Chemical shifts, aromatic carbon, indole

The proton-decoupled 13C NMR spectrum of pergolide mesylate indicates the presence of twenty carbons, which are further identified as four non-protonated carbons, six methine carbons, and ten methyl and methylene carbons via a Distortionless Enhancement by Polarization Transfer (DEPT) spectrum. The eight aromatic and olefinic carbon resonances in the 134 ppm to 106 ppm region have chemical shifts consistent with an indole structure (2). The H NMR spectrum contains four aromatic proton resonances, from 6.88 to 7.22 ppm, whose chemical shifts are also indicative of an indole-type structure. The assignment of these carbon and proton resonances were confirmed with IH correlation spectroscopy (CDSY) and 3C-1H heteronuclear correlation spectroscopy (HETCORR). The UV spectrum also indicates the presence of an indole chromophore. 

Scheme 3. Chemical shifts of the aromatic carbons in the most common chromophoric groups in indole alkaloids [d = doublet (CH), s = singlet (C)]...

Numerous papers have been published with " C-NMR data on alkaloids, as well as several reviews and a book with data on a number of alkaloids (Shamma and Hindenlang 1979). Based on these spectral data, it is possible to draw general conclusions about the chemical shifts of certain structural features, which might be helpful in structure elucidation. For example, the type of indole moiety of indole alkaloids can easily be determined on the basis of the shifts of the aromatic carbons, also the substituents and the substituent pattern can be deduced from these data (Verpoorte et al. 1984 Scheme 3). 

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