8-Oxa-1,4-dithiaspiro[4.5]decane-7,9-dione and 9-oxa-1,5-dithiaspiro[5.5]undecane-8,10-dione, cyclic anhydrides formed in situ from the respective dicarboxylic acids in the presence of acetic anhydride, possess an enhanced reactivity and display a broad substrate scope in the Castagnoli-Cushman reaction with imines.