Pyrroles from ketoximes and acetylene - I. Structural specificity of the reaction with unsymmetrical ketoximes. PMR spectra of 1-vinylpyrroles and the effects of alkyl substituents

The reaction between ketoximes CH₃(RCH₂)C = NOH and acetylene in the presence of KOH and dimethyl sulfoxide at 120°C leads exclusively to 1-vinyl-2-methyl-3-R-pyrroles in 73-87% yields. The regiospecificity of the reaction is disrupted when the temperature is raised, and the fraction of a second isomer (1-vinyl-2-RCH₂-pyrrole) reaches 20-50% at 140°C. Regioselectivity is not observed for R¹CH₂(R²CH₂)C = NOH (R¹ and R² = n-alkyl). The relative shifts of the signals of the ring protons and the vinyl group for a number of 2-alkyl-1-vinyl- and 2,3-dialkyl-1-vinylpyrroles were measured. Alkyl substituents have a distinct effect on the chemical shifts of the protons of the 4-5 bonds. As the volume of the 2-alkyl substituent increases the β protons of the N-vinyl group are deshielded by 0.10-0.13 ppm, and the 4-H ring proton is shielded by 0.05-0.16 ppm; this is explained by steric inhibition of the p-π conjugation in the N-vinyl group during an s-trans(anti)-gauche conformational transition.