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Definitive assignment of the 13C NMR signals of the indole C‐3 and C‐3a carbons of the ajmaline, picraline and sarpagine alkaloids

Comparison of a 1H coupled natural abundance 13C NMR spectrum of 1‐oxo‐1,2,3,4‐tetrahydrocarbazole with a single‐frequency experiment in which the hydrogens three bonds away from C‐3 (indole numbering) were irradiated allowed the unambiguous distinction of the C‐3 and C‐3a signals. The results can be extrapolated for the assignment of the same signals in the ajmaline, picraline and sarpagine alkaloids, which also contain a carbonyl group as the substituent at the C‐2 position of the indole skeleton. The effect of this carbonyl group on the chemical shifts of the benzenoid carbons is also discussed.