Dieter Rewicki

The crystal and molecular structure of the Li2O complex of 2,6-dimethoxyphenyllithium, (C8H9O2Li)6Li2O

Abstract The 2,6-dimethoxyphenyllithium molecule has the crystallographic site symmetry 3 . All lithium atoms of the six formula units of the complex are combined together with the Li2O to form a long cluster, Li8O, in the centre of the molecule. This cluster is composed of two Li4 pyramids, each of which is connected to the oxygen atom via its Li3 base in such a way that the oxygen atom has a nearly octahedral coordination with very short LiO distances. The remaining six Li3 faces of the Li4 pyramides are occupied by the organic residues as in the tetramers of ethyllithium [1] and methyllithium [2]. The final Fo  Fc map suggests weak 4-centre bonds in each of the Li4 pyramids and, as expected, between each of the metallated ring atoms, C(1), and the pyramid face to which it is attached. At the ring atoms C(1) and C(3) there are additional charges which extend in the sp2 and pz areas. The ring is planar but the CCC angles deviate systematically from 120°; the biggest deviation, of −8°, is found at C(1).

Maillard Reaction of Free and Nucleic Acid-Bound 2-Deoxy-D-ribose and D-Ribose with ω-Amino Acids

The Maillard reaction of free and nucleic acid-bound 2-deoxy-D-ribose and D-ribose with ω-amino acids (4-aminobutyric acid, 6-aminocaproic acid) was investigated under both stringent and mild conditions. Without (with) amines 2-deoxy-D-ribose (D-ribose) displays the strongest browning activity, and DNA is much more reactive than RNA. From stringent reaction between 2-deoxy-D-ribose (or DNA) and methyl 4-aminobutyrate, methyl 4-[2-[(oxopyrrolidinyl)methyl]-1-pyrrolyl]-butyrate (12) was identified by GC/MS and NMR as a new 2-deoxy-D-ribose specific key compound trapped by pyrrolidone formation. Levulinic acid-related N-substituted lactames 13-15 were identified as predominant products from DNA with amino acids, whereas RNA paralleled the reaction with D-ribose. α-Angelica lactone (2), a significant degradation product of DNA, and thiols leads under mild conditions to new addition products (e.g., 17 with glutathione). Probable reaction pathways considering activating effects of the polyphosphate backbone of nucleic acids are discussed.

Heat Generated Flavors and Precursors

The essential progress in the chemistry of heat generated flavors and their precursors since 1960 as well as the most important topics in this field are summarized. Starting with J. E. Hodge’s fundamental Maillard reaction scheme, presented first in 1953 and specified in a symposium “The Chemistry and Physiology of Flavors” in 1967, the improvements of the postulated pathways from reducing sugars to flavor compounds, reductones, and melanoidins are described. These improvements are based on the isolation and the chromatographic and spectroscopic characterization of many amino acid specific compounds in foods and Maillard model systems. Their formation could be explained within Hodge’s scheme from different precursors on specific routes, mainly on the basis of labeling experiments using 13C-labeled hexoses, pentoses, and disaccharides.

The complete 1H spectral analysis of styrene

Abstract The high-resolution 1 H NMR spectrum of styrene has been analyzed as an eight-spin system by means of the computer program LACX. Assignment of 180 experimental lines results in a rms value of 0.0116 Hz. In order to obtain appropriate input parameters for the computations, the spectrum of α-deuterated styrene has also been analyzed, simplifying the complex proton signals additionally by decoupling the vinylic protons. The signs of certain long-range coupling constants have been proved by selective decoupling experiments.

13C{1H} off-resonance decoupling with variable decoupler field strength

A 13C{1H} off-resonance decoupling technique by varying the decoupler field strength at a constant frequency offset is discussed. The usefulness of the method for spectrometers equipped with only a stepwise variation of the decoupler frequency is demonstrated by the carbon spectrum of fluorene. This leads to an unambiguous assignment of all tertiary carbon signals.

Modification of peptide lysine during Maillard reaction of D-glucose and D-lactose

Abstract In model experiments of 4-aminobutyric acid and N α -acetyl- l -lysine with [1- 13 C]- d -glucose and [1- 13 C]- d -lactose cleavage reactions into N e -formyllysine and N e -carboxymethyllysine (CML) were elucidated. The α-dicarbonyl cleavage reactions of the pyrraline and furosine route involve amine-assisted transformations, which easily undergo retro aldol cleavage. Analogous reactions were investigated in peptides by heating a synthesized heptapeptide ( N α -Ac-Lys-Lys-β-Ala-Lys-β-Ala-Lys-Gly) with d -glucose, [1- 13 C]- d -glucose and d -lactose, respectively. The modified peptides were separated by RP-HPLC and were analyzed by high resolution MALDI-TOF mass spectrometry. In both experiments, the first lysine residue contained the Amadori reaction product. The second lysine residue was transformed into the Amadori reaction product or into N e -carboxymethyllysine and N e -formyllysine, respectively. By collision-induced decomposition (CID)-ESI-MS/MS sequencing of the modified peptides, the corresponding lysine residues were identified and characterized by 1 H-, 13 C-, 2 D - 1 H/ 13 C- and 1 H/ 1 H-NMR. The volatile reaction products of the model reactions were investigated by capillary GC/MS. I am comparing the results and the distribution of the key compounds in both model systems different formation pathways can be described.

The analysis of the high-resolution 1H NMR spectrum of indene

The long-range coupling constants in unsaturated systems are still a subject of interest because of their sensitivity to conformational and other structural changes (1). Proton magnetic resonance studies in this field, however, usually do not enter into a complete analysis of the spectra avoiding the tedious work necessary to measure extremely high-resolved spectra and to analyze very complex spin systems. At this time even the spectra of simple unsaturated or aromatic compounds have seldom been analyzed if the resulting spin system involves more than six protons. We started our studies of complex proton magnetic resonance spectra with an analysis of the ‘H NMR spectrum of styrene (2). In this note we report the complete analysis of the A&MWXYZ eight-spin system of indene, which would appear to be one of several compounds related to styrene by a fixed orientation of the vinyl and phenyl moieties. Indene has been the subject of several proton magnetic resonance studies. Elleman and Manatt (3) observed a long-range coupling between one of the aromatic ring protons and those of the five-membered ring, which could be identified as the coupling between the protons 3 and 7 (4). Douris and Mathieu (5) tried to explain the ‘H NMR spectrum as a superposition of an ABCD and an AKXz spectrum. They could detect some long-range couplings, but they could not determine the magnitudes or signs. The ABCD spin system of the aromatic protons of 1,1,3-d,indene has been analyzed by Luzikow et al. (6), who were able to assign the protons by means of proton selectively decoupled 13C NMR spectra. The FT NMR spectrum of distilled indene in a 2 M solution in acetone-d6/TMS (98:2) was measured at 25°C using a Bruker WH-270 spectrometer. The solution was thoroughly degassed by several freeze-pump-thaw cycles and sealed under vacuum in a 5-mm sample tube. In order to get a high digital resolution, the spectrum was recorded in three separate parts: the aromatic protons were observed with a spectral width of 400 Hz, the olefinic protons with a spectral width of 130 Hz (filter width: 140 Hz), and the methylene protons with a spectral width of 80 Hz (filter width: 100 Hz) resulting in digital resolutions of 0.024, 0.008, and 0.010 Hz/Pt, respectively. The best linewidth was 0.05 Hz. The spectral analysis was performed with the program PANIC (7) on a Bruker Aspect 2000 computer. In

Mechanistic Studies on the Formation of Pyrroles and Pyridines from [1-13C]-D-Glucose and [1-13C]-D-Arabinose

The Maillard reaction of [1- 13 C]-D-glucose and [1- 13 C]-D-arabinose with 4-aminobutyric acid (representing peptide bound lysine as well as a Strecker inactive amino acid) and L-isoleucine (representing a Strecker active amino acid) was investigated to get more insight into the reaction pathways involved. The extent and position of the labeling were determined by MS data. The results support 3-deoxyaldoketose as intermediate of N-alkyl-2-formyl-5-hydroxymethyl- and N-alkyl-2-formyl-5-methylpyrroles (1, 3, 7, 9) and disqualify 4-deoxy- and 1-deoxydiketose routes to N-alkyl-2-hydroxyacetyl- and N-alkyl-2-acetyl-pyrroles (2, 4, 8, 10), respectively. The 1, 3-dideoxy-1-amino-2, 4-diketose C is postulated as a new key intermediate in the formation route to 2, 4, 8, and 10 from D-glucose. In addition, this β-dicarbonyl route is correlated to the 3-deoxy-aldoketose route by keto-enol tautomerism as demonstrated by Maillard experiments in deuterium-oxide. The D-exchange ratios of compounds 1, 3, 7, 9, 13, and 14 (which were examined by MS- and 1 H NMR spectroscopy) indicated incorporation of D-atoms at C4 of glucose. The β-dicarbonyl pathway of [1- 13 C]-D-glucose/L-isoleucine (glycine) Maillard experiments generates 2-acetyl-[5- 13 C]-pyrrole and 2-methyl-3-[6- 13 C]pyridinol via a Strecker active intermediate. Based on the results of these labeling experiments and on the results of Maillard experiments in deuteriumoxide a revised scheme of the Maillard reaction of D-glucose with amines and α-amino acids is presented.

Mechanistic Studies on the Formation of Maillard Products from [1-13C]-D-Fructose

Ketoses are known to react with amino compounds via ketimines to form the so-called Heyns compounds (2-aminoaldoses), which are assumed to undergo subsequent transformations parallel to those observed with the corresponding Amadori compounds. Until now, this assumption was not established by separate mechanistic studies. Therefore, we prepared [1- 13 C]-D-fructose from [1- 13 C]-D-glucose by enzymatic methods. In a series of model experiments [1- 13 C]-D-fructose was heated with 4-aminobutyric acid (Strecker inactive), L-isoleucine (Strecker active), and L-proline (secondary amine type), respectively. The labeled products were analyzed by capillary GC/MS and NMR spectroscopy and the labeling characteristics were examined from MS data. Compared to corresponding experiments with [1- 13 C]-D-glucose, the significant results are: (1) With 4-aminobutyric acid only trace amounts of 3-deoxyaldoketose products are formed in the D-fructose system, whereas 1-deoxydiketose products were generated in comparable amounts from D-glucose and D-fructose; and (2) With L-isoleucine both D-glucose and D-fructose form 3-deoxyaldoketose- and 1-deoxydiketose products in comparable amounts; but with D-fructose the most effective reaction is the formation of pyrazines initiated by a retro aldol cleavage into C 3 +C 3 fragments. This cleavage is also responsible for the formation of mixtures of isotopomeric products in D-fructose systems.

Microbial desaturation of bis(1-chloro-2-propyl) ether into a dichloro vinyl ether

Bis(chloropropyl)ethers are undesirable by-products of the industrial production of propylenoxide and epichlorohydrine by the two-step chlorohydrine process. Such by-products have been released into the environment. They are considered an important class of environmental pollutants due to their persistence and toxicity. Compounds containing an ether bond are poorly biodegradable because the ether bridge is very stable. In addition, metabolites resulting from ether transformation may be toxic for organisms or the organisms may lack the necessary enzymes for further degradation. The aerobic biodegradation of bis(1-chloro-2-propyl)ether (1, DDE) by the Rhodococcus species strain DTB occurs by scission of the ether bridge resulting in the formation of chloroacetone (4), 1-chloropropan-2-ol (5) and the transient formation of a so far unknown metabolite. The possible involvement of a flavin-containing monooxygenase and the formation of chloroacetone and 1-chloropropan-2-ol may indicate that scission of the ether bond is initiated by hydroxylation of DDE at C-2, which results in a hemiketal structure 3 that is unstable in aqueous solution and predisposed to spontaneous scission. It has been suggested by several groups that scission of ether compounds under aerobic conditions occurs via the formation of hemiacetal intermediates, the formation of which was proposed exclusively on the identification of the cleavage products, for example, the resulting alcohols and aldehydes. It was suggested that the conversion of the gasoline compound methyl tert-butyl ether into tert-butyl alcohol and formaldehyde occurs by hydroxylation, which leads to the formation of hydroxymethyl tert-butyl ether. It was proposed that the scission of diethyl ether by the Graphium sp. strain ATCC 58400 occurs by a cytochrome P450-mediated hydroxylation of the carbon atom adjacent to the ether bridge, which leads to the formation of a hemiacetal. However, ether-cleaving methane monooxygenase (MMO) is known to catalyze hydroxylation but also desaturation reactions, which after hydration will also result in a hemiacetal structure with ether as substrate. In contrast to hydroxylation of the carbon atom adjacent to the ether bridge, hemiacetal structures can also be achieved by hydroxyl shifts or by the addition of water to vinyl ethers. 8] For example, such a vinyl ether mechanism was proposed for isochorismate pyruvate hydrolase (EC 3.3.2.1), which catalyzes the conversion of isochorismic acid to 2,3-dihydro2,3-dihydroxybenzoic acid and pyruvate, while a hydroxyl shift route is operative with glycol monoethyl ether. In order to get further hints for the mechanism of ether scission with DDE, the unknown metabolite was isolated and its structure was determined by MS and NMR spectroscopy. Here we report the transient formation and characterization of an as yet unknown dichloro vinyl ether (DVE) 2 by the Rhodococcus sp. strain DTB during growth on DDE 1 (Figure 1).