Covalent hydration of 5-substituted pyrimidines

5-Substituted pyrimidines in which the substituent is electron-donating or weakly electron-withdrawing, form normal cations in aqueous acidic solutions. Those in which the substituent is strongly electron-withdrawing form 3,4-hydrated cations, which are usually rather prone to undergo ring cleavage reactions.¹H NMR, ¹³C NMR and u.v. spectroscopy are used in the study.     

Synthesis of 5-substituted pyrimidines. ortho-directed lithiation of pyrimidine derivatives

ortho-Directed lithiation of some pyrimidines has been investigated. Treatment of 2- and/or 4-alkoxy or acylaminopyrimidine with lithium 2,2,6,6-tetramethylpiperidide in ether at 0°, followed by quenching with various electrophiles afforded the corresponding 5-substituted pyrimidines.     

Proton magnetic resonance studies of tautomerism in substituted pyrimidines

Studies of line width and chemical shift vs. temperature for amide and hydroxyl proton magnetic resonance signals from: barbituric acid, dialuric acid, parabanic acid, alloxan and alloxan monohydrate dissolved in anhydrous dimethyl sulfoxide-d₆ are reported. The behavior of the amide signals shows that, between 20 and 65°C, none of the compounds listed exhibits lactim-lactam tautomerism. The amide proton resonance in uracil has also been investigated. The signal is a closely spaced, equal intensity, doublet due to the non-equivalence of H(1) and H(3). Again, no evidence of tautomerism is observed. Activation energies for the hydroxyl resonances in dialuric acid and alloxan monohydrate indicate hydrogen bonding between solute and solvent. The results of simple LCAO-MO calculations are in accord with the experimental conclusions concerning tautomeric equilibria.     

Proton magnetic resonance study of the configurations of severalΒ-substituted vinylcarbonyl compounds

PMR spectra of several -substituted vinylcarbonyl compounds were studied. Analysis of the spin-spin interaction constants and the chemical shifts leads to the conclusion that the molecules of methyl cis- and trans--chlorovinyl ketone, methylmalondialdehyde, and malondialdehyde exist in a cissoidal conformation. The molecule of cis--chloroacrolein has a transoidal conformation, and a rotation around the C-C bond takes place in trans--chloroacrolein.The results of the experiments are compared with calculated chemical shifts, which are determined by the effect of diamagnetic anisotropy and electrical charges of the C=C, C=O, and C-Cl bonds.     

Proton magnetic resonance study of rotational isomerism in vinylcycloalkanes

The vicinal coupling constant, J(12), between the vinyl CH and the ring CH protons in vinylcyclohexane was calculated from a ‘partial molecule’ six-spin system. The 100 and 270 MHz results are in good agreement; those at 60 MHz were, however, still inaccurate in this approximation. J(12) increases with increasing solvent polarity and decreasing temperature. The energy difference between the s-trans and gauche conformers in both C₂Cl₄ and perdeuterioacetone solvents is 879 ± 167 J mol^?1 (210±40 cal mole^?1). The s-trans conformer is the most stable, in contrast to the isoelectronic cyclohexylcarboxyaldehyde where the gauche rotamers are lower in energy.     

Proton magnetic resonance spectra of some 6- and 7-substituted 2-amino-4-hydroxypteridines

The conformations in solution of 6-phenyltetrahydropterin, 6,7-diphenyltetrahydropterin and 6-7, dimethyltetrahydropterin have been determined from their proton magnetic resonance spectra. The 6-phenyltetrahydropterin has the reduced pyrazine ring in a half chair conformation with the phenyl ring in the equatorial position. Both the 6,7-diphenyl- and 6,7-dimethyltetrahydropterins are of the cis configuration and have one substituent in an axial position. The enzymatic activity of these cofactors may be related to these conformations.     

NMR study of 5-substituted pyrazolo[3,4-c]pyridine derivatives

Substituted pyrazolopyridines are potent inhibitors of phosphodiesterases and cyclin-dependent kinases. In this study, NMR was used to investigate the potential N1-H and N2-H tautomerism of 5-substituted pyrazolo[3,4-c]pyridine derivatives. Six compounds were fully characterized by using (1)H, (13)C, and (15)N chemical shifts and indirect (1)H--(13)C and (1)H--(15)N coupling constants. The (1)H NMR spectra were measured over a broad range of temperatures. All of the compounds were shown to exist predominantly in the N1-H tautomeric form. Complementary quantum-chemical calculations of the chemical shieldings and indirect spin-spin couplings support the structural conclusions drawn.     

Proton magnetic resonance spectra of the atropisomers of ortho-hexaphenylene

The structures of the atropisomers of o-hexaphenylene are confirmed by their proton magnetic resonance spectra. The centrosymmetrical isomer 1a gives rise to an AA′BB′ spectrum, whereas the helical isomer 1b displays an AA′BB′ spectrum for two of its aromatic rings and an ABCD for the remaining four rings. Long-range coupling between the AA′BB′ and ABCD parts of 1b leads to a tentative assignment of the protons. An analysis of the spectrum of o-tetraphenylene ( 2 ) is also given.     

Proton nuclear magnetic resonance and infra-red studies of the outer-sphere hydration of pentane-2,4-dionato metal complexes in benzene solution

Studies of hydrogen bonding between water and bis(pentane-2,4-dionato)-beryllium(II) (Be(acac)₂), bis(3-methylpentane-2,4-dionato)-beryllium(II) (Be(Meacac)₂), and tris(pentane-2,4-dionato)cobalt(III) (Co(acac)₃) have been undertaken in [²H]₆-benzene solution using¹H NMR and infra-red spectroscopy. Equilibrium constants for 1:1 water-metal complex hydrates, and approximate enthalpies and entropies of reaction, have been measured from the¹H NMR data. The equilibrium constants are larger for Co(acac)₃ than for the Be(II) complexes, in line with previous data for the partition of these chelates between water and aliphatic hydrocarbon solvents.     

Atranes. V. Proton magnetic resonance spectra of organyl and organoxysilatranes

The proton magnetic resonance spectra of eight organyl- and organoxysilatranes, (R = CH₃, C₂H₅, i-C₃H₇, CH₂=CH, C₆H₅, CH₃O, C₂H₅O, n-C₃H₇O) are investigated. The PMR resonance signals are interpreted, taking into account magnetic anisotropy of substituents and d-p interaction effects. The present communication gives results of proton magnetic resonance (PMR) studies of organyl- and organoxysilatranes (I). The table shows the compounds studied and the chemical shifts of the magneto-equivalent groups of protons in their spectra (see figure).For part IV see [1].