Unfortunately the scalar couplings observed in the HMBC experiment have similar values for 2JCH and 3JCH correlations. Thus they can not be distinguished. Currently proposed methods [1,2] to circumvent this problem rely on a Proton bound to the observed Carbon nucleus. Different from scalar couplings dipolar couplings (as in [3,4]) depend on up two geometric properties of the observed molecule: The distance between the correlated nuclei and the orientation of the vector between them relative to the dipolar tensor. Hence they could be used for the identification of 2JCH correlations. Sadly the number of independent dipolar couplings observable in natural products turned out to be to small for reliable interpretation.
For the identification of 2JCH correlations we have developed a selective experiment that is based on heteronuclear NOE. The experimental setup depends on the accessability of the Proton signals and the signals observed in the HMBC experiment. The interpretation is carried out by overlaying the resulting heteronuclear spectra with the HMBC experiment, as will be shown on the poster.
Additionally, the impact of this results on computer aided structure elucidation will be shown.
The following literature references are used for this poster: