Glycosides from the sea cucumber Cucumaria frondosa. III. Structure of frondosides A₂-1, A₂-2, A₂-3, and A₂-6, four new minor monosulfated triterpene glycosides

Frondosides A 2 -1 (1), A 2 -2 (2), A 2 -3 (3), and A 2 -6 (4) are new monosulfated holostane pentaosides isolated from the sea cucumber Cucumaria frondosa. Their structures have been elucidated on the basis of spectral data (2D NMR and MS). Frondoside A 2 -2 (2) contains an α,β-unsaturated ketone fragment in the side chain of the aglycon, unique for triterpene glycosides of sea cucumbers.