Steric effects of the fused ring in the¹H and¹³C NMR spectra of 2,3-fused 1-vinylpyrroles

Increase in the size of a saturated ring fused at the 2,3-positions of l-vinylpyrrole from sixmembered to seven-membered is accompanied by enhancement of the steric effect on the vinyl group and increase of the torsion angle between the vinyl group and pyrrole ring. On further increase in the size of the fused ring from eight-membered to twelve-membered, the steric effect on the vinyl group somewhat weakens. Two stereoisomeric 4-methyl-6,7-tetramethylene-l-vinylpyrrolo[3,2-c]piperidines differing in the mode of ring junction in the tetramethylenepiperidine fragment are characterized by different steric effects on the vinyl group and hence different torsion angles. 2-[l-(4,5,6,7-Tetrahydro-l-indolyl)ethyl]-l-vinyl4,5,6,7-tetrahydroindole and structurally related compounds were found to involve a weak intramolecular hydrogen bond between the α-hydrogen atom of the vinyl group and π system of the pyrrole ring attached through the methine bridge.