1960 年 80 巻 1 号 p. 72-78
Ultraviolet spectra of the aqueous solution of compounds related to meconic acid showed varying stability and some labile substances were found. In order to find relationship between structure and stability of ultraviolet spectrum, and its reason, variation in molar extinction of over 20 kinds of compounds related to meconic acid was measured in aqueous, 0.05N hydrochloric acid, and 0.05N sodium hydroxide solutions after leaving for 24 hours. The ultraviolet spectrum in the aqueous solution was extremely labile in a compound with hydroxyl in the 5-position which is considered to take the keto form easily. However, even this compound became stable in 0.05N hydrochloric acid solution in which the γ-pyrone ring would hardly undergo ring opening. The compound was completely stabilized when derived to the enol form by chelating with a metal. The compounds with stable ultraviolet spectrum in aqueous solution became labile in 0.05N sodium hydroxide solution when their γ-pyrone ring was opened. It was thereby found that the compounds not only underwent ring cleavage in 0.05N sodium hydroxide solution but also underwent decomposition after ring cleavage to a β-diketone compound. From such facts, the reason why compounds whose hydroxyl in 5-position easily takes the keto form alone show extremely labile ultraviolet spectrum in aqueous solution was explained as shown in Chart 1.
Decomposition rate of these compounds in 0.05N sodium hydroxide solution was measured from the ultraviolet spectrum measured after leaving in 0.05N sodium hydroxide solution for 24 hours and the solution acidified with hydrochloric acid to effect cyclization. By this means, effect of 5-OH, 5-OCH3, and 2-COOH on the stability of γ-pyrone ring in meconic acid derivatives was examined. In this case, γ-pyrone ring becomes more easily liable to alkaline attack as the symmetry around ketone group in 4-position of γ-pyrone ring is destroyed.