4-Pyrones from 1,3,5-triketones

The cyclization of 1,3,5-triketones to pyrones will not be discussed here, although this is a related reaction, because pyrones are not true pyrylium salts. Mention will be made, however, of the formation of 2,6-diphenyl-4-pyrone from 1,3-dibenzoylallene this ketone adds secondary amines leading to 3-dialkylamino-l,5-diphenyl-4-penten-l,5-diones, which arc cyclized by hydrogen chloride in acetic acid to 2,6-diphenyl-4-dialkylaminopyrylium chlorides (R = Me and/or Ph) (see Scheme. 5). 

Although the Cpyridones W-230 can be prepared from the pyrones, it is more convenient to obtain them directly from triketones by treatment with ammonia in ethanol (see SectionI.l.C.,p. 0D. l-Phenyl-l,3,S-hexanetrione is acetylated and cyclized by boron trifluoride and acetic anhydride to the... 

Pyridones from 1,3,5 triketones, 46,60 Pyrolysis, diketene to ketene, 46, SO a-PYRONE, 46,101 in Diels Alder reactions, 46, 104 4-Pyrones, 2,6 disubstituted, from 1,3,5 pentanetnones by acid cycliza tion, 46, 61... 

Birch and cowoikers fully elaborated the polyketide hypothesis and examined possible biomimetic syntheses. In one example, treatment of 7-pyrone (129) with base was found to give a mixture of resorcinols (131 dihydropinosylvin) and (132), presumably from the two modes of intramolecular al-dolization of triketone (130 equation 149). ... 

Products formed from 1,3,5-triketones (XII-765) or 7-pyrones and p-nitro-phenylhydrazine had originally been assigned the structures XII-766 and XII-767.7 799 However, it has been shown that 4-pyrone and p-nitrophenyl-hydrazine give the p-nitrophenylhydrazone of the pyrazole (XII-768, R = H). ... 

Additionally, unsubstituted and 6-substimted 2-(perfluoroalkyl)-4/f-pyran-4-ones 4 have been prepared using alkyl enolates derived from p-dicarbonyl compounds. The reaction of acetylacetone enol ether with ethyl perflnoroalkanoates in the presence of i-BuOK, followed by p-TsOH catalyzed cychzation in benzene afforded pyrones 4a,b in 57-75 % yields. Similarly, the parent compounds 4c,d were obtained from the for-mylacetone derivative in 40-64 % yields [4]. Analogue 4e was accessible in low yield from the corresponding triketone [5] (Scheme 2). 

A variety of procedures have been used to obtain the 2,6-bis(polyfluoroalkyl)-4-pyrones 57 from the corresponding 1,3,5-triketones [H2SO4, PPA, HCl/MeOH, (Mc3SiO)3PO]. Ethyl polyphosphate appeared to be the most effective dehydrating agent with regard to the isolation and yield of products formed [25] (Scheme 18). 

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