A new β -carboline-type alkaloid from the flowers of Calotropis gigantea (L.) W.T.Aiton.

In our antioxidant screening of some Vietnamese plant extracts, the CHCl ₃ -soluble fraction from Calotropis gigantea (L.) W.T.Aiton flowers showed moderate DPPH free radical scavenging activity with an IC ₅₀ value of 55.8  μ g/mL. Thus, a further phytochemical study was carried out to obtain five alkaloids, including a new β -carboline-type alkaloid, caloside H ( 1 ). These known compounds were identified as 5-hydroxy-(2-methoxymethyl)pyridine ( 2 ), nicotinic acid ( 3 ), p -(acetylamino)phenol ( 4 ), and thymine ( 5 ). These structures were determined based on the NMR spectroscopic analysis. In antioxidant assay, caloside H at concentration of 100  μ M showed DPPH radical scavenging capacity with a percentage of inhibition of 40.2%. In addition, a plausible biosynthetic pathway for the formation of caloside H was proposed based on the Schiff base formation and Mannich-like reaction.