2-Pyrone ring opening

Ethylenecarboxylic acids from enollactones Reductive 2-pyrone ring opening... 

Pyrone ring opening, reductive 18, 57 Pyrones s. a. Polypyrono compounds... 

Attempted annulation of 3-iodoflavones, for example 428, by internal alkynes catalyzed by Pd(OAc)2 in the presence of NaOAc and LiCl yielded a mixture of two compounds (Scheme 73) <1998JOC2002>. While the expected benzoxanthenes (e.g., 429) were isolated as the minor compound, tetrasubstituted furans (e.g., 430) resulting from the electrophilic attack on the carbonyl oxygen atom by the vinylpalladium intermediate followed by pyrone ring opening were the major product. 

Kizhner377,378 showed that 2,6-dimethyl-y-pyrone and hydrazine react in a different way, the pyrone ring opening and a more stable... 

The nature of the substituent at the 5-position of the chromone system influences the form of existence of the reaction products, which can be either ring or open. The attack by the amine on the C-2 atom of 130 for R =H is accompanied by the pyrone ring opening and yields aminoenones 157 when R the process stops after the nucleophilic addition of the amine to give stable chromanones 158 [74] (Scheme 49). 

Thus, the reaction of 2-R -chromones with amines usually starts with the attack by the amino group on the C-2 atom. In the case of secondary amines or in the presence of a substituent at the 5-position, the reaction can stop after 1,4-nucleophilic addition however, in most cases, it is accompanied by pyrone ring opening giving the corresponding aminovinyl ketones, whose structural features and subsequent transformations provide a variety of products. An exception is the reaction of 2-R -chromones with 2-aminoethanol pointing to the possibility of an attack by the amine on the carbonyl group. 

The reaction of 8-aza-5,7-dimethyl-2-(trifluoromethyl)chromone (133a) with alkyl mercaptoacetates afforded bicycles 181a,b. When the reaction time and the amount of EtsN were increased, acyclic derivatives 182a,b were isolated [91]. A similar reaction of pyranopyrazole 135 proceeds at the C-6 atom followed by pyrone ring opening and intramolecular condensation of the aldol type to give compound 183, from which heterofused coumarin 184 was obtained [59] (Scheme 59). 

Reactions of chromones 230 with acetoacetamide and ethyl acetoacetate in ethanol in the presence of ammonium acetate proceed at the C-2 atom of the chro-mone system with pyrone ring-opening and subsequent cycUzation to 252. Similar reaction with p-aminocrotononitrile gave 5-hydroxy-2-methyl-5-(polylluoroalkyl)-5ff-chromeno[4,3- ]pyridine-3-carbonitriles (253a) [129]. Three-component reaction between chromones 230, dimedone, and AcONH, is accompanied by detrifluoroacetylation and leads to 254 in low yields [130] (Schane 82). 

In principle, complex hydrides (NaBHj, LiAlH ) ought to react similarly with 4-pyrones and lead after treatment with Bronsted or Lewis acids to 4-unsubstituted pyrylium salts. This reaction has not been reported the reduction of 2-pyrones with LiAlH4 results in ring opening. " ... 

A similar synthesis of mechanistic interest rather than preparative value involves the thermal reaction of dimethyl 2,5-bisdiazo-3,4-diketoadipate (89, Scheme 23) with benzofuran (91)." The presumed intermediate is the pyrone cation 90 produced from the adipate 89 by the Wolff rearrangement, cyclization, and loss of nitrogen. Electrophilic substitution then affords the benzofuran 92, which can be isolated. Ring opening and cyclization of the resultant ketene 93 then affords the dibenzofuran 94 in poor (0.4%) yield. 

Pyrones are also formed from D-fhreo-2,5-hexodiulose ( 5-keto-D-fructose ) (67) when it is heated in aqueous solution.89 Compounds 68 and 69 are formed in high yield, with 68 preponderating. Kojic acid (68) results from /3-eliminations in which the ring remains intact, but the formation of 5-hydroxymaltol (69) requires ring opening, because of the absence of a proton on C-2. 

Fig. 26. Reactions observed during the enantioselective hydrogenation of pyrone 1 catalyzed by Pd/ TiO2 modified by CD. After the fast hydrogenation resulting in 2 (target reaction), a considerably slower second hydrogenation reaction follows. Once lactone 3 is formed, it can undergo ring opening to yield the corresponding acid 4, which then adsorbs in various ways 5 on the catalyst support (48).

Although pyridones are usually resistant to alkali, pyrone rings are often easily opened. Pyran-2-ones are reversibly ring-opened by aqueous alkali to acid anions (222). Hydroxide ions convert coumarins (223) reversibly into salts of coumarinic acids (224) which can be converted into the trans isomers (225), and chromones (226) into 3-dicarbonyl compounds (227). 

Bromo-2-pyrones and 3-bromocoumarins give furan- and benzofuran-2-carboxylic acids by ring fission and subsequent closure, e.g. (228) — (229) (116) or (117) — (230). Pyrone rings are opened by aqueous acid in some cases, probably by successive protonation and attack of a water molecule, e.g. dehydroacetic acid (231) gives (233) which immediately forms (232) or (234) with HC1 or H2S04, respectively. 

However, chromones react differently, because the phenolic hydroxy group in the ring-opened intermediate is unreactive. Thus, isoxazoles (264) result from the reaction with hydroxylamine, and a pyrazole is formed with hydrazine. y-Pyrones also give pyrazoles with hydrazine. 

The method described is that of Miles, Harris, and Hauser2 and is an improvement over the earlier procedure of Hauser and co-workers.3,4 In the earlier method the dianion of benzoylacetone, formed by the action of alkali amide in liquid ammonia, was treated with methyl anisate to yield 1 -(/>-methoxyphenyl)-5-phenyl-l,3,5-pentanetrione (61% based on the ester). This compo ir d has also been prepared by the base-catalyzed ring opening of 2 - f -mcthoxyphenyl) -6-phenyl -4 pyrone however, no yield is reported.5... 

The mechanism of the conversion of 4-pyrone to 4-pyridone involves an initial Michael reaction followed by ring-opening. Tautomerisation of enol 9.25 to aldehyde 9.26, followed by cyclisation, affords 4-pyridone 5.23. 

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