Pyrones 4-hydroxy- from

Baeyer and Piccard were the first to prepare crystalline monocyclic pyrylium salts without hydroxy or alkoxy substituents, from y-pyrones and Grignard reagents in 1911. They ascribed a correct structure to these salts, although the bonds in the ring and the valency of the oxygen heteroatom remained contested topics for the next 20 years. The discussions around the formula of pyrylium... 

Dihydro-2f/-pyran-2-one has been prepared by reductive cycliza-tion of 5-hydroxy-2-pentynoic acid [2-Pentynoic acid, 5-hydroxy-], which is obtained in two steps from acetylene [Ethyne] and ethylene oxide [Oxirane] 3 and by the reaction of dihydropyran [277-Pyran, 3,4-dihydro-] with singlet oxygen [Oxygen, singlet].4,5 2ff-Pyran-2-one has been prepared by pyrolysis of heavy metal salts of coumalic acid [2//-Pyran-5-carboxylic acid, 2-oxo-],8 by pyrolysis of a-pyrone-6-carboxylic acid [211 - Pyran-6-carboxyl ic acid, 2-oxo-] over copper,7 and by pyrolysis of coumalic acid over copper (66-70% yield).8... 

KREUZALER, F., HAHLBROCK, F., Enzymatic synthesis of aromatic compounds in higher plants. Formation of fe-noryangonin (4-hydroxy-6[4-hydroxystyryl]2-pyrone) from p-coumaroyl-CoA and malonyl-CoA, Arch. Biochem. Biophys., 1975, 169, 84-90. 

Fig. 9. Normalized solubilities of l,2-dimethyl-3-hydroxy-4-pyridinone, dmppH, of its aluminum(III) and iron(III) complexes AKdmppla and Fe(dmpp)s, and of the 3-hydroxy-4-pyronate complexes Ga(malt)3 and In(etmalt)3 in methanol-water mixtures at 298.2 K (data from Refs. (114) and (234)).

Meconic acid (3-hydroxy-Y-pyrone-2,6-dicarboxylic acid) [497-59-6] M 200.1, m 100 (-H2O). Crystd from water and dried at 100° for 20min. 

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. 

Hydroxy benzo[6]thiophene-4-carboxaldehyde, which is prepared from 5-hydroxybenzo[6]thiophene by the Duff reaction 338 or, better, by a modified Gattermann reaction,340 reacts with diethyl bromo-malonate to give the thienobenzofuran (282), and undergoes the Perkin reaction to give the thienobenzo-a-pyrone (283).338... 

Dimethoxyoxetanes.1 In the presence of ZnCl2, this ketal reacts with 0-hydroxy aldehydes to give 2,2-dimethoxyoxetanes, which can be converted into 4-hydroxy-3-methyl-8-lactones and 5,6-dihydro-2-pyrones. In one case, an optically active lactone was obtained in 80% ee from an optically active y-hydroxy aldehyde. 

The preparation of furans from a-pyrones has been reported.74-76 For ring contraction to occur it is necessary that a suitable leaving group should be present in the 3-position. An example is given by Broy and Mayer,76 where ring opening on 3-hydroxy-a-pyrone occurred in alkaline solution, followed by ring closure to the 2-furoic acid. This reaction was also applied to 3-bromo-a-pyrone. 

Fig. 12. 600-MHz NMR spectra of (A) Streptomyces antibioticus Met tyrosinase in 100 mM sodium phosphate, pH 6.8, at 280 K (B) same as (A) but in the presence of 500 mM NaCl (C) same as (A) but in the presence of 2 mM 2-hydroxymethyl-5-hydroxy-y-pyrone (kojic acid). Asterisks indicate the N8H of the six coppercoordinating histidine residues. Reprinted with permission from Bubacco et al. (2000).

Another type of pyridazine synthesis from pyrones involves coupling of the latter with diazonium salts and rearrangement of the intermediate hydrazones with either aqueous base or acid to pyrid-azinones. Thus, 4-hydroxy 6-methyl-2-pyrone (triacetic lactone) (44)... 

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