Ascorbigen – Occurrence, Synthesis, and Analytics

Abstract

Ascorbigen (2-C-(3-indolyl)-methyl--L-xylo-hex-3-ulofuranosono-1,4-lactone, 1) is a breakdown product of glucobrassicins found in cruciferous vegetables, such as cabbage, cauliflower, Brussel sprouts, or broccoli, formed by direct C-alkylation of ascorbic acid (vitamin C) by (3-indolyl)-methyl intermediates under physiological conditions. It is a labile compound, and direct synthesis approaches are hampered by the oxidative instability of the indol moiety and ring transformations of the ascorbyl part. The latter have also impeded ascorbigen analytics for decades, and only recently different ascorbigens have been shown to represent mixtures of compounds rather than individual ascorbigen-type compounds. Following a review of pertinent literature, timely aspects of ascorbigen chemistry, including the most recent synthesis approaches and in-depth characterization of the compound, are presented.

Ascorbigen A—NMR identification

“…The 13 C chemical shifts measured in DMSO and water (Table , Figure ) are in reasonable agreement with those measured previously in CD 3 OD solutions . The appearance of 1 H NMR spectra of ascorbigen A, however, is distinctly different in different solvents (Table , Figure ).…”

“…It should be mentioned that the relative positions of the C5 and C6 signals in 13 C spectra are, beyond any doubt, interchanged in DMSO and D 2 O solutions (see the relevant parts of gHSQCAD spectra in Figure S6). The two carbons have the same chemical shifts in some CD 3 OD solutions, whereas in other solutions, the chemical shift of C5 has a shift larger by 0.04 ppm than the C6 carbon …”

“…In the course of identification of a metabolite in rapeseed ( Brassica napus ), the program for structure elucidation (CMC‐se) suggested ascorbigen A as a structure in compliance with the metabolite summary formula and the 1 H, 13 C, HSQC, and HMBC spectra. (For recent reviews of ascorbigen A occurrence, synthesis, analysis, and use, see Opietnik et al and Wagner and Rimbach due to the presence of quaternary carbons and heteroatoms breaking the proton coupling pathways.…”

“…A literature survey revealed that the structure of ascorbigen A has not been determined either by NMR or by X‐ray diffraction. The NMR signals were self‐consistently assigned to the respective atoms in the widely accepted molecular formula previously suggested in 1966. Moreover, the solvents used in these NMR studies (methanol, chloroform + methanol, pyridine, and D 2 O) precluded observation of OH and NH proton signals; their positions on the carbon skeleton have thus remained ambiguous.…”

Ascorbic acid metabolism and functions: A comparison of plants and mammals

“…During attack by insects, glucosinolates are broken down to release isothiocyanates or nitriles, the former acting as a feeding deterrent for generalist herbivores or as an attractant for specialized herbivores [40] . Ascorbate and indole-3-carbinol, a breakdown product of indole glucosinolates, react to form ascorbigen [36] , [41] . Glucosinolates are hydrolysed by myrosinase, a thioglucosidase which has an ascorbate requirement.…”