NMR-guided isolation of alkaloids from Nigella glandulifera seeds and their bioactivities

NMR-guided investigation of Nigella glandulifera Freyn et Sint seeds led to the isolation and characterization of 11 previously undescribed alkaloids, designated nigelladine D-N (1-11). These include three skeletons: (+)/(-)-nigelladine D (1a/1b), and nigelladine E-F (2-3); three piperidines: nigelladine G (4), (+)/(-)-nigelladine H (5a/5b), and nigelladine I (6); three indole alkaloids: (+)/(-)-nigelladine J (7a/7b) and nigelladine K-L (8-9); as well as two pyrrole alkaloids: nigelladine M-N (10-11). In addition, three known 1,4-diazaindane-type pyrrole alkaloids were obtained: agrocybenine (12), the enantiomeric pairs (±)-flavascensine (13a/13b), and (±)-howlumine (14a/14b). Their structures were elucidated using 1D and 2D NMR (Nuclear Magnetic Resonance) spectroscopic analyses, electronic circular dichroism (ECD), quantum chemical calculations, and single-crystal X-ray diffraction (Cu Kα) analysis. Nigelladine D (1) skeleton arises from a piperidine ring and a pyrrole ring fusion through a shared nitrogen atom. Nigelladine E (2) exhibits an unprecedented 6/6/6/6 dipiperidine-condensed cyclopentane framework, likely formed through dimerization of two piperidine derivatives, whereas nigelladine F (3) consists of two piperidine units linked by a pyrrole ring. Furthermore, all compounds exhibited weak inhibitory activity against phosphodiesterase 5 (PDE5) and menin-mixed lineage leukemia 1 (menin-MLL1) protein-protein interaction. Compound 2 significantly increased the melanin content, comparable to that of the positive control 8-methoxypsoralen (8-MOP) (134.50 ± 3.91 % and 134.80 ± 10.24 %, respectively).