1,2-Cycloadditions of 3-methyl-1-(methylthio)-1-butenyl isocyanate with N-t-butyl-α-toluenimine N-oxide and with in situ-generated N-t-butyl-α-methyl-α-lactam afforded an oxadiazolidinone derivative and a hydantoin derivative in 100 and 46% yields, respectively.