13C, 15N CPMAS NMR and GIAO DFT calculations of stereoisomeric oxindole alkaloids from Cat's Claw (Uncaria tomentosa)

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 C-13 and N-15 CPMAS NMIR spectra were recorded for mitraphylline, isomitraphylline, pteropodine (uncarine C), isopteropodine (uncarine E), speciophylline (uncarine D), rhynchophylline and isorhynchophyl line. Theoretical GIAO DFI-calculations of shielding constants provide arguments for identification of asymmetric centers and proper assignment of NMIR spectra. These alkaloids are 7R/7S and 20R/20S stereoisomeric pairs. Based on the C-13 CP MAS chemical shifts the 7S alkaloids (delta C3 70-71 ppm) can be easily and conveniently distinguished from 7R (delta C3 74.5-74.9 ppm), also 20R (delta C20 41.3-41.7 ppm) from the 20S (delta C20 36.3-38.3 ppm). The epiallo-type isomer (3R, 20S) of speciophylline is characterized by a larger N-15 MAS chemical shift of N4 (64.6 ppm) than the allo-type (3S, 20S) of isopteropodine (delta N4 53.3 ppm). N-15 MAS chemical shifts of N1-H in pentacyclic alkaloids are within 131.9-140.4 ppm. (c) 2008 Elsevier Inc. All rights reserved.