Ring synthesis flavone

There is no published mechanistic study on the Auwers flavone synthesis. The mechanism may involve the nucleophilic addition of oxonium 7, derived from 1, with hydroxide to give 8. Base-promoted ring opening of 8 could provide the putative intermediate 9, which then could undergo an intramolecular Michael addition to form 10. Expulsion of bromide ion from 10 would then give flavonol 2. 

The increased development of transition metal-catalyzed cross-coupling methods to form C-C bonds has served as an impetus to find methods to synthesize 3-halochromones and 3-haloflavones. The synthesis of 3-halochromones and flavones can be achieved with the addition of halogens across the double bond of the pyrone ring by reaction with a halogenating reagent (e.g., Br2) followed by spontaneous, or base-induced, elimination (Scheme 48). Synthesis of these important compounds has been recently reviewed <2003RCR489>. 

We have also developed an alternative synthesis [97JCS(P1) 601] of this ring system that utilizes the von Strandtmann flavone annellation procedure. 

A number of heterocyclics such as coumarins, flavones, and quino-lones have been prepared from o-subst. co-(methylsulfinyl)aceto-phenones . A simple isoindole synthesis with hydrazines has been published Phenolic coupling with PdCl2 has been used efficiently to produce carpanone whereby five contiguous asym. centers with the correct stereochemistry have been introduced in a single step . A facile synthesis of the tetracyclic skeleton of rheadan alkaloids, whidi have a characteristic azepine ring, has been reported Benzimidazoles and benzoxazoles have been prepared by a novel ring closure of a-amino-/9,7-ethylenenitriles . ... 

Naturally Occurring Intermediates.—The possibility that o-hydroxydibenzoyl-methanes are intermediates in flavone synthesis, particularly of those flavones which lack B-ring substitution, was suggested by the earlier discovery of a compound formulated as 2,4,6-trihydroxydibenzoylmethane 4-glucoside in... 

Garcia, H., Iborra, S., Miranda, M. A., and Primo, J., 6-Endo-dig versus 5-exo-dig ring closure in o-hydroxyaryl phenylethynyl ketones a new approach to the synthesis of flavones and aurones, /. Org. Chem., 51, 4432, 1986. 

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