13C chemical shielding tensor orientations in a phosphoenolpyruvate moiety from 13C rotational-resonance MAS NMR lineshapes

The absolute orientations of the three ¹³C chemical shielding tensors in the phosphoenolpyruvate (PEP) moiety in a PEP-model compound with known crystal structure are reported. The study uses a fully ¹³C-enriched polycrystalline sample of triammonium phosphoenolpyruvate monohydrate, (NH₄)₃(PEP)⋅H₂O, and ¹³C MAS NMR experiments fulfilling various different ¹³C rotational-resonance conditions. The absolute ¹³C chemical shielding tensor orientations are derived by iterative fitting, employing numerically exact simulations, of various rotational-resonance ¹³C MAS NMR lineshapes of the three-¹³C-spin system in fully ¹³C-enriched (NH₄)₃(PEP)⋅H₂O. The implications of the results of this study for future, biochemically oriented solid-state NMR studies on the PEP moiety are outlined.     

13C-NMR. Study on Isoalloxazine and Alloxazine Derivatives

A series of isoalloxazine and alloxazine derivatives have been investigated by ¹³C-NMR. The synthesis of selectively ¹³C-enriched derivatives made it possible to assign unambiguously the signals due to the quaternary carbon atoms at position 4, 4a and 10a of the isoalloxazine ring system. The assignment of the other resonances was ensured by the use of selectively deuteriated and chemically modified compounds as well as by decoupling techniques. The assignments differ in part from those published by Breitmaier & Voelter [2] on FMN and FAD. The solvent dependence of the resonances has been studied in dioxan/water mixtures. The experimental data are compared with published MO calculations and discussed.     

13C,13C coupling constants from a mixture of isotopomers (13C) of substituted aromatic compounds; 1-nitronaphthalenes

¹³C Homonuclear decoupling experiments led to the assignment of the carbon-carbon coupling constants of ¹³C-enriched 1-nitronaphthalene obtained by nitration of [1-¹³C]naphthalene. The effects of substituents on coupling constants can be explained on the basis of the electronegativity of the first atom of the substituent, and the observed substituent effects are shown to be parallel to previous data for oxygen containing substituents.     

Application of biosynthetic 13C-enrichment using [1-13C]-, [2-13C]- and [1,2-13C2]-acetate as precursor for 13C NMR spectral assignment of higher plant metabolites. The assignments of some bryonolic acid derivatives

The ¹³C NMR spectra of some derivatives of bryonolic acid (1) (D:C-friedoolean-8-en-3β-ol-29-oic acid) were assigned by means of ¹³C-enrichment, lanthanide-induced shifts (LIS) and comparison of chemical shift data between derivatives. The ¹³C-enriched species of 1, i.e., 1a, 1b and 1c were biosynthesized by Luffa cylindrica (Cucurbitaceae) callus fed with [1-¹³C]-, [2-¹³C]- or [1,2-¹³C₂]-acetate, respectively. Methyl acetylbryonolates 2, 2a, 2b and 2c, methyl bryonolates 3, 3a, 3b and 3c, methyl bryononates 4 and 4a, diacetyl-3β,29-diols (3,29-diacetyl-D:C-friedoolean-8-en-β,29-diol) 5, 5a, 5b and 5c, and 3-acetyl-3β,29-diols 6, 6a and 6b were prepared from 1, 1a, 1b and 1c, and their ¹³C NMR spectra were recorded. The ¹³C concentration of the ¹³C-enriched species was high enough to exhibit the satellite peaks clearly, and the analysed data were very useful for this study. Thus, total assignments for 2, 3, 4, 5 and 6 were established. It was found that conversion of the methoxycarbonyl group at C-29 into an acetoxymethyl group caused complex changes in the chemical shifts of the C, D- and E-ring carbons and those of the methyl carbons linked to these rings.     

Carbon-13 Nuclear Magnetic Resonance Spectroscopy Study of the Glucose Stress of Human Lenses

The sorbitol theory in diabetic cataractogenesis was based on sorbitol accumulation under glucose stress. Sorbitol accumulation was examined by ¹³C nuclear magnetic resonance spectroscopy (NMR) for the first time in matched human lenses incubated in 5.5 mM and 35.5 mM C-1 ¹³C-enriched glucose up to 28 hours. The results showed that sorbitol and lactate in human tens can be detected at 35.5 mM, but not in 5.5 mM glucose solution. The glycolysis metabolic pathway of human lenses may be quite different from that of animals. The accumulation of metabolites can be traced and quantified by the intensities of ¹³C NMR peaks. Therefore, ¹³C NMR spectroscopy can be used as a valuable tool to investigate human lens carbohydrate metabolism non-interventionally.     

The ansamycin antibiotic actamycin. II. Determination of the structure using carbon-13 biosynthetic labelling

The structure of the ansamycin antibiotic actamycin is defined by analysis of biosynthetically ¹³C-enriched and natural abundance n.m.r. spectra, in conjunction with previous degradative evidence.     

Solid-phase synthesis of oligosaccharides and on-resin quantitative monitoring using gated decoupling (13)C NMR

A general strategy for solid-phase oligosaccharide synthesis capable of nondestructive quantitative monitoring has been developed. The synthesis was carried out on TentaGel using thioglycosides as glycosylating agents and dimethylthiomethylsulfonium triflate as the activator. An acylsulfonamide linker was introduced to cleave the oligosaccharide from the resin. The solid-phase reactions were monitored quantitatively by using the inverse gated decoupling technique of (13)C NMR, where two (13)C-enriched markers were used to monitor the reactions: one was (13)C-enriched glycine incorporated as a part of the linker and as an internal standard, and the other was a (13)C-enriched acetyl group used as a protecting group of the glycosylation reagent. A representative synthesis of sialyl Lewis X branched tetrasaccharide was demonstrated.     

Synthesis and NMR spectra of tetrahydrofuran-2-13C

An original method was developed for the synthesis of THF labeled selectively with the ¹³C isotope at the α-carbon atom. The effective reduction of symmetry brought about by the insertion of the isotopic label removes the excess degeneracy of the spin systems describing the multiplet structure of the ¹H and ¹³C NMR spectra, making it possible to undertake a detailed analysis. Exact values for the ¹H–¹H spin-spin coupling constants through four bonds and also the ¹³C–¹H and ¹³C–¹³C spin-spin coupling constants involving the α-carbon atom were determined for the first time. The isotopic chemical shifts of the protons and ¹³C nuclei caused by replacement of the ¹²C nucleus by ¹³C were determined. These data can be used to construct a quantitative model of the conformational behavior of THF as a molecular system in which pseudorotation in terms of vibrations with large amplitude is realized.     

13C13C spin coupling constants in phenanthrene derivatives

The synthesis of six 1-X- (X = OH, OCH₃, OCOCH₃, CH₃, CHO and CN) phenanthrene derivatives with a ¹³C label at C-1 is described. An analysis of the ¹³C¹³C spin coupling constants shows the importance of π-interaction for the coupling constant transmission. Small ¹³C¹³C spin coupling constants over 6 bonds are reported.     

Synthetic approaches to fused heteroaromatic compounds by the condensation reactions of functional pyrroles

Condensation reactions ol the “functionalized” pyrroles which were obtained by one pot synthesis of aziridines and acetylenic dipolarophiles were discussed. On treatments of 3,4-di- and 2,3,4-tribenzoylpyrroles with hydrazine hydrate and phosphorus pentasulfide, the several pyrrolo-pyridazine derivatives and fused thiophenes, respectively, were prepared. The structure proofs for the products of the reaction of the 2,3,4-tribenzoylpyrrole ( 9a ) with hydrazine hydrate were based on the ¹³C FT-nmr spectrum of the corresponding ¹³C-enriched compounds.     

Heterogeneous 1H and 13C Parahydrogen-Induced Polarization of Acetate and Pyruvate Esters

Magnetic resonance imaging of [1-¹³C]hyperpolarized carboxylates (most notably, [1-¹³C]pyruvate) allows one to visualize abnormal metabolism in tumors and other pathologies. Herein, we investigate the efficiency of ¹H and ¹³C hyperpolarization of acetate and pyruvate esters with ethyl, propyl and allyl alcoholic moieties using heterogeneous hydrogenation of corresponding vinyl, allyl and propargyl precursors in isotopically unlabeled and 1-¹³C-enriched forms with parahydrogen over Rh/TiO₂ catalysts in methanol-d₄ and in D₂O. The maximum obtained ¹H polarization was 0.6±0.2 % (for propyl acetate in CD₃OD), while the highest ¹³C polarization was 0.10±0.03 % (for ethyl acetate in CD₃OD). Hyperpolarization of acetate esters surpassed that of pyruvates, while esters with a triple carbon-carbon bond in unsaturated alcoholic moiety were less efficient as parahydrogen-induced polarization precursors than esters with a double bond. Among the compounds studied, the maximum ¹H and ¹³C NMR signal intensities were observed for propyl acetate. Ethyl acetate yielded slightly less intense NMR signals which were dramatically greater than those of other esters under study.     

Synthesis and utility of the natural type of porphyrin selectively labeled with carbon-13 at α-meso site

A concise synthesis of the 2,4-dimethyldeuteroporphyrin regiospecifically labeled with ¹³C at the α-meso site was developed. The starting material of ethyl 3,4,5-trimethylpyrrole-2-carboxylate was first converted to a ¹³C-labeled 5,5′-dimethyldipyrromethene with ¹³C formic acid, and the resulting dipyrromethene was coupled with 5,5′-dibromodipyrromethene to afford the ¹³C-labeled porphyrin in 26% yield. The paramagnetic ¹³C NMR measurements of myoglobin with the inversion-recovery method allowed us to detect selectively the α-meso-carbon signal of the iron complex. The heme is symmetric about the α,γ-meso carbon axis to prevent the orientational disorder in protein pocket. These results indicate that the ¹³C-enriched 2,4-dimethyldeuteroporphyrin is a new promising tool to elucidate the structure-function relationship of many hemoproteins.     

Chemistry of the phenoxathiins and isosterically related heterocycles. XV. Synthesis of 1-nitrophenoxathiin and the relation of the 13C-Nmr chemical shift of the alpha-carbon to the dihedral angle

The synthesis of the previously unreported 1-nitrophenoxathiin is described. The ¹³C-nmr spectrum of the title compound is assigned based on chemical shift additivities and ¹H-¹³C spin-coupling constants. From the observed ¹³C-nmr chemical shift of the alpha-carbon the title compound was predicted to have a molecular dihedral angle, Ø = 143.3°, based on a recently described interrelation between the alpha-carbon ¹³C-nmr chemical shift and the dihedral angle. Independent measurement of the dihedral angle in a crystallographic study has shown that there are two independent molecules contained in the assymetric unit which posess dihedral angles, Ø = 145.7 and 163.4°, the former in excellent agreement with the angle predicted by the assigned ¹³C-nmr data.     

Synthesis of macrolide antibiotics. 19. Synthesis of the C7-C13 fragment of erythronolide A

A stereocontrolled synthesis was carried out for the C⁷-C¹³ fragment of erythronolide A in 33 steps with an overall yield of 13% relative to starting levoglucosan.For previous communication, see [1].Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 1, pp. 186–195, January, 1990.     

An Efficient Synthesis of Optically Active [4-13C] Labelled Quorum Sensing Signal Autoinducer-2

A new synthetic route for the quorum sensing signal Autoinducer-2 (AI-2) is described and used for the preparation of [4-¹³C]-AI-2 starting from [1-¹³C]-bromoacetic acid. The key step in this process was the enantioselective reduction of an intermediate ketone. This synthesis provides, selectively, both enantiomers of the labelled or unlabelled parent compound, (R) or (S)-4,5-dihydroxypentane-2,3-dione (DPD) and was used for an improved synthesis of [1-¹³C]-AI-2.     

SiGeAl-ITQ-13和SiAl(B)-ITQ-13分子筛的合成及其甲醇转化制烃催化性能

采用直接水热合成和后合成两种方法制备Al-ITQ-13分子筛,用于催化甲醇转化制备烃类化合物。采用XRD、²⁷Al MAS NMR、NH₃-TPD、Py-FTIR和SEM等技术对所合成样品进行了表征。结果表明,两种方法制备的Al-ITQ-13结晶度都较高,且晶体形貌为薄片。与ZSM-5相比,Al-ITQ-13在甲醇转化反应中显示出较高的丙烯选择性、较高的丙烯/乙烯产物比以及较好的催化稳定性。同时,由于后合成所得到的SiAl（B）-ITQ-13强酸量减少,其催化性能优于直接合成的SiGeAl-ITQ-13。     

Highly selective 13C separation by CO2-laser-induced IRMPD of CF2Cl2/HI and CF2ClBr/HI mixtures

The CO₂-laser-induced IRMPD of CF₂CL₂/HI and CF₂ClBr/HI mixtures produced ¹³C-enriched CF₂HCl. In CF₂Cl₂/HI mixtures, the CF₂HCl underwent secondary IRMPD in the continuing pulse irradiation at the same wavenumber and fluence, yielding CF₂H₂ with a ¹³C content of 97%. Because of efficient decomposition at relatively low fluences, CF₂ClBr seems to be a promising starting molecule in two-step ¹³C enrichment.     

Concise Synthesis of (+)-[13C4]-Anatoxin-a by Dynamic Kinetic Resolution of a Cyclic Iminium Ion

An asymmetric total synthesis of [¹³C₄]-anatoxin-a ([¹³C₄]- 1 ) has been developed from commercially available ethyl [¹³C₄]-acetoacetate ([¹³C₄]- 15 ). The unique requirements associated with isotope incorporation inspired a new, robust, and highly scalable route, providing access to 0.110 g of this internal standard for use in the detection and precise quantification of anatoxin-a in freshwater. A highlight of the synthesis is a method that leverages a cyclic iminium ion racemization to achieve dynamic kinetic resolution in an enantioselective Morita–Baylis–Hillman (MBH) cyclization.     

Synthesis and 13C NMR study of some N-substituted 4-iodo- and 3,4-diiodopyrazoles

The synthesis of some novel N-1 substituted 4-iodo- and 3,4-diiodopyrazoles starting from the corresponding NH-pyrazoles is described. On basis of their ¹³C nmr data and those of already known related congeners the influence of iodo- as well as N-1 substituents on pyrazole ¹³C chemical shifts and ¹³C,¹H spin coupling constants is investigated.     

Synthesis of macrolide antibiotics. 22. Synthesis of the C8-C13 fragment of oleandonolide

The stereospecific synthesis of the C⁸-C¹³ fragment of oleandonolide was carried out in 15 steps with an overall yield of 4.48%.For previous communication, see [1].Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 2136–2142, September, 1991.