Bulbimidazoles A–C, Antimicrobial and Cytotoxic Alkanoyl Imidazoles from a Marine Gammaproteobacterium Microbulbifer Species

Three new alkanoyl imidazoles, designated bulbimidazoles A–C (1–3), were found from the culture extract of the gammaproteobacterium Microbulbifer sp. DC3-6 isolated from a stony coral of the genus Tubastraea. The absolute configuration of the anteiso-methyl substitution in 1 was established to be a mixture of (R)- and (S)-configurations in a ratio of 9:91 by applying the Ohrui–Akasaka method. Compounds 1–3 displayed unique broad-spectrum antimicrobial activity against Gram-positive and -negative bacteria and fungi with MICs ranging from 0.78 to 12.5 μg/mL. They also exhibited cytotoxicity toward P388 murine leukemia cells with IC50 in the micromolar range.