On the configuration of five-membered rings: a spin-spin coupling constant approach

Five-membered rings are clearly among the most common structural motifs found in chemistry and biology. Nevertheless, the configuration of conformationally mobile five-membered rings is often difficult to assign from nuclear magnetic resonance (NMR) data. A simple, reliable, and efficient approach for the stereochemical analysis of five-membered rings based on the measurement of NMR coupling constants is presented. Density functional theory calculations using representative conformations of the full conformational space available to rings with different substitution patterns were used to identify differences between the accessible coupling constant values for cis and trans relative orientations of the substituents. The calculations were assessed experimentally using NMR data obtained from a number of models. This approach can be easily used to analyze different five-membered rings, such as oxolanes, cyclopentanes, furanosides and pyrrolidines, and their relative configuration can be determined without the need for making further conformational considerations.