Chromone 5-alkoxy

The photo-cycloaddition of ethylene to 3-alkoxy chromones such as 315 has been applied to the synthesis of marine sesquiterpene filiformin and congeners (Scheme 43) <1996JOC4391>. Tandem [2+2] 7i-photo-cycloaddi-tion and y-hydrogen abstraction provided tetracyclic intermediate 316 which was converted to terpene 317 by subsequent oxetane ring reduction and acid-catalyzed rearrangement. 

The first pyrylium salt without hydroxy or alkoxy substituents made by the protonation or alkylation of 4-pyrones was obtained in 1911 by Baeyer and Piccard fi"om 4-pyrones and Grignard reagents. Isolation of these compounds was often difficult, and their characterization was confiised by a lack of a clear understanding of their electronic structure. However, there was much interest in benzopyrenes (coumarins, flavones, and chromones) and benzopyrylium (flavylium, chromylium) salts isolated fi om plants (anthocyanins, anthocyanidines) to which they conferred a large diversity of colors. 

Several methods are available for the preparation of chromone-2-carboxylic acid and its analogues. Those which lead to a carboxyl or an alkoxy- or aryloxy-carbonyl group at C-2 or C-3 are discussed in this section and those which lead to the formation of derivatives such as amides are described in the section on the chemical properties of the acids. Methods of synthesising chromone-2-carboxylic acids (or esters) may be divided into two main types (1) those in which the C-2 substituent is present at the cyclisation stage (the direct synthesis) and (2) those in which the substituent is formed from another group after the formation of the pyrone ring (the indirect synthesis). The former type is by far the most commonly used and is considered first. 

When the alkyl aryl ketone contains more than one phenolic group, an additional molar equivalent of acyl chloride should be added and the ester group so formed may be hydrolysed at the end to regenerate the hydroxyl group [65]. This method has been used in the synthesis of the 2-carboxylated derivatives of many naturally-occurring [56, 66-78] and pharmaceutically interesting [79-83] 3-arylchromones. Its application to the preparation of 5-alkoxy- [47] and 3-alkyl (or araklyl)-chromone-2-carboxylic adds and esters has been described [50, 84-90]. 

Ethyl dichloroethoxyacetate, Et0 CCl2-C02Et, prepared [107] by the action of phosphorus pentachloride and phosphorus oxychloride on ethyl oxalate, reacts with 2-hydroxyacetophenone in the presence of a small amount of palladium to give ethyl chromone-2-carboxylate in 52-5 per cent yield [108]. Other noble metal catalysts may be used but usually result in a lower yield in the absence of a catalyst, the yield is even lower. A number of 3-alkyl-, 3-aryl- [109-111] and 5-alkoxy-[47] chromone-2-carboxylic esters and a bischromone [50] prepared by this method are covered by patents. 

A suspension of 7-methoxy-3-(6-methoxy-3,4-dioxocyclohexa-l,5-dienyl)chromone in 1 1 dioxane-acetone treated with water and some coned. H2SO4, then warmed with stirring until dissolved -> 7-methoxy-3-(4-hydroxy-3,6-dioxocyclohexa-l,4-dienyl)diromone. Y 81%. Also formation of alkoxy-p-quinones ith 0-alkyl migration s. C. A. Weber-Sdiilling and H.-W. Wanzlidc, B. 104, 1518 (1971). 

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