Highly Conjugated Norditerpenoid and Pyrroloquinoline Alkaloids with Potent PTP1B Inhibitory Activity from Nigella glandulifera

Three norditerpenoid alkaloids, nigelladines A–C (1–3), and one pyrroloquinoline alkaloid, nigellaquinomine (4), all possessing new skeletons with highly conjugated systems, were isolated from Nigella glandulifera. The 8aS-configuration for 1 and 2 was determined by comparison of the experimental and calculated electronic circular dichroism spectra. These alkaloids exhibited potent protein tyrosine phosphatase 1B (PTP1B) inhibitory activity but are devoid of cytotoxicity against the A431 cell line at 100 μM.