New Rufomycins from <i>Streptomyces atratus</i> MJM3502 Expand Anti-<i>Mycobacterium tuberculosis</i> Structure-Activity Relationships.

Four new rufomycins, compounds <b>1</b>-<b>4</b>, named rufomycins 56, 57, 58, and 61, respectively, exhibiting new skeletal features, were obtained from <i>Streptomyces atratus</i> strain MJM3502 and were fully characterized. Compounds <b>1</b> and <b>2</b> possess a 4-imidazolidinone ring not previously encountered in this family of cyclopeptides, thereby resulting in a [5,17] bicyclic framework. The in vitro anti-<i>Mycobacterium tuberculosis</i> potency of compounds <b>3</b> and <b>4</b> is remarkable, with minimum inhibitory concentration values of 8.5 and 130 nM, respectively.