Antispasmodic saponins from bulbs of red onion, Allium cepa L. var. Tropea

A phytochemical analysis of the polar extract from the red bulbs of Allium cepa L. var. Tropea, typical of Calabria, a southern region of Italy, was performed extensively for the first time, leading to the isolation of four new furostanol saponins, named tropeoside A1/A2 (1a/1b) and tropeoside B1/B2 (3a/3b), along with the respective 22-O-methyl derivatives (2a/2b and 4a/4b), almost certainly extraction artifacts. High concentrations of ascalonicoside A1/A2 (5a/5b) and ascalonicoside B (6), previously isolated from Allium ascalonicum Hort., were also found. This is the first report of furostanol saponins in this A. cepa variety. The chemical structures of the new compounds were established through a combination of extensive nuclear magnetic resonance, mass spectrometry and chemical analyses. High concentrations of quercetin, quercetin 4(I)-glucoside, taxifolin, taxifolin 7-glucoside, and phenylalanine were also isolated. The new saponins were found to possess antispasmodic activity in the guinea pig isolated ileum; such an effect might contribute to explaining the traditional use of onion in the treatment of disturbances of the gastrointestinal tract.