Quinones and related compounds in higher plants. XXI. New findings on the proton and carbon-13 nuclear magnetic resonance spectra of shikonin.

Abstract

Inspection of the proton and carbon-13 nuclear magnetic resonance (1H- and 13C-NMR) spectra of shikonin (1), cycloshikonin (9) and their derivatives led to the conclusion that, in solution, they exist neither as a semihydroquinone structure nor as tautomeric structures as reported recently, but as the commonly accepted 2-substituted 5, 8-dihydroxy-1, 4-naphthoquinone structure. Further, these compounds were shown to assume a comformation which allows the C-3 hydrogen and the O-functional group of the side chain to be in close proximity.