253> and the former also by Mn(OAc)3 <05SC2723>. Various 3-(perhaloacyl)chromones, which exist as a mixture with their covalent hydrates, have been obtained from 2-hydroxy-2-(perhaloalkyl)chroman-4-ones on heating with the formylating agent, diethoxymethyl acetate <05SL1164>. 

Data mainly from 13. Other compounds besides those mentioned were found in individual species. Traces of medicarpin have been found in Vlcla faba, but the major response is furanoacetylene production. Lathyrus odoratus yields chromones and a-hydroxy dihydrochalcones as... 

Bis-chromones of the type (169) and related compounds have been synthesized from hydroxy-stilbenes/ ... 

Accordingly the tautomeric forms do not differ significantly from each other except for the charge on the oxygen (position 4) which is not at all surprising since this atom is in different binding states in the two forms. Although we assume the hydroxy form to be present in the protein-bound state, all compounds were calculated as quinolones for better comparison with chromones and naphthoquinones in Table VIII. 

Elimination of the ring C as a hydroquinone derivative with the formation of chromones from rings A and B. This type of fission is most prominent with flavonoids whose A ring hydroxy groups are blocked, e.g., by methylation. 

The syntheses of chromones and flavones [60] usually start from (2-hydroxy) ---- acetophenones. 

The most frequently used method for the synthesis of chromones 10 is the acid-catalyzed cyclization of (2-hydroxy)aryl-l,3-diketones 9 which are obtained from (2-hydroxy)acetophenones (advantageously in their O-silyl protected form [61]) by a Claisen condensation. Bis-(trichloromethyl)carbonate/DMF (dimethylformamide) effects the cydization 9 10 very efficiently [62]. 

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