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A one-pot three-component sequential synthetic protocol produces structurally and biologically important phthalidyl fused spiro N,N′ -disubstituted thiohydantoins from readily available aromatic isothiocyanates, primary amines and ninhydrin. In this three-step synthesis while the initial two steps are catalyst-free, in the final step silica sulfuric acid (SSA) induces an oxidative rearrangement in [3.3.0]-bicyclic 1,2-diol adducts of ninhydrin and thioureas under solvent-free condition to generate the final products spiro-fused thiohydantoins. The adequate acidity of SSA in cooperation with moderate oxidizing property promotes a facile oxidative rearrangement in 1,2-diol intermediates to produce the spiro-fused thiohydantoins with diverse functionalities. Easy recyclability of SSA, good to excellent yield of the products, wider substrate scope, shorter reaction time, solvent-free two steps out of three and high atom economy make this method attractive and practicable. Image 1 • Synthesis of spiro-fused thiohydantoin through SSA induced oxidative rearrangement. • Easy recyclability of SSA, good to excellent yield of products, wider substrate scope. • Shorter reaction time and high atom economy. • Spiro-fusion of two biologically active heterocycles, thiohydantoin and phthalide. [ABSTRACT FROM AUTHOR]