Carbon-13 NMR assignments in 1,3-cyclohexadiene

Unambiguous ¹³C-NMR assignments for the olefinic carbons of 1,3-cyclohexadiene (in CDCl₃: δ_C(1) = 126.02 ppm and δ_C(2) = 124.30 ppm downfield from TMS) have been made by comparison with regioselectively synthesized 2-deuterio-1,3-cyclohexadiene (in CDCl₃: δ_C(1) = 125.88, δ_C(3) = 124.22 and δ_C(4) = 126.02 downfield from TMS). ¹H-NMR chemical shifts and splittings are shown for both compounds.     

A fluorogenic 1,3-dipolar cycloaddition reaction of 3-azidocoumarins and acetylenes

Copper(I)-catalyzed 1,3-dipolar cycloaddition reaction of nonfluorescent 3-azidocoumarins and terminal alkynes afforded intense fluorescent 1,2,3-triazole products. The mild condition of this reaction allowed us to construct a large library of pure fluorescent coumarin dyes. Since both azide and alkyne are quite inert to biological systems, this reaction has potential in bioconjugation and bioimaging applications. [reaction: see text]     

13C NMR spectra of 1,3-dipyridyl- and pyridylphenylthioureas. Chemical shift assignments and conformational implications

The ¹³C NMR spectra of a series of 1,3-dipyridyl- and pyridylphenylthioureas have been obtained. Complete analyses of the experimental spectra have provided the chemical shifts and coupling constants. The spectra of dipyridylthioureas over a temperature range showed important changes which could be attributed to an intramolecular conversion between the two equivalent E,Z and Z,E conformations. The coalescence temperature of the ¹³C signals leads to a ΔG* of ∼ 58.0 kJ mol⁻¹ for the dynamic process involved. The results show that pyridylphenyl thioureas exist in a single conformation at ambient temperature.     

Direct13C-13C spin-spin coupling constants for the triple bond in activated acetylenes

Conclusions ¹³C-¹³C SSCC values across the triple bond in activated acetylenes are determined primarily by the -electronic properties of substituents. The enhanced sensitivity of¹J_cc in acetylene derivatives to substituent effects also asserts their possible utility in estimating the electronegative properties of substituents.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1287–1292, June, 1988.     

Determination of configuration of products of nucleophilic addition to activated acetylenes by the NMR method

Conclusions It was shown that the stereospecificity of SSCC³ _C,H ^trans > ³J _C,H ^cis is applicable as a configurational test for di- and trisubstituted alkenes R₂E(Y)C=C(X)H with various heteroatomic substituents (E=N, P, S) and activating groups (X=CO₂Alk, CN at Y=H, and X=Y=CO₂Alk, CN, CF₃).Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 779–783, April, 1988.     

Synthesis and thermal transformations of pyrazolines obtained by 1,3-dipolar addition of diazocyclopropane to maleimides

1-Pyrazolines, obtained by 1,3-dipolar cycloaddition of diazocyclopropane to N-phenyl- and N-cyclohexylmaleimides, undergo complete dediazoniation at 175 °C for 10–16 h with the formation of spiro[3-azabicyclo[3.1.0]hexane-6,1′-cyclopropane]-2,4-diones 3 (80–89%) and isomeric 3-cyclopropyl-1H-pyrrole-2,5-diones 4. On the example of 3-cyclopropyl-1-phenyl-1H-pyrrole-2,5-dione, it was shown that compounds 4 are able again to enter into 1,3-dipolar cycloaddition with diazomethane or diazocyclopropane with the reaction in the case of diazocyclopropane being nonselective and leading to two regioisomeric pyrazolines in the ratio ∼1.7: 1, thermolysis of which, conversely, proceeds with high selectivity and exclusively affords a spiropentane derivative. An action of the aqueous methanol solution of sodium hydroxide on the spiropentanes fused with succinimide fragment and subsequent acidification of the salts obtained lead to stable cis-amidoacids of spiropentane series. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1680–1685, August, 2008.     

13C-13C NMR spin-spin coupling constants in saccharides: structural correlations involving all carbons in aldohexopyranosyl rings

13C-13C Spin-spin coupling constants (JCC) have been measured in a group of aldohexopyranoses and methyl aldopyranosides singly labeled with 13C at different sites to confirm and extend prior correlations between JCC magnitude and sign and saccharide structure. Structural correlations for 2JC1,C3, 2JC2,C4, 2JC4,C6, and 2JC1,C5 have been confirmed using density functional theory calculations to test empirical predictions. These geminal couplings depend highly on the orientation of C-O bonds appended to the terminal coupled carbons, but new evidence suggests that 2JCCC values are also affected by intervening carbon structure and C-O bond rotation. 3JC1,C6 and 3JC3,C6 values show Karplus-like dependences but also are affected by in-plane terminal hydroxyl substituents. In both cases, rotation about the C5-C6 bond modulates the coupling due to the alternating in-plane and out-of-plane O6. 3JC3,C6 is also affected by C4 configuration. Both 3JC1,C6 and 3JC3,C6 are subject to remote effects involving the structure at C3 and C1, respectively. New structural correlations have been determined for 2JC3,C5, which, like 3JC3,C6, shows a remote dependence on anomeric configuration. Investigations of dual pathway 13C-13C couplings, 3+3JC1,C4 and 3+3JC2,C5, revealed an important additional internal electronegative substituent effect on 3JCC in saccharides, a structural factor undocumented previously and one of importance to the interpretation of trans-glycoside 3JCOCC in oligosaccharides.     

Modern valence-bond description of chemical reaction mechanisms: the 1,3-dipolar addition of diazomethane to ethene

The electronic mechanism for the gas-phase concerted 1,3-dipolar cycloaddition of diazomethane (CH2N2) to ethene (C2H4) is described through spin-coupled (SC) calculations at a sequence of geometries along the intrinsic reaction coordinate obtained at the MP2/6-31G(d) level of theory. It is shown that the bonding rearrangements occurring during the course of this reaction follow a heterolytic pattern, characterized by the movement of three well-identifiable orbital pairs, which are initially responsible for the pi bond in ethene and the C-N pi bond and one of the N-N pi bonds in diazomethane and are retained throughout the entire reaction path from reactants to product. Taken together with our previous SC study of the electronic mechanism of the 1,3-dipolar cycloaddition of fulminic acid (HCNO) to ethyne (C2H2) (Theor. Chim. Acc. 1998, 100, 222), the results of the present work suggest strongly that most gas-phase concerted 1,3-dipolar cycloaddition reactions can be expected to follow a heterolytic mechanism of this type, which does not involve an aromatic transition state. The more conventional aspects of the gas-phase concerted 1,3-dipolar cycloaddition of diazomethane to ethene, including optimized transition structure geometry, electronic activation energy, activation barrier corrected for zero-point energies, standard enthalpy, entropy and Gibbs free energy of activation, have been calculated at the HF/6-31G(d), B3LYP/6-31G(d), MP2/6-31G(d), MP2/6-31G(d,p), QCISD/6-31G(d) and CCD/6-31G(d) levels of theory. We also report the CCD/6-311++G(2d, 2p)//CCD/6-31G(d), MP4(SDTQ)/6-311++G(2d,2p)//CCD/6-31G(d) and CCSD(T)/6-311++G(2d, 2p)//CCD/6-31G(d) electronic activation energies.     

13C NMR assignments of nonionic and anionic ethoxylated surfactants

¹³C chemical shift assignments have been made for the ethoxy region of nonionic and anionic ethoxylated surfactants. The lanthanide shift reagent Eu(fod)₃ was used to assign the shifts of nonionics because of some ambiguities in the literature.     

On the assignments of carbon-13 NMR spectra of cyclic nucleotides

Two conflicting assignments have been proposed for the ¹³C NMR signals of 3′,5′-cyclic nucleotides. This communication describes selective decoupling experiments for several cyclic nucleotides which provide the correct assignment based on unambiguous experimental evidence.     

(R)-4-menthenone in reactions of 1,4-conjugate and 1,3-dipolar addition

(R)-4-Menthenone compared with common cyclic enones exhibits considerably lower reactivity in 1,4-conjugate addition of organometallic reagents, is inert in Michael reactions and pyrazoline formation evidently due both to the distorted polarization in the enone system and to steric hindrances from the α-isopropyl group in the cyclohexene ring.     

Carbon-13 and proton NMR assignments of a new agathisflavone derivative

The (1)H and (13)C NMR spectra of 5-acetyl-7,4'-dimethoxyflavone-(6-8')-5'-acetyl-7',4'-dimethoxyflavone, a new agathisflavone derivative, were completely assigned on the basis of 1D and 2D NMR techniques.     

1H and 13C NMR spectral assignments of novel chromenylchalcones

Several types of chalcones containing 2H‐chromen group were synthesized. Claisen–Schmidt condensation of 2H‐chromen‐3‐carbaldehydes (I) with methoxy substituted acetophenones afforded (E)‐3‐(2H‐chromen‐3‐yl)‐1‐(methoxyphenyl)prop‐2‐en‐1‐ones (chromenylchalcones, 1–7). Other types of chromenylchalcone, (E)‐1‐(6‐methoxy‐2H‐chromen‐3‐yl)‐3‐(methoxyphenyl)prop‐2‐en‐1‐ones (8–13) were also obtained between reaction of methoxy substituted benzaldehydes and 1‐(6‐methoxy‐2H‐chromen‐3‐yl)ethanone (II). Dichromenylchalcones (14–16) were also synthesized through the same reaction between aldehydes (I) and ketone (II). Their complete ¹H‐NMR and ¹³C‐NMR assignments are reported here and more polysubstituted chromenylchalcones synthesized or isolated from the natural sources in the future can be identified on the basis of the NMR data reported here. Copyright © 2012 John Wiley & Sons, Ltd.