Purpuramine R, a New Bromotyrosine Isolated from Pseudoceratina cf. verrucosa Collected in the Kingdom of Tonga

Sponges in the verongiid genus Pseudoceratina Carter are well-known producers of bioactive secondary metabolites. Chemical screening of a Tongan P. cf. verrucosa Bergquist using NMR highlighted the presence of aromatic natural products. Subsequent extraction and purification of P. cf. verrucosa yielded a new bromotyrosine, purpuramine R (1), that exhibits moderate (MIC 16 µg/mL) antibacterial activity against Gram-positive Staphylococcus aureus. The E-geometry of the oxime was confirmed using a combination of NMR and computational approaches. Additionally, computational conformational analysis indicates that purpuramine R adopts a hairpin orientation, stabilized by intramolecular hydrogen and halogen bonds. Knowledge of this stabilized conformation can inform synthetic approaches to make analogues of the purpuramines for future SAR studies.