Oxindole alkaloids, isolated from the bark of
Uncaria tomentosa [Willd. ex Schult.] Rubiaceae, are considered to be responsible for the biological activity of this herb. Five pentacyclic and two tetracyclic alkaloids were studied by solid-state NMR and theoretical GIAO DFT methods. The
13C and
15N CPMAS NMR spectra were recorded for mitraphylline, isomitraphylline, pteropodine (uncarine C), isopteropodine (uncarine E), speciophylline (uncarine D), rhynchophylline and isorhynchophylline. Theoretical GIAO DFT calculations of shielding constants provide arguments for identification of asymmetric centers and proper assignment of NMR spectra. These alkaloids are 7
R/7
S and 20
R/20
S stereoisomeric pairs. Based on the
13C CP MAS chemical shifts the 7
S alkaloids (
δ C3 70–71 ppm) can be easily and conveniently distinguished from 7
R (
δC3 74.5–74.9 ppm), also 20
R (
δC20 41.3–41.7 ppm) from the 20
S (
δC20 36.3–38.3 ppm). The
epiallo-type isomer (3
R, 20
S) of speciophylline is characterized by a larger
15N MAS chemical shift of N4 (64.6 ppm) than the
allo-type (3
S, 20
S) of isopteropodine (
δN4 53.3 ppm).
15N MAS chemical shifts of N1–H in pentacyclic alkaloids are within 131.9–140.4 ppm.
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