Anti-Tuberculosis Constituents from the Stem Bark of Micromelum hirsutum

Anti-TB bioassay-directed fractionation led to the isolation of six carbazole alkaloids, as well as the γ-lactone derivative of oleic acid, from the CH₂Cl₂ extract of the stem bark of Micromelum hirsutum. The carbazoles include the new micromeline (2) and five known alkaloids: lansine (3), 3-methylcarbazole (4), methyl carbazole-3-carboxylate (5), 3-formylcarbazole (6), and 3-formyl-6-methoxycarbazole (7). Compound 1 was identified as the lactone derivative of oleic acid, (-)-Z-9-octadecene-4-olide, for which the trivial name micromolide (1) is suggested. It showed potent in vitro anti-TB activity against H37Rv (MIC: 1.5 μg/mL), a selectivity index (SI) of 63, and exhibited activity against the Erdman strain of M. tuberculosis in a J774 mouse macrophage model (EC₉₀: 5.6 μg/mL). Thus, 1 appears worthy of further evaluation as a potential new anti-TB agent. Isolates 2, 3, 6 and 7 had anti-TB MIC values between 14.3 and 42.3 μg/mL, while compounds 4 and 5 were considered inactive (MIC > 128 μg/mL). Structure elucidation and identification were based on spectroscopic analysis, including MS, 1D/2D NMR, and a full ¹H spin system analysis of 1.