Fig. 1.

Chemical structures of key indole and oxindole kratom alkaloids. Structures of mitragynine and mitraphylline are numbered to serve as references for the indole and oxindole alkaloids, respectively. Stereochemical differences at C3 and C20 of the indole alkaloids are indicated by the colored boxes. These differences may explain the pharmacokinetic differences observed between those with the 3S configuration (mitragynine, speciogynine, paynantheine) and the 3R configuration (mitraciliatine, speciociliatine, isopaynantheine) in human adult participants.