Protonenresonanzspektren von Pteridinen V. 2-Amino-4-oxo-3,4-dihydropteridine

The proton magnetic resonance spectra of 20 pteridines, i.e. 2-amino-4-oxo-3,4-dihydropteridine and 19 derivatives, have been measured at 100 Mc/s and 60 Mc/s in CF₃COOH under standardized conditions as the mono-cations. The chemical shifts of the vinyl-, methyl- and NH₂-protons are tabulated, and some typical spectra are illustrated. The influence of the substrate concentration and of solvent effects upon the NH-signals have been studied. Protonation always takes place at N(1) or N(3), and an attempt is made to correlate δ(NH₂) and pK_a values.     

Pteridine. Teil XCIV. Synthese und Eigenschaften von 5,6-Dihydro-6-(1,2,3-trihydroxypropyl)pteridinen: Kovalente intramolekulare Addukte

Pteridines: Synthesis and Characteristics of 5,6-Dihydro-6-(1,2,3-trihydroxypropyl)pteridines: Covalent Intramolecular Adducts Various 5,6-diaminopyrimidines ( 1, 15, 24, 33 ) were condensed with the phenylhydrazones of L -( 2 ) and D -arabinose ( 3 ) in acidic medium under N₂ to give formal 5,6-dihydro-6-(1,2,3-trihydroxypropyl)pteridines (see, e.g., 4 and 5 ), the latter turned out to exist preferentially as intramolecular adducts, the hexahydropyrano-[3,2-g]pteridines 6, 7, 16, 17, 25, 26 , and 34 , formed subsequently by addition of the terminal OH group of the side-chain to the C(7)?N(8) bond of the pteridine moiety. Spectroscopically, the isomeric hexahydrofuro-[3,2-g]pteridines 10,11,18,19 , and 35 were also detected as minor components in the equilibrium mixtures. In the 4-amino-2-(methylthio)pteridine series, crystallization of 6 and 7 led to the stereochemically pure (3S,4R,4aR, 10aS)-6-amino-3,4,4a,5,10,10a-hexahydro-8-(methylthio)-2H-pyrano[3,2-g]pteridine-3,4-diol ( 8 ) and its corresponding enantiomer 9 , respectively Structure 8 was proven by X-ray analysis. Acylation of the hexahydropyrano[3,2-g]pteridines yielded the more stable tri-, tetra-, and pentaacetyl derivatives 12–14, 20–23, 27–32 , and 37–39 which were characterized and of which the absolute and relative configurations were determined (¹H- and ¹³C–NMR and UV spectra, chiroptical measurements, elemental analyses).     

Nitrogen-15 nuclear magnetic resonance spectroscopy. Differential rates of N?H proton-exchange reactions of hydrazine-carbothioamide and 4-methylhydrazine-carbothioamide in dimethyl sulfoxide

The proton-coupled nitrogen-15 NMR spectra of hydrazinecarbothioamide and 4-methylhydrazinecarbothioamide have been taken at the natural-abundance level in neutral, basic and acidic solutions at 25°C. The N? H proton-exchange reactions of the hydrazino-NH₂ groups in both compounds were found to be very rapid in the presence of acid, but quite slow in the presence of base. The hydrazino-NH protons of hydrazinecarbothioamide exhange six times and 200 times faster than the amide protons in the presence of either base or acid, respectively. Similarly, acid- and base-catalyzed N? H proton exchanges of the hydrazino-NH group of 4-methylhydrazinecarbothioamide were found to be two to three orders of magnitude faster than those of N-methylamido protons. These results can be rationalized by consideration of the effect of the lone pair on the hydrazino? NH₂ group on the reactivity of the adjacent ? NH? group.     

Synthesis of chiral calix[4]arenes bearing tartaric ester moieties

The synthesis of chiral calix[4]arenes with tartaric acid ester moieties has been achieved by the reactions of tartaric ester chloroacetates with calix[4]arenes in moderate yields. All the chiral calix[4]arene derivatives are in a cone conformation according to the ¹H NMR doublet–doublet pattern of the protons of the methylene groups between the phenol rings. The results of NMR and specific rotations indicate that the molecules have C₂ symmetry with asymmetric features.     

Proton magnetic resonance studies of compounds with bridgehead nitrogen atoms—XX: The NMR spectra and stereochemistry of some hexahydropyrido[2, 1-c] [1, 4] oxazin-3(4H)-ones and related compounds

A series of substituted hexahydropyrido [2,1-c] [1,4] oxazin-3(4H)-ones has been synthesised, and the configurations of these bicyclic lactones assigned utilising chemical and spectral data. All the compounds adopt trans-fused conformations and the conformation of the lactone ring is discussed with reference to the magnitude of the geminal coupling constant of the N? CH₂? C(O)? O protons, and the vicinal couplings between the angular proton and the methylene protons adjacent to the ring oxygen atom. The lactone ring conformation is shown to differ slightly from the half chair conformation described for some monocyclic δ -lactones. The synthesis and NMR spectra of some related compounds possessing the bridgehead N? CH₂? C(O)? O system are discussed and these compounds are also shown to adopt a trans-fused ring conformation.     

Charakterisierung der Protonen in polykristallinen Parawolframaten durch 1H‐MAS‐NMR‐Untersuchungen

Characterization of the Protons in Polycrystalline Paratungstates using ¹H MAS NMR Investigations ¹H MAS NMR experiments are used to characterize the non‐acid protons of the anions in polycrystalline paratungstates by means of the measured isotropic chemical shift values. The investigation of various hydrates of ammonium paratungstate allows a direct proof of protons in NH₄ ions and in water molecules while protons of the anions are not detectable. However, for both the potassium and the sodium paratungstates ¹H MAS NMR investigations detected the protons of water molecules and the non‐acid protons of the paratungstate anions. Additional ¹H broad‐line NMR experiments at 173 K support the interpretation of the results obtained by the ¹H MAS NMR investigations. For the NMR signal of the non‐acid protons of the paratungstate anion in the ¹H MAS NMR spectra of the potassium salt line‐splitting appears. This refers to the existence of two nonidentical positions of the protons in the crystal lattice and is in agreement with the results of the X‐ray structural analysis.     

Détermination par Résonance Magnétique Nucléaire de la Configuration d'alcools Bicycliques [4.2.0] et Tricycliques [6.4.0.02,7] ou [5.4.0.02,6]

The configuration of various bicyclo[4.2.0]octanols has been established by ¹H NMR spectroscopy, with Eu(dpm)₃ as shift reagent. The intrinsic parameters Δ and K have been obtained and used as structural probes. Moreover, detailed data analysis showed that, contrary to the generally accepted concept, the vicinal coupling constant between trans cyclobutanic protons can be larger than that between cis protons. The chemical shifts obtained by ¹³C NMR spectroscopy are consistent with the proposed structures. The results have been extensively used to determine unambiguously the configuration of tricyclo[6.4.0.0^2,7]dodecanols and tricyclo[5.4.0.0^2,6]undecanol.     

Über Pterinchemie. 14. Mitteilung. Neues Verfahren zur Darstellung und Stabilisierung von Tetrahydrofolsäure und Tetrahydropterinen

A new method for preparing hydrogenated pterines by catalytic hydrogenation in halogenated acids is described. The best results are obtained with platinum or rhodium in trifluoracetic acid. By this procedure the complete and selective hydrogenation of the pyrazine nucleus of pterine, its N-methylated derivatives and of folic acid is effected in short time. For the first time the oxygen sensitive hydrogenation products have been isolated as hydrochlorides or sulphates. Some UV., NMR. and pK_a data of 5,6, 7,8-tetrahydrogenated pterines are given.     

NMR studies of polymer solutions. II. Polyethylene

The intramolecular structure of polyethylene in solution was studied by a high-resolution nuclear magnetic resonance technique. Highly purified n-alkanes (99.5%) from C₅H₁₂ to C₃₆H₇₄ were used as its oligomers. The NMR spectra of the polyethylenes (oligomers) are very sensitive to the solvents used. The internal methylene protons of all polyethylenes of various chain length resonance at an identical frequency in carbon tetrachloride. A sharp transition in the NMR spectrum of polyethylene in α-chloronaphthalene at 35°C. was observed at n-C₁₇H₃₆, above which there exist two distinguishable NMR peaks for internal methylene protons, and below which (fewer carbons) only a single peak was seen. The NMR spectra of the internal methylene protons of the polyethylenes (oligomers) taken in benzene are very similar to those taken in pyridine. They are not as easily resolved as those NMR spectra taken in α-chloronaphthalene solutions. The effect of the size of the aromatic solvent molecule on the NMR spectra of the internal methylene protons of the polyethylenes (oligomers) in solutions was demonstrated by using aromatic solvents of various sizes, such as chlorobenzene, α-chloronaphthalene, and 9-chloronathracene. The results indicate that the formation of polymeric structure of the internal methylene groups in the polyethylene chain is very sensitive to the size of the solvent used. The interaction of the solvent with the methylene groups of the polyethylenes varies as a function of chain length; it is stronger for those low member n-alkanes and decreases gradually to an asymptotic value.     

Proton NMR studies of reduced porphyrin compounds

Detailed ¹H NMR studies of ms tetraphenylchlorins (H₂TPC), new amino- and hydroxypyrroline substituted ms tetraphenylchlorins and ms tetraphenylisobacteriochlorins (H₂TPisoB) are presented and discussed. The results obtained are consistent with the general aspects of the ring current models as applied to the parent porphyrins. According to proton chemical shifts a gradual reduction in the magnitude of the ring current is observed in the order ms tetraphenylporphyrin (H₂TPP) > H₂TPC> ms tetraphenylbacteriochlorin (H₂TPB) > H₂TPisoB. The ¹H NMR spectra show chemical non-equivalence of the pyrroline ring protons due to adjacent substituents, and effects of steric constraints on the aminoalkyl substituents due to the close vicinity to the meso phenyl rings. The non-equivalence of the methylene protons of the pyrroline ring produces geminal coupling between the two methylene protons and vicinal coupling with the adjacent pyrroline proton, more pronounced in H₂TPC? OH and in . Restricted configuration of the methylene groups in the ethyl groups of H₂TPC? C(H)(CH₃)N(CH₂CH₃)₂ produces observable geminal coupling between the methylene protons. ¹H NMR reveals the difference between two types of meso phenyls in the chlorins, and three types of meso phenyls in isobacteriochlorins, as reflected in the chemical shifts of the o-phenyl protons.     

Die Kernresonanzspektren von N-substituierten 2.6-Dimethylmorpholinen

The NMR spectra of N-substituted 2,6-dimethylmorpholines after separation into cis- and trans-isomers by VPC have been investigated. The spectral parameters (chemical shift and coupling constants) are evaluated by a mathematical analysis. The spectra of the cis-compounds show some special features so far unexplainable. The difference of the chemical shift of the protons in 3-position of the cis-isomers depends strongly on the nature of the N-substituent.     

1H NMR studies of intramolecular hydrogen bonding in N-(pyridyl)amides of 6-methylpicolinic acid N-oxide

The ¹H NMR spectra of seven N-(pyridyl)amides of 6-methylpicolinic acid N-oxide in chloroform were obtained. The influence on the chemical shifts of the N? H protons of temperature, concentration and the CH₃ substituent in the pyridine ring was studied. The N? H protons were found to be shifted to low fields (～14 ppm) owing to the formation of strong intramolecular hydrogen bonding. The influence of the pyridine ring on the chemical shift of the N? H proton is comparable with the inductive effect of the p-nitrophenyl group. The hindered rotation around the N-pyridyl bond of N-(α-pyridyl)amides of 6-methylpicolinic acid in solution is discussed.     

Der Einfluss von D-Aminosäureresten auf das konformative Verhalten von Kollagenmodellpeptiden

The influence of D -amino acid residues upon the conformative behaviour of collagen model peptides. (L -Pro-L -Pro-Gly)₁₀ and (L -Pro-D (L )-Pro-Gly)₁₀ with 10 percent D -Prolin residues have been synthesized by the Merrifield technique. Further Poly-(Gly-Pro-Pro) and Poly-(Ala-Gly-Pro) with various amounts of D -amino acid residues have been synthesized by polycondensation of the proper tripeptides. The conformational behaviour of all these polypeptides has been studied by measurements at varying temperatures of optical rotation, circular dichroism and molecular weight. The results show that limited amounts of D -amino acid residues do not impedecollagen-like triple helical conformations. The denaturation temperatures, however, are considerably lowered by D -amino acid residues. The implications of these findings for the understanding of the properties of partially racemized degraded gelatins with relatively low gel melting points are discussed.     

1H-NMR-Spektroskopische Untersuchungen an methylsubstituierten Cyclohexen-(4)-dicarbonsäure-(1,2)-anhydriden

Chemical shifts and coupling constants of methyl substituted 4-cyclohexene-1,2-dicarboxylic anhydrides were determined. The NMR data demonstrate the boat conformation of the cis-anhydrides. The orientation of the cis-anhydride ring relative to the cyclohexene boat depends on the constitution and configuration of the compounds, and can be deduced from coupling constants and δ-values of the H₁- and H₂-protons. Methyl groups in the neighbourhood of these protons shift the resonance frequencies dependent upon molecular conformation.     

Conformational analysis. Part 20—conformational analysis of 4-deoxy-4-fluoro-D-glucose and 6-deoxy-6-fluoro-D-galactose in solution

The ¹H and ¹⁹F NMR spectra of the α- and β-pyranose anomers of 4-deoxy-4-fluoro-D -glucose (4FG) and 6-deoxy-6-fluoro-D -galactose (6FGA) in methanol-d₄, DMSO-d₆, acetone-d₆ and D₂O solution are reported. Computer analysis of the ABMX spectra of the CH CH₂F fragments gives accurate vicinal HH and HF coupling constants. An iterative computational analysis of the observed vicinal couplings in this fragment for 6FG, 6FGA and other molecules allows the determination of both the individual rotamer couplings and the rotamer populations. Consideration of the derived rotamer couplings strongly suggests that the correct assignment for the prochiral C-6 methylene protons in 6FG is that with the 6S proton having the larger coupling to H-5. This is the reverse of the assignment of these protons in D -glucose. In contrast, the assignment of these protons in 6FGA follows that given previously for D -galactose. The relative energies for the conformations about the C-5 C-6 bond for 4FG, 6FG and 6FGA are given from the derived rotamer populations. For 6FGA the rotamer in which the fluorine is antiperiplanar to C-4 is particularly favoured. For 4FG the rotamer with OH anti-periplanar to the ring O is highly unfavoured, but the other two rotamers are of almost equal energy. Consideration of the effect of replacing hydroxyl by fluorine in these molecules indicates that any hydrogen bonding involving the C-4 or C-6 hydroxyls plays little part in determining the conformer energies of glucose or galactose in polar solutions.     

Protonation of amino and hydroxypyrimidines. NMR-spectra and structures of mono and dications

Proton-NMR. spectra of amino- and hydroxypyrimidines including biologically important bases have been measured in four solvents: CF₃COOH, CF₃COOH? SO₂, FSO₃H and FSO₃H? SbF₅–SO₂ at 27° and ?55°C. In CF₃COOH mono-cations are formed, whereas in FSO₃H and FSO₃H? SbF₅–SO₂ double protonation occurs. In each case the structures of the protonated species are derived from the chemical shifts of CH, NH and OH protons and proton-proton spin coupling constants. A combination of the measurements described leads to a complete assignment of all proton resonances of the protonated pyrimidines. This approach is also recommended for the structural determination of heterocyclic compounds.     

Acetylene Derivatives of Cationic Diazaoxatriangulenes and Diaza [4]Helicenes - Access to Red Emitters and Planar Chiral Stereochemical Traits

Cationic triangulenes, and related helicenes, constitute a rich class of dyes and fluorophores, usually absorbing and emitting light at low energy, in the orange to red domains. Recently, to broaden the scope of applications, regioselective late-stage functionalizations on these core moieties have been developed. For instance, with the introduction of electron-donating groups (EDGs), important bathochromic shifts are observed pushing absorptions towards or in the near-infrared (NIR) spectral domain while emissive properties disappear essentially completely. Herein, to upset this drawback, acetylene derivatives of cationic diazaoxa triangulenes (DAOTA) and [4]helicenes are prepared (16 examples). Contrary to other EDG-functionalized derivatives, C≡C− functionalized products remain broadly fluorescent, with red-shifted absorptions (Δλ_abs up to 25 nm) and emissions (Δλ_em up to 73 nm, Φ_PL up to 51 %). Quite interestingly, a general dynamic stereoisomerism phenomenon is evidenced for the compounds derived from achiral DAOTA cores. At low temperature in ¹H NMR spectroscopy (218 K), N−CH₂ protons become diastereotopic with chemical shifts differences (Δδ) as high as +1.64 ppm. The signal coalescence occurs around 273 K with a barrier of ∼12 kcal mol⁻¹. This phenomenon is due to planar chiral conformations (S_p and R_p configurations), induced by the geometry of the alkyl (n-propyl) side-chains next to the acetylenic substituents. Ion pairing studies with Δ-TRISPHAT anion not only confirm the occurrence of the chiral conformations but evidence a moderate but definite asymmetric induction from the chiral anion onto the cations. Finally, DFT calculations offer a valuable insight on the geometries, the corresponding stereodynamics and also on the very large difference in NMR for some of the diastereotopic protons.     

Anionic graft polymerization of ethylene oxide on starch. II. Structure of the graft polymers

Starch–poly(ethylene oxide) graft polymers were prepared in DMSO at various monomer and starch alkoxide concentrations. Complimentary and varied information on the structure of the graft polymers was obtained from NMR and periodic acid oxidation of the polymers. From the NMR spectra of the graft polymers in pyridine containing a trace of HCl, which causes shifting of the resonance of the internal ? CH₂O? protons from the terminal ? CH₂OH protons, the polyethylene oxide content, the DP _n of the grafted side chains, and the efficiency of the alkoxides were calculated. With increase of the alkoxide concentration there was a small decrease in ? DP _n, and in the efficiency of the alkoxides in initiating graft polymerization. With increase of monomer concentration, there was only a small increase in ? DP _n but a large increase in the efficiency, indicating the existence of transfer reactions between the growing anions and the free hydroxyl groups on the starch. The results of he periodic acid oxidation showed that with increase of alkoxide concentration there was no significant change in the per cent oxidation of the graft polymers, but with increase of monomer, there was an increase in the participation of the secondary hydroxyl groups in initiation. This supports the NMR evidence for the existence of transfer reactions leading to ? DP _n values much lower than those calculated from [monomer]/[catalyst] ratios.     

Di-isotacticity of poly(methyl propenyl ether) and double-bond opening of methyl propenyl ether in cationic polymerization

The di-isotacticity of poly(methylpropenyl ether) obtained by the cationic polymerization has been studied by NMR spectra. The NMR spectra of β-methyl protons of the polymer are decoupled from the β-methine proton spectra to determine the di-isotactic fraction in a polymer. The signals of β-methyl protons at 8.78 and 8.89 τ are estimated as spectra based on threo- and erythro-di-isotactic diads, respectively. With BF₃·O(C₂H₅)₂ as a catalyst, the trans monomer yields a crystalline polymer and its structure is threo-di-isotactic. Otherwise, cis monomer produces an amorphous polymer, and it is a mixture of threo- and erythro-di-isotactic structure. From these results, it is concluded that the double bond in trans monomer is opened exclusively in the cis type, and in cis monomer cis- and trans-openings take place at almost the same rate.     

Crystal and Molecular Structure of Tricarbonyl-[(1R*, 4S*, 4aS*, 9aR*)-1,4-epoxy-2, 3-dimethylidene-1,2,3,4,4a,9,9a,10-octahydroanthracene]iron

The thermal cyclodimerization of 5,6-dimethylidene-7-oxabicyclo[2.2.1]hept-2-ene assisted by Fe₂ (CO)₉ gives the title complex 1 , a precursor for the synthesis of antitumoral anthracyclinones. The crystal structure of 1 has been determined by X-ray diffraction: a = 11.188 (1); c = 26.968 (3) Å; space group tetragonal, P4₁2₁2, Z = 8; R = 0.041; R_W = 0.033. The tricarbonyliron group is in the exo-position and the coordination polyhedron is tetragonal pyramidal. The NMR coupling constants are well-related to the observed dihedral angles between the non-aromatic protons and now give a reliable criterion for assigning the stereochemistry of the metal in d⁸-complexes of 2,3-dimethylidene-7-oxanorbornane derivatives.