Asmarines A-F, novel cytotoxic compounds from the marine sponge Raspailia species

Three pairs of nitrogen-containing metabolites, asmarines A-F (1-6), were isolated from the Red Sea sponge Raspailia sp., collected in the Dahlak Archipelago, Eritrea. Although the first pair could fully be separated to give compounds 1 and 2, the other two pairs could only be enriched up to about 80% of one isomer. The structures of the new compounds were established by spectroscopic means. Besides the asmarines, methyl 3-oxo-cholan-24-oate (12) was also isolated. The absolute configuration of asmarine A (1) was determined on the basis of CD measurements of its unstable 18-oxo derivative (7) and mainly the cotton effect of the dicarbonyl derivative (9) of chelodane (8). A O,N(7')-dimethyl derivative (10) and a second, unexpected, methylated product (11) were obtained from 2.