Effective assignment of positional isomers in dimeric shikonin and its analogs by 1H NMR spectroscopy

An approach for distinguishing two types of positional isomers of dimeric shikonin and its analogs was explored with 4JC,H long-range correlation by prolonging the acquisition time at 2,3JC,H values of 2.0 and 8.0 Hz. Furthermore, the 1H (proton) nuclear magnetic resonance (NMR) pattern of phenolic hydroxyl protons was developed as a “diagnosis signal” to ascertain the relative location of each side chain in DMSO-d6 at sample concentrations of 0.022–0.034 mol/L. The chemical shift differences of 0.6 ppm between OH-5′ and OH-1 and between OH-8′ and OH-4 are assigned to Type A and Type B, respectively. All reported ambiguous structures were corrected by this pattern. Additionally, the steric structures of isolated compounds were elucidated by quantum chemical calculations of electronic circular dichroism (ECD) spectra.