Synthetic modifications of abietane diterpene acids to potent antimicrobial agents

Among abietane type semisynthetic diterpenoids, a series of quinopimaric and maleopimaric acid derivatives modified at the carboxyl and carbonyl groups, and in ring E were synthesised to obtain new compounds with antimicrobial potency against Mycobacterium tuberculosis H₃₇Rv and key ESKAPE pathogens. It was found that compound 8 exhibited low toxicity to human embryonic kidney cell line HEK-293 (> 32 μg/mL) and showed significant bacteriostatic activity against methicillin-resistant Staphylococcus aureus (MRSA) (MIC ≤ 0.25 µg/mL) and excellent antifungal activity against Cryptococcus neoformans var. grubii (MICs ≤0.25 µg/mL) being ≈4 and ≈30 fold more active than vancomycin and fluconazole. It also showed moderate activity against fungus Candida albicans (MIC ≤ 0.25 µg/mL). Compound 9 inhibited M. tuberculosis H₃₇Rv with MIC of 1.25 µg/mL. The docking studies suggest possible interactions of the leading compounds with the molecular targets.