A Convenient Synthesis of 3-Hydroxy-4H-pyran-4-one Derivatives Having a Halo or Hydroxy Group at the 5-Position

The reaction of 4,5-epoxy-4-halo-6-methoxy-2-methyl- tetrahydropyran-3-ones (4b and 5b) in acidic media was examined and the following results were found: The reaction of 4b or 56 with 1% sulfuric acid afforded a mixture of 3,5-dihydroxy-4Hpyran-4-one ( l b ) and 5-halo-3-hydroxy-4H-pyran-4-one (2b or 3 b ) , but the use of concentrated acid formed the corresponding 5-halo-4H-pyran-4-one (2b or 3b) oniy.The reaction was applicable for a general method for synthesizing 3,5-disubstituted 4H-pyran-4ones (1, 2, and 3 ) .Various studies of 4H-pyran-4-one derivatives have been reported in connection with natural products,'r2 but those of 5-substituted 3-hydroxy-4H-pyran-4-ones are few and their properties have not been clearly elucidated.For example, 3,5-dihydroxy-4H-pyran-4-one ( l b ) was synthesized by the pyrolysis of D-threo-2,5-he~odiulose,~ but this method was not suitable for the synthesis because of low yield.Therefore, the synthesis of 5-substituted 3-hydroxy-4H-pyran-4-ones was studied first in order to survey their biological activities.In this paper, we wish to describe a convenient method for synthesizing 3-hydroxy-4H-pyran-4-ones (1-3) with a halo or hydroxy group at the 5-position and their derivatives.Generally, it has been known that esters of a-haloglycidic acids, obtained from dihaloacetates and ketones by the Darzens reaction, are converted into a-haloketo ester or a-hydroxyketo ester by the pyrolysis4 or solvolysis with