Isosteroidal alkaloids from the bulbs of Fritillaria tortifolia

Three new isosteroidal alkaloids, frititorines A-C (1–3), were isolated from the bulbs of Fritillaria tortifolia, together with ten known ones (4–13). Their structures were elucidated by extensive spectroscopic analyses, chemical methods, and single-crystal X-ray crystallographic analysis. Compound 1 is the first 5β-cevanine alkaloid with a cis A/B ring junction from the Fritillaria genus. Compound 2 is the first example of glycosylated isosteroidal alkaloid N-oxide. Compound 1 showed significant relaxant effect on Ach-induced tracheal contraction with pA2 and EC50 values equivalent to those of aminophylline.