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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).

An enzyme is extracted from the red peel of Amanita muscaria which cleaves the C(2)-C(3) and the C(4)-C(5) bond of the aromatic ring of L-dopa (1) to form a mixture of 4,5-secodopa ( = salt of 6-amino-2-hydroxy-4-(2'-oxoethylidene)hept-2-enedioic acid; 2) and 2,3-secodopa ( = salt of 7-amino-5-formyl-2-hydroxyocta-2,4-dienedioic acid; 3), two hitherto hypothetical biosynthetic intermediates (see Scheme). Though isolation of these products has not been possible, structural evidence is inferred from reaction products, kinetics, and spectroscopical characteristics in comparison with known compounds. Secodopas 2 and 3 are characterized in dilute solution by HPLC and UVjVIS spectroscopy (anions: I,,, 424 and 414 nm, resp., E~~~ = 25 500; on acidification, shift to 380 and 372 nm, resp.). They cyclize without enzyme catalysis, optimally at pH 4.5-5; 3 produces muscaflavin (5) and 2 betalamic acid (4). The products are identified by direct comparison with authentic samples in HPLC, by 'H-NMR of 5, and by condensation of 4 with L-proline to form the well known betalain indicaxanthin (7). The enzymatic conversion of L-dopa (1) via 2 to betalamic acid (4; (5')) and its condensation with L-proline leads to pure natural indicaxanthin (7; (2S, 1 1s)); correspondingly, the enzymatic conversion of o-dopa to (R)-betalamic acid and its condensation with L-proline produces isoindicaxanthin ((2S,IlR)) which is unknown in nature. Particularly relevant is the fact that the same enzyme cleaves pyrocatechol to produce a solution of the enolate form of the known 2-hydroxy-6-oxohexa-2,4-dienoate (secopyrocatechol; 9; see Fig. 5 ) . Dissociation constants of the corresponding enolic functions in the cleavage products are determined by spectrometric titration and compared to those of known systems.

Summary. The synthesis of the methylester of cyclodopa (5.6-dihydroxy-indoline-2-carboxylic acid) is achieved by oxidative conversion of dopa methyl ester to dopachrome methyl ester and its subsequent reduction a t pH 8, using respectively potassium hexacyanoferrate(II1) and sodium dithionite. The efficiency of the oxidation step was found to be dependent on the rather low concentration of the reactants and the very short reaction time involved. After its production cyclodopa methyl ester was stabilized by immediate protonation followed by acetylation leading in a high yield to O,O, N-triacetyl cyclodopa methyl ester. By partial hydrolysis this derivative gives an 0, N-diacetyl cyclodopa methyl ester and further N-acetyl cyclodopa methyl ester. Cyclodopa methyl ester and cyclodopa are obtained by prolonged anaerobic hydrolysis. Micro-separation procedures, nmr-, uv-spectra and chiral-optical properties of cyclodopa and its derivatives are reported and discussed. It is shown by nmr evidence that under acidic conditions the aromatic proton on C(7) of cyclodopa slowly exchanges with deuterium. When an alcoholic solution of cyclodopa methyl ester and semicarbazide is allowed to oxidize in the air, the semicarbazone of dopachrome methyl ester forms.Einleitung. -Cyclodopa I a ist unter dem Namen ((Leukodopachrom H schon langer

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