Synthesis of Antimicrobial Norlabdane Compounds with Rearranged Cycle B and Molecular Docking Studies

The synthesis of tetra- and pentanorlabdane compounds with rearranged cycle B based on commercially available (+)-sclareolide is reported. Desired compounds were prepared from intermediate ketones via Baeyer-Villiger oxidation. The structures of synthesized compounds were confirmed by spectral IR, 1D (¹H, ¹³C, and DEPT), and 2D (H-COSY, H,C-HSQC, H,C-HMBC, H,N-HMBC, NOESY) NMR analyses, mass-spectrometry and single crystal X-rays diffraction. Two out of the four synthesized compounds showed high antifungal and antibacterial activities comparable to and exceeding standard antifungal (caspofungin) and antibacterial (kanamycin) agents. DFT calculations show that in gas and DCM, compound 4 is more stable than 3 with a difference in the Gibbs free energy of 23.3 kJ/mol and 20.7 kJ/mol, respectively. In water and methanol, compound 3 is slightly more stable, by 2.4 kJ/mol and 2.78 kJ/mol, respectively. Molecular docking to four targets DNA gyrase from E. coli (1KZN), Fabz from P. aeruginosa (1U1Z), dihydrofolate reductase from C. albicans (3QLS) and MurB from E. coli (2Q85) showed good agreement with the results of in vitro evaluation and confirmed the biological activity of compounds 3 and 4, with binding affinities comparable and for some targets exceeding that of Caspofungin and Kanamycin.