Antibacterial activity of two canthin-6-one alkaloids from Allium neapolitanum.

The emergence of multidrug-resistant strains of many human pathogens has led to an urgent need for the discovery and development of new antimicrobial agents. As part of an ongoing investigation into the antibacterial properties of the Alliaceae, the isolation of 1 (canthin-6-one), 2 (8-hydroxy-canthin-6-one) and 3 (5(zeta)-hydroxy-octadeca-6(E)-8(Z)-dienioc acid)) from A. neapolitanum, a perennial bulbous herb found in open pastures of the Mediterranean is reported. Compounds 1 and 2 were isolated by Sephadex LH-20 from fractions exhibiting a positive reaction with Dragendorff's reagent on TLC, compound 3 was isolated after HPLC purification of Sephadex fractions. Structures were elucidated by extensive 1D and 2D NMR experiments and are in accordance with published data, however, the 13C NMR data for compound 2 and the 1H and 13C NMR data for compound 3 are reported here for the first time. Canthin-6-one alkaloids are well-known constituents of the Simaroubaceae and Rutaceae, and display a wide range of biological activities. These metabolites are reported as constituents of the Alliaceae here for the first time, and displayed minimum inhibitory concentrations (MICs) in the range 8-32 microg/mL against a panel of fast-growing Mycobacterium species and 8-64 microg/mL against multidrug-resistant (MDR) and methicillin-resistant (MRSA) strains of Staphylococcus aureus. Compound 3 displayed antimycobacterial activity in the range of 16-32 microg/mL.