Dynamic NMR Study and Detailed 1H and 13C Assignments of an ADAR Precursor in Solvent

A nitrogen-containing compound, named methyl-4-phenyl-2-(2,4,6-triisopropyl-phenylsulfonamido)but-3-enoate (MPSN), is used as a precursor for inverse-electron-demand aza-Diels–Alder reaction. Its 1H and 13C nuclear magnetic resonances (NMR) were fully assigned by a combination of one- and two-dimensional NMR experiments. It was found that this compound had two conformers (a and b) in the process of assignments. Exchange spectroscopy (EXSY) was used to study the exchange dynamics of this compound in solvent. On the basis of 1H NMR and EXSY experiments at various temperatures, thermodynamic and dynamic parameters of the conformational exchange process were deduced for this compound. The free-energy activation (ΔG≠) at 298 K is 15.07 ± 0.6 kcal mol−1, which suggests that the conformational exchange might be the result of C=N bond rotation.