Synthesis and Anticancer and Antiviral Activities of New 2‐Pyrazoline‐Substituted 4‐Thiazolidinones

2‐(4,5‐Dihydropyrazol‐1‐yl)‐thiazol‐4‐ones ( 2–5 ) have been synthesized starting from 3‐phenyl‐5‐aryl‐1‐thiocarbamoyl‐2‐pyrazolines via [2+3]‐cyclization with 2‐bromopropionic acid, maleic anhydride, N ‐arylmaleimides, and aroylacrylic acids. The in vitro anticancer activity of 2a , 3a , 4a , 5b , and 5c were tested by the National Cancer Institute. Compounds 4a , 5b , and 5c demonstrated selective inhibition of leukemia cell lines growth at a single concentration (10 −5 M). The screening of antiviral activity for a broad panel of viruses revealed that N ‐(4‐methoxyphenyl)‐2‐{2‐[5‐(4‐methoxyphenyl)‐3‐phenyl‐4,5‐dihydropyrazol‐1‐yl]‐4‐oxo‐4,5‐dihydrothiazol‐5‐yl}‐acetamide 4a was highly active against Tacaribe TRVL 11 573 virus strain (EC 50 = 0.71 μg/mL, selectivity index = 130).