Abstract 1 H and 13 C NMR spectroscopy showed that both isomers of 6-ethylindolizidine adopt predominantly the trans-fused conformation in CDCl 3 at 298 K. ΔG math image for the cis-⇌ tram-fused conformational equilibrium in indolizidine must accordingly be not less than 2.8 kcal mol− 1.