Three novel approaches to the synthesis of carfentanil were investigated that did not involve the hydrolysis of the corresponding a- aminonitrile. Starting with commercially available 1-benzyl-4-piperidone, regioselective cleavage of an a,B-exoxysulfone, an aza (3.3) sigmatropic rearrangement and aminolysis of a 1-bromo-1,2-diene were studied. Of these, the most promising was the sigmatropic rearrangement.... Carfentanil, Aza sigmatropic rearrangement, Piperidine.