2.4- Dioxopentanoic acid

Carbonation of the dianion derived from benzoylacetone occurs at the methyl rather than the methylene group (58JA6360). Sodamide was shown to be the reagent of choice for generation of the dianion. Cyclization of the resulting dioxopentanoic acid by polyphosphoric acid gives 4-hydroxy-6-phenylpyran-2-one. This route has been utilized in the synthesis of a number of naturally occurring 4-hydroxypyran-2-ones acetic anhydride at room temperature is the preferred cyclization medium and the initial formation of a mixed anhydride... 

The simplest derivative of this ring system known at present is the triketone 304 (Scheme 47).320) Condensation of 4,5-dioxopentanoic acid with dimethyl /3-keto-glutarate in aqueous sodium carbonate solution was shown to give 302 in 80 % yield. 

Phenyl-3,5-dioxopentanoic acid dissolved in polyphosphoric acid on a steam bath, and the product isolated after 90 min. 6-phenylpyronone. Y 98%. G. R. Hauser and T. M. Harris, Am. Soc. 80, 6360 (1958). 

Other new syntheses of the isomers of the dolichodials, chrysomelidial (364A) and dehydroiridodial (364B), have been published. Methyl 2,4-dioxopentanoate and isoprene react together on irradiation to give two products, one of which, 376, underwent ring closure to 377 with titanium dichloride (Scheme 32). Both aldehydes were available by this route because the monoacetal 378 equilibrates on aluminum oxide. Furthermore, the acid of 377 has been resolved as the menthyl ester, making other double-bond isomers available. A somewhat... 

Titanium(ii)-chloride-induced reductive cyclization of one of the photo-adducts (43) of isoprene and methyl 2,4-dioxopentanoate produced a mixture of diastereo-meric diols (44), which are useful precursors in the synthesis of iridoids. The Lewis-acid-catalysed reaction of unsaturated carbonyl compounds such as (45) shows a strong dependence on the strength and quantity of Lewis acid used. 

The reaction of ethyl 2,4-dioxopentanoate with ethyl trifluoroacetate in the presence of NaOEt leads to ester 21a. This ester is smoothly hydrolyzed to acid 21b by reflux in 20 % HCl, whereas its treatment with 20 % ammonia depending on conditions applied affords amides 22 and 23 in high yields [15], Decarboxylation of 6-(ttifluoromethyl) comanic acid (21b) gave 2-(trifluoromethyl) H-pyran-4-one (4c) [4,16] (Scheme 9). 

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