No abstract

When betanin (III) was treated with S‐proline (IV), a base exchange took place which produced indicaxanthin (I). In this way this yellow pigment of the cactus fruit was partially synthesized by a method which should be generally applicable for the mutual interconversion of betacyanins and betaxanthins. For it was also possible to transform indicaxanthin into betanidin (II) by a base exchange with 5,6‐dihydroxy‐2,3‐dihydroindole‐2S‐carboxylic acid (V).

The base exchange of betanidin and isobetanidin with 5,6‐dihydroxy‐2,3‐dihydroindole‐2R‐carboxylic acid (named R‐Cyclodopa, III) led to the formation of enantio‐isobetanidin and enantio‐betanidin, respectively. The enantiomers exhibited rotatory dispersion curves which were the mirror images of those of betanidin and isobetanidin. Thus it is shown that, in agreement with our proposed structure for betanidin, there exists in the molecule another, and only one other, asymmetric centre in addition to that present at C2, and furthermore, that isobetanidin differs from betanidin only in its configuration at this second centre (C15).

Summary. During studics on the biogenesis of betalains (I) in cactus fruits (Opuntia sp.). DL- dopa-l-[14C] and -Z-[14C]were incorporated into betanin (111) which was obtained radiopure after crystallization. The specific activity remained constant after conversion to betanitlin (IV) and to a neobctanidin derivative (IX) . Reaction of radiobetanin with proline afforded indicaxanthin (V) carrying more than 90% of the radioactivity. Dopa (VI) is thus an efficient precursor for betalamic acid (VIII) but not for cyclodopa (VII) . Decarboxylation of radiobetanidin and radioindicaxanthin showed that the carboxyl group of dopa remained a carboxyl group in the biotransformation to betalaniic acid. It is concludcd that the aromatic ring of dopa is cleavcd and that re-cyclization involving the nitrogen generates the dihydropyridine moiety. Under the same conditions mcvalonic acid, aspartic acid and phenplalanine showed low incorporations.

dim et hy I -2,Z'b ia n t h race n e-8 (7H) ,8' (7'H)d i o n e ( XI1 Ia) .