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 (EC50 = 0.71 μg/mL, selectivity index = 130).