Glycosides from the sea cucumber Cucumaria frondosa. III. Structure of frondosides A(2)-1, A(2)-2, A(2)-3, and A(2)-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 alpha,beta-unsaturated ketone fragment in the side chain of the aglycon, unique for triterpene glycosides of sea cucumbers.