A New Structural Theme in the Imidazole-Containing Alkaloids from a Calcareous Leucetta Sponge

Further study of the Fijian sponge Leucetta sp., a source of (+)-calcaridine A (4) and (−)-spirocalcaridines A (5) and B (6), has yielded (−)-spiroleucettadine (8), the first natural product to contain a fused 2-aminoimidazole oxalane ring along with the known compounds N,N-dimethylnaamidine D (3) and isonaamine B (7). NMR analysis allowed the unambiguous 2D structural assignment of 8, and its relative stereochemistry was determined by ROESY data. Good antibacterial activity was observed for 8 against Enterococcus durans with an MIC of less than 6.25 μg/mL.