Benzo-y-pyrone

The parent substance, 3-phenyl-benzo y-pyrone, was recognised and named flavone by Kostanecki and Tambor, and synthesised by Kostanecki and W. Feuerstein before it was found in nature. F. Kehrmann and A. Duttenhofer discovered an addition compound of dimethylpyrone and methyl iodide, proposing two formulae and preferring (X). Baeyer showed that (XI) better expresses the chemical properties (both formulae are here given in modern ionic form) ... 

The Simonis chromone synthesis is the reaction of a phenol la-c with a P-keto ester 2 using an appropriate acid promoter to generate a chromone or benzo-y-pyrone 3 (also called a benzo-l,4-pyrone). While compound 3 is actually a chromenone, for this article, whether the double bond is present or not, the system will be characterized as a chromone. The condensation is related to the Pechmann-Duisberg reaction, which yields coumarins from the condensation of a phenol with a P-keto ester and like its relative, the reaction conditions require the loss of water from the ketone moiety and alcohol from the ester moiety. 

About 8,000 phytochemical compounds have been reported as flavonoids, which are formed by the combination of multiple hydroxyl, methoxy, and 0-glycoside groups on the basic benzo-y-pyrone skeleton... 

Bio active polyphenols are present in a variety of plants and are used as important components of human and animal diets [27]. Flavonoids are a broad class of low-molecular-weight secondary plant polyphenolics, which are benzo-y-pyrone derivatives consisting of phenolic and pyrane rings. Flavonoids are usually subdivided according to their substituents into fla-vanols, flavones, flavanones, chalcones, and anthocyanidines. 

The simple y-pyrones are quite stable crystalline substances, whereas the a-pyrones are much less stable, a-pyrone itself, which has the smell of fresh-mown hay, polymerising slowly on standing. There are relatively few simple pyrone natural products in great contrast with the widespread occurrence and importance of their benzo-derivatives, the coumarins and chromones, in nature. 2- and 4-Hydroxypyrylium salts are quite strongly acidic and are therefore much better known as their conjugate bases, the 2- and 4-pyrones. 

Methyl propiolate and triethylamine added to refluxing 3 -hydroxy-4, 5 -di-methoxy-l,2-benzo-l-cyclohepten-4-one in abs. benzene-terf-pentanol, refluxed 45 min., K-fert-butoxide soln. added, and refluxing continued for 30 min. -> hydroxydimethoxy-2-pyrone derivative. Y over 70%. Also isolation of intermediates s. J. Schreiber et al., Helv. 44, 540 (1961). 

---