The use of potassium ethyl xanthate (EtOCS2k) as a sulfur atom donor enabled the transition-metal-free [3 + 1 + 1] cascade annulation of isopropene derivatives with NH4I in DMSO/H2O, affording various 4-substituted isothiazoles in moderate to good yields with good functional group compatibility. Furthermore, Pd and Ag-catalyzed C5–H-selective direct oxidation dimerization of 4-substituted isothiazoles afforded 5,5′-bisisothiazoles. This series of reactions included the formation of new C–S, C–N, N–S, and C–C bonds via cascade addition, annulation, and direct C–H activation under mild conditions.