2-Pyrone from coumalic acid

Scheme 13. One-step synthetic protocol for 3,5-dibromo-2-pyrone from coumalic acid...

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... 

Both 3-bromo-2-pyrone 4 and 5-bromo-2-pyrone 5 can also be prepared from 2-pyrone-3-carboxylic acid 40 and coumalic acid 39 in one step, albeit in moderate yields, utilizing the same protocol, varying only the equivalents of NBS employed in the reaction (Scheme 14). 

The long known synthesis of coumalic acid from treatment of malic acid with hot sulfuric acid illustrates this route decarbonylation produces formylacetic acid, in situ, which serves as both 1,3-aldehydo-acid component and the second component. Decarboxylation of coumalic acid is still used to access 2-pyrone itself. ... 

---