Antimycobacterial Activities of Dehydrocostus Lactone and Its Oxidation Products

In an attempt to study the structural dependence of antimycobacterial activity of the guaianolide dehydrocostus lactone and its derivatives, m-chloroperoxybenzoic acid oxidations of dehydrocostus lactone (1a) were performed. Three new monoepoxides, one previously synthesized diepoxide, and two new diepoxides were obtained. Two of the monoepoxides are C-10 epimers (3a, 3b), while the 4(15)-monoepoxide (2) has the 4alpha-O-configuration. The known diepoxide (4a) contains a C-10 alpha-epoxide and a beta-epoxide at C-4. The diepoxides 4b and 4c, each with a C-4 alpha-epoxy group, differ in the configuration of the epoxide ring at C-10. Allylic oxidation of dehydrocostus lactone (1a) with selenium dioxide/tert-butyl hydroperoxide afforded the known 3-epizaluzanin C (1b). The relative configurations of compounds 1b-4c were established by 1D and 2D NMR techniques (1H, 13C, COSY, NOESY, HMQC, and HMBC) as well as comparison with literature data. The molecular structures of lactones 1b, 4a, and 4c were determined by single-crystal X-ray diffraction. In radiorespirometric bioassays against Mycobacterium tuberculosis and Mycobacterium avium, dehydrocostus lactone (1a) exhibited minimum inhibitory concentrations of 2 and 16 microgram/mL, respectively. In contrast, its monoepoxides (2, 3a, and 3b) and diepoxides (4a-c), as well as its hydrogenated derivatives and other analogues (1b, 1c, 5, and 6), showed significantly lower activities against M. tuberculosis.