Solid phase approaches to N-heterocycles using a sulfur linker cleaved by SmI2

A sulfur HASC (alpha-hetero-atom substituted carbonyl) linker has been utilized in solid-phase approaches to oxindoles and tetrahydroquinolones. The route to oxindoles employs the first Pummerer cyclizations on solid phase, whereas the route to tetrahydroquinolones involves a microwave-assisted Heck reaction followed by a Michael cyclization. In both cases, the linker is cleaved in a traceless fashion by electron transfer from samarium(II) iodide. The routes illustrate the compatibility of the linker system with a number of reaction types and its utility for library synthesis.     

Nitrogen NMR studies on some fused ring N-heterocycles

¹⁵N and¹⁴N NMR data are reported for a number of fused ring N-heterocycles.¹⁴N line widths,¹⁵N-¹H couplings and nitrogen shielding additivity rales are used for¹⁵N signal assignments. The results obtained lead to some progress in the generalization of the effects of nitrogen-nitrogen interactions on nitrogen shieldings.     

Consequences of the dynamical approach to the nuclear spin relaxation induced by molecular rotations in an external magnetic field

The evolution of two nuclear spins, interacting between themselves and with an external magnetic field, and situated on molecules of a fluid, is treated, under the assumption of the mixing property of molecular rotations, by the method of limit theorems for random differential equations. The limit diffusion behaviour of the system is derived and the results on relaxation times and (non-thermodynamical) equilibrium are obtained and discussed.     

A quantitative N NMR investigation of tautomeric equilibria of two 1-hydroxybenzimidazoles

¹⁵N nuclear shielding data are used in a study of the tautomeric equilibria of two 1-hydroxybenzimidazoles. The nitrogen signal assignments are supported by INDO-S/SOS shielding calculations. The observation of the N-3 signal is shown to give a quantitatively reliable estimate of the position of equilibrium. In the unsubstituted 1-hydroxybenzimidazole the amount of the hydroxy from present in the equilibrium mixture is found to be proportional to the pK_a value of the solvent employed.     

Solvent-Induced Deuterium Isotope Effects in 13C- and 15N-NMR. Spectra of Enaminones

The effect of deuterium on the ¹³C and ¹⁵N chemical shifts of enaminones has been investigated. D/H isotope shifts are reported for neutral and protonated species, i.e., when the isotope is exchanged on the C(2)-, N-, or O-atoms. In cases of slow exchange the isotope shifts were obtained from solutions containing both isotopomers, whereas for fast exchange (acidic solutions) either separate NMR. sample tubes (¹⁵N-NMR.) or coaxial tubes (¹³C-NMR.) were used. In neutral molecules the isotope effects δ_C(D, H) are intrinsic in nature. In acidic solutions, the enaminocarbonyl cations formed exhibit δ_C(D, H)- and δ_N(D, H)-values which are discussed in terms of the proton transfer. The mesomeric character of the cations is reflected by characteristic features in the δ_C(D, H)- and δ_N(D, H)-values, which can be ascribed to isotopic perturbation of resonance. O-Protonation shifts in the ¹⁵N-resonance, observed for the first time, are large and positive (+60 to +76 ppm), in contrast to amides, where the effects are of the same sign but an order of magnitude smaller. Both protonation shifts and solvent-induced isotope effects are discussed in connection with the nucleophilic character of the reactive centers in the enaminone synthon.     

A H and C NMR study of the prototropic equilibria of 2-indolinone

¹³C NMR data are reported for 0.5 M solutions in CDCl₃ of 2-indolinone and six related compounds. The question of prototropic equilibria is discussed involving three possible tautomers, A, B, C. Form A is found to predominate in the dynamic equilibria established for three of the compounds studied. In the case of a fourth compound slow exchange between forms B and C results in the finding of both of these forms in the approximate ratio 1:1.8. These findings are supported by ¹H NMR measurements. Corresponding ¹H and ¹³H C NMR measurements on methanol solutions are essentially the same.     

Reductive alkylation of thioureas: a highly practical synthesis of unsymmetrical N,N′-disubstituted thioureas

A highly practical synthesis of unsymmetrical N,N′-disubstituted thioureas by the reductive alkylation of N-monosubstituted thioureas with aldehydes is described. N-Monosubstituted thioureas can in turn be synthesized by the reductive amination of thiourea with an appropriate aldehyde. This reductive alkylation methodology was also extended to carbamates.     

14N and 13C NMR of tautomeric systems of hydroxy-pyridines

¹⁴N NMR is shown to be an efficient method of investigation of the tautomeric equilibria in hydroxy-and related substituted pyridines. Large relative ¹⁴N chemical shifts within tautomeric pairs estimated from N-Me and O-Me derivatives make possible quantitative determinations of the equilibria. Qualitative conclusions may be drawn from ¹⁴N line widths as well as from ¹³C and proton chemical shifts.