**Synthesis, Tyrosinase Inhibiting Activity and Molecular Docking of Fluorinated Pyrazole Aldehydes as Phosphodiesterase Inhibitors**

**Vesna Rastija 1, Harshad Brahmbhatt 2, Maja Molnar 2, Melita Lonˇcari´c 2, Ivica Strelec 2, Mario Komar 2 and Valentina Pavi´c 3,\***


Received: 6 March 2019; Accepted: 19 April 2019; Published: 25 April 2019

**Abstract:** A series of fluorinated 4,5-dihydro-1 *<sup>H</sup>*-pyrazole derivatives were synthesized in the reaction of corresponding acetophenone and di fferent aldehydes followed by the second step synthesis of desired compounds from synthesized chalcone, hydrazine hydrate, and formic acid. Structures of all compounds were confirmed by both 1H and 13C NMR and mass spectrometry. Antibacterial properties of compounds were tested on four bacterial strains, *Escherichia coli*, *Pseudomonas aeruginosa*, *Bacillus subtilis,* and *Staphylococcus aureus.* Among synthesized compounds, the strongest inhibitor of monophenolase activity of mushroom tyrosinase (32.07 ± 3.39%) was found to be *5-(2-chlorophenyl)-3-(4-fluorophenyl)-4,5-dihydro-1H-pyrazole-1-carbaldehyde.* The PASS program has predicted the highest probable activity for the phosphodiesterase inhibition. To shed light on molecular interactions between the synthesized compounds and phosphodiesterase, all compounds were docked into the active binding site. The obtained results showed that the compound with the dimethoxyphenyl ring could be potent as an inhibitor of phosphodiesterase, which interacts in PDE5 catalytic domain of the enzyme. Key interactions are bidentate hydrogen bond (H-bond) with the side-chain of Gln817 and van der Waals interactions of the dimethoxyphenyl ring and pyrazole ring with hydrophobic clamp, which contains residuals, Val782, Phe820, and Tyr612. Interactions are similar to the binding mode of the inhibitor sildenafil, the first oral medicine for the treatment of male erectile dysfunction.

**Keywords:** fluorinated pyrazole aldehydes; tyrosinase inhibition; phosphodiesterase inhibition; antibacterial activity; molecular docking
