13C-NMR-Studien von geschützten Aminosäuren

The¹³C-NMR spectra of a series of protected amino acids, as frequently used by peptide chemists, are discussed. The signals of the synthetic products are assigned on the basis of known chemical shift rules, spectral comparison and off-resonance spectroscopy. The value of pulse Fourier transform¹³C-NMR spectroscopy for chemists engaged in peptide synthesis is clearly demonstrated.     

13C-NMR of ABS in the solid state. Characterization and relaxation studies

¹³C-NMR spectra of bulk acrylonitrile-butadiene-styrene (ABS) resin are reported. The styrene-co-acrylonitrile (SAN) carbons can be observed selectively by using high-power proton decoupling, cross polarization, and magic angle spinning (MAS). The polybutadiene (PBd) carbons can be observed selectively by using low-power proton decoupling, with or without MAS. Methods and conditions for using ¹³C-NMR to quantify the amount of PBd in solid ABS resins are developed. The ¹³C-NMR results and results from iodine monochloride titrations for PBd content are in good agreement. Resonances of PBd in ABS in the solid state are assigned on the basis of triad sequences of cis, trans, and vinyl-1,2 geometric isomers of butadiene. Computational methods are set forth for measuring directly from the ¹³C-NMR spectra the percent of cis, trans, and vinyl-1,2 isomers in PBd in the ABS resin. The NMR measurement of PBd microstructure appears to be preferable to infrared (IR) determinations. Relaxation parameters (T₁ and NOE) are reported for PBd in bulk ABS as a function of temperature and are analyzed in terms of a distribution of rotational correlation times. These relaxation parameters are similar to those of nongrafted PBd and indicate that the local motions in grafted and nongrafted PBd are similar.     

Nylon 6 structure in solid state as studied by high-resolution 13C-NMR spectroscopy

High resolution ¹³C-NMR spectra of nylon 6 samples crystallized under various conditions and of a drawn sample were measured at room temperature by the cross polarization/magic angle spinning (CP/MAS) and pulse saturation transfer/magic angle spinning techniques. Additionally, ¹³C-NMR spectra of the drawn sample were measured at temperatures from 20 to 100°C by the CP/MAS technique and at 20 and 100°C by the low-power decoupling/magic angle spinning technique. The nylon 6 structure in the solid-state is discussed on the basis of these results. The solid-state ¹³C chemical shift data are used for reference in a study of conformation in solution.     

Identification of microbial inhibitory functional groups in corn stover hydrolysate by carbon-13 nuclear magnetic resonance spectroscopy

Dilute-acid biomass hydrolysates contain biomass degradation products that are inhibitory to cell growth and fermentation. Overliming with Ca(OH)₂ has been found to be one of the most effective methods for detoxifying the dilute-acid hydrolysate for ethanol production. However, the mechanism of overliming is not well understood. Carbon-13 nuclear magnetic resonance (¹³C-NMR) spectroscopy was used to elucidate the functional groups involved in the overliming reaction. The ¹³C-NMR spectra showed that the major functional groups removed during the overliming process were aliphatic and aromatic acids or esters, and other aromatic and aliphatic compounds. Ketone and aldehyde functionalities were not detected in the spectra. This is the first time that ¹³C-NMR has been used to elucidate the overliming reaction.     

Identification of Initiator Fragments in Polyisobutylene by Nmr Spectroscopy

Abstract

Deuterated polyisobutylenes carrying protonated initiator fragments were prepared by the living polymerization technique employing perdeuterated isobutylene [CD₂=C(CD₃)₂] and select protonated initiators (see Scheme 1). The polymers were analyzed by ¹H- and ¹³C-NMR spectroscopy, and the resonances due to the protic initiator fragments were unequivocally assigned. The assignments of ¹³C-NMR signals were affected by the distortionless enhancement by polarization transfer mode of spectra accumulation.     

A 13C-NMR. Study of cis-trans isomeric vitamins A, carotenoids and related compounds.

The ¹H-decoupled ¹³C-NMR. spectra of 35 all-trans, 17 mono-cis vitamin A compounds (acetates, alcohols, aldehydes, acids and esters) and of one 11, 13-di-cis compound (11, 13-di-cis retinol) are reported. Included in this investigation are desmethyl-, desmethylethyl, and aryl-vitamin A analogues and others as well as 30 reference compounds of smaller molecular weight. Furthermore, the ¹³C-NMR. spectra of 23 β-apo- and other carotenoids were studied. A complete assignment of the signals of all 106 compounds to the specific carbon atoms was achieved by extensive application of lanthanide shift reagents, mainly Yb(dpm)₃, by CW-offset and selective ¹H-decoupling experiments, by comparison of the shifts of related compounds, and in three cases by utilization of specifically deuteriated compounds (11, 12-D₂-retinol and retinyl acetate, 15, 15′-D₂-β-carotene). The chemical shift differences between the cis- and trans-vitamin A compounds and the applicability of the shift reagents for the assignment of the ¹³C-NMR. spectra are discussed.     

Grindstone Chemistry: Design,One-Pot Synthesis,and Promising Anticancer Activity of Spiro[acridine-9,2′-indoline]-1,3,8-trione Derivatives against the MCF-7 Cancer Cell Line

In this study, the synthesis of one-pot 10-phenyl-3,4,6,7-tetrahydro-1H-spiro [acridine-9,2′-indoline]-1,3,8-trione derivatives was achieved via a four-component cyclocondensation reaction, which was carried out in solvent-free conditions, and using p-toluenesulfonic acid (p-TSA) as a catalyst. The product was confirmed by FT-IR, ¹H-NMR, ¹³C-NMR, mass spectra, and elemental analysis. Furthermore, the anticancer activity was screened for all compounds. Among these compounds, compound 1c was more effective (GI₅₀ 0.01 µm) against MCF-7 cancer cell lines than standard and other compounds. Therefore, the objective of this study was achieved with a few promising molecules having been demonstrated to be potential anticancer agents.     

Synthesis and NMR elucidation of pentacycloundecane-derived hydroxy acid peptides as potential anti-HIV-1 agents

The synthesis and NMR elucidation of eight novel peptides incorporating the pentacycloundecane (PCU)-derived hydroxy acid are reported. The PCU cage amino acids were synthesized as racemates and the incorporation of the PCU-derived hydroxy acid with natural (S)-amino acids produced inseparable diastereomeric peptides. A series of overlapping signals from the cage and that of the peptide side chain was observed in the ¹H- and ¹³C-NMR spectra, complicating the elucidation thereof. Two-dimensional NMR techniques proved to be a very useful tool in overcoming these difficulties. These compounds are potential HIV protease inhibitors.     

Control synthesis of isocyanate and alkoxy-silane terminated macromers

The kinetics of the reactions in bulk of 4,4′-dicyclohexyl methane diisocyanate (H₁₂ MDI) and 5-isocyanato-1,3,3-trimethylcyclohexylmethyl isocyanate or isophorone diisocyanate (IPDI) with benzylic alcohol (BZA) and α-hydroxy-ω-methyl ether-terminated polyethylene oxide PEO (M?_w = 350) were studied by size exclusion chromatography (SEC) and ¹³C nuclear magnetic resonance (¹³C-NMR). The substitution effect is exhibited in the case of H₁₂ MDI reactivity. The kinetic constants were calculated by a numerical method. The second-order kinetic mechanism was shown to be valid. In the IPDI case, the cycloaliphatic isocyanate group is shown to be more reactive than the aliphatic group in our conditions, without catalysis, in agreement with previous results from the literature, in our obtained by ¹H-and ¹³C-NMR without any catalyst. The reactivity ratio is found to be on the order of 3. This difference in reactivity of the two isocyanate groups is used for the control synthesis of isocyanate and alkoxy-silane-terminated macromers.     

Biosynthesis of the cytochalasans. Part III. C-NMR. of cytochalasin B (phomin) and cytochalasin D. Incorporation of [1-13C]- and [2-13C]-sodium acetate

Sequential single frequency decoupling and partially relaxed Fourier transform (PRFT) are used to assign the natural abundance ¹³C-NMR. spectra of cytochalasin B (phomin) ( 1 ) and cytochalasin D ( 2 ). Cultures of Phoma spec. S 298 were fed [2-¹³C]-sodium acetate, and the distribution of this precursor in cytochalasin B (phomin) ( 1 ) was determined by ¹³C-NMR. spectroscopy. Likewise, the labelling patterns in cytochalasin D (zygosporin A) ( 2 ) from Zygos-posium masonii could be identified after incorporation of [2-¹³C]-sodium acetate and [l-¹³C]-sodium acetate. The results confirm previous proposals for the biogenesis of the cytochalasans from phenylalanine, methionine, and a C₁₈, or C₁₆, polyketide part.     

A 13c-NMR study of stoving alkyd systems

¹³C-NMR has been used to characterise solutions of alkyd binders used in heat-cured paints. The insoluble products formed between alkyds and urea-formaldehyde and melamine-formaldehyde resins were swollen in deuterochloroform to provide good quality ¹³C-NMR spectra. Using this technique, the changes in the alkyd resin during the curing process were monitored.     

Stereoregularity of poly(2-vinylpyridine) and poly-(4-vinylpyridine)

In order to determine the stereoregularity of poly(4-vinylpyridine), 4-vinylpyridine-β,β-d₂ was synthesized from 4-acetylpyridine. The ¹H-NMR spectra of the deuterated and nondeuterated polymers were measured and analyzed. From the ¹H-NMR spectra of poly(4-vinylpyridine-β,β-d₂), triad tacticity can be obtained, while the ¹H-NMR spectra of nondeuterated poly(4-vinylpyridine) give the fraction of isotactic triad. The ¹³C-NMR spectra of poly(4-vinylpyridine) were also observed, and the spectra of C₄ carbon of polymers were assigned by the pentad tacticities. The fraction of isotactic triad of poly(2-vinylpyridine) and poly(4-vinylpyridine) obtained under various polymerization conditions were determined. The radical polymerization and anionic polymerizations with phenylmagnesium bromide and n-butyllithium as catalysts of 4-vinylpyridine gave atactic polymers.     

Piperaceae alkaloids: Part IV. Structure and Synthesis of Cyclostachine A,Cyclostachine B and Cyclopiperstachine

Three new alkaloids designated as cyclostachine A (2), cyclostachine B (7) and cyclopiperstachine (10) have been isolated from Piper trichostachyon C. DC. Their structures have been derived on the basis of spectral and degradative studies and confirmed by synthesis. The ¹H- and ¹³C-NMR. spectra of these compounds are discussed.     

A 13C-NMR investigation of plasma polymerized ethane,ethylene, and acetylene

Solid-state ¹³C-NMR spectra were obtained by cross-polarization and magic angle spinning of polymers prepared by injecting ethane, ethylene, and acetylene into a radiofrequency plasma. By use of the delayed decoupling technique to suppress protonated carbon peaks and difference spectroscopy five resolved spectral bands can be distinguished. These bands are assigned to (I) unsaturated nonprotonated, (II) unsaturated CH and CH₂, (III) quaternary, (IV) methine and methylene, and (V) methyl carbons by comparison with standard ¹³C shifts compiled for organic materials. The relative amounts of these structural features in the polymers were determined quantitatively and the possible sources of errors considered.     

Die NMR-Spektren von CF3I,CF3IF2 und CF3IF4

The NMR Spectra of CF₃I, CF₃IF₂, and CF₃IF₄ The ¹⁹F-NMR and ¹³C-NMR spectra of CF₃I, CF₃IF₂ and CF₃IF₄ were recorded in acetonitrile solution. The chemical shifts of the CF₃-groups are strongly dependent on the oxidation state of the iodine atom. With increasing oxidation state the resonances of the CF₃-groups in the ¹⁹F-NMR spectra are characteristically shifted to high field, whereas in the ¹³C-NMR spectra a characteristic shift to low field is measured. The absolute value of the coupling constants ¹J(¹⁹F? ¹³C) increases with increasing oxidation state from 344 Hz (CF₃I) via 354 Hz (CF₃IF₂) to 359 Hz (CF₃IF₄).     

Peak area measurements of cross-polarization magic-angle-spinning 13C-NMR spectra of crosslinked polystyrenes

Cross-polarization magic-angle-spinning ¹³C-NMR spectra of polystyrenes crosslinked with 1–20% of methine vinyl carbon ¹³C-labeled p-divinylbenzene and of Friedel–Crafts crosslinked poly(chloromethylstyrene)s have been obtained with both glossy solid and CDCl₃-swollen gel samples. The spectra of natural abundance, uncrosslinked, glassy polystyrene, and the spectra of the solid labeled networks give aliphatic and aromatic peak areas only 0.7 times as large per ¹³C atom as that of poly(oxymethylene). Similarly the crosslinked poly(chloromethylstyrene) gave peak areas about 0.6 times that of internal poly(oxymethylene). The labeled gels give peak areas 0.2–0.6 times as large per ¹³C atom as glassy polystyrene, and the peak areas in spectra of gels increase with the divinylbenzene content     

13C-NMR研究丁二烯-异戊二烯共聚合的竞聚率和共聚物的序列分布

 用¹³C-NMR方法研完了不同配料比的丁二烯-异戊二烯本体共聚和溶液共聚物的结构,定量计算出了共聚物二元组的浓度和数均序列长度,采用T(?)DO″S法计算出了本体共聚和溶液共聚的竞聚率,并证明各种共聚产物的序列分布都服从一级Markov统计模型。     

13C- and 1H-NMR. Assignments for colchicine derivatives

The ¹³C-NMR. spectra of a number of colchicine derivatives are given comprising examples of the normal series ( 4→10 ), iso series ( 11→16 ) and colchicine series ( 17 ), which were either reported in the literature or obtained by partial synthesis or degradation reactions. The ¹³C-NMR. assignments were made by comparisons with known compounds and selective single-frequency offresonance decoupling experiments. Selective proton decoupling experiments have also allowed assignments of the H —C(11) and H —C(12) protons of the iso and colchicine series.     

Hydrogenolysis of 3,5-0-benzylidene acetals with the LiAlH4-AlCl3 reagent in methyl d-xylofuranosides

The hydrogenolysis of methyl 3,5-0-benzylidene-α- and -β-D-xylofuranoside derivatives with the LiAlH₄-AlCl₃ reagent gave 5-benzyl ethers as main products. In some cases the attack of the reagent occured at the ring oxygen of the furenoside skeleton to yield 5-0-benzyl-1-0-methylxylitol derivatives. The structure of the synthesized compounds was proved by ¹³C-NMR spectroscopy. Unambiguous assignment of lines in the ¹³C-NMR spectra of numerous partially methylated methyl α-and β-d-xylofuranoside derivatives has been made.     

Stereoregularity of poly(α-methylvinyl alkyl ether)s determined by 1H- and 13C-NMR spectroscopy

α-Methylvinyl methyl ether, ethyl ether, and isobutyl ether were polymerized under various polymerization conditions and the structure of the polymers was determined by ¹H- and ¹³C-NMR spectroscopy. α-Methyl and β-methylene carbon spectra of poly(α-methylvinyl isobutyl ether) showed splitting and were analyzed by triad and tetrad sequences. β-Methylene carbon spectra of poly(α-methylvinyl ethyl ether) also showed splitting. When Eu(fod)₃ was added, α-methyl and methoxy proton spectra in benzene of poly(α-methylvinyl methyl ether) showed splitting assigned to triad tacticities. All the polymers obtained in polar solvents exhibited an increase in syndiotacticity. The polymerization mechanism is discussed.