AbstractCycloaddition of sulfene toN,N‐disubstituted 4‐amino‐3‐methyl‐3‐buten‐2‐ones(III)occurred in fair to good yield only in the case of aliphaticN‐substitution to give 4‐dialkylamino‐3,4‐dihydro‐5,6‐dimethyl‐1,2‐oxathiin 2,2‐dioxides, whereasN,N‐disubstituted 1‐amino‐1‐penten‐3‐ones(II)did not react at all. Cycloaddition of dichloroketene toII, IIIandN,N‐disubstituted 4‐amino‐3‐buten‐2‐ones occurred only in the case of the methylphenylamino derivative, giving in good to moderate yield 6,(5)(di)alkyl‐3,3‐dichloro‐3,4‐dihydro‐4‐methylphenylamino‐2‐Hpyran‐2‐ones, which were dehydrochlori