Summary The 3-chlormethyl-5-trifluoromethyl-l, 3, 4-thiadiazolone 3 undergoes a ring transformation to 3-acylated 2, 3-dihydro-l, 3, 4-thiadiazoles 4 with many nucleophiles. Upon formal replacement of the chloromethyl group in the 3-position of 3 by an extended bromoalkyl chain (+ 9a-c), the reaction with nucleophiles yields 4-acylated 5, 6-dihydro-