CHEMISTRY OF A COLOR REACTION OF RETINOIC ACID (VITAMIN A ACID) IN SULFURIC ACID
Retinoic acid (vitamin A acid) (I) has long been recognized as one of the most interesting members of the retinoid family (l). A quantitation method of I was proposed by KAWASAKI et al. who estimated an absorptivity of coloration developed by 740 sulfuric acid (2). The color was found to be stable in sulfuric acid, persisting for several hours at room temperature, so they used the method for assay of I in animal tissues. However, since no exact chemical approaches have been done on this color reàction and many aspects remain mostly unexplored, it seems desirable that the mechanism of this reaction should be better understood, so that the method may be qualified as a highly selective method of analysis. Furthermore, the investigation on this subject is expected to afford a great deal of knowledges on the chemistry of conjugated polyenoic acids, because little is known about their behavior toward strong acids. As an extension of model reactions, which were carried out previously on conjugated trienoic acid (Il) (3) and tetraenoic acid (Ill) (4), this report describes the first chemical explanation on the titled reaction and a brief survey on the chemistry of a homologous series of I (I, Il, and Ill) toward sulfuric acid.
Treatment of the all-trans conjugated pentaenoic acid I using chloroform (3 vol) with 740 0 sulfuric acid (d I .6574) (70 vol) at room temperature for a period of a few minutes, pouring the aqueous acidic layer onto ice-water (210 vol), extracting with ether three times, and methylating with diazomethane in an absolute ethereal solution led to the formation of a methylated mixture consisting of at least four to five quenched products. Serious difficulties were encountered in a separation of these products by conventional method such as preparative gas-liquid chromatography (PGLC) employed in our previous studies (3, d). For example, none of the fractions obtained by PGLC furnished any clean NMR spectral data even after laborious purification was repeated. After a preliminary purification on an alumina (50 0 1-120) column, however, an application of a recent high-performance liquid chromatographic technique (HPLC) on a Zorbax SIL column employing 3 0 ether in n-hexane
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