Indole alkaloids from Tabernaemontana australis (Müell. Arg) Miers that inhibit acetylcholinesterase enzyme

In an enzymatic TLC survey, among the 10 alkaloids identified in Tabernaemontana australis, 1– 4 inhibited the acetylcholinesterase enzyme at the same detection limit concentration of the reference compounds physostigmine and galanthamine, showing that iboga alkaloids are promising anti-Alzheimer agents. Ten indole alkaloids from the chloroform extract of stalk of Tabernaemontana australis (Müell. Arg) Miers were tentatively identified by GC–MS, viz., coronaridine ( 1), voacangine ( 2), voacangine hydroxyindolenine ( 3), rupicoline ( 4), ibogamine ( 5), ibogaine ( 6), ibogaline ( 7), desethyl-voacangine ( 8), voachalotine ( 9), and affinisine ( 10). Of these, the first four were isolated by silica gel open column chromatography, identified by uni- and bidimensional NMR, IR, MS and showed anti-cholinesterasic activity at the same concentration as the reference compounds physostigmine and galanthamine (detection limit of 0.01 mM) by TLC assay using the modified Ellman’s method.