Simonis chromone cyclization

Chromones from phenols and p-ketoesters using phosphorus pentoxide. 

P2O5 actually exists as P4O10, an adamantane-like structure. 

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Simonis chromone cyclization. Formation of chromones from phenol and (3-keto esters in the presence of phosphorus pentoxide, phosphorus oxychloride, or sulfuric acid. Coumarins may also be formed. 

A more recent report on the use of PPE to promote cyclization of a P-naphthol has appeared. The reagent allowed product formation in excellent yield at room temperature. Such results were a great improvement to the reaction conditions and yield typically obtained in the Simonis chromone synthesis. 2,6-Dihydroxy-naphthol 35 gave chromone 36 in 98% yield. 

The Simonis chromone synthesis was reported as a key reaction in the preparation of the natural product visnagin 48, a component found in fruits. Badawi and Fayez reported the conversion of 44 and 46 using heat in diphenyl ether in satisfactory yield. The aldehydic product 47 was derived from phloroglucin aldehyde 45 via similar thermal reaction conditions and was subsequently converted into visnagin 48. Note that the cyclization occurred under thermal conditions with no acid catalysis. 

Although there is evidence that chromone syntheses which proceed by the cyclization of phenyl esters under Friedel-Crafts conditions may involve a Fries rearrangement and hence require the formation of one bond adjacent to the heteroatom, syntheses of chromones from phenols will be considered together in this section. The Simonis reaction (530R(7)l)... 

There has been some controversy about the mechanisms of the Simonis and Pechmann reactions, which still remain in doubt. It has been suggested (50BSF1132) that the condensations proceed through a common oxonium ion (467). Dehydration to the phenoxyacrylic ester (468) is followed by cyclization to the chromone whilst a rearrangement to the substituted phenol (469) subsequently affords the coumarin (Scheme 171). 

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