Synthesis, Antifungal, and Antioxidant Evaluation of New Class of Thiazolo[3,2-a]pyrimidine and Pyrimido[5,4-d]thiazolo[3,2-a]pyrimidine Derived from α,α-Ketene Dithioacetals as Five and Six-membered Heterocycles Analogues

Abstract: A novel series of heterocyclic compounds containing thiazolo[3,2-a]pyrimidine and pyrimido[5,4-d]thiazolo[3,2-a]pyrimidine analogues were designed and synthesized starting from α,α-ketene dithioacetals (I), and their structures were confirmed by elemental analysis, IR, ¹H NMR, and ¹³C NMR. In our study, we also present optimization of the synthetic method along with a biological evaluation of these newly synthesized compounds as antioxidants and antifungal. Preliminary bioassay showed that most of them exhibited certain-to-excellent antioxidant and antifungal activity. Among all the evaluated compounds, it was found that some showed significant antioxidant activity at 10 µg/mL while the others exhibited better antifungalal activity at 100 µg/mL. Thiazolo[3,2-a]pyrimidines (IVa–IVb) and (IVc–IVd) showed higher than 80% inhibition activity against C. arachidicola and P. infestans respectively whereas the thiazolo[3,2-a]pyrimidines (VIc–VId) showed maximum activity against B. cinerea and C. arachidicola, as for the tested pyrimido[5,4-d]thiazolo[3,2-a]pyrimidines (IXc–IXd) against G. zeae, P. piricola, and P. sasakii. In particular, thiazolo[3,2-a]pyrimidines (IVc–IVd) exhibited outstanding activity (> 90%) against G. zeae. Among all the evaluated compounds, it was found that some showed significant antioxidant activity at 10 µg/mL and the maximum DPPH radical scavenging activity was observed with synthetic thiazolo[3,2-a]pyrimidines (VIIc–VIId) and pyrimido[5,4-d]thiazolo[3,2-a]pyrimidine (IXc–IXd), especially pyrimido[5,4-d]thiazolo[3,2-a]pyrimidine (IXc) which possessed the highest antioxidant activity in comparable to that of standard vitamin C at similar concentration.