Deuterierung von Betanidin und Indicaxanthin, (E/Z)‐Stereoisomerie in Betalainen

Abstract

SummaryPreviously unexplained 'H-NMR signals of the red-violet betanidine and of the yellow indicaxanthine in CF,COOH are interpreted with the help of deuteration experiments in CF,COOD. They confirm the 1,7-diazaheptarnethinium chromophore of these pigments and further show an (E/Z)-stereoisomerism at one of the partial double bonds: both betanidine and isobetanidine, in CF,COOH solution, consist of a -75:25 mixture of the ( 1 2 4 -(1 resp. 12) and the (122)

First 13C‐NMR Assignments of Betaxanthins

“…For the (8E)-and (8Z)-isomers, shift differences A C H T U N G T R E N N U N G of Dd(H) = À 0.29 for HÀC(7), + 0.29 for HÀC (8), and À 0.26 for HÀC (14) were reported earlier [12]. Comparison of the magnitude of these values with the respective shift differences of (1E,8E)-1a and 1c (Dd(1a -1c): À 0.32 for HÀC(7), + 0.33 for HÀC(8), À 0.30 for HÀC (14)), as well as for (1Z,8E)-1b and 1d (Dd(1b -1d): À 0.32 for HÀC(7), + 0.34 for HÀC(8), À 0.36 for HÀC (14)) suggest the (1E,8Z)-and (1Z,8Z)-configurations for 1c and 1d, respectively.…”

“…In aqueous solution, indicaxanthin mainly undergoes double-bond isomerization at N(1)=C(7). In contrast, in neat TFA, only (8E)-and (8Z)-indicaxanthin were reported, in a ratio of 65 : 35 [12]. By comparing the 1 H-NMR chemical shifts and shift differences of especially HÀC(7), HÀC(8), and HÀC (14), assignment of the two diastereoisomers based on anisotropy effects was achieved [12].…”

“…By comparing the 1 H-NMR chemical shifts and shift differences of especially HÀC(7), HÀC(8), and HÀC (14), assignment of the two diastereoisomers based on anisotropy effects was achieved [12]. However, no (E/Z)-stereoisomerism at the partial double bonds N(1)=C(7) and C(7)=C(8) could be observed due to fast rotation and steric conditions in TFA, respectively [12].…”

“…Consid- ering the 1,7-diazaheptamethinium substructure of indicaxanthin, a total of eight stereoisomers are theoretically possible due to double-bond isomerism at N(1)=C (7) and C(8)=C (9), as well as based on s-cis/s-trans conformations of the dienyl moiety caused by rotation about the C (7)ÀC (8) bond [3] [12]. NOESY Correlations and chemical-shift considerations allowed the unambiguous identification of the major stereoisomers 1a and 1b.…”

“…However, due to the inherent liability of betacyanins [5] [6] and betaxanthins [7 -9] under acidic conditions, 13 C-NMR measurements requiring prolonged data acquisition have been exclusively reported for neobetanin [10] and, just recently, for acylated and non-acylated betacyanins under neutral conditions [11]. Apart from an earlier report by Wyler and Dreiding [12] acquiring NMR signals for indicaxanthin over two years in neat CF 3 …”