Kainic acid (I) comes as colorless needle crystals of m.p. 251° (decomp.), [α]D29: -14.8° (c=1, H2O); it is soluble in water and acetic acid, sparingly soluble in methanol, insoluble in ether, petroleum ether, and benzene, and soluble in acids and alkalis; pK′ of the aqueous solution are 2.05, 4.30, and 10.08; it discolors bromine and poassium permanganate solutions; forms a salt sparingly soluble in water with heavy metals and barium. Its ninhydrin reaction is yellow, coloration with sodium naphthoquinone-4-sulfonate is orange red, and with ferric chloride, yellowish brown. Its infrared absorption curve shows the absorption peculiar to acidic amino acid. Molecular formula corresponds to C10H15O4N, molecular weight 213, and does not contain N-methyl, carbonyl, acetyl, or methoxyl group. In order to examine the properties of the functional groups in (I), the following derivatives were prepared by the routes shown in Fig. 1. Dihydrokainic acid (II), m.p. 272° (decomp.), C10H17O4N. Zinc kainate (III), m.p. over 300°, C10H13O4NZn⋅H2O. Dimethyl kainate (IV), b.p4 145°, C12H19O4N. Dimethyl N-methylkainate methiodide (VI), m.p. 188-194° (decomp.), C14H24O4NI. Monomethyl kainate (V), m.p. 250-252° (decomp.), C11H19O4N. Kainic acid betaine (VII), m.p. 205-210° (decomp.), C13H21O4N. Picrate of (VII), m.p. 174° (decomp.). Dimethyl dihydrokainate (VIII), b.p3 125°, C12H21O4N. Hydrochloride of (VIII), m.p. 158°. Diethyl dihydrokainate (IX), b.p13 105°, C14H25O4N. From the properties of these derivatives, it was seen that (I) possessed one unsaturated group and two carboxyl groups.
