Conformational equilibria and barriers to rotation in some novel nitroso derivatives of indolizines and 3- and 5-azaindolizines - An NMR and molecular modeling study

Conformational equilibria in novel C-nitroso derivatives of indolizines and 3- and 5-azaindolizines have been studied by NMR. ¹³C chemical shifts of the carbon alpha to the nitroso group confirmed that these compounds are present in solution as monomers. The conformers arising from restricted rotation about the C-NO bond in monomers were identified by the chemical shifts of the carbon beta to the nitroso group. Barriers to rotation in these compounds were unusually high, particularly for substituents in position 3 of indolizine. Ethyl 2-(methylamino)-1-nitrosoindolizine-3-carboxylate displayed conformers arising from the restricted rotation about the C-COOR bond. Molecular modelling demonstrated that in 1-nitrosoindolizines, the position of the conformational equilibrium is due to steric effects, while for 3-nitrosoindolizines electronic effects prevail.