Totalsynthese von Decarboxybetalainen durch photochemische Ringöffnung von 3‐(4‐Pyridyl)alanin

Abstract

A photochemical approach is presented for the total synthesis of the decarboxybetalaines, which were previously known from the mild decarboxylation of the natural plant colorants, the betalaines: Irradiation of rac-3-(4-pyridyl)alanine (1) yielded the rac-2-decarboxybetalamic-acid-imine (4, 86%), presumably via a Dewar pyridine 2, a cyclic aminal 3 and an electrocyclic ring opening. The imine-zwitterion 4 was treated with three amines, namely (S)-cyclodopa (6), (S)-proline (7), and indoline (S), to afford three decarboxybetalaines, namely (2S)-17-decarboxybetanidine (9, red, 34%), (2S)-13-decarboxyindicaxanthine (10, yellow, 56%), and ruc-16-decarboxyindobetalaine (11, orange, 78%), respectively. The structures of these coloring matters were confirmed by their electrophoretic behavior and their spectroscopic properties. 17-Decarboxybetanidine 9 was shown to be a ca. 1: 1 mixture of two C( 15)-epimers 9a and 9b, separable by chromatography. The configuration of 9a was determined as (2S,15S) and that of 9b as (2S,15R), by correlating their optical rotations with those of betanidine (12a) and isobetanidine (12b), respectively. The decarboxybetalaines 9, 10, and 11 did not show the double-bond isomerism at C@),(Cy) of the chromophore which had been found characteristic for the corresponding betalaines 12, 13, and 14.

Structural investigations on betacyanin pigments by LC NMR and 2D NMR spectroscopy

“…Inversely, neobetanin is stable under acidic conditions allowing spectrum acquisition without progressing degradation. Actually, in previous investigations, the pH issue was found to be the most crucial parameter for the structural characterisation of betacyanins by NMR because not only did acidification of the most frequently used solvents DMSO-d 6 and methanol-d 4 by TFA or DCl ensure high solubility, but at the same time cause rapid and undesirable C-15 and C-12/C-13-isomerisations, and finally total betalain decomposition (Minale et al, 1966;Wyler and Dreiding, 1984;Hilpert et al, 1985;Schliemann et al, 1996;Kobayashi et al, 2000). Furthermore, in acidified methanol acylglucosides substituted with a dicarboxylic acid such as malonic acid may form methyl ethers (Fossen et al, 2001) and thus prevent structural elucidation of the respective genuine molecular structures.…”

“…1). The Econfiguration of the C-12/C-13 double bond was Table 1 LC-1 H NMR data of betanin (1), isobetanin (2), phyllocactin (3), hylocerenin (4), and isohylocerenin (6) deduced by comparison of the 1 H chemical shifts of H-11, H-12 and H-18 with reported data from the literature (Hilpert et al, 1985;Hilpert and Dreiding, 1984;Wyler and Dreiding, 1984). Unfortunately, the direct proof of the E-configuration of the C-12/C-13 double bond by ROESY (H-12, H-18; H-11, H-14) as well as the assignment of the remaining carbons C-13, 17, 18, 19 and C-20 (Ring C) by gHMQC was not possible, as no ROESY-and gHMQC-correlations could be observed.…”

“…Additionally, screening for a optimum solvent system was essential to achieve high solubility of betalains together with minimum overlapping of residual NMR solvent and analyte signals. As stability criteria, the time-dependent formation of additional 1 H NMR signals for H-4 and H-7 (Wyler and Dreiding, 1984;Hilpert and Dreiding, 1984;Hilpert et al, 1985) due to C-12/C-13-isomerism and the appearance of two additional anomeric protons arising from glucose released by hydrolytic betalain cleavage were monitored. Betacyanins were insoluble in organic solvents such as DMSO, ethanol, isopropanol and methanol unless the pH was set below 3.…”

“…During NMR experiments in acidified solutions, besides signals associated with the genuine pigment, the spectrum is superposed by an array of additional structures that may be produced by configurational epimerisation at C-15 (Wilcox et al, 1965;Wyler et al, 1963) as well as E/Z double-bond isomerism at C-12, C-13 (Hilpert and Dreiding, 1984;Hilpert et al, 1985;Heuer et al, 1992). Therefore, one aim of the present study was to find a solvent system affording at the same time the chemical stability and avoiding rapid isomerisation of betalains thus permitting improved and prolonged data collection including 1 H-13 C NMR correlations.…”

First 13C‐NMR Assignments of Betaxanthins

“…Furthermore, treatment of synthetic indicaxanthin with dilute alkali induces epimerization at C(11) to yield genuine (2S,11S)-indicaxanthin and (2S,11R)-isoindicaxanthin [19 -21]. Unfortunately, these diastereoisomers could not be differentiated by NMR spectroscopy [7], but have been distinguished by HPLC separation [19] [22]. Miraxanthin V (2) was first identified in flowers of Mirabilis species, and its structure was elucidated by 1 H-NMR analysis on a 60-MHz spectrometer [9].…”