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The Chemistry of Kratom [Mitragyna speciosa]: Updated Characterization Data and Methods to Elucidate Indole and Oxindole Alkaloids
journal contribution
posted on 2020-06-29, 13:04 authored by Laura Flores-Bocanegra, Huzefa A. Raja, Tyler N. Graf, Mario Augustinović, E. Diane Wallace, Shabnam Hematian, Joshua J. Kellogg, Daniel A. Todd, Nadja B. Cech, Nicholas H. OberliesTwo separate commercial products
of kratom [Mitragyna speciosa (Korth.) Havil. Rubiaceae]
were used to generate reference standards
of its indole and oxindole alkaloids. While kratom has been studied
for over a century, the characterization data in the literature for
many of the alkaloids are either incomplete or inconsistent with modern
standards. As such, full 1H and 13C NMR spectra,
along with HRESIMS and ECD data, are reported for alkaloids 1–19. Of these, four new alkaloids (7, 11, 17, and 18)
were characterized using 2D NMR data, and the absolute configurations
of 7, 17, and 18 were established
by comparison of experimental and calculated ECD spectra. The absolute
configuration for the N(4)-oxide (11) was established by comparison of NMR and ECD spectra of its reduced
product with those for compound 7. In total, 19 alkaloids
were characterized, including the indole alkaloid mitragynine (1) and its diastereoisomers speciociliatine (2), speciogynine (3), and mitraciliatine (4); the indole alkaloid paynantheine (5) and its diastereoisomers
isopaynantheine (6) and epiallo-isopaynantheine (7); the N(4)-oxides mitragynine-N(4)-oxide (8), speciociliatine-N(4)-oxide
(9), isopaynantheine-N(4)-oxide (10), and epiallo-isopaynantheine-N(4)-oxide
(11); the 9-hydroxylated oxindole alkaloids speciofoline
(12), isorotundifoleine (13), and isospeciofoleine
(14); and the 9-unsubstituted oxindoles corynoxine A
(15), corynoxine B (16), 3-epirhynchophylline
(17), 3-epicorynoxine B (18), and corynoxeine
(19). With the ability to analyze the spectroscopic data
of all of these compounds concomitantly, a decision tree was developed
to differentiate these kratom alkaloids based on a few key chemical
shifts in the 1H and/or 13C NMR spectra.