Stereoselective synthesis of the lichen metabolite, (+) montagnetol and its congeners as antimicrobial agents

In view of structural diversity, (+ ) montagnetol, the major metabolite of the fruticose lichen, Roccella montagnei was synthesized along with three of its congeners by employing highly efficient protocols. (+) Montagnetol (2R, 3S; 11) and (-) montagnetol (2S, 3R; 5) were synthesized in 7 and 9 steps, respectively, from L-ascorbic acid. The two new congeners 3 (2R, 3R) and 6 (2S, 3S), which differ in configuration at C-2 and C-3 positions of the (+) montagnetol, were synthesized from (-) diethyl D-tartrate and (+) diethyl L-tartrate, respectively. The synthesized compounds were evaluated in vitro for antimicrobial activity against two Gram-positive (S. aureus and E. coli) and two Gram-negative (S. typhi and P. aeruginosa) bacteria and one fungal strain Candida albicans. Interestingly, the congener 3 showed promising anti-bacterial activity (MIC: 0.062mg/ml) against P. aeruginosa, whereas the congener 6 displayed potent anti-fungal activity (MIC: 0.062 mg/ml) against C. Albicans.