H. Wyler

The conversion of betanidin and betanin to neobetanindin derivatives

Abstract Details are given for the conversion of betanin, the pigment of the red beet, and its aglucone, betanidin, to a number of derivatives of neobetanidin trimethyl ester. The conversion occurred, presumably by air-oxidation, when the basic conjugated system was deprotonated under the influence of diazomethane, pyridine and acetic anhydride, amines or sodium acetate. The neobetanidin-esters turned out to be 14, 15-dehydrobetanidin derivatives and were the key for the structure elucidation of the betacyanins and betaxanthins. Their constitutions could be derived essentially by comparison of the NMR spectrum of the 5,6-di-O-acetyl derivative with those of two degradation products from betanidin, namely N-acetyl-5,6-diacetoxy-2,3-dihydroindole-2-carboxylic-acid methyl ester (now called triacetylcylodopa methyl ester) and 4-methyl-pyridine-2,6-dicarboxylic-acid methyl ester. A separate proof for the vinylene group connecting the two heterocycles was available through its reduction in a Pd catalyzed reaction at the expense of a dehydrogenation of the dihydroindole system. The same type of disproportionation reaction could also be applied ot N-styrylindolin, which was converted to N-(β-phenylethyl) indole. The neobetanidin structures contain a “1,7-diazaheptamethin” system. This new expression, which is explained briefly, allows a rationalization of the absorption spectra (color), the halochromism (about 100 nm), the pKa-values (1·6 to 2·4) and certain NMR signals of the neobetanidin derivatives.

Chemical Review and Evolutionary Significance of the Betalains

The betalains, which include the red-violet betacyanins and the yellow betaxanthins (Fig. 10.1), represent a structurally and biosynthetically distinct class of naturally-occurring pigments. These pigments are found in flowering plants only in members of certain families of the order Caryophyllales (Centrospermae) (Table 10.1), though betaxanthins have also been shown to occur in the fly agaric, Amanita muscaria (Dopp et al. 1982). Both betacyanins and betaxanthins possess a dihydropyridine moiety which is attached via a vinyl group to another nitrogenous group. In the betacyanins the latter is a glyeosyl-hydroxylated dihydroindole ring; in the betaxanthins it is any one of several amino acids or amines. The basic chromophore of the betalain pigments is the 1,7-diazaheptamethinium system, as discussed by Mabry et al. (1967) and Mabry and Dreiding (1968). The different spectral properties of the betacyanins and betaxanthins are due to the extension of this conjugated system through the dihydroindole moiety in the betacyanins. The term “betalains” was introduced by Mabry and Dreiding (1968) to emphasize the common biogenetic and structural features of these two groups, which clearly distinguish this class of pigments from the more common group of plant pigments, the anthocyanins.