Derivatives of (–)-Cytisine with Thiourea Fragment. Synthesis and Antiviral Activity

New derivatives of the quinolizidine alkaloid, (–)-cytisine, with a substituted 2-pyridone ring and a thiourea moiety in the bispidin fragment of the molecule were synthesized. The ability of the synthesized cytisine-containing thioureas to inhibit the reproduction of human parainfluenza virus type 3 was assessed. It was found that the derivatives obtained by the reaction of benzoyl or phenyl isothiocyanate with (–)-cytisine, as well as its 9-bromo or 9,11-derivative, effectively suppress the reproduction of human parainfluenza virus type 3 (their selectivity indices are 56, 58 and 95, respectively), which confirms the promise of the chosen approach to synthetic modifications of the alkaloid (–)-cytisine in order to obtain effective antiviral agents on its basis.