Additional Sarasinosides from the Marine Sponge Melophlus sarasinorum Collected from the Bismarck Sea

In our continuing efforts to describe the biological and chemical diversity of sponges from Kimbe Bay, Papua New Guinea, the known 30-norlanostane saponin sarasinoside C-1 (1) was identified along with six new analogues named sarasinosides C-4, C-5, C-6, C-7, C-8, and C-9 (2-7) from the sponge Melophlus sarasinorum. The structures of the new compounds were elucidated by analysis of 1D and 2D NMR and HRMS data, as well as comparison with literature data. All new compounds are characterized by the same tetraose moiety, beta-d-Xylp-(1 -> 6)-beta-d-GlcNAcp-(1 -> 2)-[beta-d-GalNAcp-(1 -> 4)]-beta-d-Xylp, as described previously for sarasinoside C-1, but differed in their aglycone moieties. When comparing NMR data of sarasinoside C-8 with those of known analogues, a misassignment was identified in the configuration of the C-8/C-9 diol for the previously described sarasinoside R (8), and it has been corrected here using a combination of ROESY analysis and molecular modeling.