Pseudoceratins A and B, antifungal bicyclic bromotyrosine-derived metabolites from the marine sponge Pseudoceratina purpurea.

Pseudoceratins A (1) and B (2) have been isolated from the marine sponge Pseudoceratina purpurea. Pseudoceratins are unique among the large class of bromotyrosine-derived sponge metabolites with respect to the substitution pattern of aromatic rings and the presence of spiroacetal and oxime ether functional groups as well as an 1,5-diamino-1,5-dideoxy-D-arabitol tether. Pseudoceratins A (1) and B (2) are epimeric differing in the orientation of the tether. Their structures were determined by analysis of spectral data. The rigidity of the compounds arising from their bridged structure gave ROESY/NOESY spectra with many transannular cross-peaks, which allowed the assignment of the relative stereochemistry of 1 and 2 to be established. The d-configuration of the 1,5-diamino-1,5-dideoxyarabitol unit was determined by converting the fragment isolated from the acid hydrolysate to the Mosher's ester and compared the 1H NMR spectrum with those of standard samples. Pseudoceratins exhibited significant antifungal activity against Candida albicans.