N-methoxy-β-carboline alkaloids with inhibitory activities against Aβ42 aggregation and acetylcholinesterase from the stems of Picrasma quassioides.

Nine new N-methoxy-β-carboline alkaloids (NMCAs) (1a/1b-3a/3b and 4-6) and two known NMCAs (7 and 8) were isolated from the stems of Picrasma quassioides. Their structures were elucidated by spectroscopic data analyses, quantum chemical calculations, and single-crystal X-ray crystallographic data. An analysis of the 13C NMR chemical shifts of the N-methoxy groups in these NMCAs and 41 gathered known compounds reveals the phenomenon that the chemical shifts of all these N-methoxy groups are greater than δC 62, which can be used to recognize the N-methoxy group rapidly. In addition, the acetylcholinesterase (AChE) and Aβ42 aggregation inhibitory activities of 1-8 were evaluated. Compounds 1, 2, 7, and 8 displayed AChE inhibitory activity with IC50 values of 14.9, 13.2, 17.6, and 43.9 μM, respectively. Compound 2 showed inhibition activity against Aβ42 aggregation with an IC50 value of 10.1 μM.