The reaction of amidoximes 1 with 1,1′-thiocarbonyldiimidazole (TCDI) followed by treatment with silica gel or boron trifluoride diethyl etherate (BF3·OEt2) provided 3-substituted 4,5-dihydro-5-oxo-1,2,4-thiadiazoles 2 in moderate yields. The Lewis acids are considered to promote the rearrangement of the thioxocarbamate intermediates 5 to the thiol carbarn ate intermediates 7, which cyclize to afford