Antimicrobial Indolequinones from the Mid-Intestinal Gland of the Muricid Gastropod Drupella fragum

Abstract Three new indolequinones, 6-methoxyindole-4,7-quinone (1), 5-methoxyindole-4,7-quinone (2) and 5-methylindole-4,7-quinone (3) were isolated from the mid-intestinal gland of the muricid gastropod Drupella fragum. The structures of 1 and 2 were established by spectroscopic methods and total synthesis, whereas the structure of 3 was elucidated mainly by NMR spectroscopic analyses. Compounds 1 ∼ 3 exhibited moderate antimicrobial activities against Staphylococcus aureus, Bacillus subtilis, and Escherichia coli with MIC = 6.25 ∼ 50 μg/mL.