Cascade reactions of ortho-carbonyl-substituted benzonitriles with ((chloromethyl)sulfonyl)benzenes as pronucleophiles led to new isoindolin-1-ones with a tetrasubstituted C-3 position or to (Z)-3-(sulfonyl-methylene)isoindolin-1-ones. The reactions start from readily available materials, are carried out under mild conditions, and do not require metal catalysis. Promoted only by the cheap and environmentally benign K2CO3 as the base, up to six elemental steps can be combined in a single pot. Hence, a sequential one-pot cascade/β-elimination/alkylation furnished useful intermediates for the synthesis of aristolactam natural products. The observed selectivity and the mechanism were investigated by DFT studies.