Analysis of relative configuration of acyclic compounds based on long-range carbon-proton coupling constants determined by two dimensional NMR

A new NMR methodology was devised for configurational assignments of acyclic structures in natural products or synthetic compounds. Conventional methods based on NOEs often suffer from the ambiguity in assigning the configurations of compounds with acyclic structures and conformational alternation. Unlike these NOE analyses, the new methodology is based on the dihedral angle dependency of long-range carbon-proton coupling constants (^2,3J^C.H). The combination of the coupling constants (^2,3J_C.H, ³J_H,H) which are categorized to large, medium, or small allows to determine the relative stereochemistry among adjacent asymmetric centers or those separated by a methylene group. Hetero half-filter TOCSY (HETLOC) was shown to be the most effective for measuring ^2,3J_C,H of protonated carbon system. For structures with quaternary carbons or weakly coupling protons, in which TOCSY in HETLOC fails to correlate relevant protons, phase-sensitive HMBC (HMQC optimized for long-range coupling) worked effectively. To evaluate the utility of the method in structural analysis of natural products, the relative configurations of acyclic portions of maitotoxin, the largest secondary metabolite known to date, were assigned by this method, which evidently demonstrated the validity of ^2,3J_C,H in the configurational and conformational analysis of organic compounds.