Structure and total synthesis of fungal calpinactam, a new antimycobacterial agent.

A new fungal metabolite designated calpinactam (1) was isolated from the culture broth of Mortierella alpina FKI-4905, and its structure was elucidated by spectroscopic analyses including NMR experiments. Calpinactam was found to be a hexapeptide with a caprolactam ring at its C-terminal. Its absolute stereochemistry was determined by amino acid analysis and total synthesis. Calpinactam selectively inhibited the growth of mycobacteria among various microorganisms. The MIC values of calpinactam against Mycobacterium smegmatis and M. tuberculosis were 0.78 and 12.5 microg/mL, respectively.