Patent US3141823 (http://l2.espacenet.com/dips/viewer?PN=US3141823&CY=gb&LG=en&DB=EPD)
Patent US3164600 (http://l2.espacenet.com/dips/viewer?PN=US3164600&CY=gb&LG=en&DB=EPD)
Also, FR 2 430 M
Something like this:
1-benzyl-4-piperidone + aniline --LAH--> 4-anilino-1-benzylpiperidine
4-anilino-1-benzylpiperidine + propionic anhydride --H2/Pd-C--> N-(4-piperidyl)-propioanilide
N-(4-piperidyl)-propioanilide + 2-phenylethylchloride --Na2CO3--> Fentanyl